A4.1 Evolution and speciation

Evolution is the unifying idea of all biology: it explains both why living things are so diverse and why, underneath, they are so similar. The syllabus defines evolution as the change in the heritable characteristics of a population over time — note that populations evolve, not individuals. A4.1 asks you to marshal the evidence that evolution has occurred, to explain the mechanism (natural selection) that drives it, and to show how one species can split into two. The thread running through it all is that evolution is a consequence of inherited variation combined with differences in survival and reproduction.

Evidence for evolution

No single observation proves evolution; the case is built from many independent lines of evidence that all point the same way:

• Fossils show that life has changed over time and reveal transitional forms linking older and newer groups. The relative ages of rock strata place these changes in sequence.
• Selective breeding of domesticated animals and crop plants shows that heritable characteristics can change dramatically within a species over just a few generations — artificial selection demonstrates in miniature what natural selection can do.
• Homologous structures such as the pentadactyl (five-fingered) limb of mammals, birds, reptiles and amphibians share the same basic bone arrangement despite different functions, pointing to a common ancestor.
• Patterns of variation such as continuous geographical change, and molecular similarities in DNA and proteins, also support common ancestry.

Contrast homologous structures with analogous structures (like an insect wing and a bird wing), which look alike because of similar function rather than shared ancestry — these are the result of convergent evolution.

Natural selection: the mechanism

Darwin and Wallace proposed natural selection as the mechanism of evolution. It can be reasoned out as a chain:

• Populations produce more offspring than the environment can support, so there is a struggle for survival.
• Individuals show heritable variation — differences that can be passed to offspring.
• Some variants are better suited to the environment and so are more likely to survive and reproduce (differential reproductive success).
• These individuals pass on the favourable alleles, so over generations the frequency of those alleles increases in the population.

It is crucial to state that selection acts on the variation that already exists; it does not create new features on demand. The environment is the selection pressure, and the result is adaptation — populations become better matched to their surroundings. A classic example is antibiotic resistance in bacteria, where resistant variants survive treatment and pass resistance on.

The sources of heritable variation

Natural selection can only work if there is variation to select from, so the syllabus emphasises where heritable variation comes from:

• Mutation is the original source of new alleles. Random changes in DNA create the raw material on which selection acts.
• Meiosis shuffles alleles through crossing over and the independent assortment of chromosomes, producing genetically unique gametes.
• Sexual reproduction then combines alleles from two parents at fertilisation, generating new combinations in the offspring.

Because of this, sexually reproducing species generate far more variation than those reproducing asexually, which gives populations a greater capacity to adapt to changing conditions.

Speciation through isolation

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, and it happens when populations become reproductively isolated so that gene flow between them stops and they diverge.

The clearest route is geographic isolation leading to allopatric speciation: a physical barrier such as a sea, river or mountain range splits a population. The two groups then experience different selection pressures and accumulate different mutations, so they gradually diverge. If, after the barrier is removed, they can no longer interbreed to produce fertile offspring, they have become separate species.

Speciation can also occur without a physical barrier (sympatric speciation), for example through differences in behaviour, timing of reproduction or, in plants, sudden changes in chromosome number (polyploidy). In every case the essential step is the same: a barrier to interbreeding allows two gene pools to evolve independently.