A4.1 Evolution and speciation

What evolution means

Evolution is change in the heritable characteristics of a population over generations. The key word is heritable: features gained during a lifetime, such as larger muscles from training, are not passed on through genes and so are not evolution. Lamarck's idea of inheriting acquired characteristics is rejected for this reason, while Darwin's natural selection explains change through differences in survival and reproduction.

Evidence for evolution

• Sequence data. DNA base sequences and protein amino acid sequences can be compared between species. The more differences, the longer ago two lineages are likely to have split. Similar sequences in very different organisms point to common ancestry.
• Selective breeding. Humans have produced dramatic changes in crops and domestic animals within a few thousand years by choosing which individuals breed. This shows that heritable variation exists and that selection can change a population quickly.
• Homologous structures. The pentadactyl limb appears in the arm of a human, the wing of a bat and the flipper of a whale. Different functions but the same underlying bone pattern suggests descent from a shared ancestor followed by adaptive radiation.

Convergent evolution

Analogous structures do similar jobs but have different underlying structures and origins, such as the wings of insects and birds. They arise when unrelated groups face similar selection pressures. Analogous features are evidence of natural selection, but not of close relationship.

Species and speciation

A species is often defined as a group of organisms that can interbreed to produce fertile offspring. Speciation is the formation of a new species from an existing one. Its essential step is reproductive isolation, which stops gene flow between two groups. Once gene flow stops, mutation, natural selection and chance act separately in each group and the gene pools drift apart.

flowchart TD
  A["One interbreeding population"] --> B["Barrier stops gene flow"]
  B --> C["Different mutations arise in each group"]
  C --> D["Different selection pressures act"]
  D --> E["Allele frequencies diverge"]
  E --> F["Groups can no longer interbreed to give fertile offspring"]
  F --> G["Two separate species"]

Barriers can be pre-zygotic (preventing mating or fertilisation, such as different breeding seasons or courtship) or post-zygotic (hybrids are sterile or weak, such as the mule).

Allopatric and sympatric speciation (HL)

• Allopatric: a geographical barrier such as a mountain range, river or sea separates populations.
• Sympatric: populations share the same area but become isolated by behaviour (different courtship), timing (different breeding times) or by sudden genetic change.

Adaptive radiation, hybrids and polyploidy (HL)

Adaptive radiation is the rapid evolution of many species from one ancestor, each adapted to a different niche, often after a group reaches new habitats with little competition, such as finches on islands. Hybrids between species are often sterile because their chromosomes cannot pair properly in meiosis. In plants, polyploidy (extra whole sets of chromosomes) can give an individual that is fertile with others like it but cannot produce fertile offspring with the parent population. This produces a new species in a single generation, an example of sympatric speciation. Many crops, such as wheat, are polyploid.

Common mistakes

• Saying individuals evolve. Populations evolve; individuals do not.
• Treating analogous structures as evidence of close relationship.
• Saying that a barrier "causes mutations". The barrier only stops gene flow; mutations happen randomly anyway.
• Writing that species "try to adapt" or "need" to change. Variation comes first, selection follows.

Exam tips

• Explain speciation questions score marks for a clear sequence: isolation, no gene flow, different selection, different allele frequencies, cannot interbreed.
• Distinguish homologous from analogous with one example of each.
• Outline sequence evidence by stating that the number of differences correlates with time since divergence.
• When an HL question mentions plants and a sudden new species, think polyploidy.