Genes and Variation Before we continue, let’s review some genetics vocabulary: Genotype Genetic make-up of an organism Ex: TT, Tt or tt Phenotype Physical characteristics of an organism Ex: tall plants or short plants Allele One of a number of different forms of a gene Ex: (T) for tall plants or (t) for short plants

Genes and Variation Genotype and Phenotype in Evolution Natural Selection acts on PHENOTYPES It does NOT directly act on genes In any population, some individuals some individuals have phenotypes (traits) that are better fit for their environment Those individuals have higher FITNESS. Higher fitness = more offspring & more of their genes are passed to the next generation

Genes and Variation Sources of Genetic Variation Mutations Sexual Reproduction Crossing Over

Natural Selection on Polygenic Traits Evolution of Populations

Natural Selection on Polygenic Traits Traits that are controlled by more than one gene produce a range of phenotypes. This range can often be shown by a bell curve. Natural selection can act on polygenic traits in one of three ways: Directional Selection Stabilizing Selection Disruptive Selection

Natural Selection on Polygenic Traits Directional Selection Individuals at one end of a distribution curve have higher fitness than individuals in the middle or at the other end of the curve

Natural Selection on Polygenic Traits Imagine a population of birds that live on an island. The island has many large seeds with thick shells, but few small seeds. Birds with bigger beaks would have more to eat than birds with smaller beaks. Birds with bigger beaks would live longer and have more offspring.

Natural Selection on Polygenic Traits What would happen to this population over time?

Natural Selection on Polygenic Traits Natural selection would move in one direction—toward larger beaks.

Natural Selection on Polygenic Traits Stabilizing Selection Individuals near the center of the distribution curve have higher fitness than individuals at either end of the curve.

Natural Selection on Polygenic Traits Large spiders are easily seen by predators. Small spiders have difficulty eating large insects they catch. Spiders of average size are not easily seen and can easily eat insects they catch.

Natural Selection on Polygenic Traits What would happen to this population over time?

Natural Selection on Polygenic Traits In this example, the fitness of the extremes is lower than that of the average.

Natural Selection on Polygenic Traits Disruptive Selection Individuals at the upper and lower ends of distribution curve have higher fitness than individuals near the middle of the curve.

Natural Selection on Polygenic Traits A type of crab ranges in color from almost white to dark brown. Light-colored crabs are not easily seen on sand. Dark brown crabs are not easily seen on dark rocks. Tan crabs can be seen on either dark or light surfaces.

Natural Selection on Polygenic Traits What would happen to this population over time?

Natural Selection on Polygenic Traits The population might split into two groups: one with smaller beaks and one with larger beaks.