Macroevolution: Speciation, Radiations, and Coevolution

Macroevolution is evolution at or above the species level over long time scales, including the origin of new species and groups, adaptive radiations, convergent and divergent evolution and coevolution, whereas microevolution is the change in allele frequencies within a population. In brief: the same processes (mutation, selection, drift, gene flow) underlie both. In adaptive radiation, a single ancestral species rapidly diversifies into many species that occupy different niches, as with Darwin’s finches or the mammals after the extinction of the dinosaurs. Convergent evolution produces similar traits in unrelated groups, and coevolution is the reciprocal evolutionary change in interacting species.

These large-scale patterns are the result of many small changes accumulating over time, together with events such as mass extinctions. This guide defines micro- and macroevolution, explains adaptive radiation, convergent and divergent evolution and coevolution, introduces evolutionary developmental biology (evo-devo), compares the patterns in a table, works through a rate-of-diversification calculation and corrects common mistakes.

How do microevolution and macroevolution differ?

Microevolution is a change in the frequencies of alleles in a population over generations. It can be observed directly, for example the evolution of antibiotic resistance. Macroevolution refers to large-scale changes, such as the origin of species, the evolution of new body plans or groups (the origin of birds from dinosaurs, of whales from land mammals) and patterns of extinction and diversification over millions of years.

Most evolutionary biologists regard macroevolution as the accumulation of microevolutionary processes over time, with extra factors such as mass extinctions and the evolution of key innovations (such as wings or flowers). Some, such as Stephen Jay Gould and Niles Eldredge with their theory of punctuated equilibrium (1972), argued that the fossil record often shows long periods of little change (stasis), interrupted by rapid bursts of change at the time of speciation, in contrast to the idea of constant slow change (gradualism). Both patterns are found, depending on the group.

What is adaptive radiation?

Adaptive radiation is the rapid evolution of many species from a common ancestor, each adapted to a different ecological niche. It is typically triggered by:

• New habitats: colonization of islands or lakes with many empty niches.
• Mass extinctions: the loss of competitors opens up niches.
• Evolutionary innovations: a new trait that opens up new ways of life.

Examples:

• Darwin’s finches on the Galápagos: about 15 species, from one ancestor in the last 2 to 3 million years, with beaks adapted to seeds, insects, cactus and even blood.
• Hawaiian honeycreepers and fruit flies: more than 50 species of honeycreepers from a single finch ancestor, and about a thousand species of Drosophila.
• African cichlid fish: several hundred species in each of Lakes Victoria, Malawi and Tanganyika, many arising in only 15 000 to several million years, depending on the lake.
• Mammals: after the extinction of the non-avian dinosaurs 66 million years ago, mammals diversified into bats, whales, primates, hoofed animals and carnivores within about 10 to 15 million years.
• Flowering plants: a rapid diversification in the Cretaceous, which Darwin called an “abominable mystery”.
• Anoles lizards in the Caribbean: ecomorphs adapted to the ground, trunks, twigs and canopy evolved independently on different islands.

What are convergent and divergent evolution?

Divergent evolution occurs when related populations or species become more different as they adapt to different environments. It is the process by which adaptive radiation occurs. The forelimbs of vertebrates (homologous structures) are an example.

Convergent evolution occurs when unrelated species independently evolve similar traits because they have similar ecological roles or face the same selection pressures. The resulting traits are analogous structures.

• Wings of birds, bats and insects.
• Streamlined bodies and fins of sharks (fish), dolphins (mammals) and ichthyosaurs (extinct reptiles).
• Camera-type eyes of vertebrates and cephalopods (octopus, squid).
• Succulent, spiny desert plants: cacti in the Americas and euphorbs in Africa look alike.
• Marsupial and placental mammals: the Tasmanian wolf (thylacine), a marsupial, resembled wolves, and the sugar glider resembles the flying squirrel.

Convergence shows that natural selection leads to similar solutions to the same problems. It is also a reason that appearance alone cannot be used to work out relationships.

What is coevolution?

Coevolution occurs when two or more species exert selection pressures on each other, so that changes in one lead to evolutionary changes in the other, in a reciprocal way. Types include:

• Predator and prey (arms races): faster predators select for faster prey and vice versa. Garter snakes and newts in western North America: newts with more potent toxin (tetrodotoxin) select for snakes with greater resistance.
• Host and parasite: the host evolves defenses and the parasite evolves ways to overcome them, a pattern known as the Red Queen hypothesis (after Lewis Carroll’s character, who has to run to stay in the same place). It may explain the advantage of sexual reproduction, which produces variation.
• Plants and pollinators (mutualism): Darwin predicted in 1862 that a Madagascan orchid with a nectar spur 30 centimeters long must be pollinated by a moth with an equally long tongue. Such a hawk moth (Xanthopan morganii praedicta) was discovered in 1903 and observed pollinating the orchid in 1992.
• Plants and herbivores: plants evolve toxins and thorns, and herbivores evolve detoxification or avoidance. Milkweed and monarch butterflies.
• Mutualistic symbioses: mitochondria and chloroplasts are the descendants of formerly free-living bacteria, according to the endosymbiotic theory.

What is evolutionary developmental biology?

Evolutionary developmental biology (evo-devo) studies how changes in the genes that control embryonic development produce differences in body form between species. A discovery was that many key control genes are shared by animals as different as flies and mice, for example the Hox genes, which determine the identity of body segments along the head-to-tail axis. Differences in body plan often arise from changes in when, where and how much these genes are expressed, not by new genes. Small changes in regulatory DNA can have large effects, which allow rapid macroevolutionary change: for example, the loss of hindlimbs in snakes and the lengthening of the neck in giraffes involve changes in developmental timing and gene regulation. Changes in the timing of development (heterochrony) can explain features such as the rounded skull and flat face of adult humans, which resemble those of juvenile apes.

Evolutionary patterns compared

Worked example: diversification rate

A lineage of cichlid fish colonizes a lake that formed 500 000 years ago. Today there are 400 species. Assume that the lineage started as one species and that species number grew exponentially, with no extinction.

1. Number of doublings: 2ⁿ = 400, so n = log₂ 400 = ln 400 ÷ ln 2 = 5.99 ÷ 0.693 ≈ 8.64 doublings.
2. Time per doubling: 500 000 years ÷ 8.64 ≈ 57 900 years, so the number of species doubled approximately every 58 000 years.
3. Net diversification rate (r): N = e^(rt), so r = ln 400 ÷ 500 000 = 5.99 ÷ 500 000 = 1.2 × 10⁻⁵ per lineage per year, which is about 12 per lineage per million years.
4. Check: e^(1.2 × 10⁻⁵ × 500 000) = e^6 = 403. ✓

The fast rate is typical of adaptive radiations in lakes, and is much higher than the long-term rate in the fossil record of many groups, which is often less than 1 per million years per lineage. The model is simple: in reality the diversification rate slows as niches fill, and extinctions occur. Some estimates suggest the cichlid flock in Lake Victoria is even younger, about 15 000 years, which would require a still faster rate, though this age is debated and may reflect an older genetic ancestry.

How did the concepts develop?

Darwin’s observations of finches and other island species led to the concept of divergence. Henry Fairfield Osborn used the term “adaptive radiation” in 1902. George Gaylord Simpson analyzed patterns in the fossil record in Tempo and Mode in Evolution (1944). Paul Ehrlich and Peter Raven coined “coevolution” in 1964 from butterflies and plants. Eldredge and Gould proposed punctuated equilibrium in 1972. The discovery of Hox genes in the 1980s by Edward Lewis, Christiane Nüsslein-Volhard and Eric Wieschaus (Nobel Prize 1995), and the work of Sean Carroll and others, created evo-devo. Dolph Schluter’s studies of stickleback fish and other adaptive radiations have tested the theory in nature.

Which mistakes are common?

Mistake 1: Saying that macroevolution is a different process from microevolution

It is the same processes continued over long times, with additional factors such as mass extinctions.

Mistake 2: Confusing convergent evolution with common ancestry

Convergent evolution produces similar traits in unrelated species. Shared traits from a common ancestor are homologous.

Mistake 3: Believing that coevolution means cooperation only

It also includes antagonistic relationships such as predator and prey, and host and parasite.

Mistake 4: Saying that adaptive radiation occurs because species “need” to fill empty niches

Empty niches give opportunity. Selection acts on variation that arises by chance.