TYPES OF NATURAL SELCTION How Evolution Happens NATURAL

TYPES OF NATURAL SELCTION How Evolution Happens

NATURAL SELECTION: RECALL: • Result of the environment selecting for individuals with certain traits makes them better suited to survive and reproduce • Over many generations, frequency of alleles of many different genes may change, resulting in significant changes

1. Stabilizing Selection • Natural selection that favours intermediate phenotypes and acts against extreme variants • Reduces variation and improves adaptation when enviro conditions are stable • What examples can you think of?

Examples

2. Directional Selection • Natural selection that favours the phenotypes at one extreme over the other • Distribution curve of phenotypes shifts in the direction of the extreme • What examples can you think of?

Examples

3. Disruptive Selection • Natural selection that favours the extremes of a range of phenotypes rather than intermeds. • Can result in elimination of intermediate phenotypes • Could eventually cause speciation • What examples can you think of?

Examples

Ticket out of class • Suppose there is a population of rabbits. The color of the rabbits is governed by two incompletely dominant traits: black fur, represented by “B”, and white fur, represented by “b”. A rabbit in this population with a genotype of “BB” would have a phenotype of black fur, a genotype of “Bb” would have grey fur (a display of both black and white), and a genotype of “bb” would have white fur. • This population of rabbits occurred in an environment that had areas of black rocks as well as areas of white rocks. • Which type of selection would occur? Draw a graph and justify your answer.