D4.1 Natural selection

Natural selection is differential survival and reproduction caused by heritable differences; over generations it changes populations and drives evolution.

Heritable variation exists → an environmental or biological pressure affects individuals differently → some genotypes leave more surviving offspring → their alleles become more frequent → population characteristics change.

Operating continuously over billions of years, repeated selection has contributed to life's biodiversity. It can also be observed over short timescales when pressures are strong.

Darwin supplied a convincing selection mechanism and displaced Lamarckian explanations based on inherited acquired characteristics. Replacing a dominant explanatory framework in this way is a scientific paradigm shift.

Individuals do not evolve because they need to. Selection acts on existing phenotypic differences, while evolutionary change is measured across generations.

Mutation creates new alleles by changing DNA sequence; sexual reproduction creates new combinations of existing alleles.

In a sexually reproducing plant, a mutation may introduce a drought-tolerance allele; meiosis and fertilization then place that allele into varied genetic backgrounds.

Selection does not create a needed mutation. Mutations arise without regard to usefulness, and sexual reproduction reshuffles rather than invents alleles.

Populations can produce more offspring than the environment can support, so individuals of the same species compete for limiting resources.

Food, water, light, mineral ions, territory, nesting sites or mates can limit carrying capacity. Individuals that obtain these resources more successfully are more likely to survive and reproduce.

Potential offspring exceed available resources → intraspecific competition occurs → heritable differences affect resource access → reproductive success differs.

When food limits a population, individuals whose inherited feeding traits increase food acquisition may leave more offspring than competitors.

Overproduction means reproductive potential exceeds long-term support; it does not require the population to remain permanently above carrying capacity.

Abiotic conditions can act as selection pressures when they affect survival or reproduction differently among heritable phenotypes.

High or low temperature, drought, salinity, pH and other physical conditions may affect individuals regardless of population density, so they are density-independent pressures.

Abiotic condition changes → physiological performance differs → survival or reproduction differs → alleles associated with advantageous traits become more frequent.

During repeated low-temperature events, plants with inherited frost tolerance may survive and set more seed, increasing the frequency of tolerance alleles.

A useful response is evolutionary adaptation only if it is heritable and changes reproductive contribution; temporary acclimatization alone does not alter allele frequency.

Biological fitness is a genotype's relative contribution of surviving, reproducing offspring to the next generation in a particular environment.

Intraspecific competition exposes differences in adaptation. Survival has evolutionary importance when it increases reproductive opportunity, and reproduction passes the responsible alleles onward.

Compare individuals in the same population: adaptation to current conditions → survival and mating differences → different numbers of fertile offspring → different fitness.

A genotype that survives well but produces no fertile offspring has lower fitness than a competing genotype that leaves many reproducing descendants.

Fitness does not mean strength, health or lifespan in isolation; it is relative, environment-dependent reproductive success.

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.

John Endler modelled natural and sexual selection in Trinidadian guppies by experimentally controlling predation pressure and gravel background.

After dangerous predators were introduced, mean spot number decreased; with no or weak predation it continued to increase. Coarse gravel favoured larger spots and fine gravel smaller spots when predators were present.

A field transfer from a dangerous-predator site to a weak-predator site produced more colourful males over 15 generations, consistent with sexual selection becoming stronger relative to predation.

The experiment shows a trade-off: conspicuous colour can improve mating success yet reduce survival. Correlation alone is weaker evidence than Endler's controlled manipulation of selection pressures.