16-1 Genes and Variation

How Common Is Genetic Variation? Many genes have at least two forms, or alleles. All organisms have genetic variation that is “invisible” because it involves small differences in biochemical processes. An individual organism is heterozygous for many genes.

Variation and Gene Pools A population is a group of individuals of the same species that interbreed. A gene pool consists of all genes, including all the different alleles, that are present in a population.

The relative frequency of an allele is the number of times the allele occurs in a gene pool, compared with the number of times other alleles for the same gene occur. Relative frequency is often expressed as a percentage, and it is not related to whether an allele is dominant or recessive.

Variation and Gene Pools Gene Pool for Fur Color in Mice Sample Population Frequency of Alleles allele for brown fur allele for black fur

Variation and Gene Pools In genetic terms, evolution is any change in the relative frequency of alleles in a population.

Sources of Genetic Variation The two main sources of genetic variation are mutations and the genetic shuffling that results from sexual reproduction.

Sources of Genetic Variation Mutations A mutation is any change in a sequence of DNA. Mutations occur because of mistakes in DNA replication or as a result of radiation or chemicals in the environment. Mutations do not always affect an organism’s phenotype.

Sources of Genetic Variation Gene Shuffling Most heritable differences are due to gene shuffling. Crossing-over increases the number of genotypes that can appear in offspring. Sexual reproduction produces different phenotypes, but it does not change the relative frequency of alleles in a population.

Single-Gene and Polygenic Traits The number of phenotypes produced for a given trait depends on how many genes control the trait.

Copyright Pearson Prentice Hall A single-gene trait is controlled by one gene that has two alleles. Variation in this gene leads to only two possible phenotypes.

Single-Gene and Polygenic Trait A bell-shaped curve is typical of polygenic traits. A bell-shaped curve is also called normal distribution.

16-2 Evolution as Genetic Change Natural selection affects which individuals survive and reproduce and which do not. Evolution is any change over time in the relative frequencies of alleles in a population. Populations, not individual organisms, can evolve over time Evolution as Genetic Change

Natural Selection on Single-Gene Traits

Natural Selection on Polygenic Traits Natural selection can affect the distributions of phenotypes in any of three ways: directional selection stabilizing selection disruptive selection

Natural Selection on Polygenic Traits Directional Selection When individuals at one end of the curve have higher fitness than individuals in the middle or at the other end, directional selection takes place.

Natural Selection on Polygenic Traits Stabilizing Selection When individuals near the center of the curve have higher fitness than individuals at either end of the curve, stabilizing selection takes place.

Natural Selection on Polygenic Traits Disruptive Selection When individuals at the upper and lower ends of the curve have higher fitness than individuals near the middle, disruptive selection takes place.

Genetic Drift Genetic drift may occur when a small group of individuals colonizes a new habitat. Individuals may carry alleles in different relative frequencies than did the larger population from which they came.

Genetic Drift

Descendants Population A Population B When allele frequencies change due to migration of a small subgroup of a population it is known as the founder effect.

Copyright Pearson Prentice Hall Evolution Versus Genetic Equilibrium The Hardy-Weinberg principle states that allele frequencies in a population will remain constant unless one or more factors cause those frequencies to change. When allele frequencies remain constant it is called genetic equilibrium.

Five conditions are required to maintain genetic equilibrium (aka Non-evolution) from generation to generation: there must be random mating, the population must be very large, there can be no movement into or out of the population, there can be no mutations, and there can be no natural selection.

Hardy Weinberg Conditions Allelic / genotypic frequencies in a population remain constant from generation to generation unless specific disturbing influences are introduced.

A Model Gene Pool Imagine a gene pool containing alleles R,r. Frequency of R =.9 Frequency of r =.1

Allele Frequencies The sum of the frequencies, by definition, must equal 1: This can be stated p + q = 1 Or… R + r = = 1

Genotype Frequencies When dealing with alleles, we are selecting two at a time. The probability of which pair will be realized is equal to the product of their individual probabilities: p + q = 1 p 2 + 2pq + q 2 = 1 (.9) 2 + 2(.9)(.1) + (.1) 2 = 1 RR + Rr + rR + rr = 1

Evolution Versus Genetic Equilibrium Random Mating Random mating ensures that each individual has an equal chance of passing on its alleles to offspring. In natural populations, mating is rarely completely random. Many species select mates based on particular heritable traits.

Evolution Versus Genetic Equilibrium Large Population Genetic drift has less effect on large populations than on small ones. Allele frequencies of large populations are less likely to be changed through the process of genetic drift.

Evolution Versus Genetic Equilibrium No Movement Into or Out of the Population Because individuals may bring new alleles into a population, there must be no movement of individuals into or out of a population. The population's gene pool must be kept together and kept separate from the gene pools of other populations.

Evolution Versus Genetic Equilibrium No Mutations If genes mutate, new alleles may be introduced into the population, and allele frequencies will change.

Evolution Versus Genetic Equilibrium No Natural Selection All genotypes in the population must have equal probabilities of survival and reproduction. No phenotype can have a selective advantage over another. There can be no natural selection operating on the population.