Macroevolution, Speciation and Evidence

1Overview

Arctic foxes and ptarmigans (a bird) turn white in winter. They are not close relatives, yet both have solved the same problem of hiding in snow. Whale flippers, bat wings and human arms look nothing alike in function but contain the same set of bones. Hawaiian honeycreepers have beaks shaped for seeds, nectar or insects, yet all descend from a single ancestor that arrived on the islands. These patterns are the footprints of macroevolution: major evolutionary change over long periods of time, including the origin of new species. This lesson covers how species form, how evolution is described at large scale, and the evidence that supports it.

2From microevolution to macroevolution

The previous lesson described microevolution, changes in allele frequencies within a population. Macroevolution is the accumulation of such changes over long periods until populations become separate species, and eventually into new genera, families and larger groups. The modern synthesis links the two: no special process is required beyond the ordinary mechanisms of mutation, selection, drift and gene flow operating over very long timescales and with isolation between populations.

3What is a species, and how do new ones arise?

By the biological species concept, a species is a group of individuals that interbreed in nature and produce fertile, viable offspring. Speciation occurs when one ancestral species splits into two descendant species that can no longer interbreed. Hybrids between different species may occur but are usually infertile.

Reproductive isolation

Speciation depends on reproductive isolation, barriers that stop gene flow between populations.

Allopatric speciation

In allopatric speciation, geographic separation stops gene flow. The separated populations experience different selection pressures and different random drift, so over time their gene pools diverge until they can no longer interbreed even if they meet again. Separation can come from dispersal of a few individuals to a new place or from a barrier that splits a range, such as a rising mountain range or a changing river. Northern and Mexican spotted owls are an example of populations separated geographically.

Sympatric speciation

Sympatric speciation occurs within a shared location without physical barriers. In plants it often happens through polyploidy, where extra sets of chromosomes arise, either within one species (autopolyploidy) or from hybridization between species (allopolyploidy). Wheat, cotton and tobacco are examples. In animals, sympatric speciation can occur when groups exploit different food sources, as is described for cichlid fish in some lakes.

4Models of the pace of speciation: neo-Darwinism and punctuated equilibrium

• Neo-Darwinism (phyletic gradualism) holds that change is continuous and occurs at a steady rate, with speciation involving no mechanisms beyond ordinary natural selection.
• Punctuated equilibrium (proposed by Eldredge and Gould in 1972) holds that large changes happen mainly during speciation events, with long periods of little change (stasis) in between. Stasis is attributed to stabilizing selection or developmental constraints. Because new species arise in small peripheral populations, transitional forms are rarely found.

The fossil record offers examples of both. Some lineages fit punctuated equilibrium: Caribbean bryozoans barely changed over 5 to 15 million years, and horseshoe crabs changed greatly early in their history and then hardly at all. Others show steady change, such as Welsh trilobites, horse teeth and foraminifera. Evolutionary rates therefore vary greatly: rapid change tends to occur in empty habitats under strong selection, and slow change in stable habitats. The distinction between the two models is probably a continuum, and nothing in the evidence requires speciation to involve processes beyond conventional natural selection.

Hybrid zones and incomplete speciation

Speciation is a process, not an instant. Hybrids between similar species occur in nature, which may mean that speciation is not yet complete. Where two populations meet and interbreed to some degree, biologists can study how strongly the barriers act and whether they are strengthening or weakening. This is one reason the boundary between "one species" and "two species" is sometimes difficult to draw, and why the biological species concept, useful as it is, does not apply neatly to organisms that reproduce asexually, such as bacteria.

5Other forces in speciation: genetic drift and sexual selection

Genetic drift, random change in allele frequencies, is strongest in small populations. Small founding populations at the edge of a range, or survivors of a bottleneck, can diverge quickly from the main population by chance. Drift can therefore speed up the divergence that leads to speciation.

Sexual selection favours traits that help individuals win mates. Elaborate ornaments, displays or songs can make mate choice very specific, so populations that develop different mating signals may stop recognizing one another as mates, a form of behavioural isolation. Traits such as a peacock's tail help reproduction while hurting survival, and the handicap principle and the good genes hypothesis are two proposals for why such traits can signal quality.

Adaptive radiation

Adaptive radiation is the diversification of one ancestral species into many species that fill different ecological roles. It often happens when a founder reaches an empty habitat, such as an island, or after a change opens up new resources. Hawaiian honeycreepers, which have beak shapes evolved for seeds, nectar or insects, and Darwin's finches are island examples. Galapagos finches diversified into 14 species in about 500,000 years, and rift-lake cichlids show rapid change in feeding structures and coloration.

6Processes of macroevolution: divergent, convergent and co-evolution

Divergent evolution

In divergent evolution, species descended from a common ancestor evolve in different directions. Adaptive radiation is a clear case. Flowering plants share a basic reproductive structure, but their forms differ as a result of varied environments and pollinators. The result is homologous structures: the limb bones of humans, dogs, birds and whales share the same basic layout despite differences in size and shape, pointing to a shared ancestral limb.

Convergent evolution

In convergent evolution, unrelated species in similar environments evolve similar traits because similar selection pressures favour them. Arctic foxes and ptarmigans both turn white in winter, and insect, bat and bird wings all serve flight. These are analogous structures: similar function but different construction and developmental origins, so the similarity does not indicate close relationship.

Co-evolution

Co-evolution is reciprocal evolutionary change between interacting species, in which each exerts selection pressure on the other. The yucca moth pollinates the yucca flower while laying its eggs in it, so each depends on the other. Interactions between predators and prey, and between flowers and pollinators, are other settings where reciprocal change can occur. Over long periods, the interaction can lead to close specialization. A cuckoo's egg in a reed warbler's nest is often cited as a case in which hosts and a brood parasite push each other to change.

7Evidence for macroevolution

Several independent lines of evidence support macroevolution. The strongest conclusions come from agreement among them.

The fossil record

Fossils show that past organisms differ from living ones and document gradual change. Dated fossils for horses and humans reveal how body form changed over millions of years. Fossils also show transitions and patterns of stasis and rapid change, as described above.

Embryology

Early vertebrate embryos, including human embryos, develop gill slits and tails. Land-living adults lose these, while aquatic species such as fish keep the gills. Great ape embryos, including humans, form a tail that disappears before birth. Similar early development in groups with very different adults points to shared ancestry.

Anatomy: homologous, analogous and vestigial structures

Homologous structures indicate shared ancestry, and analogous structures indicate convergence. Vestigial structures have little or no function and appear to be leftovers from ancestors: wings on flightless birds, leaves on some cacti and hind leg bones in whales.

Molecular evolution and mitochondrial DNA

Shared DNA and a shared genetic code across life suggest a common ancestor, and greater similarity in DNA sequences generally tracks closer evolutionary relationship. Some genes or proteins accumulate mutations at a roughly constant rate, forming a molecular clock: the number of sequence differences between two lineages can be used to estimate when they diverged from a common ancestor. Mitochondrial DNA is useful for this purpose because it is inherited only from the mother, does not recombine, mutates relatively fast and is present in high copy number, so it is best suited to closely related or recently diverged groups. Caveats apply: different genes change at different rates (haemoglobin faster than cytochrome c), rates can differ between groups, selection can alter the rate, and over long periods later changes can reverse earlier ones, reducing accuracy.

One consequence of this pattern is worth noting. Molecular evolution also reveals how new functions arise: through gene duplication, one copy of a gene can evolve a new function while the other copy keeps its original role. This provides a way for evolution to build new features without losing the old ones, and it is detectable by comparing DNA sequences.

Biogeography

The distribution of organisms matches the history of the continents. Proteaceae plants in Australia, southern Africa and South America reflect their presence on the former supercontinent Gondwana, and Australia's marsupials and island species found nowhere else reflect long isolation.

8Summary

• Macroevolution is evolutionary change over long periods, including the origin of new species, through the same mechanisms as microevolution.
• Speciation requires reproductive isolation: allopatric (geographic separation) or sympatric (for example, polyploidy).
• Gradualism and punctuated equilibrium describe the pace of change; both occur in the fossil record.
• Genetic drift and sexual selection can speed divergence; adaptive radiation produces many species from one ancestor.
• Divergent, convergent and co-evolution produce homologous structures, analogous structures and reciprocal adaptation.
• Evidence: the fossil record, embryology, anatomy, molecular evolution (including mitochondrial DNA and molecular clocks) and biogeography.

?Quick check

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