Copyright Pearson Prentice Hall 16-1 Genes and Variation 16-1 Genes and Variation Photo credit: ©MURRAY, PATTI/Animals Animals Enterprises Copyright Pearson Prentice Hall

How Common Is Genetic Variation? Many genes have at least two alleles. All organisms have genetic variation. An individual organism is heterozygous for many genes. Copyright Pearson Prentice Hall

Copyright Pearson Prentice Hall Variation and Gene Pools Population – group of individuals of the same species that interbreed. Gene pool – all genes present in a population. Relative frequency – The number of times the allele occurs in a gene pool (usually a %). 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 When scientists determine whether a population is evolving, they may look at the sum of the population’s alleles, or its gene pool. This diagram shows the gene pool for fur color in a population of mice.  Copyright Pearson Prentice Hall

Variation and Gene Pools Evolution is any change in the relative frequency of alleles in a population. Copyright Pearson Prentice Hall

Sources of Genetic Variation Two main sources of genetic variation- mutations genetic shuffling that results from sexual reproduction. Copyright Pearson Prentice Hall

Sources of Genetic Variation Mutations Any change in a sequence of DNA. 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. Copyright Pearson Prentice Hall

Sources of Genetic Variation 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. Copyright Pearson Prentice Hall

Single-Gene and Polygenic Traits REVIEW Single-gene trait – controlled by one gene that has two alleles. Variation in this gene leads to only two possible phenotypes. In humans, a single gene with two alleles controls whether a person has a widow’s peak (left) or does not have a widow’s peak (right). As a result, only two phenotypes are possible. The number of phenotypes a given trait has is determined by how many genes control the trait. Copyright Pearson Prentice Hall

Single-Gene and Polygenic Trait REVIEW Polygenic traits – controlled by two or more genes. One polygenic trait can have many possible genotypes and phenotypes. Examples-height, skin color, hair Copyright Pearson Prentice Hall

16-2 Evolution as Genetic Change Photo credit: ©MURRAY, PATTI/Animals Animals Enterprises Copyright Pearson Prentice Hall

16-2 Evolution as Genetic Change REVIEW 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! Copyright Pearson Prentice Hall

Natural Selection on Single-Gene Traits Natural selection on single-gene traits can lead to changes in allele frequencies and then to evolution. Copyright Pearson Prentice Hall

Natural Selection on Single-Gene Traits Example- Organisms of one color may produce fewer offspring than organisms of other colors. For example, a lizard population is normally brown, but has mutations that produce red and black forms. Red lizards are more visible to predators, so they will be less likely to survive and reproduce. Therefore, the allele for red color will become rare. Copyright Pearson Prentice Hall

Natural Selection on Single-Gene Traits Black lizards may warm up faster on cold days. This may give them energy to avoid predators. In turn, they may produce more offspring. The allele for black color will increase in relative frequency. Copyright Pearson Prentice Hall

Natural Selection on Single-Gene Traits Natural selection on single-gene traits can lead to changes in allele frequencies and thus to evolution. Organisms of one color, for example, may produce fewer offspring than organisms of other colors. Copyright Pearson Prentice Hall

Natural Selection on Polygenic Traits- Occurs 3 ways- Directional Selection Stabilizing Selection Disruptive Selection In your book, draw these 3 graphs and a brief description of each!!! Copyright Pearson Prentice Hall

Evolution Versus Genetic Equilibrium Will evolution ever NOT occur? 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. Copyright Pearson Prentice Hall

Copyright Pearson Prentice Hall There must be random mating! The population must be large! There can be no movement into or out of the population! There may be no mutations! There is no natural selection! Copyright Pearson Prentice Hall

16-3 The Process of Speciation Photo credit: ©MURRAY, PATTI/Animals Animals Enterprises 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: formation of new species. Species: a group of organisms that breed with one another and produce fertile offspring. Copyright Pearson Prentice Hall

Copyright Pearson Prentice Hall Reproductive Isolation: when members of 2 populations cannot interbreed. Can develop by: Behavioral isolation (different rituals) Geographic isolation (separated by lakes/rivers) Temporal isolation (reproduction at different times) Copyright Pearson Prentice Hall

Speciation in Darwin's Finches Speciation in the Galápagos finches occurred by: founding of a new population geographic isolation changes in new population's gene pool reproductive isolation ecological competition Copyright Pearson Prentice Hall

Speciation in Darwin's Finches Founders Arrive  A few finches—species A—travel from South America to one of the Galápagos Islands. There, they survive and reproduce. Speciation in the Galápagos finches occurred by founding of new populations, geographic isolation, gene pool changes, reproductive isolation, and ecological competition. Small groups of finches moved from one island to another, became reproductively isolated, and evolved into new species. Copyright Pearson Prentice Hall

Speciation in Darwin's Finches Geographic Isolation Some birds from species A cross to a second island. The two populations no longer share a gene pool. Speciation in the Galápagos finches occurred by founding of new populations, geographic isolation, gene pool changes, reproductive isolation, and ecological competition. Small groups of finches moved from one island to another, became reproductively isolated, and evolved into new species. Copyright Pearson Prentice Hall

Speciation in Darwin's Finches Changes in the Gene Pool Seed sizes on the second island favor birds with large beaks. The population on the second island evolves into population B, with larger beaks. Speciation in the Galápagos finches occurred by founding of new populations, geographic isolation, gene pool changes, reproductive isolation, and ecological competition. Small groups of finches moved from one island to another, became reproductively isolated, and evolved into new species. Copyright Pearson Prentice Hall

Speciation in Darwin's Finches Reproductive Isolation If population B birds cross back to the first island, they will not mate with birds from population A. Populations A and B are separate species. Copyright Pearson Prentice Hall

Speciation in Darwin's Finches Ecological Competition As species A and B compete for available seeds on the first island, they continue to evolve in a way that increases the differences between them. A new species—C—may evolve. Copyright Pearson Prentice Hall

Speciation in Darwin's Finches Continued Evolution 13 different finch species eventually evolved to today!! Copyright Pearson Prentice Hall