Evolution of Populations Chapter 16

Gene and Variation Although Mendel and Darwin both worked in the 1800’s, they were not able to share information as scientists do today. By the 1930’s evolutionary biologists started to focus on genes as a way to understand evolutionary change.

Gene and Variation When the double helix model of DNA was presented in the 1950s, evolutionary biologists were able to demonstrate the molecular nature of mutation and genetic variation.

Genetic Variation Genetic variation refers to differences in genes that produce heritable variation in traits. (different geneotypes = different phenotypes) The two main sources of genetic variation are mutations and the normal shuffling of genes as a result of sexual reproduction.

Genetic Variation Most mutations involve only minor changes in DNA and do not lead to visible changes in the phenotype. Some minor changes in DNA do have major effects on the organism.

Genetic Variation Genetic variation is studied in populations. A gene pool consists of all the genes, including the different alleles that are present in a population.

Genetic Variation The relative frequency of an allele is the number of times that the allele occurs in a gene pool; compared with the number of times other alleles for the same gene to occur. It has nothing to do with whether an allele is dominant or recessive!

Genetic Variation In genetic terms, evolution occurs when there is a change in the relative frequency of alleles in a population = EVOLUTION!! Sexual reproduction produces many different combinations of genes but does not change the relative frequencies of each type of allele in the population. (the alleles are still there, just shuffled!)

Genetic Variation The number of phenotypes produced for a given trait depends on how many genes control that trait. Single-gene traits have two alleles– dominant or recessive.

Genetic Variation Polygenic traits are controlled by two or more alleles usually have many genotypes and a range of phenotypes. Graphing polygenic traits usually results in a bell shaped curve!

Evolution as Genetic Change Natural Selection on single- gene traits can lead to changes in allele frequencies and thus to evolution. Organisms of one color may be more visible to predators and therefore less likely to survive and reproduce than organisms of another color.

Evolution as Genetic Change Natural selection on polygenic traits are more complex and can affect the distribution of phenotypes in any of three ways: directional selection, stabilizing selection or disruptive selection.

Evolution as Genetic Change Think of the bell shaped curve representing a range of phenotypes, each of these three changes result in changes in the bell shaped curve!

Directional Selection Directional selection is a form of natural selection in which the entire curve moves. This occurs when individuals at one end of the curve have a higher fitness than those at the other end or in the middle.

Stabilizing Selection Stabilizing selection is a form of natural selection by which the center of the curve remains in its current position because those in the middle have higher fitness than those at either end.

Disruptive Selection Disruptive Selection is a form of natural selection in which a single curve splits into two! This occurs when individuals at the upper and lower ends of a distribution curve have higher fitness than individuals near the middle resulting in a population splitting into two sub groups.

Genetic Drift Genetic drift is a random change in allele frequency and may occur when a small group of individuals or more migrate to colonize a new habitat.

Genetic Drift This group may carry different allele frequencies than the larger population that they came from had. The cause here is NOT natural selection, but the chance that certain alleles were in this small subgroup. The resulting change in allele frequencies is known as founder effect.

GENETIC DRIFT Start with five different alleles Due to random events, only two different alleles are left after several generations.

Gene Flow Gene flow, or genetic immigration, is the process in which genes "flow" from one population to another. This movement of genes helps maintain genetic diversity, or genetic variation within populations.

Genetic Equilibrium Genetic Equilibrium is a situation in which allele frequencies remain constant and evolution does not occur. The Hardy-Weinberg Principle states that allele frequencies in a population will remain constant unless one or more factors cause those frequencies to change.

The five conditions required to maintain genetic equilibrium from generation to generation are: 1.There must be random mating 2.The population must be very LARGE 3.There can be no movement into or out of the population 4.No mutations 5.No natural selection These conditions may be met for long periods of time, but eventually the equilibrium will be disrupted and the populations will evolve.