Evolution and speciation

📚The biology

What evolution is

Evolution is change in the heritable characteristics of a population over time. Two words in that definition do all the work:

• Heritable — the characteristic must be encoded in genes and passed to offspring. A bodybuilder’s muscles, a sun tan, or a tree bent by the wind are changes acquired during an individual’s life, not in its genes, so they are not evolution.
• Population — individuals do not evolve. A population evolves when the frequency of heritable characteristics changes across generations.

This definition separates Darwinian evolution from Lamarckism, the earlier idea that organisms change in response to need during their lives and pass these acquired changes on — for example that giraffes stretched their necks and their offspring inherited longer necks. Acquired characteristics are not inherited, so Lamarckism is rejected.

Evolution by natural selection is called a theory not because it is doubtful but because of how science works. It explains and predicts a vast range of observations, and nothing has falsified it. But no scientific theory can be formally proved true, only supported; it is accepted as a pragmatic truth that works.

Evidence from sequences

Comparing base sequences of DNA or RNA, or amino acid sequences of proteins, gives some of the most powerful evidence of common ancestry. Proteins such as cytochrome c and haemoglobin are found in a very wide range of species, and their sequences are similar but not identical. The pattern of differences matches the pattern of relatedness predicted by other evidence: species thought to be closely related have very few differences, and distantly related species have many. Mutations accumulate over time after lineages split, so the number of differences reflects how long ago they shared an ancestor (A3.2).

Evidence from selective breeding

Humans have bred domesticated animals and crop plants by choosing which individuals reproduce. Over relatively few generations this has produced enormous variation:

• all dog breeds, from chihuahuas to Great Danes, descend from wolves;
• cabbage, broccoli, cauliflower, kale, kohlrabi and Brussels sprouts are all varieties of one wild species, Brassica oleracea;
• maize was bred from teosinte, a wild grass with tiny cobs of a few hard kernels.

The differences between breeds, and between breeds and their wild ancestors, show that heritable characteristics of a population can change rapidly when there is consistent selection. If artificial selection can achieve this in thousands of years, natural selection acting over millions of years can produce the diversity of life.

Evidence from homologous structures

Homologous structures have the same basic structure, inherited from a common ancestor, even if they now have different functions.

The standard example is the pentadactyl limb, the five-digit limb of tetrapod vertebrates. A human arm, a bat wing, a whale flipper, a bird wing and a lizard leg all contain the same arrangement of bones:

These limbs do very different jobs — grasping, flying, swimming, walking. If each had been designed from scratch for its function, there is no reason they would share a bone plan. The shared plan is best explained by descent from a common ancestor, with the limb then modified by adaptive radiation for different uses.

Convergent evolution and analogous structures

Analogous structures have the same function but different evolutionary origins. They arise by convergent evolution: unrelated organisms facing similar selection pressures evolve similar solutions.

• The wings of birds and the wings of insects: both used for flight, but a bird wing is a modified pentadactyl limb with bones, while an insect wing is an outgrowth of the exoskeleton with no bones.
• The eyes of octopuses and vertebrates: similar camera-like eyes, but they develop from different tissues and differ in structure.
• The streamlined bodies of sharks (fish) and dolphins (mammals).

Speciation by splitting

Speciation is the formation of new species by the splitting of an existing species into two or more. It is the only way new species have ever appeared. Two consequences follow:

• Speciation increases the total number of species on Earth, and extinction decreases it. Biodiversity at any time reflects the balance between the two.
• A species that changes gradually over time into a different form, without splitting, has evolved but has not undergone speciation: the number of species has not changed.

Reproductive isolation and differential selection

For one species to become two, two things must happen:

1. Reproductive isolation. The populations must stop interbreeding, so that genes no longer flow between them. The commonest way is geographical isolation: a barrier such as a river, mountain range or stretch of sea separates them.
2. Differential selection. The separated populations experience different conditions and so different selection pressures, and each accumulates different heritable characteristics. Chance events and different mutations add to the divergence. Eventually the populations are so different that they could not interbreed even if they met.

Bonobos and common chimpanzees are a specific example. Their common ancestor lived in central Africa. The formation of the Congo River, around one to two million years ago, separated the populations: bonobos now live only south of the river and chimpanzees to the north. Neither species swims, so the river is an effective barrier. The habitats on either side differ — notably, gorillas compete with chimpanzees for food north of the river but are absent from the bonobos’ range — and different selection pressures led to divergence in body form and in behaviour: bonobos are more slender and their social groups are generally less aggressive and led by females. They are now regarded as separate species.

Sympatric and allopatric speciation AHL

Reproductive isolation can be:

• Geographic — a physical barrier separates the populations (allopatric).
• Behavioural — the populations are in contact but no longer respond to each other’s courtship signals, such as songs, displays or pheromones.
• Temporal — the populations breed or flower at different times of day or year, so they never mate even in the same place.

Similarities: both result in reproductive isolation, stop gene flow, and lead to the divergence of gene pools by differential selection. Difference: whether the populations are physically separated.

Adaptive radiation AHL

Adaptive radiation is the rapid evolution of many species from one ancestral species, each adapted to a different way of life. It typically happens when a species reaches an environment with many vacant niches — an isolated island or lake, or a world emptied by a mass extinction.

Examples include the finches of the Galápagos Islands, whose beak shapes are adapted to different foods, the Hawaiian honeycreepers, and the hundreds of cichlid fish species in the lakes of East Africa. Because each species exploits a different resource, closely related species can coexist without competing, so adaptive radiation increases biodiversity within an ecosystem.

Barriers to hybridization and sterile hybrids AHL

Once species have formed, several mechanisms keep their gene pools from mixing:

• Barriers before mating. In animals, courtship behaviour is often species-specific: a female responds only to the song, display or scent of males of her own species, so mating between species rarely happens.
• Sterility of hybrids. If two species do mate and produce offspring, the interspecific hybrid is often sterile. A mule, the offspring of a male donkey and a female horse, is strong and healthy but cannot reproduce. Its chromosomes come from two different sets that cannot pair properly in meiosis, so it cannot produce viable gametes. Its alleles are therefore not passed on, and the gene pools of horses and donkeys remain separate.

Abrupt speciation by hybridization and polyploidy AHL

Most speciation is gradual, but in plants a new species can arise in a single generation.

• Polyploidy is having more than two sets of chromosomes. It can arise when a cell’s chromosomes are duplicated but the cell does not divide.
• When two different species hybridize, the hybrid is usually sterile, because its two sets of chromosomes are not homologous and cannot pair in meiosis.
• If the hybrid’s chromosome number then doubles, every chromosome has an identical partner. Meiosis can proceed normally and the plant becomes fertile.
• The new polyploid plant can breed with itself or with similar polyploids but not with either parent species: offspring of a cross back to a parent would have an odd number of chromosome sets and be sterile. It is instantly reproductively isolated — a new species.

The knotweeds and smartweeds of the genus Persicaria are a good example: the genus contains many species that have arisen by hybridization and polyploidy, and their chromosome numbers are multiples of a basic set. Many crop plants, including wheat, are also polyploids.