13.16 to 13.18

 Struggle for Existence & Survival of the fittest ◦ Because there are limiting resources (food, shelter, competition, predation, etc…) organisms struggle to survive. ◦ Organisms that are best suited for their environment will have a better chance to survive allowing them to reproduce and pass on their traits that made them better suited for survival.

 Fitness ◦ The contribution an individual makes to the gene pool of the next generation, relative to the contribution of other individuals in the population ◦ Those with higher fitness will make a greater contribution to the gene pool; those with lower fitness may not contribute to the gene pool at all

Stabilizing SelectionExample  Type of natural selection that favors intermediate variants by acting against extreme population  Human birth weight is usually in the range of 6.5lbs to 9lbs. Smaller babies have increased health risks. Larger babies lead to health risks for the mother.

Directional SelectionExample  Type of natural selection that acts against the relatively rare individuals at one end of the phenotype range.  Pesticide resistance in insects.

Disruptive SelectionExample  Type of natural selection that favors extremes over intermediate phenotypes  Coloration of African Swallowtail – extremes mimic unpalatable species; most common / non-mimic becomes easy prey

 Sexual selection is natural selection for mating success  It can result in sexual dimorphism, marked differences between the sexes in secondary sexual characteristics

 Intrasexual selection is competition among individuals of one sex (often males) for mates of the opposite sex  Intersexual selection, often called mate choice, occurs when individuals of one sex (usually females) are choosy in selecting their mates  Male showiness due to mate choice can increase a male’s chances of attracting a female, while decreasing his chances of survival