Genes and VariationBefore we continue, let’s review some

genetics vocabulary:Genotype

Genetic make-up of an organism Ex: TT , Tt or tt

PhenotypePhysical characteristics of an organismEx: tall plants or short plants

Allele One of a number of different forms of a geneEx: (T) for tall plants or (t) for short plants

Genes and VariationGenotype and Phenotype in Evolution

Natural Selection acts on PHENOTYPES

It does NOT directly act on genesIn 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 VariationSources of Genetic Variation

MutationsSexual Reproduction

123

Crossing Over

Natural Selection on Polygenic Traits

Evolution of Populations

Natural Selection on Polygenic TraitsTraits 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 SelectionStabilizing SelectionDisruptive Selection

Natural Selection on Polygenic TraitsDirectional 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 TraitsImagine 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 TraitsWhat would happen to this population over

time?

Natural Selection on Polygenic TraitsNatural selection would move in one

direction—toward larger beaks.

Natural Selection on Polygenic TraitsStabilizing Selection

Individuals near the center of the distribution curve have higher fitness than individuals at either end of the curve.

Natural Selection on Polygenic TraitsLarge 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 TraitsWhat would happen to this population over

time?

Natural Selection on Polygenic TraitsIn this example, the fitness of the extremes

is lower than that of the average.

Natural Selection on Polygenic TraitsDisruptive 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 TraitsA 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 TraitsWhat would happen to this population over

time?

Natural Selection on Polygenic TraitsThe population might split into two groups:

one with smaller beaks and one with larger beaks.