Mutations in Animals: Types, Causes, and Effects

Mutation is a change in the sequence of DNA, the molecule that carries the instructions for building and running an animal. A mutation can be as small as a single letter of the genetic code swapped for another, or as large as a whole chromosome gained or lost. Every animal alive today carries mutations, and every animal is the product of mutations that happened in its ancestors. Without mutations there would be no variation, and without variation there would be no evolution, no new species, and no difference between one animal and the next. A mutation is the raw material from which all of the diversity of the animal kingdom is made.

That does not mean mutations are always good. A mutation is a random change, and most random changes to a finely tuned machine do nothing, or do harm. A single changed letter can stop a protein from working, and a missing chromosome can prevent an embryo from developing at all. But a few mutations help, and those few are the ones that natural selection keeps. A mutation that darkens a moth's wings can hide it from birds. A mutation that changes one amino acid in haemoglobin can protect its carrier from malaria. A mutation that shortens a dog's legs can be kept by breeders who like the look. The same kind of event, a change in DNA, can be harmless, harmful, or helpful, and which one it is depends on the change itself, on where it happens, and on the world the animal lives in.

This article covers the whole subject at full depth: what a mutation is and what it is not, the difference between a mutation in a body cell and a mutation in a reproductive cell, the small changes that alter a single letter of code and the large changes that rearrange whole chromosomes, what causes mutations, how often they happen, why most of them do nothing, how a few of them harm and a few help, the famous animal examples from albino peacocks to peppered moths to sickle-cell haemoglobin, and the part mutations play in evolution. The science is explained in plain language, and the famous examples are given with the numbers and the history that make them worth knowing, because mutation is a subject where the detail is the point.

Mutations at a Glance

A mutation is a change in the DNA sequence of a cell. DNA is written in four letters, A, T, C and G, grouped into three-letter words called codons, and each codon stands for one amino acid, the building block of proteins. A gene is a stretch of DNA that codes for a protein, or for a piece of RNA, and the sequence of codons in the gene decides the sequence of amino acids in the protein, which decides what the protein does. A mutation changes that sequence, and the change can alter the protein, or leave it untouched, or stop it being made at all.

The smallest mutations change one letter. A point mutation swaps one base for another. A silent mutation changes a codon but not the amino acid it codes for, because the genetic code is redundant and several codons code for the same amino acid, so the protein is unchanged. A missense mutation changes one amino acid for another, and the effect can be tiny or serious, depending on where in the protein the change falls. A nonsense mutation turns a codon that codes for an amino acid into a stop codon, and the protein is cut short, usually destroying it. An insertion adds one or more bases, and a deletion removes them, and if the number added or removed is not a multiple of three, every codon after the change is read wrongly, which is called a frameshift.

Larger mutations rearrange the chromosome itself. A duplication copies a stretch of DNA, so the animal has two copies of a gene where one would do. An inversion flips a stretch of DNA around, so it reads backwards. A translocation moves a stretch of DNA from one chromosome to another. Aneuploidy changes the number of chromosomes, giving a cell an extra chromosome or one too few, and polyploidy gives a cell whole extra sets of chromosomes. These larger changes are rarer than point mutations, but they can have larger effects, and some of the most important events in the history of life, including the duplication of whole genomes, were large mutations of this kind.

Mutations happen for two reasons. Some are mistakes: DNA is copied billions of times in a lifetime, and the copying machinery, although it checks its own work, sometimes gets a letter wrong, and the mistake is kept. Others are caused by damage: ultraviolet light from the sun, X-rays, and certain chemicals can break or alter DNA, and if the cell repairs the damage wrongly, the repair itself becomes a mutation. The cell has repair systems that fix most damage and most mistakes, and only the ones that escape repair become mutations that last.

What a Mutation Is, and What It Is Not

The first thing to get clear is that a mutation is a change in DNA, not a change in an animal's body. A scar is not a mutation. A muscle built by exercise is not a mutation. A suntan is not a mutation. These are changes to the body that do not change the DNA, and they are not passed to offspring. A mutation changes the instructions, and the instructions are what are passed on, so a mutation in a reproductive cell is inherited, while a change to the body is not. The distinction matters, because it is the answer to one of the oldest confusions in biology: the idea that an animal can pass on what it acquires in its lifetime. It cannot, unless the acquisition changes its DNA, which exercise and suntans do not.

A mutation is also not the same as a variation. Variation is the differences between animals, and mutation is one source of variation, but not the only one. Sexual reproduction shuffles existing variation into new combinations, and that shuffling, called recombination, produces new combinations of genes without changing any single gene. A mutation creates a new variant where none existed before, and recombination rearranges the variants that already exist. Both matter, but they do different work, and the new gene that selection acts on had to come from a mutation first.

A mutation is not the same as an adaptation either, and this confusion is common enough to need its own section later in the article. A mutation is a random change in DNA. An adaptation is a trait that has been shaped by natural selection over generations, because it helped the animals that carried it survive and breed. A mutation can become an adaptation, if selection favours it, but most mutations never do, and an adaptation is not made by a single mutation in a single animal. The peppered moth's dark wings were a mutation first, and they became an adaptation only when the soot-darkened trees of industrial England made dark wings an advantage.

A mutation is not always visible, and it is not always a disease. Most mutations are invisible, because they happen in DNA that does not code for a protein, or because they change a codon without changing the amino acid, or because they change a protein in a way that does not affect what it does. Of the mutations that are visible, most are harmless, and a few are helpful. The idea that a mutation always means a monster, or a disease, or a superpower, is a story from films, not from biology. In real animals, a mutation is usually a small, quiet change, and the animal never knows it happened.

Germline Mutations and Somatic Mutations

Where a mutation happens decides whether it can be inherited. A germline mutation happens in a reproductive cell, a sperm or an egg, or in the cells that give rise to them, and it is passed to the offspring, who carries it in every cell of its body. A somatic mutation happens in a body cell, a skin cell, a liver cell, a muscle cell, and it is passed only to the daughter cells of that cell, in that one animal, and it dies with the animal. A germline mutation is inherited. A somatic mutation is not.

The difference explains why a mutation can matter to evolution in one case and not in the other. Evolution works on the genes that are passed from one generation to the next, and only germline mutations enter that stream. A somatic mutation in a skin cell may cause a patch of different-coloured fur, or a tumour, but it cannot be inherited, so it cannot spread through a population, and it cannot be selected for or against in the evolutionary sense. The mutations that built the animal kingdom, from the first vertebrates to the first mammals, were all germline mutations, because only those could be passed on.

Somatic mutations still matter to the individual animal. Most cancers are caused by somatic mutations, which happen in body cells during an animal's life and switch on genes that drive cell division, or switch off genes that hold it back. A sunburn is not a mutation, but the ultraviolet light that causes it can damage the DNA of skin cells, and if the damage is repaired wrongly, the repair becomes a somatic mutation, and if that mutation happens in the right gene, the cell can become cancerous. The same logic applies to the whole body: an animal accumulates somatic mutations throughout its life, and the number it carries grows with age, which is one reason cancer is more common in old animals than in young ones.

The two kinds of mutation are measured differently. A germline mutation is measured per generation, and in humans the number is about sixty to seventy new mutations in every child, most of them inherited from the father, because sperm are made throughout a man's life and the cells that make them keep dividing, accumulating copying errors, while eggs are made before birth and divide far fewer times. The same pattern, more mutations from the father than from the mother, is found in other mammals, and it is one reason the children of older fathers carry slightly more new mutations. A somatic mutation is measured per cell division, and the number is small, about one mistake in every hundred million bases copied, because the copying machinery is accurate and the repair systems catch most of the errors that slip through.

Point Mutations: Silent, Missense and Nonsense

A point mutation changes one base of DNA into another, and it is the smallest kind of mutation. Its effect depends on where it falls and on what it does to the codon it sits in. There are three kinds, and the difference between them is the difference between a change that does nothing, a change that alters a protein, and a change that destroys it.

A silent mutation changes a codon without changing the amino acid it codes for. The genetic code is redundant: there are sixty-four codons and only twenty amino acids, so most amino acids are coded for by more than one codon, and a change in the third letter of a codon often leaves the amino acid unchanged. A silent mutation changes the DNA but not the protein, and the animal is unaffected, which is why silent mutations are the most common kind found when scientists compare the DNA of two animals. They are not always completely silent, because a changed codon can affect how fast the protein is made, or how the messenger RNA is folded, but in most cases a silent mutation is invisible to selection, and it spreads or disappears by chance alone.

A missense mutation changes one amino acid for another. The effect depends on the amino acid and on where it sits in the protein. A change on the surface of a protein, far from the part that does the work, may do nothing at all. A change in the active site, the part of the protein that does the chemistry, can destroy it. A change that swaps one amino acid for a chemically similar one, such as one small hydrophobic amino acid for another, may do little harm, while a change that swaps a small amino acid for a large one, or a neutral one for a charged one, can twist the protein out of shape. The most famous missense mutation in animals is the one that causes sickle-cell disease, in which a single base change swaps one amino acid, glutamic acid, for another, valine, in the beta chain of haemoglobin, and that one change makes the haemoglobin clump together and bend the red blood cell into a sickle shape. The example is covered in full in its own section below, because it shows how much a single letter can do.

A nonsense mutation turns a codon that codes for an amino acid into a stop codon, and the protein is cut short. A protein that is cut short is usually useless, because it is missing the part that comes after the stop, and the cell often destroys it. Nonsense mutations are among the most damaging point mutations, and they cause many genetic diseases in animals, including some forms of muscular dystrophy in dogs and some forms of blindness in cats. The severity depends on where the stop falls: a stop near the end of the gene may leave a protein that still works, while a stop near the beginning destroys it completely.

Type of point mutation	What happens to the DNA	What happens to the protein	Usual effect
Silent	One base changes, codon still codes for the same amino acid	Unchanged	Usually harmless
Missense	One base changes, codon codes for a different amino acid	One amino acid swapped	Harmless to serious, depends on the site
Nonsense	One base changes, codon becomes a stop codon	Protein cut short	Often serious, protein usually destroyed

Insertions, Deletions and Frameshifts

An insertion adds one or more bases to the DNA, and a deletion removes them. If the number added or removed is a multiple of three, the reading frame is kept, and the protein gains or loses one or more amino acids, but the rest of the protein is read correctly. If the number is not a multiple of three, the reading frame shifts, and every codon after the change is read wrongly. This is a frameshift mutation, and it is usually far more damaging than a point mutation, because it does not change one amino acid but every amino acid after the change, and it usually creates a stop codon soon after, cutting the protein short.

The classic example of a frameshift is the mutation that causes Tay-Sachs disease in humans, in which a four-base insertion in the HEXA gene shifts the reading frame and destroys the enzyme that breaks down a fatty substance in the brain. In animals, frameshifts cause many of the same kinds of disease, and they are one reason insertions and deletions are more dangerous than point mutations: a single extra base can wreck a whole gene, while a single swapped base may do nothing at all.

Not all insertions and deletions are harmful. An insertion of three bases adds one amino acid, and if the amino acid falls in a part of the protein that can tolerate an extra one, the protein may still work. The mutation that causes Huntington's disease in humans is an insertion of a repeated three-base sequence, CAG, in the huntingtin gene, and the number of repeats decides whether the disease appears and how early: a few repeats are harmless, and many repeats cause the disease, because the extra amino acids, glutamines, make the protein clump together in brain cells. The same kind of repeat expansion causes diseases in animals, and it is one way a small insertion can have a large effect.

Insertions have a second source, and it is one of the most interesting in genetics. The genome is full of mobile pieces of DNA called transposons, or jumping genes, which can copy themselves and paste the copy into a new place in the genome. When a transposon lands inside a gene, it disrupts the gene, and the disruption is a mutation. Transposons were discovered by Barbara McClintock in maize in the 1940s, and they are now known to be present in every animal genome, making up nearly half of the human genome and a large part of the genome of every other animal. Most transposons are inactive, switched off by the cell, but the ones that are still active are a steady source of new mutations, and some of the most famous mutations in animals, including the short legs of the dachshund, were caused by a transposon-like insertion, as the section on dog breeds explains.

Bigger Changes: Duplications, Inversions and Translocations

A duplication copies a stretch of DNA, so the animal has two copies of a gene where one would do. The extra copy is free to change, because the original copy still does the job, and over time the extra copy can gain a new function, or be switched on in a new tissue, or be lost. Duplication is how new genes are born, and it is one of the most important kinds of mutation in evolution. The genes for colour vision in primates, for example, were made by duplication: an ancestral gene for seeing red and green was duplicated, and the two copies diverged, one becoming sensitive to red light and the other to green, and the result was trichromatic colour vision, the ability to see red, green and blue, which most other mammals lack. Without that duplication, primates would see the world in two colours, as dogs do.

The largest duplications are whole-genome duplications, in which the entire genome is doubled, and these have happened twice in the ancestry of vertebrates, an event called the two-round duplication, which gave the ancestors of all vertebrates four copies of every gene, and the extra copies were the raw material from which the complexity of the vertebrate body was built. Whole-genome duplication is rare in animals, and it is usually fatal, because doubling the genome upsets the balance of the cell, but when it succeeds, it can be the start of a new lineage, and some fish, including salmon and carp, carry the marks of a whole-genome duplication in their own ancestry.

An inversion flips a stretch of DNA around, so that it reads backwards. An inversion does not usually change the genes themselves, but it changes their order, and it can break a gene at the point where the flip happens, or it can move a gene next to a different control region, so that it is switched on at the wrong time or in the wrong tissue. Inversions also suppress recombination: when a chromosome with an inversion pairs with a normal chromosome during meiosis, the two cannot exchange pieces cleanly in the inverted region, so the genes inside the inversion are inherited together, as a block. This matters in evolution, because an inversion can hold a set of genes together that work well as a combination, and the fruit fly Drosophila carries famous inversions that have been studied for a century for exactly this reason.

A translocation moves a stretch of DNA from one chromosome to another. A balanced translocation, in which two chromosomes swap pieces without losing or gaining any DNA, may do no harm to the animal that carries it, but it causes problems in its offspring, because the chromosomes do not pair cleanly during meiosis, and the gametes can end up with too much or too little DNA. The best-known translocation in animals is the Robertsonian translocation in cattle, in which two chromosomes fuse into one, and the animals that carry it are healthy but less fertile, because their gametes are often unbalanced. Translocations also cause some cancers, including the leukaemia in which a piece of chromosome 9 moves to chromosome 22, creating a gene that drives the cancer.

Chromosomal change	What happens	Usual effect	Animal example
Duplication	A stretch of DNA is copied	Extra gene copy, free to evolve a new function	Opsin genes in primates, giving colour vision
Inversion	A stretch of DNA is flipped around	Gene order changed; recombination suppressed	Inversions in Drosophila, studied for a century
Translocation	A stretch of DNA moves to another chromosome	Carrier healthy, offspring often infertile	Robertsonian translocation in cattle
Deletion (large)	A stretch of DNA is lost	Genes lost; often serious	Large deletions causing disease in dogs

Chromosome-Number Changes: Aneuploidy and Polyploidy

Aneuploidy is a change in the number of chromosomes: a cell has an extra chromosome, or is missing one. It happens when chromosomes fail to separate properly during meiosis, a mistake called nondisjunction, and the gamete that results carries two copies of a chromosome instead of one, or none at all. When such a gamete is fertilised, the offspring has three copies of the chromosome, called trisomy, or one copy, called monosomy, instead of the normal two.

Most aneuploidies are fatal before birth, because having the wrong number of a chromosome unbalances the whole cell: the genes on the extra chromosome are made in larger amounts, and the genes on the missing chromosome in smaller amounts, and the balance of the cell is lost. The aneuploidies that survive are the ones involving the smallest chromosomes, which carry the fewest genes, or the sex chromosomes, which the cell can partly silence. In humans, the best-known surviving aneuploidy is trisomy 21, Down syndrome, in which the person has three copies of chromosome 21, the smallest human chromosome. In animals, the same pattern holds: aneuploidies of small chromosomes can be survived, and aneuploidies of large ones cannot, and most aneuploid embryos are lost early in pregnancy, which is one reason many pregnancies in mammals end before they are even detected.

Polyploidy is the gain of whole sets of chromosomes, so that a cell has three or four sets instead of two. It is common in plants, where it has created many of the crops we eat, including wheat, which is hexaploid, with six sets, but it is rare in animals, because animals have two sexes, and a polyploid animal usually cannot breed with a normal one, so the polyploid lineage dies out. The exceptions are mostly fish and frogs: some carp and salmon are polyploid, and some frogs of the genus Xenopus carry extra sets, and in these animals polyploidy has been a route to new species, because a polyploid population can breed with itself and become a separate lineage. Polyploidy is a reminder that a mutation that is usually fatal can, in the right circumstances, be the start of something new.

What Causes Mutations?

Mutations have two sources: mistakes in copying, and damage to the DNA. Copying mistakes happen every time a cell divides, because the machinery that copies DNA, although it is accurate to about one mistake in every hundred million bases, is not perfect, and it sometimes puts the wrong base in. The copying machinery checks its own work, and a second system, called mismatch repair, checks it again, and together they catch almost all of the errors, but a few escape, and those become mutations. The number of copying errors grows with the number of divisions, which is why the germ cells of older fathers carry more new mutations than those of younger fathers, and why a cell that divides many times in a lifetime, such as a skin cell, accumulates more mutations than a cell that rarely divides.

Damage comes from outside and from inside the cell. Ultraviolet light from the sun is the most familiar cause: it joins two adjacent thymine bases together into a structure called a pyrimidine dimer, which distorts the DNA and can cause a copying mistake when the cell tries to read past it. X-rays and other ionising radiation break the DNA backbone, and the cell's repair of the break can lose or gain bases. Certain chemicals, called mutagens, damage DNA directly: the chemicals in tobacco smoke, for example, attach themselves to DNA bases and cause copying mistakes, and some industrial chemicals do the same. Inside the cell, the chemistry of life itself causes damage: the oxygen that cells use to make energy produces reactive molecules that can alter DNA bases, and the cell's own enzymes sometimes make mistakes in handling DNA.

Viruses can also cause mutations, by inserting their own DNA into the host's genome, and transposons, the jumping genes described above, can do the same. Both are insertions, and both can disrupt a gene or switch it on at the wrong time. Some viruses cause cancer in animals by inserting near a gene that controls cell division and switching it on permanently, and the same logic applies to transposons that land in the wrong place. These are not the commonest causes of mutation, but they are the most dramatic, because a single insertion can move a whole piece of DNA into a new context.

The cell defends itself with repair systems, and they are remarkably good. Proofreading by the DNA polymerase catches most copying errors as they happen. Mismatch repair catches most of the ones that slip through. Base excision repair removes damaged bases and replaces them. Nucleotide excision repair cuts out a whole patch of DNA around a bulky lesion, such as a pyrimidine dimer, and replaces it. When these systems fail, the animal pays the price: the disease xeroderma pigmentosum, in which the nucleotide excision repair system is broken, makes the skin unable to repair ultraviolet damage, and the animal, or the person, develops skin cancers from ordinary sunlight. The repair systems are the reason most damage does not become a mutation, and the reason a mutation, when it does happen, is the damage that escaped repair.

How Often Do Mutations Happen?

The mutation rate is the number of new mutations per base per generation, and it is small but not zero. In humans, the rate is about one change in every hundred million bases per generation, which sounds tiny, but the human genome has three billion bases, so every child is born with about sixty to seventy new mutations that neither parent carried. Most of those are in DNA that does not code for a protein, and most of the ones in genes are harmless, so the child is healthy, but the number shows that mutation is not rare: it happens in every generation, in every family, in every species.

The rate varies between species, and the variation has a pattern. Animals with large genomes and long generations, such as mammals, have lower mutation rates per base, because a high rate would load each offspring with too many damaging changes. Animals with small genomes and short generations, such as bacteria, have higher rates, because they can afford to lose many offspring to bad mutations. The fruit fly Drosophila, the workhorse of genetics, has a mutation rate of about the same order as a human, and the roundworm Caenorhabditis elegans is similar. The rate is not fixed: it can be raised by mutagens, and it can be lowered by better repair, and it has been tuned by evolution to a level that balances the benefit of new variation against the cost of new damage.

The rate also varies within the genome. Some stretches of DNA mutate faster than others, because they are rich in a particular sequence that is easy to miscopy, or because they sit in a part of the chromosome that is copied late, when the copying machinery is tired, or because they are exposed to more damage. The repeated sequences called microsatellites, short stretches of DNA repeated many times, mutate far faster than ordinary DNA, because the copying machinery slips on the repeats, and they are used by geneticists to track parentage and population history for exactly that reason. The fastest-mutating parts of the genome are not the genes themselves but the repeats around them, and the slowest are the genes that are most important to survival, which are protected by repair and by selection.

Neutral Mutations: Most Changes Do Nothing

Most mutations are neutral: they change the DNA but not the animal. A silent mutation changes a codon without changing the amino acid, so the protein is the same. A mutation in a stretch of DNA that does not code for anything, and does not control anything, changes nothing at all. A mutation that swaps one amino acid for a chemically similar one, in a part of a protein that does not matter, leaves the protein working as before. These mutations are invisible to natural selection, because they do not change the animal's survival or its breeding, and they spread or disappear by chance alone, a process called genetic drift.

The neutral theory, put forward by the Japanese geneticist Motoo Kimura in 1968, says that most of the molecular evolution that happens, the changes in DNA sequences over time, is driven by neutral mutations and drift, not by selection. The theory was controversial when it was proposed, because it seemed to leave natural selection with little to do, but it has been confirmed by the sequencing of genomes: when two species are compared, most of the differences in their DNA are silent changes, or changes in non-coding DNA, which do not affect the animal, and the differences that do affect the animal are a minority. Selection is still the force that shapes adaptations, but drift is the force that shapes most of the genome.

Neutral mutations are not useless to science. Because they accumulate at a roughly steady rate, they can be used as a molecular clock, a way of dating the split between two species by counting the neutral differences between their DNA. If two species differ at ten neutral sites, and the rate is one change per million years, the split happened about ten million years ago. The clock is not perfect, because the rate varies, and because some sites mutate faster than others, but it has dated the great splits in animal history, including the split between humans and chimpanzees at about six million years ago, and the split between birds and mammals at about three hundred million years ago, and it is one of the most useful tools in evolutionary biology.

Harmful Mutations: When a Change Breaks Something

A harmful mutation is one that reduces the animal's survival or its breeding, and the harm can range from a small disadvantage to death before birth. A nonsense mutation that cuts a protein short usually destroys it, and if the protein is essential, the animal dies. A missense mutation in the active site of an enzyme can stop the enzyme working, and if the enzyme makes something the animal needs, the animal is ill. A deletion that removes a whole gene leaves the animal without that gene's product, and a duplication that makes too much of a protein can be as harmful as making too little.

The harm depends on whether the mutation is dominant or recessive. A dominant mutation causes harm even when only one copy of the gene is mutated, because the faulty protein interferes with the normal one, or because half the normal amount is not enough. A recessive mutation causes harm only when both copies are mutated, because one good copy is enough to do the job, and the animal that carries one copy is a healthy carrier. Most harmful mutations are recessive, which is why they can hide in a population for generations, carried by healthy animals, and only appear when two carriers breed and produce an offspring with two faulty copies. This is the reason inbreeding is dangerous: it brings together two copies of the same faulty gene, which would otherwise stay hidden.

Natural selection removes harmful mutations, but it cannot remove them all, because new ones keep appearing, and because recessive ones hide in carriers. The balance between new mutations appearing and selection removing them is called mutation-selection balance, and it is the reason every population carries a load of harmful recessive mutations, called the genetic load. In humans, every person carries several harmful recessive mutations, and in animals the number is similar. The load is the price of mutation, and it is paid in the offspring of related parents, and in the rare animals that inherit two copies of a faulty gene.

Some harmful mutations are removed quickly, and some linger. A mutation that kills the embryo is removed in one generation, because the embryo never breeds. A mutation that causes a disease late in life, after the animal has already bred, is removed slowly, because the animal has already passed it on. A mutation that is harmful in one environment and harmless in another can persist, because selection only acts in the environment the animal actually lives in. The sickle-cell mutation is the classic case of a harmful mutation that persists, because it is harmful in one form and helpful in another, and it is covered in its own section below.

Albinism and Coat Colour: Mutations You Can See

The most visible mutations are the ones that change an animal's colour, because colour is made by pigments, and pigments are made by genes, and a mutation in a pigment gene changes the colour. Albinism is the best known: it is caused by a mutation in a gene needed to make melanin, the dark pigment of skin, fur, feathers and eyes, and the best-studied gene is tyrosinase, or TYR, which makes the enzyme that starts melanin production. When tyrosinase does not work, no melanin is made, and the animal is white, with pink eyes, because the blood vessels show through the unpigmented iris, and pink skin, because there is no melanin to darken it.

Albino animals are found in almost every group, from mice to snakes to penguins, and they are usually at a disadvantage: without melanin, their skin burns in the sun, their eyes are sensitive to light and their vision is poor, and their white colour makes them easy for predators to spot. In the wild, most albino animals die young, which is why albinism is rare, about one in ten thousand births in many species, and why the albino animals that people see are usually in zoos or in captivity, where they are protected. In some species, though, albinism is kept by humans: white mice and white rats are bred for laboratories, and white tigers, which are not truly albino but carry a mutation in a different pigment gene, are kept in zoos, although the breeding that produces them is controversial, because it relies on inbreeding.

White peacocks are often called albino, but they are not: they are leucistic, which means they have reduced pigment in their feathers but normal pigment in their eyes, and their eyes are blue, not pink. Leucism is caused by a mutation in a gene that controls the migration of pigment cells during development, so the feathers have no pigment cells, but the eyes, which get their pigment from a different source, are normal. The distinction matters, because leucism and albinism are different mutations, with different causes and different effects, and the white peacock, with its fully spread white tail, is a leucistic bird, not an albino one. Both are mutations, and both are visible, and both show how a single change in a pigment gene can transform an animal's appearance.

Coat-colour mutations are not always a disadvantage. In some animals, a colour mutation is an advantage, and selection keeps it: the dark form of the peppered moth, the black panther, which is a leopard or a jaguar with a mutation that makes extra melanin, and the white winter coat of the Arctic fox, which is not a mutation but a seasonal change, all show that colour is a trait selection acts on. The difference between a harmful colour mutation and a helpful one is the environment: white fur is a disadvantage in a green forest, and an advantage in snow, and the same mutation can be harmful in one place and helpful in another.

Short Legs and Flat Faces: Mutations Humans Selected

Some of the most familiar mutations in animals are the ones humans have selected in domestic breeds. The short legs of the dachshund, the corgi and the basset hound are caused by a mutation called chondrodysplasia, in which the long bones of the legs stop growing early, while the rest of the body grows normally. The mutation was found in 2009 to be caused by a retrogene, a copy of the gene FGF4 that was pasted into a new place in the genome by a retrotransposon, and the extra copy is switched on in the growing bone, where it stops the bone growing. The short-legged breeds all carry the same retrogene, which shows that the mutation happened once, in a single dog, and was then kept by breeders who liked the look, and spread through the breeds that carry it.

The flat face of the bulldog and the pug is a similar story: a mutation that shortens the bones of the face, selected by breeders who liked the look, and kept even though it causes breathing problems, because the shortened face leaves too little room for the airway. The folded ears of the Scottish Fold cat are caused by a mutation in a gene for cartilage, which folds the ears and also weakens the cartilage of the joints, causing arthritis. The hairlessness of the Sphynx cat is caused by a mutation in the gene for keratin, the protein of hair. In each case, a mutation that would be a disadvantage in the wild, and would be removed by selection, is kept by humans, because humans like the look, and the animal pays the price in health.

These breeds are a warning about selection, and about the difference between natural selection and artificial selection. Natural selection removes mutations that harm the animal, because the animal that carries them breeds less. Artificial selection can keep them, because the breeder chooses the animal for its looks and breeds it anyway, and the harm is passed on with the look. The result is a set of breeds that carry mutations no wild animal would carry, and the veterinary problems of those breeds, from the dachshund's back problems to the bulldog's breathing, are the price of selecting for a mutation.

The Peppered Moth: A Mutation That Changed History

The peppered moth, Biston betularia, is the most famous example of a mutation in action, and it is the textbook case of natural selection. The moth has two forms: a pale, speckled form, called typica, which is camouflaged against lichen-covered tree bark, and a dark form, called carbonaria, which is caused by a mutation in a gene that controls melanin production in the wings. The dark form was first recorded near Manchester in 1848, and by 1895 it made up about ninety-eight percent of the moths in the industrial areas of northern England, while the pale form was still common in the unpolluted countryside.

The reason was the soot. The factories of the Industrial Revolution covered the trees with soot, killing the lichen and darkening the bark, so that the pale moths, which had been camouflaged, became conspicuous, and the dark moths, which had been conspicuous, became camouflaged. Birds, which hunt moths by sight, ate the pale moths and left the dark ones, and the dark form spread, because the moths that survived were the ones that carried the mutation. The change was fast: within fifty years, the dark form had gone from rare to almost universal in the industrial areas, and the speed of the change was the first clear proof that natural selection could work in a human lifetime, not only over geological time.

The story was tested in the 1950s by the biologist Bernard Kettlewell, who released marked pale and dark moths in a polluted wood and in an unpolluted wood, and recaptured them, and found that the dark moths survived better in the polluted wood and the pale moths in the unpolluted wood, which confirmed that birds were doing the selecting. Kettlewell's methods were later criticised, because he released the moths during the day, when they would normally be resting, and because he placed them on tree trunks, where they may not normally rest, but later work, including a large study by Michael Majerus published in 2012, confirmed the result with better methods: the dark form survived better on soot-darkened trees, and the pale form on clean ones, and the selection was real.

The story has a second half, which is as important as the first. After the Clean Air Act of 1956, the air became cleaner, the soot washed off the trees, the lichen came back, and the pale form, which had been rare in the industrial areas, became common again, while the dark form fell. The rise and the fall of the dark moth, following the pollution and then the clean air, is the clearest demonstration in nature that selection follows the environment, and that a mutation which is an advantage in one world can be a disadvantage in another. The gene behind the dark form was identified in 2016 as a transposon insertion in the cortex gene, which controls wing patterning, so the famous mutation is, like the dachshund's short legs, a jumping gene that landed in the wrong place, and changed the animal's colour.

Beneficial Mutations: When a Change Helps

A beneficial mutation is one that increases the animal's survival or breeding, and it is rarer than a harmful one, because most random changes to a working machine make it worse, not better. But beneficial mutations do happen, and they are the ones that natural selection keeps, and they are the reason evolution can build new traits. The dark peppered moth is one. The sickle-cell mutation, in its heterozygous form, is another. There are others, and they are worth knowing, because they show what a beneficial mutation looks like.

The best-known beneficial mutation in a mammal is the one that lets adult humans drink milk. Most mammals stop making lactase, the enzyme that digests the sugar in milk, after weaning, but a mutation in the control region of the lactase gene, which keeps the gene switched on into adulthood, spread through the populations that kept cattle, sheep and goats, because the people who carried it could drink milk and get its nutrition, while those who did not could not. The mutation happened several times, in different populations, and each time it spread, because it was useful. The same logic applies to animals: a mutation that lets a bird digest a new food, or a fish tolerate salt water, or a mammal resist a disease, will spread, because the animals that carry it breed more.

Gene duplication can be a beneficial mutation in itself. When a gene is duplicated, the extra copy is free to change, and the change can be useful. The duplication of the opsin gene gave primates colour vision. The duplication of the amylase gene, which makes the enzyme that digests starch, gave humans, and also dogs, the ability to eat starchy food, and dogs carry more copies of the amylase gene than wolves, because they have lived on human food, which is rich in starch, for thousands of years. The duplication of the genes for antifreeze proteins in Antarctic fish let them live in water below freezing, because the proteins stop ice crystals growing in their blood. In each case, the duplication was a mutation, and the new function was the benefit, and selection kept the duplication.

Beneficial mutations are not always dramatic. A mutation that makes an enzyme slightly more efficient, or a muscle slightly stronger, or a sense slightly sharper, can be enough, because selection works on small differences, and a small advantage, repeated over many generations, becomes a large one. The mistake is to think that a beneficial mutation must make a super-animal. It does not. It must only make the animal slightly better at surviving and breeding than its neighbours, and that is enough for selection to spread it.

Pesticide Resistance: Selection on New Variation

Pesticide resistance is the clearest modern example of mutation and selection working together, and it is also a warning. When a pesticide is sprayed, most of the insects die, but a few survive, because they carry a mutation that makes them resistant, and those survivors breed, and their offspring carry the mutation, and the next time the pesticide is sprayed, more of the insects survive, until the pesticide stops working. The mutation was not caused by the pesticide: it was already there, in a few insects, before the spraying began, and the pesticide only selected for it, by killing the insects that did not carry it. This is the point that is most often misunderstood: the pesticide does not create the resistance, it reveals it.

The mutations that cause resistance are of several kinds. The best studied is the knockdown resistance mutation, called kdr, in the gene for the voltage-gated sodium channel, which is the target of the insecticides DDT and the pyrethroids. A single amino acid change in the channel stops the insecticide binding to it, and the insect survives. The mutation has appeared independently in many species of mosquito, including the malaria mosquito Anopheles gambiae, and it has spread through mosquito populations wherever DDT and pyrethroids are used, which is one reason malaria control is harder than it was. The same logic applies to the warfarin resistance of rats, in which a mutation in the VKORC1 gene, which makes the enzyme that warfarin blocks, makes the enzyme resistant to the poison, and the rats that carry it survive the poisoned bait and breed.

Pesticide resistance shows why mutation matters to humans, and why it cannot be wished away. A pesticide that works today will stop working tomorrow, because the insects carry mutations, and selection will find the ones that help, and the only answer is to use pesticides in rotation, so that no single mutation is favoured for long, and to use other methods, such as biological control, alongside them. The insects are not evolving on purpose, and they are not trying to defeat us: they are carrying random mutations, and we are selecting the ones that help them survive, and the result is the same as it has always been, from the peppered moth to the mosquito, which is that selection follows the environment, and the environment now includes our chemicals.

Sickle-Cell Haemoglobin: One Letter, Two Stories

The sickle-cell mutation is the most studied single mutation in animals, and it is the best example of how one change can be both harmful and helpful. The mutation is a point mutation, a single base change in the gene for the beta chain of haemoglobin, the protein that carries oxygen in red blood cells. The change swaps one amino acid, glutamic acid, for another, valine, at the sixth position of the beta chain, and the valine makes the haemoglobin clump together when oxygen is low, bending the red blood cell into a sickle shape, which blocks small blood vessels and causes pain, anaemia and organ damage. The disease is called sickle-cell disease, and it is one of the most common genetic diseases in the world.

The disease appears in animals that carry two copies of the mutation, one from each parent. Animals that carry one copy, called sickle-cell trait, are healthy, because they have one normal copy of the gene as well, and their red blood cells are mostly normal. But they have an advantage that normal animals do not: they are protected against malaria. The malaria parasite, Plasmodium falciparum, lives inside red blood cells, and in the cells of a carrier, the sickle haemoglobin makes the cell sickle when the parasite infects it, and the sickled cell is destroyed by the spleen, taking the parasite with it. The carrier is therefore less likely to die of malaria, and in the parts of the world where malaria is common, carriers have more children than non-carriers, and the mutation spreads, even though it causes a terrible disease in the animals that inherit two copies.

This is called heterozygote advantage, or balancing selection, and it is the reason a harmful mutation can persist in a population. The mutation is harmful in the homozygote, who has two copies, and helpful in the heterozygote, who has one, and the balance between the two decides how common the mutation is. In the parts of Africa where malaria is common, about one person in four carries one copy of the sickle-cell mutation, and about one in a hundred is born with the disease, and the numbers have stayed stable for thousands of years, because the advantage to the carriers balances the cost to the diseased. The same pattern is found in animals: the mutation is common where malaria, or a similar parasite, is common, and rare where it is not.

The sickle-cell story is also a lesson in how a mutation works at the level of the molecule. A single base change, A to T, changes one codon, GAG to GTG, which changes one amino acid, glutamic acid to valine, which changes the shape of the haemoglobin, which changes the shape of the cell, which changes the course of a disease, which changes the survival of the animal, which changes the frequency of the gene in the population. The chain from a letter of DNA to the fate of a species is unbroken, and it is the chain that genetics is built on.

Mutations and Evolution: The Raw Material

Mutation is the ultimate source of variation, and variation is the raw material of evolution. Without mutation, there would be no new genes, and without new genes, there would be no new traits, and without new traits, there would be no new species. Sexual reproduction shuffles the variation that already exists, and it is a powerful force, but it cannot create a new gene, and it cannot create a new trait that no animal has ever had. Only mutation can do that, by changing the DNA itself.

The role of mutation in evolution is not to create adaptations directly. A mutation is random, and it does not know what the animal needs, and it does not happen because the animal needs it. The dark peppered moth did not mutate because the trees were dark: the mutation happened first, in a moth that lived on a clean tree, and it spread only when the trees became dark and the mutation became useful. The sickle-cell mutation did not happen because malaria was present: it happened by chance, and it spread only where malaria made it useful. The distinction is the heart of the modern understanding of evolution: mutation proposes, and selection disposes, and the proposal is random while the disposal is not.

Mutation also sets the pace of evolution. A species with a high mutation rate evolves faster, because it has more variation for selection to work on, but it also pays a higher price, because most mutations are harmful, and the species carries a larger genetic load. A species with a low mutation rate evolves more slowly, but it carries a smaller load. The balance between the two has been tuned by evolution itself, and the mutation rate of every species is a compromise between the benefit of new variation and the cost of new damage. The rate is not zero, because a rate of zero would stop evolution, and it is not high, because a high rate would kill the species, and the rate that survives is the one that balances the two.

The largest events in the history of life were mutations. The duplication of the whole genome, twice, in the ancestors of vertebrates, gave them the extra genes from which the complexity of the vertebrate body was built. The duplication of the opsin gene gave primates colour vision. The insertion of a transposon gave the peppered moth its dark wings. The single base change in the haemoglobin gene gave some humans protection from malaria. None of these was planned, and none of them was caused by the need they met, but all of them were mutations, and all of them changed the course of evolution, which is why the study of mutation is the study of the origin of diversity itself.

Mutation Versus Adaptation: Clearing Up the Confusion

The confusion between mutation and adaptation is one of the commonest in biology, and it is worth clearing up directly. A mutation is a change in DNA. An adaptation is a trait that has been shaped by natural selection over generations, because it helped the animals that carried it survive and breed. A mutation is an event, in one animal, in one generation. An adaptation is a pattern, in a population, over many generations. A mutation can become an adaptation, if selection favours it, but most mutations never do, and an adaptation is not made by a single mutation.

The confusion comes from the way the two are talked about. People say that the peppered moth "adapted" to the soot, as if the moth changed itself, or as if the mutation happened because the moth needed it. Neither is true. The mutation happened by chance, in a moth that lived before the soot, and the moth did not change itself: the population changed, because the dark moths survived and the pale moths did not, and the change in the population is the adaptation, not the change in the individual. An individual animal does not adapt in the evolutionary sense: it either survives or it does not, and the population adapts, by changing its composition, over generations.

The same confusion appears in the phrase "the animal mutated to survive", which is wrong in two ways. First, the mutation did not happen to help the animal survive: it happened at random, and it happened to help, which is a different thing. Second, the animal did not mutate: its DNA changed, and the change was in its germ cells, and the change was passed to its offspring, and the offspring, not the animal itself, were the ones that benefited. The language of purpose, of animals mutating in order to survive, is a habit of speech, and it is misleading, because evolution has no purpose, and mutation has no direction, and the only direction in evolution is the one that selection gives it, by keeping the changes that help and removing the ones that harm.

Can Mutations Be Repaired?

The cell repairs most DNA damage before it becomes a mutation, and the repair systems are among the most important machinery in the cell. Proofreading by the DNA polymerase catches copying errors as they happen, and mismatch repair catches the ones that slip through, and base excision repair removes damaged bases, and nucleotide excision repair cuts out a patch of DNA around a bulky lesion and replaces it. Together, these systems reduce the error rate from about one mistake in every hundred thousand bases, which is the rate of the copying machinery on its own, to about one in every hundred million, which is the rate that survives repair.

When repair fails, the consequences are visible. The disease xeroderma pigmentosum, in which the nucleotide excision repair system is broken, makes the skin unable to repair ultraviolet damage, and the animal develops skin cancers from ordinary sunlight. The disease ataxia telangiectasia, in which the system that repairs double-strand breaks is broken, makes the animal sensitive to radiation and prone to cancer. The gene BRCA1, in which a mutation raises the risk of breast and ovarian cancer in humans, and of mammary cancer in dogs, makes a protein that repairs double-strand breaks, and when it does not work, the breaks accumulate, and the cell becomes cancerous. These are not mutations in the ordinary sense: they are mutations in the repair machinery itself, and they raise the mutation rate of the whole genome, which is why they cause cancer.

Repair is not perfect, and it is not meant to be. A repair system that fixed every mistake would stop evolution, because there would be no new mutations, and no new variation, and no new traits. A repair system that fixed nothing would kill the animal, because the mutations would pile up faster than selection could remove them. The repair systems that survive are the ones that balance the two, fixing most of the damage and letting a few mutations through, and the few that get through are the raw material of evolution, which is why the repair systems, and the mutation rate they set, are themselves products of evolution.

How Scientists Study Mutations

Scientists study mutations in three ways: by sequencing DNA, by breeding animals, and by watching mutations happen in the laboratory. Sequencing is the most direct: the DNA of two animals is read, and the differences are the mutations that have happened since they shared an ancestor, and the pattern of the differences, which ones are silent and which ones change proteins, tells the scientist which mutations were neutral and which ones were selected. The sequencing of whole genomes, which is now routine, has shown that every animal carries thousands of mutations that its parents did not, and that most of them are neutral, and that the few that matter are the ones that change a protein or its control.

Breeding is the oldest method, and it is still used: an animal with a new trait is crossed with a normal animal, and the pattern of inheritance in the offspring tells the scientist whether the trait is caused by one gene or by many, and whether it is dominant or recessive, and where in the genome it sits. The fruit fly Drosophila, the roundworm Caenorhabditis elegans, the zebrafish and the mouse are the animals in which most of this work is done, because they breed fast, they are easy to keep, and their genomes are known, and the mutations found in them are the ones that have taught us most of what we know about how genes work.

The laboratory can also make mutations on purpose, by exposing animals to mutagens, such as X-rays or chemicals, and looking for the offspring with new traits, a method called mutagenesis, which was used to find most of the genes that control development in the fruit fly, and which won the Nobel Prize in 1995. The modern version is gene editing, in which a specific change is made to a specific gene, using tools such as CRISPR, and the effect of the change is observed, which lets the scientist test, directly, whether a particular mutation causes a particular trait. Gene editing is not a mutation in the natural sense, because it is done on purpose, but it is the most powerful tool for studying mutations that exists, and it is the tool that is now used to test the mutations found by sequencing, to see whether they really do what the sequence suggests.

Key Takeaways

• A mutation is a change in the DNA sequence, and it can be as small as one base swapped for another or as large as a whole chromosome gained or lost.
• Point mutations are silent, missense or nonsense: a silent mutation changes a codon without changing the amino acid, a missense mutation swaps one amino acid for another, and a nonsense mutation turns a codon into a stop codon and cuts the protein short.
• Insertions and deletions that are not a multiple of three bases cause a frameshift, which changes every codon after the change and usually destroys the protein.
• Duplications copy a gene and free the extra copy to evolve a new function, inversions flip a stretch of DNA and suppress recombination, and translocations move DNA between chromosomes.
• Aneuploidy, an extra or missing chromosome, is usually fatal before birth, and polyploidy, whole extra sets of chromosomes, is rare in animals but common in plants.
• Germline mutations happen in reproductive cells and can be inherited, while somatic mutations happen in body cells, cannot be inherited, and are the cause of most cancers.
• Mutations are caused by copying mistakes and by damage from ultraviolet light, radiation, chemicals, viruses and transposons, and the cell repairs most of the damage before it becomes a mutation.
• Every human child is born with about sixty to seventy new mutations, and most of them are neutral, changing the DNA without changing the animal.
• Most mutations are neutral and spread or disappear by chance, and the neutral theory of Motoo Kimura says that most molecular evolution is driven by neutral mutations and drift, not by selection.
• Albinism is caused by a mutation in a gene for making melanin, such as tyrosinase, while white peacocks are leucistic, not albino, because their eyes are blue, not pink.
• The short legs of dachshunds and corgis are caused by a retrogene, a copy of the FGF4 gene pasted into a new place, and the trait was kept by breeders who liked the look.
• The peppered moth's dark form, caused by a transposon insertion in the cortex gene, spread when soot darkened the trees and fell again when the air became clean, which is the clearest demonstration of natural selection in a human lifetime.
• The sickle-cell mutation swaps one amino acid in haemoglobin, and carriers of one copy are protected against malaria, which is why a harmful mutation persists, a case of heterozygote advantage.
• Pesticide resistance, such as the kdr mutation in mosquitoes, is not created by the pesticide but revealed by it, because the mutation was already present and the pesticide only selected for it.
• A mutation is a random change in DNA, while an adaptation is a trait shaped by natural selection over generations, and a mutation can become an adaptation only if selection favours it.
• DNA repair systems fix most damage, and when they fail, as in xeroderma pigmentosum, the animal develops cancers from ordinary sunlight.

Short Questions

1. What is a mutation in simple words?

See answer

A mutation is a change in the sequence of DNA, the molecule that carries the instructions for building and running an animal. It can be as small as one letter of the genetic code swapped for another, or as large as a whole chromosome gained or lost. Mutations happen by copying mistakes, when the machinery that copies DNA gets a letter wrong, and by damage, when ultraviolet light, radiation, chemicals or viruses alter the DNA. Most mutations do nothing, because they happen in DNA that does not code for a protein, or because they change a codon without changing the amino acid, and the animal never knows they happened. A few mutations harm the animal, and a few help it, and the ones that help are the ones that natural selection keeps.

2. What is the difference between a germline and a somatic mutation?

See answer

A germline mutation happens in a reproductive cell, a sperm or an egg, or in the cells that give rise to them, and it is passed to the offspring, who carries it in every cell of its body. A somatic mutation happens in a body cell, such as a skin cell or a liver cell, and it is passed only to the daughter cells of that cell, in that one animal, and it dies with the animal. Only germline mutations can be inherited, and only they can be acted on by evolution, because evolution works on the genes passed from one generation to the next. Somatic mutations still matter to the individual animal, because most cancers are caused by somatic mutations that switch on genes for cell division or switch off genes that hold it back.

3. What is a frameshift mutation?

See answer

A frameshift mutation is caused by an insertion or a deletion of a number of DNA bases that is not a multiple of three, which shifts the reading frame of the codons after the change. DNA is read in three-letter words called codons, and each codon stands for one amino acid, so adding or removing one or two bases moves every codon after the change by one or two positions, and the protein is read wrongly from that point on. A frameshift usually creates a stop codon soon after the change, cutting the protein short and destroying it, which is why frameshifts are usually far more damaging than point mutations, which change only one amino acid.

4. Why is the peppered moth famous?

See answer

The peppered moth is famous because it is the clearest demonstration of natural selection in a human lifetime. The moth has a pale speckled form and a dark form, caused by a mutation in a gene that controls melanin in the wings. During the Industrial Revolution, soot darkened the trees and killed the lichen, so the pale moths became conspicuous and the dark moths became camouflaged, and birds ate the pale moths and left the dark ones, so that by 1895 the dark form made up about ninety-eight percent of the moths in industrial areas. After the Clean Air Act of 1956, the air became cleaner, the trees became pale again, and the pale form returned, which showed that selection follows the environment.

5. How can one mutation protect against malaria?

See answer

One mutation protects against malaria because it changes the haemoglobin in red blood cells in a way that the malaria parasite cannot tolerate. The sickle-cell mutation is a single base change that swaps one amino acid, glutamic acid, for another, valine, in the beta chain of haemoglobin, and the changed haemoglobin clumps together when oxygen is low, bending the red blood cell into a sickle shape. In an animal that carries one copy of the mutation, the parasite infects a red blood cell, the cell sickles, and the spleen destroys the sickled cell, taking the parasite with it, so the carrier is less likely to die of malaria. The protection is the reason the mutation persists, even though animals that inherit two copies suffer sickle-cell disease.

6. Can mutations be good for an animal?

See answer

Yes, mutations can be good for an animal, although beneficial mutations are rarer than harmful ones, because most random changes to a working machine make it worse, not better. The dark form of the peppered moth was a beneficial mutation when the trees were darkened by soot, because it hid the moth from birds. One copy of the sickle-cell mutation is beneficial where malaria is common, because it protects the carrier. A duplication of the opsin gene gave primates colour vision, and a duplication of the amylase gene let dogs digest starchy human food. A beneficial mutation only needs to make the animal slightly better at surviving and breeding than its neighbours, and natural selection will spread it over generations.

Frequently Asked Questions

What is a mutation in animals?

A mutation in animals is a change in the sequence of DNA, the molecule that carries the instructions for building and running an animal. It can be as small as one base swapped for another, which is a point mutation, or as large as a whole chromosome gained or lost, which is aneuploidy. Mutations happen by copying mistakes, when the machinery that copies DNA gets a letter wrong, and by damage from ultraviolet light, radiation, chemicals, viruses and transposons, and the cell repairs most of the damage before it becomes a mutation.

Are all mutations harmful?

No, most mutations are neutral: they change the DNA without changing the animal, because they happen in DNA that does not code for a protein, or because they change a codon without changing the amino acid it codes for. Of the mutations that do change the animal, most are harmful, because most random changes to a working machine make it worse, but a few are helpful, and those are the ones that natural selection keeps. The dark peppered moth, one copy of the sickle-cell mutation, and the duplication that gave primates colour vision are all beneficial mutations.

What causes mutations in animals?

Mutations are caused by copying mistakes and by damage to DNA. Copying mistakes happen when the machinery that copies DNA puts the wrong base in, and the cell's proofreading and mismatch repair systems catch almost all of them, but a few escape. Damage comes from ultraviolet light from the sun, which joins adjacent thymine bases together, from X-rays and other ionising radiation, which break the DNA backbone, from chemicals called mutagens, which alter DNA bases, and from viruses and transposons, which insert DNA into new places. The cell repairs most damage, and only the damage that escapes repair becomes a mutation.

Can mutations be passed to offspring?

A mutation can be passed to offspring only if it happens in a reproductive cell, a sperm or an egg, which is called a germline mutation, and the offspring then carries it in every cell of its body. A mutation that happens in a body cell, such as a skin cell or a liver cell, is called a somatic mutation, and it is passed only to the daughter cells of that cell, in that one animal, and it dies with the animal. Most cancers are caused by somatic mutations, which is why cancer is usually not inherited, while the mutations that built the animal kingdom were all germline mutations, because only those could be passed on.

What is the most famous example of a mutation in animals?

The most famous example is the peppered moth, Biston betularia, whose dark form is caused by a mutation in a gene that controls melanin in the wings. During the Industrial Revolution, soot darkened the trees and killed the lichen, so the pale moths became conspicuous and the dark moths became camouflaged, and birds ate the pale moths and left the dark ones, so that by 1895 the dark form made up about ninety-eight percent of the moths in industrial areas. After the Clean Air Act of 1956, the air became cleaner and the pale form returned, which showed that selection follows the environment. The gene behind the dark form was identified in 2016 as a transposon insertion in the cortex gene.

Can a mutation create a new species?

A single mutation cannot create a new species on its own, because a species is a population, not an individual, and speciation takes many mutations accumulating over many generations, together with reproductive isolation, which stops the populations breeding with each other. But mutation is the ultimate source of the variation that speciation works on: without new mutations, there would be no new genes, and without new genes, there would be no new traits, and without new traits, there would be no new species. The largest mutations, such as whole-genome duplications, have created new lineages in the past, and polyploid fish and frogs show that a single large mutation can be the start of a new species.

Related reading

• DNA Structure and Replication Explained Simply
• Mendel's Laws of Inheritance Explained
• All Genetics Articles

MCQs

1. Which of the following best defines a mutation?

• A. A change in an animal's behaviour learned during its life
• B. A change in the sequence of DNA
• C. A change in the number of chromosomes only
• D. A change caused by natural selection

Answer: B

Explanation: A mutation is a change in the DNA sequence, and it can be as small as one base swapped for another or as large as a whole chromosome gained or lost, while a learned behaviour, a chromosome number alone, and natural selection are not definitions of a mutation.

2. A silent mutation is one that...

• A. Deletes an entire gene
• B. Stops the ribosome from reading the gene
• C. Adds a whole chromosome
• D. Changes a codon without changing the amino acid it codes for

Answer: D

Explanation: The genetic code is redundant, so several codons code for the same amino acid, and a silent mutation swaps one codon for another that codes for the same amino acid, so the protein is unchanged and the animal is unaffected.

3. What does a frameshift mutation do?

• A. It shifts the reading frame of the codons after the mutation
• B. It swaps one amino acid for another
• C. It duplicates a whole chromosome
• D. It repairs a damaged gene

Answer: A

Explanation: An insertion or a deletion of a number of bases that is not a multiple of three moves every codon after the change by one or two positions, so the protein is read wrongly from that point on and a stop codon usually appears soon after.

4. Which statement about germline and somatic mutations is correct?

• A. Somatic mutations are passed to offspring, germline mutations are not
• B. Both kinds are passed to offspring
• C. Germline mutations occur in reproductive cells and can be inherited; somatic mutations occur in body cells and cannot
• D. Germline mutations only happen in plants

Answer: C

Explanation: A germline mutation happens in a sperm or an egg and is passed to the offspring, who carries it in every cell, while a somatic mutation happens in a body cell, is passed only to that cell's daughters, and dies with the animal.

5. Why did the dark (carbonaria) form of the peppered moth become common near industrial towns in the 1800s?

• A. The moths chose to become darker
• B. Soot-darkened tree trunks made dark moths harder for birds to spot, so more of them survived and bred
• C. Dark moths could fly faster
• D. The light moths migrated away

Answer: B

Explanation: Soot from factories darkened the tree bark and killed the lichen, so the pale moths became conspicuous and the dark moths became camouflaged, and birds ate the pale moths and left the dark ones, so the dark form spread to about ninety-eight percent of the moths in industrial areas by 1895.

6. Albinism in animals is usually caused by...

• A. Too much sunlight
• B. A diet without pigment
• C. A lack of melanocytes in the skin
• D. A mutation in a gene needed to make melanin, such as the tyrosinase gene

Answer: D

Explanation: Albinism is caused by a mutation in a gene needed to make melanin, and the best-studied gene is tyrosinase, or TYR, which makes the enzyme that starts melanin production, so when it does not work, no melanin is made and the animal is white with pink eyes.

7. What is the advantage of carrying one copy of the sickle-cell mutation?

• A. It protects against malaria
• B. It makes red blood cells larger
• C. It prevents all infections
• D. It increases muscle strength

Answer: A

Explanation: In a carrier of one copy of the sickle-cell mutation, the malaria parasite infects a red blood cell, the cell sickles, and the spleen destroys the sickled cell, taking the parasite with it, so the carrier is less likely to die of malaria, which is why the mutation persists where malaria is common.

8. Aneuploidy means...

• A. A gene is duplicated
• B. A chromosome is turned upside down
• C. A cell has an extra chromosome or is missing one
• D. A base is replaced by another base

Answer: C

Explanation: Aneuploidy is a change in the number of chromosomes, giving a cell an extra chromosome, called trisomy, or one too few, called monosomy, and it happens when chromosomes fail to separate properly during meiosis, a mistake called nondisjunction.

9. Which of these is a mutagen?

• A. Pure water
• B. Ultraviolet radiation
• C. Oxygen at normal levels
• D. Table salt

Answer: B

Explanation: Ultraviolet radiation damages DNA by joining two adjacent thymine bases together into a pyrimidine dimer, which distorts the DNA and can cause a copying mistake, while pure water, oxygen at normal levels and table salt do not damage DNA.

10. What is the difference between a mutation and an adaptation?

• A. A mutation is a change in DNA; an adaptation is a trait shaped by natural selection over generations
• B. They are the same thing
• C. Adaptations change DNA, mutations do not
• D. Mutations only happen in the laboratory

Answer: A

Explanation: A mutation is a random change in DNA, in one animal, in one generation, while an adaptation is a trait that has been shaped by natural selection over generations, because it helped the animals that carried it survive and breed, and a mutation can become an adaptation only if selection favours it.