II. Evidence for Evolution: evolutionary biologists look at a variety II. Evidence for Evolution: evolutionary biologists look at a variety of pieces of evidence to show the common ancestry of organisms, or to show that organisms may change over time. Important to the concept of evolution is the understanding that evolutionary changes are recognized at the population level, but not at the level of an individual organism. It is also important to understand that an individual cannot gain an adaptation that it does not already possess. Over time, a population may adapt to its environment as a beneficial trait becomes common in the population. A. Convergent evolution; Natural Selection can result in the appearance of analogous structures. Usually, convergent evolution does not show a common ancestry. Instead, it shows that organisms that are not closely related may have similar features that function for the same purpose in their environment.

Analogous structures; Have the same function but differ in structure

B. Divergent evolution; Species that once had a B. Divergent evolution; Species that once had a common ancestry have been separated allowing the organisms of each population to change as they adapt to different environments, or as the allelic frequency of their gene pool changes. Comparative anatomy gives evidence of common ancestry.

1. Homologous Structures; Structures are fundamentally 1. Homologous Structures; Structures are fundamentally similar, but they may serve a different function.

2. Vestigial structures; Structure is present in the organism, but it 2. Vestigial structures; Structure is present in the organism, but it no longer serves its original function. Human appendix- (breaks down cellulose in other animals), * Recently the ideas of an appendix as a vestigial structure has been contested by some biologists. and the coccyx (tailbone)

b. Cave animals with eyes. ← Blind tetra cave fish

c. Wings on flightless birds d. Pelvic bones in some whales and some snakes.

3. Active atavisms; an ancestral trait reappears in an organism. a. Leg bones in a snake

b. dolphin with rear fins.

c. Horse hoof atavism- reverting back to multiple toes.

d. Embryonic chickens can develop teeth for up to 18 days. At or before this time, the teeth are reabsorbed, and the chicks hatch without teeth. This is a lethal trait for chicks that do not reabsorb the teeth beyond 20 days.

4. Embryology Early embryonic stages of development for most vertebrate animals show very similar anatomical features.

5. Fossils; Show the similarities in form to present day organisms.

6. Biochemical analysis; All cells use roughly the same set of 6. Biochemical analysis; All cells use roughly the same set of 20 amino acids to build proteins. All cells rely on ATP for cellular energy 7. Genetic analysis; All organisms, including humans and bacteria use DNA as the carrier for genetic information. All cells use RNA, ribosomes and approximately the same genetic code to translate genetic information into proteins.