More Evidence of Evolution

Homologous Structures …similar structure found in more than one species that share a common ancestor. Homologous structures are revealed by comparing anatomy, cellular similarities and differences, and studying embryological development. Homologous structures are best explained as having all derived from the same body part in a common ancestor.

Mammal Forelimbs

Vestigial Structures …a type of homologous structure that is no longer used as it is in most other organisms that have it. “Vestigial” does not necessarily mean useless or nonfunctional. It literally means “not fully developed.”

Examples of Vestigial Structures Hipbones and ear muscles of whales Hipbones and rudimentary legs of some snakes Muscles in the human tailbone and human ear, human wisdom teeth, and “goose bumps” Fingernails on manatee flippers

Another Example Human appendix = small, finger-sized structure found near junction of small and large intestine. Thought to have descended from an organ (cecum) in a distant herbivorous ancestor. Modern herbivores maintain a cecum; a sac that houses bacteria essential to the digestion of cellulose in plants. Human appendix, although homologous, does not contain a significant number of these bacteria (can’t digest cellulose).

More Examples of Vestigial Structures Blind cave fish and salamanders have rudimentary eyes. Flightless beetles have functionless wings under fused wing-covers that never open. The ostrich has wings which are useless as wings. All whales have a number of small muscles devoted to nonexistent external ears.

Hind flippers on a dolphin Atavisms …traits from a distant evolutionary ancestor that have reappeared in modern-day organisms Hindlimbs with toes on a snake Hindlimb bones from a humpback whale Hind flippers on a dolphin Tails in humans

A snake with a leg!

Other Atavisms Additional nipples occurring in mammals, including humans Dew claws in many dog breeds Extra toes in horses

Let’s Make a Cladogram and a Venn Diagram of These Four Animals!