EVOLUTION Inheritable Variation

Where does variation come from? Remember that inheritable variation comes from mutations and gene shuffling Inheritable variation is the raw material for natural selection and fuels evolution Natural selection acts on variations in phenotype only

Traits Controlled by a Single Gene Traits controlled by a single gene with only 2 alleles have 3 possible genotypes This results in at most 3 possible phenotypes Traits determine fitness

Traits Controlled by Several Genes Many traits are polygenic and/or multiple alleles This leads to a wide range of phenotypes (think human traits like height, skin color, etc)

Genes, Fitness, and Adaptation A species is defined as a group of similar- looking organisms that can breed with each other to produce fertile offspring. Reproductive isolation is the idea that individuals of different species cannot interbreed.

Genes, Fitness, and Adaptation Individuals within a species share the same gene pool – group of alleles that are shared Change in one member of a species can spread through the population, but not to other species

Genes, Fitness, and Adaptation When there are no selection pressures, each allele in the gene pool occurs in a particular frequency Sexual reproduction itself does not change the relative frequency of an allele Think of a deck of cards

Evolution – Genetically Defined Using genetics, evolution is defined as any change in the relative frequencies of alleles in the gene pool of a species. Evolution is a natural consequence of the nature of genes and DNA and the interaction between organisms and their environment.

Natural Selection – Single-Gene Traits Let’s assume that there is a mutation in one copy of one gene carried by one individual If the mutation is harmful, it decreases that organism’s fitness and fewer copies of that allele will be passed on to future generations. The mutated allele will become less common in the population.

Natural Selection – Single-Gene Traits If the mutation is helpful, it increases that organism’s fitness and over time carriers of the mutated allele will produce more offspring. After a few generations, relative frequency of the allele will increase.

Natural Selection – Polygenic Traits Polygenic traits have a wide range of phenotypes (bell curve) Fitness throughout the curve varies, and natural selection can act on these traits in different ways