A4.1 Evolution and speciation

Evolution is change in the heritable characteristics of a population over generations. Two words in this definition do a lot of work.

• Heritable means coded for by genes and passed from parents to offspring in gametes. Evolution is therefore a change in the frequencies of alleles in a population’s gene pool.
• Population means a group of organisms of the same species living and interbreeding in the same area. Individuals do not evolve: they live and die with the genes they were born with.

Acquired characteristics are not evolution

During its life an organism can change in ways that are not caused by its genes. A weightlifter builds muscle, a plant grown in shade becomes tall and pale, and an injury leaves a scar. These acquired characteristics do not alter the base sequences of genes in the gametes, so offspring do not inherit them.

Before Darwin, many naturalists accepted Lamarckism: the idea that characteristics acquired through use or disuse are inherited. The standard example is a giraffe stretching to reach leaves and passing a slightly longer neck to its young. No mechanism for this has ever been found, and claimed cases have not stood up to testing. In Darwinian evolution the variation comes first, from random mutation and sexual reproduction, and natural selection then changes which alleles become common (see D4.1 Natural selection).

All living organisms store their genetic information in DNA built from the same four bases, and translate it into proteins using almost exactly the same genetic code. This unity is itself evidence that all life shares a single origin. More detailed evidence comes from comparing the sequences of the same gene or protein in different species.

How sequence differences arise

When one ancestral species splits into two, each lineage starts to accumulate its own mutations: changes to the base sequence of its DNA. Some change the amino acid sequence of the protein that the gene codes for. The longer ago two lineages separated from their common ancestor, the more differences they have accumulated. So:

• species that look closely related, such as humans and chimpanzees, have very similar base sequences;
• distantly related species, such as humans and yeast, have many more differences.

Base sequences of DNA or RNA (for example ribosomal RNA, used for comparing all forms of life) and amino acid sequences of proteins can all be compared.

The order matches the relationships worked out from anatomy and fossils. The pattern of shared and unshared differences is best explained by repeated splitting of ancestral species, and it is used to build cladograms (see A3.2 Classification and cladistics).

Domesticated animals and crop plants were developed from wild species by selective breeding (artificial selection): in each generation, people chose individuals with useful heritable traits and bred from them. Many breeds can still interbreed with their wild relatives, which shows they come from the same stock.

Examples

• Dogs were domesticated from grey wolves, at least 15 000 years ago. Breeds as different as a Chihuahua, a greyhound and a St Bernard are still one species and can interbreed.
• Wild cabbage (Brassica oleracea) has been bred into several crops by selecting for different parts of the plant:

• Maize was bred from a wild grass, teosinte, in Mexico about 9000 years ago. Teosinte ears hold a few hard-cased grains; modern maize cobs hold hundreds of exposed kernels.
• Carrots: wild carrot roots are thin and pale, while cultivated varieties are thick and, in most modern varieties, orange because of their high carotene content.

Why this counts as evidence

The differences between varieties, and between them and the wild species, have arisen in only a few thousand years — a short time in evolutionary terms. This shows that heritable variation exists and that selection can change a population quickly. If artificial selection can do this so fast, natural selection acting over millions of years can plausibly account for the differences between species.

Homologous structures have the same underlying structure in different organisms because they were inherited from a common ancestor, even when they are now used for different things.

The classic example is the pentadactyl limb (“five-fingered” limb), found in amphibians, reptiles, birds and mammals. Whether a limb is used for grasping, flying, swimming, running or digging, its bones follow the same plan:

Why homology is evidence for evolution

There is no engineering reason why a bat’s wing, a dolphin’s flipper and a mole’s spade should be built from the same bones. Different designs would work at least as well. The shared plan makes sense only if all these animals descended from an ancestor that had a pentadactyl limb. As its descendants spread into different habitats and ways of life, natural selection modified the limb in each group:

• bones became longer (bat fingers), shorter and flatter (dolphin), fused (bird carpometacarpus) or reduced (the horse’s splint bones);
• digits were lost (horse, bird) or gained extra phalanges (dolphin).

This modification of one inherited structure in different directions in different lineages is called divergent evolution: natural selection in each lineage adapts the shared plan to a different function.

Analogous structures do the same job but have different structures and different evolutionary origins. They arise by convergent evolution: unrelated lineages face similar selection pressures, so natural selection favours similar solutions.

Examples

• Wings of birds and insects. Both produce lift for flight. A bird wing is a modified pentadactyl limb with bones, muscles and feathers. An insect wing has no bones: it is a thin outgrowth of the exoskeleton, made of cuticle containing chitin and stiffened by veins, and it is not a limb at all (insects also have six legs).
• Camera eyes of octopuses and vertebrates. Both have a lens that focuses an image on a retina. They develop differently, and in vertebrates the nerve fibres run across the front of the retina and leave through it, making a blind spot; in the octopus they run behind the retina, so there is no blind spot. Their last common ancestor, over 500 million years ago, did not have a camera eye.
• Streamlined bodies of sharks (fish) and dolphins (mammals), and the tail of a fish compared with the flukes of a whale: a fish’s tail fin is vertical and beats from side to side, while whale flukes are horizontal, are stiffened mainly by tough fibrous tissue rather than bony fin rays, and beat up and down.

Compare the insect wing with the bird and bat wings in the limb comparator above.

A species is a group of organisms that can interbreed to produce fertile offspring (see A3.1 Diversity of organisms for this and other ways of defining species). Speciation is the formation of new species, and it happens in only one way: an existing species splits into two or more populations that evolve separately until they become distinct species.

Speciation, extinction and biodiversity

• Speciation increases the total number of species on Earth.
• Extinction is the opposite process: it decreases the number of species.
• The number of species at any time therefore depends on the balance between the two. Periods of rapid speciation and mass extinctions both show up in the fossil record.

Gradual change is not speciation

A single species can change a great deal over time. Fossils may show its body size or tooth shape changing slowly over millions of years, and palaeontologists sometimes give the earlier and later forms different names. But at any one time there is still only one species, so the number of species has not increased. Gradual evolutionary change within one lineage is not speciation. Only splitting adds species.

Darwin’s notebooks of 1837 already sketched evolution as a branching tree: each fork is a speciation event, and each branch that stops short is a lineage that went extinct.

For one species to split into two, two things are needed.

1. Reproductive isolation. The populations must stop interbreeding. While they interbreed, alleles move between them (gene flow) and keep their gene pools similar. Isolation stops gene flow, so each gene pool can change independently. The most obvious cause is geographical isolation by a barrier such as a mountain range, a wide river or a stretch of sea.
2. Differential selection. If natural selection acted in the same way on both populations, they would stay alike. Speciation needs the selection pressures to differ — for example in climate, predators or competition for food — so that different traits are favoured in each. Genetic drift, random change in allele frequencies, adds to the divergence, especially in small populations.

Eventually the populations are so different that taxonomists judge them to be separate species.

Bonobos and common chimpanzees

Bonobos (Pan paniscus) and common chimpanzees (Pan troglodytes) are our closest living relatives. Their ranges do not overlap: chimpanzees live north and east of the Congo River, and bonobos only south of it. The river is wide and deep, and neither species swims.

The probable history: the common ancestor lived north of the river. Roughly one to two million years ago, during a dry period when the river was low, a small group reached the south bank and was then cut off. In the south there are no gorillas competing for plants on the forest floor, so food is more plentiful and reliable. The two populations were subject to different selection pressures, and they diverged:

• bonobos are more slender, with longer legs and smaller, rounder heads;
• bonobo groups are female-led and much less aggressive than chimpanzee groups, which are dominated by males.