Explain how natural selection, mutation, genetic drift, gene flow and sexual selection change the proportions of traits in a population, and use the Hardy-Weinberg principle to compare observed gene frequencies with expected ones

Learning goal: Explain the mechanisms of evolution: natural selection, mutation, genetic drift, gene flow (immigration and emigration) and sexual selection; shift from thinking about selection of individuals to thinking about changing proportions of a trait in a population; describe natural selection as the consequence of four interacting conditions; apply the Hardy-Weinberg principle to compare observed gene frequencies with those expected under its assumptions; and use real-world examples of each mechanism, including how natural selection spreads antibiotic and pesticide resistance and how that spread can be discouraged.

Populations evolve; individuals do not. A population is a group of individuals of one species living in one place and interbreeding. Its gene pool is all the alleles its members carry, and an allele's frequency is its share of all the copies of that gene in the pool. Evolution, in the precise sense this course uses, is a change in the allele frequencies of a population across generations. The grade 8 picture of "the fit individuals survive" is a starting point; the high school picture is about proportions: what fraction of the population carries a trait, and what makes that fraction rise or fall. Five mechanisms do this work.

Natural selection. Natural selection is the process by which traits become more or less common in a population because consistent environmental pressures affect the survival and reproduction of individuals with the trait. It is the consequence of four conditions acting together: (1) a population has the potential to increase its numbers faster than its environment can support; (2) offspring vary genetically because of mutation and the recombination of genes in sexual reproduction; (3) the resources required for life are finite; and (4) individuals survive and reproduce differentially according to their phenotypes. Remove heritable variation and selection has nothing heritable to act on: individuals may still fare differently, but nothing carries into the next generation; weaken overproduction and limited resources and being less suited costs far less, so selection becomes much weaker (though predators and disease can still select even when food is plentiful). Natural selection acts on phenotypes, but it changes allele frequencies, because phenotypes are built from alleles. Its direction is set by the environment, so the same allele can be favored in one place and selected against in another.

Mutation. Mutation is the ultimate source of new alleles; without it, selection and drift would eventually exhaust a population's variation. Mutation rates are low per gene per generation, so mutation alone changes allele frequencies very slowly; its importance is in supplying the variation that the other mechanisms act on. The heredity lessons describe the kinds of mutation and their effects on proteins.

Genetic drift. Genetic drift is change in allele frequencies caused by chance rather than by any advantage. Every generation is a sample of the previous one, and a sample of a few individuals can differ from the population by luck alone, just as ten coin flips can easily give seven heads. Drift is strongest in small populations, where a single storm, or the accident of which animals happened to breed, can shift frequencies or eliminate an allele entirely. Two named cases: a bottleneck, when a population is cut to a small number and the survivors carry only a sample of the original alleles, as happened to cheetahs, which are genetically very uniform; and a founder effect, when a few individuals start a new population and their particular alleles become common in it, as with certain rare inherited conditions that are unusually frequent in some Amish communities descended from a small number of founders. Heritage varieties of crops and livestock, zoo populations and island populations all experience drift, and managing them means keeping numbers large enough that chance does not erode diversity.

Gene flow. Gene flow is the movement of alleles between populations by immigration and emigration of individuals or their gametes. Pollen blowing from a field of genetically modified corn into an organic farmer's field is gene flow; so is a salmon returning to spawn in a river other than its birth river. Gene flow makes neighboring populations more alike and can bring in alleles that selection in the receiving population has not favored. Distinguish it from drift: drift is chance sampling inside a population; gene flow is movement across the boundary between populations.

Sexual selection. Sexual selection is differential reproduction caused by competition for mates and by mate choice. A peacock's tail lowers its survival and raises its mating success; where the second effect outweighs the first, the alleles for the tail spread. Sexual selection explains many traits, from bright plumage to antlers to elaborate songs, that natural selection for survival alone would not produce.

Hardy-Weinberg: what a population looks like when nothing is happening. If a population is very large, mates randomly, and experiences no mutation, no selection and no gene flow, then its allele frequencies do not change from generation to generation, and the genotype frequencies follow from the allele frequencies. For a gene with two alleles, A at frequency p and a at frequency q, with p + q = 1, the genotypes AA, Aa and aa occur at p², 2pq and q². This is the Hardy-Weinberg principle, and its use is as a baseline: when observed genotype frequencies depart clearly from it (by more than sampling error), one of the five assumptions is being broken and deserves investigation; when allele frequencies change over generations, the population is evolving. The two are not the same: nonrandom mating, for example, can shift genotype frequencies without changing allele frequencies. Worked in the prose: in a large population 4 percent of individuals show a recessive trait. Then q² = 0.04, so q = 0.2 and p = 0.8. Expected heterozygotes: 2 × 0.8 × 0.2 = 0.32, so 32 percent of the population are carriers of an allele that only 4 percent express. That is the general lesson about carriers of rare recessive conditions: when q is small, q² is tiny but 2pq is not, so most copies of a rare recessive allele sit unseen in heterozygotes, which is why such alleles are not quickly removed by selection against the affected homozygotes.

Resistance: natural selection in real time. When an antibiotic is used, any bacterium carrying a mutation that lets it survive the drug leaves descendants while its neighbors die; the resistant proportion of the population rises with every course of treatment that fails to clear it. The same mechanism spreads herbicide resistance in weeds sprayed repeatedly with one herbicide and pesticide resistance in insects. Selection cannot be switched off, but it can be discouraged: a medical protocol against resistance uses antibiotics only for bacterial infections, chooses the narrowest effective drug, uses the dose and duration that evidence supports, no more and no less, prevents transmission by hygiene, and tracks resistance so that prescribing guidance can change before resistant strains dominate. An agricultural protocol rotates herbicides with different modes of action, rotates crops, adds mechanical weeding, and keeps refuges of unsprayed habitat so that susceptible insects survive to breed with resistant ones, diluting resistance alleles. Every one of these steps works by reducing the reproductive advantage of the resistant phenotype or by adding gene flow from susceptible populations.

Misconceptions. "Individuals adapt" confuses acclimation of one organism with the change of proportions in a population. "Natural selection is random" is wrong; mutation and drift are random with respect to advantage, but selection is directional. "Hardy-Weinberg describes real populations" is backward; it describes what a population would look like without evolution, so that real deviations can be recognized. "Gene flow always helps" is wrong; it can carry in alleles that are harmful locally and prevent populations from adapting to local conditions.

Where the rest of the standards is taught. B.E.2 is shared with the speciation lesson. This lesson teaches variation within species through gene frequencies and the protocols for slowing resistance. Deciding whether populations are distinct species, comparing Lamarck, Darwin and Wallace, the evidence for evolution and cladograms are taught there.

Investigation you can do. Model genetic drift with beads. Put 5 red and 5 blue beads in a cup to represent a population of 10 gene copies with each allele at 0.5. Draw one bead, record its color, put it back, and repeat 10 times; the 10 records are the next generation. Refill the cup to match that generation's counts and repeat for 10 generations, plotting the red frequency each time. Then run the same procedure with 50 red and 50 blue beads and 100 draws per generation. A result in which the small population wanders widely and often loses one color entirely while the large population wanders much less (typically by about 0.1 to 0.2 over ten generations) and keeps both colors supports the claim that drift is strongest in small populations. What the model cannot show: beads have no phenotype, so the model contains no selection at all, no mutation to restore a lost allele and no gene flow, and it says nothing about how fast a real population of a given size would drift.

Worked Hardy-Weinberg with a rare allele: In a large population an autosomal recessive condition affects 1 in 2,500 births. Then q² = 0.0004 and q = 0.02, so p = 0.98. Carrier frequency 2pq = 2 × 0.98 × 0.02 = 0.0392, about 1 in 25 people. The ratio of carriers to affected individuals is 0.0392 ÷ 0.0004 = 98: for every affected person there are about 98 unaffected carriers. Selection against the condition removes only the 1 in 2,500 who express it, so the allele frequency falls very slowly, which is why rare recessive conditions persist for many generations.

Worked test for equilibrium: A sample of 200 plants is scored for a flower-color gene with incomplete dominance, so all genotypes are visible: 98 red (RR), 84 pink (RW), 18 white (WW). Allele counts: R = 2 × 98 + 84 = 280; W = 2 × 18 + 84 = 120; total 400. So p = 0.70 and q = 0.30. Expected genotype counts under Hardy-Weinberg: RR 0.49 × 200 = 98; RW 2 × 0.7 × 0.3 × 200 = 84; WW 0.09 × 200 = 18. Observed matches expected exactly, so this sample gives no evidence that any assumption is broken. If a later sample showed 120 red, 60 pink and 20 white, the allele frequencies would be p = 300 ÷ 400 = 0.75 and q = 0.25, a shift from 0.70, and the heterozygote count of 60 would be below the expected 2 × 0.75 × 0.25 × 200 = 75, which would suggest non-random mating or selection against pink and would call for further study.

Worked identification of mechanisms: (a) A hurricane leaves 12 survivors on an island from a population of 2,000 lizards; the survivors happen to carry mostly the striped allele: genetic drift, a bottleneck, because the change was chance survival, not advantage. (b) Striped lizards later prove harder for birds to see and increase from 60 to 85 percent over 20 generations: natural selection. (c) Lizards from the mainland arrive on floating debris and interbreed: gene flow by immigration. (d) Females prefer males with the largest dewlaps, and dewlap size increases even though large dewlaps attract predators: sexual selection. (e) A new allele for a paler stripe appears in one hatchling: mutation.

Worked protocol against resistance: A hypothetical hospital finds that 30 percent of a bacterial species isolated from patients resists its standard antibiotic. Protocol: confirm each infection is bacterial before prescribing; test isolates and use a different drug where resistance is found; follow evidence-based, clinician-directed choice of drug, dose and duration, aiming to cure the infection without unnecessary exposure; isolate patients with resistant strains and enforce handwashing to stop transmission; track the resistant percentage monthly. Each step lowers the reproductive advantage of resistant cells or cuts their spread, so the resistant proportion should stop rising; the protocol is evaluated by whether the monthly percentage falls.

Supported record: Make a table with the five mechanisms down the side and columns for Random or directional?, Source of new alleles?, Strongest in what kind of population?, Real example. Fill it, then draw a bar chart of the genotype frequencies p², 2pq and q² for p = 0.8, for p = 0.5 and for p = 0.98, and write one sentence about what happens to the share of heterozygotes as the recessive allele becomes rare.

Independent performance: Run the bead-drift simulation for ten generations with 10 gene copies and again with 100, then write an analysis. Success criteria: (a) allele frequencies for every generation of both runs are tabulated and plotted on one graph; (b) the analysis explains, in terms of sampling, why the small population wandered more; (c) it states which Hardy-Weinberg assumptions the model kept and which it broke; (d) it computes p, q and expected genotype frequencies for a given observed recessive frequency and explains what a departure from those expectations would mean; (e) it designs a protocol, medical or agricultural, to discourage the spread of resistance and explains each step through the reproductive advantage of the resistant phenotype; (f) it names two things the bead model cannot show.