Explain natural selection and the mechanisms that drive evolution and lead to speciation

What this dot point is asking

SCSA wants you to treat evolution as a population-level process measured by changing allele frequencies, not as individuals "trying" to change. A strong answer links the source of variation to the agents of selection, and then shows how isolation turns one gene pool into two.

Variation and the gene pool

A population is a group of interbreeding individuals of the same species. The gene pool is the total collection of alleles in that population. Evolution is defined as a change in the relative frequencies of alleles in a gene pool from one generation to the next.

Variation is the raw material. It arises from:

• Mutation, the ultimate source of new alleles.
• Meiosis, which shuffles existing alleles through crossing over and independent assortment.
• Sexual reproduction, which combines alleles from two parents through random fertilisation.

Without heritable variation there is nothing for selection to act on.

Natural selection

Natural selection follows a clear logic, often summarised as variation, selection pressure, differential survival and reproduction, and inheritance.

• Individuals in a population vary in their heritable characteristics.
• More offspring are produced than the environment can support, so there is competition for resources (a struggle for existence).
• A selection pressure (predation, disease, climate, food supply) means some variants survive and reproduce more successfully than others.
• These individuals pass their advantageous alleles to the next generation.
• Over many generations the favourable alleles become more common in the gene pool.

Other mechanisms of evolution

Natural selection is not the only driver of allele frequency change.

• Genetic drift is random change in allele frequencies, strongest in small populations. Chance events can remove or fix alleles regardless of their fitness.
• The bottleneck effect occurs when a population is drastically reduced (for example by disaster), leaving a small, less diverse gene pool.
• The founder effect occurs when a few individuals start a new population, carrying only a sample of the original gene pool.
• Gene flow is the movement of alleles between populations through migration, which tends to keep populations similar.

Selection patterns

• Directional selection favours one extreme phenotype, shifting the mean (for example, larger body size).
• Stabilising selection favours the intermediate phenotype and reduces variation (for example, human birth mass).
• Disruptive selection favours both extremes over the middle and can split a population.

Speciation

A species is commonly defined (the biological species concept) as a group of organisms that can interbreed to produce fertile offspring. Speciation is the formation of new species from existing ones, and it requires reproductive isolation so that two groups stop exchanging alleles.

Allopatric speciation is the most common pathway:

• A geographic barrier (mountain range, river, ocean, new desert) physically separates a population.
• Gene flow between the groups stops.
• Each group experiences different mutations, selection pressures and drift.
• The gene pools diverge until, even if the groups meet again, they can no longer interbreed to produce fertile offspring.

Sympatric speciation occurs without a geographic barrier, through reproductive isolation arising within the same area (for example, polyploidy in plants or a shift in food source or breeding time).

Isolating mechanisms

Reproductive isolation can be prezygotic (preventing mating or fertilisation) or postzygotic (acting after fertilisation):

• Prezygotic: temporal (different breeding times), behavioural (different courtship), geographic, morphological (incompatible body structures), and gametic incompatibility.
• Postzygotic: hybrid inviability, hybrid sterility (such as the mule), and hybrid breakdown in later generations.

Evidence for evolution

You should be able to cite supporting evidence: the fossil record showing transitional forms and change over time, comparative anatomy (homologous structures from common ancestry, analogous structures from convergent evolution, and vestigial structures), comparative embryology, biogeography, and molecular evidence such as similarities in DNA and protein sequences that allow phylogenetic trees to be built.