Homology and analogy

Comparative anatomy and morphology show likenesses and differences among present and past organisms, and tell whether they share ancestors.

The forelimbs of whales, bats, cheetah and human (all mammals) do different jobs but share one bone plan: humerus, radius, ulna, carpals, metacarpals and phalanges.

One structure developing in different directions for different needs is divergent evolution, and such structures are homologous. Homology points to common ancestry.

More homologous examples: vertebrate hearts and vertebrate brains; in plants, the thorn of Bougainvillea and the tendril of Cucurbita.

Analogy is the opposite case: structures that are not anatomically similar but do the same job, such as the wings of a butterfly and of a bird.

Analogous structures come from convergent evolution, different structures evolving towards one function. A similar habitat selects similar adaptive features in unrelated groups.

More analogous examples: the eye of an octopus and of a mammal; the flippers of penguins and dolphins; sweet potato (a root modification) and potato (a stem modification).

Biochemical likeness, similar proteins and genes doing the same function in diverse organisms, points to shared ancestry in the same way as structural likeness.

Artificial selection: humans bred plants and animals for agriculture, horticulture, sport and security, making breeds (such as dogs) that differ yet stay one group. If people did this in hundreds of years, nature could have done it over millions.