Evidence and speciation

MDCAT Biology · Biology · Evolution. The idea in plain words, the rules to remember, the mistake to avoid and a worked example, written and checked by Mega Lecture.

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The idea

Several kinds of evidence show that living species have descended, with change, from common ancestors. Fossils in rock layers record the order in which forms appeared. Comparing the structure, embryos and molecules of living species shows how closely they are related.

A species is a group whose members can interbreed and produce fertile offspring. A new species forms (speciation) when a population is split, most often by a geographical barrier. The two groups collect different mutations and face different selection, until they can no longer interbreed. This is reproductive isolation.

Evolution can also be seen happening today, as in peppered moths and antibiotic-resistant bacteria.

Rules to remember

Fossils: in undisturbed sedimentary rock, deeper layers are older. Archaeopteryx links reptiles and birds.
Homologous organs: same basic structure and origin, different function (human arm, bat wing, whale flipper); show divergent evolution from a common ancestor.
Analogous organs: same function, different structure and origin (bird wing, insect wing); show convergent evolution.
Vestigial organs: reduced and no longer used for their original function, e.g. coccyx, appendix.
Embryology: early vertebrate embryos all have gill pouches and a tail.
Biochemistry: the fewer the differences in a protein or in DNA, the more recent the common ancestor.
Adaptive radiation: one ancestor gives many species suited to different ways of life, e.g. Galápagos finches.

Common mistake

Homologous and analogous are often swapped. Homologous means the same origin; analogous means only the same job.

Worked example

A shark (a fish) and a dolphin (a mammal) both have a streamlined body and fins or flippers for swimming. What kind of evidence is this, and does it show close relationship?

The shapes do the same job, swimming, in the same environment.
A dolphin's flipper has the limb bones of a mammal; a shark's fin does not, so the origins differ.
Same function with different origin means the structures are analogous.

Answer: They are analogous structures produced by convergent evolution. They show similar selection pressures, not a recent common ancestor.

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