Mutation, gene flow, natural selection and genetic drift

Evolution is a change in the gene pool

• A gene pool is every allele carried by the breeding members of a population; an allele frequency is the share of one gene's copies that are a particular allele.
• Evolution, at the scale of a population, is a change in those allele frequencies from one generation to the next.
• Four processes change them: mutation makes new alleles, gene flow moves alleles between populations, natural selection changes them in a direction set by the environment, and genetic drift changes them at random.
• Individuals do not evolve: a possum cannot change its alleles, but the possum population's allele frequencies can change over generations.
• In Level 3 answers each process should be tied to what it does to allele frequencies and to genetic diversity, not just named.

Mutation: the only source of new alleles

• A mutation alters the DNA base sequence for good: a substitution, insertion or deletion of bases, or a larger change to a chromosome's structure or number.
• Only mutations in the germ line (cells that make eggs or sperm) reach the next generation; a mutation in a skin or liver cell dies with the individual.
• Most mutations are neutral or harmful, and a few are beneficial; whether an allele is beneficial depends on the environment, not on the mutation itself.
• Mutations are random: they are not caused by the organism needing them, so antibiotic resistance alleles exist in a bacterial population before the antibiotic arrives.
• Mutation rates per gene are tiny, so mutation adds variation slowly, but across thousands of genes and many individuals it keeps supplying the raw material that selection and drift act on.

Gene flow between populations

• Gene flow is the movement of alleles from one population to another when individuals, or their gametes such as pollen, migrate and breed.
• Immigrants can bring alleles the population lacked, raising its diversity; emigrants take alleles away.
• Gene flow makes the gene pools of connected populations more alike, so it works against divergence; populations only drift or are selected apart when gene flow between them is low.
• A barrier such as Cook Strait, a mountain range or a stretch of farmland reduces gene flow, which is the first step towards allopatric speciation.
• Conservation managers use gene flow on purpose: the Department of Conservation moves some kiwi and hihi between isolated populations so each gets new alleles.

Natural selection: stabilising, directional and disruptive

• Natural selection happens when individuals with some phenotypes survive and reproduce more than others, so the alleles behind those phenotypes become more common.
• Stabilising selection favours the average phenotype and acts against both extremes, so the mean stays the same and the variation narrows, as with human birth weight, where very light and very heavy babies historically had lower survival.
• Directional selection favours one extreme, so the mean shifts, as when an antibiotic kills all but the most resistant bacteria and the population becomes resistant.
• Disruptive selection favours both extremes over the middle, so the population can split into two forms, as in African seedcracker finches, where large-billed and small-billed birds each crack a different seed well and medium bills do neither well.
• Disruptive selection is the one that can drive sympatric speciation, if the two forms also start to mate with their own kind.

Try it: which kind of selection?

Genetic drift, founders and bottlenecks

• Genetic drift is a random change in allele frequencies because only some individuals happen to breed, and which gametes meet is chance.
• Drift does not respond to the environment, so it can make a harmful allele common or lose a useful one.
• In the founder effect a few individuals start a new population, and its gene pool holds only their alleles, at frequencies set by chance; tauhou (silvereyes) that colonised New Zealand's offshore islands carry fewer alleles than the Australian birds they came from.
• In a bottleneck a population crashes, by a disaster or by people, and the survivors' alleles are all that remain; kākāpō fell to 51 known birds in 1995, so today's birds share a narrow gene pool.
• After either event, drift keeps acting strongly while the population stays small, so alleles go on being lost or fixed for generations.

Try it: drift in a small population

How the four processes act together

• Mutation supplies new alleles; selection and drift decide which ones become common; gene flow mixes them between populations.
• In a large, connected population gene flow and size keep drift weak, so selection does most of the work and populations stay alike.
• In a small, isolated population drift is strong and gene flow is absent, so the gene pool can change quickly in random directions as well as adaptive ones.
• When two populations are cut off, each gets its own mutations, its own selection pressures and its own drift, and with no gene flow to undo the differences their gene pools diverge.
• That divergence is what can, given enough time, make them unable to interbreed: speciation.

Try it: selection in a large population

What Achieved, Merit and Excellence look like

• Achieved defines the processes; Merit explains how each one changes allele frequencies; Excellence links several processes to each other and to the evidence in the question, saying which matters more and why.