Describe patterns of evolution including divergent and convergent evolution, coevolution, adaptive radiation, and the pace of change (gradualism and punctuated equilibrium) (Ohio's Learning Standards for Science, Biology, B.E).

What this topic is asking

The Ohio Evolution strand (B.E) asks students to recognize the patterns evolution leaves across many lineages and across time. The Ohio Biology EOC turns this into items where you name a pattern from a scenario (two unrelated animals that look alike, one ancestor that became many species) and explain the selection behind it. The crosscutting idea is patterns: the same processes from natural selection and speciation produce recognizable large-scale shapes. The trick is matching the right label to the right description.

Divergent evolution and adaptive radiation

Divergent evolution happens when groups that share a common ancestor become more different over time as they adapt to different environments or ways of life. It is the pattern behind homologous structures: the forelimbs of a human, a whale, and a bat diverged from one ancestral limb into a hand, a flipper, and a wing.

Adaptive radiation is a fast, dramatic case of divergent evolution: from a single ancestor, many new species evolve in a relatively short time, each adapted to a different niche. It typically happens when many habitats are open (after a mass extinction, or when a founder reaches a new island). The classic example is Darwin's finches on the Galapagos: one ancestral finch gave rise to many species with different beaks suited to different foods.

Convergent evolution

Convergent evolution is the opposite of divergent: unrelated species (different ancestors) independently evolve similar traits because they live in similar environments and face similar selection pressures. The similar features are analogous structures, not signs of close common ancestry.

Standard examples:

• Sharks (fish) and dolphins (mammals) both have streamlined bodies and fins for fast swimming, though they are only distantly related.
• The wings of birds, bats, and insects all enable flight but are built differently.
• Cacti (Americas) and some euphorbs (Africa) both store water in thick stems with spines, having adapted separately to deserts.

When a question shows two clearly unrelated organisms that resemble each other, convergent evolution is the answer.

Coevolution

Coevolution is when two species evolve in response to each other, each acting as a selection pressure on the other. A change in one drives a change in the other, back and forth over time. Examples:

• A flower and its pollinator (a long flower tube and a long-tongued moth that feeds from it).
• A predator and its prey (faster prey select for faster predators, which select for still-faster prey).
• A plant and a herbivore (plant toxins select for insects that can tolerate them).

The signature of coevolution is a reciprocal relationship between two specific species.

The pace of evolution

Evolution does not always proceed at one speed, and the standards contrast two models of its tempo.

• Gradualism. Evolutionary change is slow and steady, accumulating in small steps over long periods. Under gradualism, the fossil record should show many small transitional forms.
• Punctuated equilibrium. Long periods of little change (stasis, or equilibrium) are interrupted by short bursts of rapid change, often linked to a new selection pressure or a new environment. Under this model, new forms can appear relatively suddenly in the fossil record.

Both are supported in different cases; they are not mutually exclusive, and the EOC may ask you to read a graph of change over time and decide which pattern it shows.

Try this

Q1. State the difference between convergent and divergent evolution. [2]

• Cue. Convergent evolution makes unrelated species more similar (similar environment, analogous structures); divergent evolution makes related species (from a common ancestor) more different (homologous structures).

Q2. A type of orchid and a single moth species have evolved a matching long flower tube and long tongue. Name the pattern of evolution and explain it. [2]

• Cue. Coevolution: the two species act as selection pressures on each other, so a change in the flower drives a change in the moth and vice versa over time.