End Show Slide 1 of 24 Copyright Pearson Prentice Hall 16-1 Genes and Variation Genes and Variation

End Show 16-1 Genes and Variation Slide 2 of 24 Copyright Pearson Prentice Hall 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.

End Show 16-1 Genes and Variation Slide 3 of 24 Copyright Pearson Prentice Hall 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.

End Show 16-1 Genes and Variation Slide 4 of 24 Variation and Gene Pools 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. In genetic terms, evolution is any change in the relative frequency of alleles in a population.

End Show 16-1 Genes and Variation Slide 5 of 24 Copyright Pearson Prentice Hall Variation and Gene Pools Gene Pool for Fur Color in Mice Sample Population Frequency of Alleles allele for brown fur allele for black fur

End Show 16-1 Genes and Variation Slide 6 of 24 Copyright Pearson Prentice Hall Sources of Genetic Variation The two main sources of genetic variation are mutations and the genetic shuffling that results from sexual reproduction.

End Show 16-1 Genes and Variation Slide 7 of 24 Copyright Pearson Prentice Hall 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.

End Show 16-1 Genes and Variation Slide 8 of 24 Copyright Pearson Prentice Hall 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.

End Show 16-1 Genes and Variation Slide 9 of 24 Single-Gene and Polygenic Traits The number of phenotypes produced for a given trait depends on how many genes control the trait.

End Show 16-1 Genes and Variation Slide 10 of 24 Copyright Pearson Prentice Hall Single-Gene and Polygenic Traits A single-gene trait is controlled by one gene that has two alleles. Variation in this gene leads to only two possible phenotypes.

End Show 16-1 Genes and Variation Slide 11 of 24 Copyright Pearson Prentice Hall Single-Gene and Polygenic Trait Many traits are controlled by two or more genes and are called polygenic traits. One polygenic trait can have many possible genotypes and phenotypes. Height in humans is a polygenic trait.

End Show 16-1 Genes and Variation Slide 12 of 24 Copyright Pearson Prentice Hall Single-Gene and Polygenic Trait A bell-shaped curve is typical of polygenic traits. A bell-shaped curve is also called normal distribution.

End Show 16-1 Genes and Variation Slide 13 of 24 Copyright Pearson Prentice Hall 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. Natural selection on single-gene traits can lead to changes in allele frequencies and thus to evolution Evolution as Genetic Change

End Show 16-1 Genes and Variation Slide 14 of 24 Copyright Pearson Prentice Hall Natural Selection on Polygenic Traits Natural selection can affect the distributions of phenotypes in any of three ways: directional selection stabilizing selection disruptive selection

End Show 16-1 Genes and Variation Slide 15 of 24 Copyright Pearson Prentice Hall 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.

End Show 16-1 Genes and Variation Slide 16 of 24 Copyright Pearson Prentice Hall 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.

End Show 16-1 Genes and Variation Slide 17 of 24 Copyright Pearson Prentice Hall 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.

End Show 16-1 Genes and Variation Slide 18 of 24 Copyright Pearson Prentice Hall 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. When allele frequencies change due to migration of a small subgroup of a population it is known as the founder effect. Descendents Founding Populations Sample of original population

End Show 16-1 Genes and Variation Slide 19 of 24 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.

End Show 16-1 Genes and Variation Slide 20 of 24 Copyright Pearson Prentice Hall Evolution Versus Genetic Equilibrium Five conditions are required to maintain genetic equilibrium 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.

End Show 16-1 Genes and Variation Slide 21 of 24 Copyright Pearson Prentice Hall 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.

End Show 16-1 Genes and Variation Slide 22 of 24 Copyright Pearson Prentice Hall 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.

End Show 16-1 Genes and Variation Slide 23 of 24 Copyright Pearson Prentice Hall 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.

End Show 16-1 Genes and Variation Slide 24 of 24 Copyright Pearson Prentice Hall Evolution Versus Genetic Equilibrium No Mutations If genes mutate, new alleles may be introduced into the population, and allele frequencies will change.

End Show 16-1 Genes and Variation Slide 25 of 24 Copyright Pearson Prentice Hall 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.

End Show 16-1 Genes and Variation Slide 26 of 24 Copyright Pearson Prentice Hall 16-3 The Process of Speciation Natural selection and chance events can change the relative frequencies of alleles in a population and lead to speciation. Speciation is the formation of new species. A species is a group of organisms that breed with one another and produce fertile offspring.

End Show 16-1 Genes and Variation Slide 27 of 24 Copyright Pearson Prentice Hall Isolating Mechanisms What factors are involved in the formation of new species? The gene pools of two populations must become separated for them to become new species. Isolating Mechanisms As new species evolve, populations become reproductively isolated from each other. When the members of two populations cannot interbreed and produce fertile offspring, reproductive isolation has occurred

End Show 16-1 Genes and Variation Slide 28 of 24 Copyright Pearson Prentice Hall Isolating Mechanisms Behavioral Isolation Behavioral isolation occurs when two populations are capable of interbreeding but have differences in courtship rituals or other reproductive strategies that involve behavior.

End Show 16-1 Genes and Variation Slide 29 of 24 Copyright Pearson Prentice Hall Isolating Mechanisms Geographic Isolation Geographic isolation occurs when two populations are separated by geographic barriers such as rivers or mountains. Abert Kaibab

End Show 16-1 Genes and Variation Slide 30 of 24 Copyright Pearson Prentice Hall Isolating Mechanisms Temporal Isolation Temporal isolation occurs when two or more species reproduce at different times.