Natural Selection is the only mechanism that causes adaptive evolution

Relative fitness in relation to Natural selection Natural selection often has “survival of the fittest” and the “struggle to survive” associated with it, but it doesn’t necessarily mean direct physical conflict between two organisms.

Frequency of distribution with natural selection

Directional Selection Individuals at one end of the curve have a higher fitness than the others. Example: In an area with thick seeds, birds with larger beaks are able to feed more easily. This in turn will show over time the average size beak of birds in that area will increase.

Stabilizing Selection Individuals in the middle of the curve have higher fitness. Example: Mass size in human babies. Babies of average mass are more likely to survive than those that are too heavy or too thin.

Disruptive Selection When individuals in the extremities of the curve have a higher fitness than the middle of the curve. In this case, if the selective pressure is left long enough, then the curve may separate into two new curves. Example: In a bird population where medium beaks are less common, the birds with drastically larger or smaller beaks may be more fit.

Sexual selection on natural selection A form of natural selection in which individuals with certain inherited characteristics are more likely than other individuals to to obtain mates. In some cases, there are visible differences in the secondary sex characteristics between the sexes. These visible characteristics are called sexual dimorphism.

Intrasexual versus intersexual selection

Balancing selection Balancing selection is a type of selection in which traits that could be unfavorable can be preserved in a population. Two types of selection that are common with balancing selection are heterozygote advantage and frequency dependent selection.

Heterozygote advantage Those with a heterozygous genotype have an advantage over homozygous individuals in the population. Example: sickle cell anemia

Frequency dependent selection The fitness of a phenotype depends on how prevalent it is in the population. Example: the left mouthed or right mouthed Perissodus microlepis Right mouthed allele is dominant over left mouthed allele.

Why Natural selection cannot fashion a perfect organism 1. Selection can act only on existing populations. 2. Evolution is limited by historical constraints. 3. Adaptations are often compromises. 4. Chance, natural selection, and the environment interact.