Selection and Evolution

State that phenotypic variation is caused by genetic factors, environmental factors, or a combination of both — and give a matching example for each (ABO blood group; hydrangea flower colour; milk yield in cool vs hot climates)

Define discontinuous variation — the data fall into a finite number of distinct categories with no intermediates; the cause is one (or a few) genes

Define continuous variation — the data fall into a smooth range with every intermediate value present; the cause is many genes, each with a small additive effect (polygenic), often modulated by environment

State the genetic basis of discontinuous variation: one (or a few) gene, each allele producing a distinct phenotype

State the genetic basis of continuous variation: many genes (polygenic) with additive effects; the phenotype is the sum of the small effects

State that the t-test compares the means of two samples of continuous data; the formula is provided in the examination

Apply the t-test: identify the two means, two standard deviations and two sample sizes from a table; compute the numerator $|\bar{x}_1 - \bar{x}_2|$ and the denominator $\sqrt{s_1^2/n_1 + s_2^2/n_2}$; divide to get t

State the t-test decision rule: if t > critical value (at the chosen p and ν = n_1 + n_2 - 2 degrees of freedom), the difference is significant and not due to chance; if t ≤ critical value, the difference is not significant

Define natural selection as the differential survival and reproduction of individuals with different phenotypes, due to a selection pressure (e.g. predation, disease, climate, competition)

State the five-step process: (1) variation exists in the population; (2) a selection pressure acts; (3) individuals with the favoured phenotype have a selective advantage and survive / reproduce more; (4) the favoured alleles are passed to offspring; (5) the allele frequency of the favoured phenotype increases in the population over generations

State the struggle for existence: more offspring are produced than can survive; competition for resources (food, mates, space) means only some survive to reproduce

Define directional selection: one extreme phenotype is favoured; the mean of the trait shifts in one direction (e.g. antibiotic resistance, longer-necked giraffes)

Define stabilising selection: the intermediate phenotype is favoured; the mean stays the same but the variance decreases (e.g. human birth weight, clutch size in birds)

Define disruptive selection: both extreme phenotypes are favoured; the mean stays the same but the variance increases (e.g. African seedcrackers with small or large beaks)

Define genetic drift: random changes in allele frequency, especially in small populations; the favoured allele is not necessarily the one that drifts to high frequency

Define the founder effect: a small group of individuals establishes a new population; the new population's gene pool is a biased sample of the original population's gene pool (drift)

Define the bottleneck effect: a population is reduced to a small number of individuals by a catastrophic event; the surviving population's gene pool is a biased sample of the original population's gene pool (drift)

Outline the evolution of antibiotic resistance: random mutation produces a resistance allele; the antibiotic acts as a selection pressure; resistant bacteria survive and reproduce; the resistance allele increases in frequency

Outline horizontal gene transfer: a bacterium can acquire a resistance allele by conjugation (plasmid transfer through a pilus), transformation (uptake of free DNA) or transduction (phage-mediated transfer)

State the Hardy-Weinberg equations: $p + q = 1$ and $p^2 + 2pq + q^2 = 1$ (provided in the examination)

Apply the Hardy-Weinberg principle in five steps: (1) compute $q^2$ from the count of homozygous recessives; (2) compute $q = \sqrt{q^2}$; (3) compute $p = 1 - q$; (4) compute $2pq$; (5) multiply $2pq$ by the population size to get the number of heterozygotes

State the conditions of the Hardy-Weinberg principle: no selection, no mutation, no migration, random mating, large population, diploid, two alleles

Interpret a deviation from Hardy-Weinberg: a difference between the expected and observed numbers indicates that one or more of the conditions is not met (e.g. negative selection against a deleterious allele makes the estimate of $q$ too low)

Outline selective breeding (artificial selection): humans select the individuals with the best expression of the desired trait; only the selected individuals are allowed to breed; the offspring inherit the desired alleles; over many generations, the population becomes more uniform in the desired trait

State the named examples: wheat (high yield, short stem, disease resistance), rice (high yield, disease resistance, drought tolerance), maize (high yield, sugar/starch content, disease resistance), cattle (high milk yield, high meat yield, fast growth rate, disease resistance)

State the consequences of selective breeding: reduced genetic variation, increased vulnerability to disease, inbreeding depression, loss of alleles from the gene pool

Define gene pool: the complete set of alleles of all the genes in a population at a given time

Define evolution: a change in allele frequency in a population's gene pool over generations

State the four mechanisms of evolution: natural selection, genetic drift, migration (gene flow), mutation

Distinguish individual vs. population: an individual cannot evolve; a population evolves; the individual's genotype is fixed at fertilisation

Describe how DNA sequence data is used to investigate evolutionary relationships: compare the base sequences of homologous genes; more similar sequences = more closely related; less similar sequences = more distantly related; count the number of base differences as a measure of evolutionary distance

Interpret a phylogenetic tree: closely-related species are placed on branches that join close to the tips; distantly-related species are placed on branches that join close to the base; a node represents a common ancestor of all the species above it

Explain why DNA evidence is more reliable than morphological evidence: DNA is not affected by convergent evolution; DNA is not affected by environmental variation; DNA provides a quantitative measure of evolutionary distance

Define species (biological species concept): a group of organisms that can interbreed to produce fertile offspring

Define allopatric speciation: speciation that occurs when a population is geographically isolated from its parent population; the two sub-populations diverge in allele frequency (different mutations, different selection, different drift) and become reproductively isolated

Define sympatric speciation: speciation that occurs without geographical isolation; the two populations become reproductively isolated by behavioural, temporal, mechanical or hybrid-infertility mechanisms

State the four mechanisms of reproductive isolation: behavioural (different courtship), temporal (different breeding times), mechanical (incompatible reproductive structures), hybrid infertility (offspring are sterile)

Distinguish cause vs. consequence of speciation: the cause is reproductive isolation; the consequence is morphological divergence — a common error is to claim that morphological difference is the criterion for speciation