A4.1 Evolution and speciation

A breeding population already contains heritable variation produced by mutation and recombination. The breeder does not create the desired allele by wanting it.

Individuals with a desired phenotype are chosen as parents, so alleles contributing to that phenotype are overrepresented among their offspring.

Repeating the choice over generations shifts the population mean and can produce striking breeds or crop forms, as in pigeons, Brassica vegetables and maize.

One ancestral species begins as populations exchanging alleles through reproduction; gene flow tends to keep their gene pools similar.

A geographic, ecological, temporal, behavioural or chromosome barrier reduces allele exchange, so the populations no longer evolve as one fully connected gene pool.

Mutation, genetic drift and different selection pressures alter allele frequencies independently on each side of the barrier.

If reproductive isolation becomes strong enough that the lineages no longer exchange genes and produce fertile offspring, one pre-existing species has split into two or more species.

Chimpanzees occur mainly north of the Congo River and bonobos south of it. A founder population crossing during a lower-water period, followed by renewed river isolation, is one evidence-based explanation for their divergence from a common ancestor.

The river restricts movement and mating, so alleles are not continually mixed between the populations. Isolation creates the conditions for divergence; it does not directly specify which traits will evolve.

Different environments and social or ecological interactions can favour different heritable variants, while mutation and genetic drift also change each gene pool independently. Over many generations these processes can establish reproductive isolation.

An ancestral population reaches an environment containing several underused ecological niches, such as different food sources or habitats.

Heritable variants that improve performance in different niches leave more offspring in those niches, so populations diverge in traits such as beak form.

Reduced gene flow and reproductive isolation turn the diverging populations into several related species. Niche differences reduce direct competition and allow the species to coexist.

A meiotic spindle failure can produce an unreduced 2n gamete instead of a haploid n gamete.

Fusion of two 2n gametes, or genome doubling in a diploid individual, produces a 4n autopolyploid containing extra chromosome sets from the same species.

A 4n plant crossed with a 2n ancestor produces 3n offspring. Three homologues cannot divide into balanced pairs during meiosis, so the triploid is usually infertile.

Two compatible 4n plants can pair homologues and produce fertile offspring with one another. The chromosome-number barrier can therefore establish a reproductively isolated lineage abruptly.

Gametes from two related species, carrying chromosome sets A and B, can fuse to form an AB hybrid. Each chromosome lacks a fully homologous partner, so meiosis is unbalanced and the hybrid is usually sterile.

Whole-genome doubling produces AABB. Every chromosome now has a homologous partner, allowing balanced pairing and potentially fertile gametes.

The fertile allopolyploid can reproduce with others carrying AABB but not successfully with either original diploid parent, creating a new reproductively isolated lineage.