5/2/ Genes and Variation. How Common Is Genetic Variation? Variation and Gene Pools

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1 16-1 Genes 16-1 and Variation Genes and Variation 1 of 24 How Common Is Genetic 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. 2 of 24 Variation and Gene Pools 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. 3 of 24 1

2 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. 4 of 24 Variation and Gene Pools Gene Pool for Fur Color in Mice Sample Population Frequency of Alleles allele for brown fur allele for black fur 5 of 24 Variation and Gene Pools In genetic terms, evolution is any change in the relative frequency of alleles in a population. 6 of 24 2

3 Sources of Genetic Variation Sources of Genetic Variation The two main sources of genetic variation are mutations and the genetic shuffling that results from sexual reproduction. 7 of 24 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. 8 of 24 Single-Gene and Polygenic Traits Single-Gene and Polygenic Traits The number of phenotypes produced for a given trait depends on how many genes control the trait. 9 of 24 3

4 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. 10 of 24 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. 11 of 24 Single-Gene and Polygenic Trait A bell-shaped curve is typical of polygenic traits. A bell-shaped curve is also called normal distribution. 12 of 24 4

5 Evolution as Genetic as 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. 13 of 24 Natural Selection on Single-Gene Traits 14 of 24 Natural Selection on Polygenic Traits Natural selection can affect the distributions of phenotypes in any of three ways: directional selection stabilizing selection disruptive selection 15 of 24 5

6 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. 16 of 24 Stabilizing Selection Natural Selection on Polygenic Traits When individuals near the center of the curve have higher fitness than individuals at either end of the curve, stabilizing selection takes place. 17 of 24 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. 18 of 24 6

7 What is genetic drift? A random change in allele frequency 19 of 24 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. 20 of of 24 7

8 22 of of 24 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. 24 of 24 8

9 Evolution Versus Genetic Equilibrium 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. 25 of 24 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. 26 of 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. 27 of 24 9

10 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. 28 of 24 Isolating Mechanisms 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. 29 of 24 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. 30 of 24 10

11 Isolating Mechanisms Geographic Isolation Geographic isolation occurs when two populations are separated by geographic barriers such as rivers or mountains. Abert Kaibab 31 of 24 Isolating Mechanisms Temporal Isolation Temporal isolation occurs when two or more species reproduce at different times. 32 of 24 Testing Natural Selection in Nature Testing Natural Selection in Nature Studies showing natural selection in action involve descendants of the finches that Darwin observed in the Galápagos Islands. The finches Darwin saw were different, but he hypothesized that they had descended from a common ancestor. 33 of 24 11

12 Testing Natural Selection in Nature 34 of 24 Testing Natural Selection in Nature 35 of 24 Testing Natural Selection in Nature Peter and Rosemary Grant tested Darwin s hypothesis, which relied on two testable assumptions: For beak size and shape to evolve, there must be enough heritable variation in those traits to provide raw material for natural selection. Differences in beak size and shape must produce differences in fitness, causing natural selection to occur. 36 of 24 12

13 Testing Natural Selection in Nature When food was scarce, individuals with large beaks were more likely to survive. 37 of 24 Speciation in Darwin's Finches 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 38 of 24 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. 39 of 24 13

14 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. 40 of 24 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. 41 of 24 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. 42 of 24 14