Mechanisms of Evolution cont Types of Natural Selection

Mechanisms of Evolution, cont. Types of Natural Selection

Evolution is the change in allele frequency ® Occurs in one of three ways: § Gene Flow § Genetic Drift § Natural Selection

3) Natural Selection ~ Most common source of evolutionary change • 3 types

1. Directional Selection ® One of the extreme phenotypes is selected against

1) Directional Selection ® There are 2 types of worms: worms that eat at night (nocturnal) and worms that eat during the day (diurnal). The birds eat during the day and seem to be eating ONLY the diurnal worms.

2. Stabilizing Selection ® BOTH of the extreme phenotypes are selected against

2. Stabilizing Selection

2. Stabilizing Selection ® EX. Birth weight in humans. Babies that are born too small can lose heat too easily and may die, whereas babies being born too large can lead to complications during childbirth and the death of the mother or the baby.

3. Disruptive Selection ® The middle phenotype is selected against

3. Disruptive Selection

3. Disruptive Selection ® Ex. A population of rabbits occurs in an environment that has areas of black rocks as well as areas of white rocks Rabbits with black fur are able to hide from predators amongst the black rocks, and the rabbits with white fur likewise amongst the white rocks. The rabbits with grey fur, however, would stand out in all areas of the habitat, and would thereby suffer greater predation.

Evolution leads to new species ® When the gene pool of two populations become separated, they become two separate species. ® Natural selection is the key mechanism of evolution that leads to speciation