Systematics: taxonomy (classification) and phylogeny (reconstructing evolutionary relationships)

Systematics: taxonomy (classification) and phylogeny (reconstructing evolutionary relationships).

Why reconstruct phylogenies? Natural selection always starts with “ last year ’ s model ” so knowing the ancestry of a species helps us understand which adaptations have and which haven ’ t evolved.

Which two species are most closely related?. Chicken bipedal lays eggs doesn’t lactate.

Using egg-laying. Chicken bipedal lays eggs doesn’t lactate.

Using lactation. This one’s correct. Why?. Chicken bipedal lays eggs doesn’t lactate.

Bipedality in humans and chickens is convergent (independently evolved in the two species): we infer this because humans are among the very few bipedal placental mammals, and the details of human vs. chicken bipedality are very different Egg-laying is a shared primitive character (present in the common ancestor of all three species): we infer this because amphibians, the outgroup to all three species, lay eggs Lactation is a shared, derived character of humans and duck-billed platypus – it characterizes all mammals, and it’s too similar in its details across the mammals to be a result of convergence.

Humans. Duck-billed platypus. Chicken. Amphibians (outgroup).

It can be difficult to distinguish between ancestral and derived characters.