Evolution and speciation

Biology · A Unity and diversity. 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

Evolution is change in the heritable characteristics of a population over time. Only characteristics coded in genes can be passed on. Changes that an organism gains during its own life are not inherited, which is why Lamarck's idea was wrong.

Evidence comes from several sources. Closely related species have very similar DNA base sequences and amino acid sequences. Selective breeding shows that large changes can happen in a short time. Homologous structures have the same basic plan because they come from a common ancestor.

New species form only when an existing species splits. Two populations must stop interbreeding, and then be changed in different ways by selection. This is speciation.

Rules to remember

Evolution happens to populations, not to individuals.
Fewer differences in base or amino acid sequence = more recent common ancestor.
Homologous structures: same structure and origin, different function (the pentadactyl limb of humans, bats and whales).
Analogous structures: same function, different structure and origin; they arise by convergent evolution.
Speciation needs reproductive isolation followed by differential selection.
Geographical barriers such as rivers, mountains and seas can isolate populations.
Speciation adds one species; extinction removes one.

Common mistake

Mixing up homologous and analogous. Look at the structure inside, not the job: the same bones mean homologous, and the same job with a different build means analogous.

Worked example

A dolphin's flipper and a shark's pectoral fin are both used for steering in water. Are they homologous or analogous?

A dolphin is a mammal, and its flipper contains the bones of the pentadactyl limb.
A shark's fin has no such bones; it is supported by cartilage rods.
Same function but different structure and different origin means the two are not inherited from a shared ancestor with that structure.
Similar selection pressures in water produced a similar shape in both: convergent evolution.

Answer: They are analogous structures, produced by convergent evolution.

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