D4.1 Natural selection

Natural selection is the process in which heritable trait differences cause some individuals to leave more surviving offspring than others.

Selection pressure changes success; successful individuals pass alleles more often; across generations the population’s trait and allele frequencies shift.

Trace variation; pressure; survival or mating; offspring; population change.; separate variation, selection, inheritance and time

In a dry habitat, beetles whose colour better matches the soil are eaten less and leave more offspring, so camouflage becomes more common. This gives a concrete prediction from the stated population.

Individuals do not evolve because they need to; selection changes inherited variation already present. Interpret the result within the stated selection model and evidence limits.

Mutation introduces new DNA variants and sexual reproduction reshuffles existing alleles, supplying variation on which selection can act.

A mutation may alter a protein or regulation; meiosis and fertilization combine alleles in new ways. Selection then filters outcomes rather than producing the initial variation.

Label each source as new sequence change or allele reshuffling before predicting population variation.; separate variation, selection, inheritance and time

A mutation conferring insecticide resistance can spread faster when resistant and susceptible alleles are reshuffled into different offspring. This gives a concrete prediction from the stated population.

Most mutations are neutral or harmful; selection does not make every new variant advantageous. Interpret the result within the stated selection model and evidence limits.

When more offspring are produced than a habitat can support, individuals compete for food, space, mates or other limiting resources.

Resource limits turn small trait differences into differences in survival and reproduction. Competition is therefore a mechanism linking population size to selection pressure.

Name the limiting resource and the trait that changes access to it.; separate variation, selection, inheritance and time

A clutch produces ten hatchlings but the nest supports four; faster-feeding hatchlings obtain more food and are more likely to fledge. This gives a concrete prediction from the stated population.

Overproduction does not mean every species always exceeds carrying capacity; the model concerns potential versus available resources. Interpret the result within the stated selection model and evidence limits.

Abiotic factors such as temperature, salinity, pH or drought act as selection pressures when they change survival or reproduction unequally.

The factor alters physiology or resource access; individuals with suitable heritable traits leave more offspring; allele frequencies can then change.

Link the environmental value to a measurable survival or reproductive consequence.; separate variation, selection, inheritance and time

During a drought, plants with deeper roots maintain water uptake and set more seeds than shallow-rooted plants. This gives a concrete prediction from the stated population.

A trait is not an adaptation merely because it is useful; inheritance and differential reproduction must be shown. Interpret the result within the stated selection model and evidence limits.

Biological fitness is relative contribution of surviving offspring to the next generation, not simply strength, speed or lifespan.

A trait increases fitness only in a specified environment and comparison. Survival matters because it affects the number of offspring that inherit the trait.

Compare individuals in the same conditions using offspring contribution as the outcome measure.; separate variation, selection, inheritance and time

A smaller bird that produces eight fledglings can have higher fitness than a larger bird producing two. This gives a concrete prediction from the stated population.

Fitness is context-dependent; a trait advantageous in one habitat can be costly in another. Interpret the result within the stated selection model and evidence limits.

For selection to produce an evolutionary change, the trait difference associated with reproductive success must be transmitted to offspring.

Environmental effects can change an individual’s phenotype without changing inherited alleles. Only the heritable component can shift population frequencies across generations.

Ask whether offspring resemble parents for the trait before attributing a long-term change to selection.; separate variation, selection, inheritance and time

If dark fur is inherited and dark mice leave more pups, dark alleles rise; if fur darkens only from soot exposure, the population need not evolve. This gives a concrete prediction from the stated population.

A trait can be heritable yet show little response when selection is weak or environments change. Interpret the result within the stated selection model and evidence limits.

Sexual selection is selection for traits that increase access to mates or fertilization success, even when they carry survival costs.

Mate choice and competition among same-sex individuals change reproductive success. The trait spreads when its mating advantage outweighs its costs.

Separate survival benefit from mating benefit before explaining a conspicuous trait.; separate variation, selection, inheritance and time

Peacock tail feathers may attract mates while making escape harder, so mating success can favour the tail despite predation risk. This gives a concrete prediction from the stated population.

Sexual selection is one component of natural selection, not a guarantee that the trait improves survival. Interpret the result within the stated selection model and evidence limits.

A selection model represents how different survival or reproduction rates alter genotype or allele frequencies through repeated generations.

Set initial frequencies and fitnesses; calculate surviving contributions; normalize the next generation; repeat. The direction depends on assumptions and starting values.

Check which class has the highest reproductive contribution before reading a model’s trend.; separate variation, selection, inheritance and time

If resistant bacteria leave twice as many offspring as susceptible bacteria under antibiotic exposure, the resistant frequency rises over successive cycles. This gives a concrete prediction from the stated population.

A model is not a forecast without its assumptions; migration, mutation and changing pressures can alter the trend. Interpret the result within the stated selection model and evidence limits.

A gene pool is the collection of all alleles present in an interbreeding population at a given time.

Counting alleles across individuals lets us describe population genetic composition and compare it across generations. It is a population property, not one organism’s genome.

Define the population boundary and count allele copies before comparing gene pools.; separate variation, selection, inheritance and time

Ten diploid individuals carry 20 copies at a locus; those copies make the population’s gene pool for that locus. This gives a concrete prediction from the stated population.

A gene pool is not the sum of every species in an ecosystem; it is tied to a breeding population. Interpret the result within the stated selection model and evidence limits.

Allele frequency is the proportion of copies of an allele among all copies at a locus in a population.

In a diploid sample, count each allele copy, divide by total copies, and compare the proportion through time. Genotype frequency and allele frequency are related but different.

Count copies rather than individuals carrying the allele, then divide by the total allele count.; separate variation, selection, inheritance and time

If 30 of 100 allele copies are A, the frequency of A is 0.30, not the fraction of individuals with at least one A. This gives a concrete prediction from the stated population.

The denominator changes with ploidy and sampling; state the population and locus. Interpret the result within the stated selection model and evidence limits.

Allele frequencies change when selection, genetic drift, gene flow or mutation changes which alleles are represented in the next generation.

Selection is non-random reproductive difference; drift is sampling chance; migration moves alleles; mutation creates new variants. Their relative strengths determine the pattern.

Identify the process and its expected direction before interpreting a frequency change.; separate variation, selection, inheritance and time

A storm randomly leaves mostly one colour of beetle (drift), whereas a predator consistently removes the visible colour (selection). This gives a concrete prediction from the stated population.

A frequency change alone cannot identify the cause without population and environmental evidence. Interpret the result within the stated selection model and evidence limits.

Directional selection shifts a trait mean; stabilizing selection favours intermediate values; disruptive selection favours extremes.

The pattern depends on which phenotypes have higher fitness. Repeated selection changes the distribution, not merely one individual’s trait.

Compare fitness at low, intermediate and high trait values before naming the pattern.; separate variation, selection, inheritance and time

If both very small and very large beaks feed better than medium beaks, disruptive selection can widen the distribution. This gives a concrete prediction from the stated population.

The labels describe the observed fitness pattern; they do not specify the genes or guarantee speciation. Interpret the result within the stated selection model and evidence limits.

For two alleles with frequencies p and q, the Hardy–Weinberg model predicts p², 2pq and q² genotype frequencies under specified assumptions.

The equation p+q=1 describes allele frequencies; p²+2pq+q²=1 follows random union of gametes. It provides a null expectation for detecting evolutionary forces.

State p and q; calculate expected genotypes; compare observed values with the model.; separate variation, selection, inheritance and time

If q=0.2, expected aa frequency is q²=0.04 and heterozygote frequency is 2pq=0.32. This gives a concrete prediction from the stated population.

The model is an expectation, not a claim that real populations never evolve. Interpret the result within the stated selection model and evidence limits.

Hardy–Weinberg equilibrium assumes a large population, random mating, no migration, no mutation and no selection.

These conditions prevent systematic changes in allele frequencies or sampling noise. Violating one condition can make observed genotype frequencies depart from expectation.

Check each assumption before using an equilibrium calculation as a biological conclusion.; separate variation, selection, inheritance and time

A small isolated population can deviate because drift acts strongly even if no selection is present. This gives a concrete prediction from the stated population.

A departure does not reveal which assumption failed; independent evidence is needed. Interpret the result within the stated selection model and evidence limits.

Artificial selection occurs when humans choose which organisms reproduce to increase a desired inherited trait.

Repeated selection raises the frequency of alleles associated with the chosen phenotype, often faster than natural environments would. Trade-offs can reduce genetic diversity or expose harmful linked traits.

Identify the human criterion, breeding decisions and inherited response across generations.; separate variation, selection, inheritance and time

Breeders repeatedly choose dairy cattle with high milk yield, increasing average yield but potentially narrowing the genetic base. This gives a concrete prediction from the stated population.

Artificial selection is not genetic engineering; it changes reproduction choices rather than directly editing DNA. Interpret the result within the stated selection model and evidence limits.