Selection acting on Populations

Selections Tool -allele – variations of a gene (blue/brown eyes) -gene pool- all of the alleles in a population -allelic frequency - the percentage of an allele in the gene pool (when allele frequency changes, evolution is happening) genetic equilibrium- is when allele frequencies do not change

Selection Types Stabilizing- selection favors an average variation (size, color, etc) ex avg. sized colony spiders 600 500 400 300 200 100 # of spiders 0 1 2 3 4 5 size (inches)

Selection Types Directional- selection favors one extreme variation. Ex. Elephant Size 60 50 40 30 20 10 # of elephants 0 3 6 9 12 15 height (feet)

Selection Types Disruptive- selection favors two extreme variations. Ex. Black bellied seed crackers 60 50 40 30 20 10 # of birds small beak large beak Beak size

Frequency Dependant Selection The fitness level of a given trait varies depending on how many organisms in the population have that trait. Ex. Scale Eating fish

Sexual Selection traits allow organisms to attract mates. results in sexual dimorphism- differences between the sexes (size, color, etc)

Intrasexual Selection Members of the same sex compete for mates **

Intersexual Selection Member of one sex choose mates, usually based on particular traits, ornaments or behaviors .**