Speciation & Patterns of Evolution

Speciation is the formation of a new species, distinct from other species, through the processes of evolution. Speciation through natural selection has one non-negotiable requirement: there must be no gene flow (no movement of alleles) between the diverging populations.

GENETIC DRIFT

Selection is not the only force that changes a gene pool. Genetic drift is random change in allele frequency — it is not the result of selection pressures. Causes include death, immigration, natural disasters and human activity, and because drift is random it can remove favourable or unfavourable alleles alike. The survivors provide the gene pool for the next generation, so allele frequencies shift by chance. Drift affects small, low-diversity populations most strongly — they may lose beneficial traits altogether, raising their risk of extinction.

Two named causes of genetic drift:

1. Bottleneck effect — a disaster wipes out most of the population, and the few survivors carry only a fraction of the original alleles.
2. Founder effect — a small group breaks away to colonise new territory. Its gene pool is not representative of the original, so as the colony grows its allele frequencies differ. Unlike a bottleneck, the original population remains largely intact. For example, some Amish communities have a higher incidence of polydactyly (extra digits) because of inter-marriage within the community.

Both cause genetic drift: a bottleneck shrinks the whole population, while the founder effect samples a small, unrepresentative group from an intact one.

ALLOPATRIC SPECIATION

Allopatric speciation begins when a population separates into at least two distinct populations. The original population becomes the common ancestor of the new species. The sequence is:

1. A geographic barrier (a mountain range, river, ocean or desert) splits the population and prevents gene flow.
2. The separated populations experience different environmental conditions, so they face different selection pressures.
3. Natural selection (and genetic drift) act differently on each side, so over many generations the populations become genetically different.
4. Eventually pre-zygotic and/or post-zygotic isolating mechanisms evolve. The populations are now reproductively isolated, and new species have formed.

Darwin's finches are the classic example: the finch populations on different islands are separated by a water barrier, which prevents gene flow between each island's population.

Allopatric speciation: barrier → no gene flow → different selection pressures → genetic divergence → reproductive isolation.

SYMPATRIC SPECIATION

Sympatric speciation produces new species from a common ancestor without geographic separation — gene flow must still be prevented in some other way. It is very rare in animals. One route is food preference: individuals that prefer a particular food stay near, and mate with, others with the same preference, limiting gene flow over many generations.

In plants, sympatric speciation occurs through polyploidy — duplication of whole chromosome sets, often after hybridisation between species growing in the same area. Polyploidy is thought to be one of the main mechanisms of plant speciation in angiosperms (flowering plants). Because the hybrid's chromosome count matches neither parent, it is reproductively isolated instantly. Many polyploid hybrids have odd chromosome sets and reproduce asexually (by mitosis) — though allopolyploids can regain sexual fertility if their chromosomes double, giving every chromosome a pairing partner for meiosis.

ADAPTIVE RADIATION

Adaptive radiation is rapid divergent evolution from one common ancestor to fill newly available niches — new food sources or resources, or changed abiotic factors. It is a form of colonisation: mutations accumulate, different niches apply different selection pressures, and natural selection adapts each population to its niche until speciation results. Darwin's finches, radiating into different feeding niches, are the standard example.

DIVERGENT AND CONVERGENT EVOLUTION

• Divergent evolution - species sharing a common ancestor accumulate different characteristics under different selection pressures. Each time one ancestral species diverges into multiple descendant species, that is speciation — so speciation is an important result of divergent evolution. Its signature is homologous structures: the same plan doing different jobs, such as the pentadactyl limb.
• Convergent evolution - unrelated species facing similar selection pressures evolve similar features. Its signature is analogous structures: the same job done by structures of different origin.

Divergent evolution branches one ancestor into many species; convergent evolution brings unrelated lines toward the same form.