Evolution of Populations Chapter 16

16-1 Genes and Variation Darwin’s handicap while developing theory of evolution Darwin’s handicap while developing theory of evolution Discoveries since Darwin Discoveries since Darwin Commonality of Genetic Variation Commonality of Genetic Variation At least 2 alleles At least 2 alleles “invisible” variation involving small differences in biochemical processes “invisible” variation involving small differences in biochemical processes Heterozygous for many genes (4-8% in mammals) Heterozygous for many genes (4-8% in mammals)

Variation and Gene Pools Population is a group of individuals of the same species that interbreed Population is a group of individuals of the same species that interbreed Share the same gene pool- all genes, including all the different alleles, that are present in a population Share the same gene pool- all genes, including all the different alleles, that are present in a population Relative frequency- the number of times that the allele occurs in a gene pool, compared with the number of times other alleles for the same gene occur Relative frequency- the number of times that the allele occurs in a gene pool, compared with the number of times other alleles for the same gene occur In genetic terms, evolution is any change in the relative frequency of alleles in a population In genetic terms, evolution is any change in the relative frequency of alleles in a population

Gene Pools 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. Calculating Here, in a total of 50 alleles, 20 alleles are B (black), and 30 are b (brown). How many of each allele would be present in a total of 100 alleles?

Sources of Genetic Variation The two main sources of genetic variation are mutations and the genetic shuffling that results from sexual reproduction The two main sources of genetic variation are mutations and the genetic shuffling that results from sexual reproduction Mutation- any change in a sequence of DNA Mutation- any change in a sequence of DNA Mistake in replication of DNA Mistake in replication of DNA Result of radiation or chemicals in environment Result of radiation or chemicals in environment Gene shuffling- different combinations of genes Gene shuffling- different combinations of genes During production of gametes During production of gametes Crossing over Crossing over Sexual reproduction does not change the relative frequency of alleles in a population Sexual reproduction 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 The number of phenotypes produced for a given trait depends on how many genes control the trait Single-gene trait- trait controlled by a single gene that has two alleles Single-gene trait- trait controlled by a single gene that has two alleles i.e. widow’s peak- 2 distinct phenotypes i.e. widow’s peak- 2 distinct phenotypes Polygenic traits- traits controlled by two or more genes that have two or more alleles Polygenic traits- traits controlled by two or more genes that have two or more alleles i.e. human height- many possible genotypes and phenotypes i.e. human height- many possible genotypes and phenotypes

16-2 Evolution as Genetic Change Evolutionary fitness/ Evolutionary adaptation Evolutionary fitness/ Evolutionary adaptation Natural selection doesn’t act directly on genes Natural selection doesn’t act directly on genes Natural selection on single-gene traits can lead to changes in allele frequencies and thus to evolution Natural selection on single-gene traits can lead to changes in allele frequencies and thus to evolution Lizard color mutations and the effects Lizard color mutations and the effects

Color Mutations 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.

Natural Selection on Polygenic Traits Effects are more complex Effects are more complex Natural selection can affect the distributions of phenotypes in any of three ways: Natural selection can affect the distributions of phenotypes in any of three ways: 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- when individuals at one end of the curve have higher fitness than individuals in the middle or at the other end Stabilizing selection- when individuals near the center of the curve have higher fitness than individuals at either end of the curve Stabilizing selection- when individuals near the center of the curve have higher fitness than individuals at either end of the curve Disruptive selection- when individuals at the upper and lower ends of the curve have higher fitness than individuals near the middle Disruptive selection- when individuals at the upper and lower ends of the curve have higher fitness than individuals near the middle

Genetic Drift Natural selection is not the only source of evolutionary change Natural selection is not the only source of evolutionary change Genetics controlled by laws of probability Genetics controlled by laws of probability In smaller pops, the results may be further from laws of probability In smaller pops, the results may be further from laws of probability Genetic drift- random change in allele frequencies that occurs in small populations Genetic drift- random change in allele frequencies that occurs in small populations

Genetic Drift cont… In small populations, individuals that carry a particular allele may leave more descendants than other individuals, just by chance. Over time, a series of chance occurrences of this type can cause an allele to become common in a population. In small populations, individuals that carry a particular allele may leave more descendants than other individuals, just by chance. Over time, a series of chance occurrences of this type can cause an allele to become common in a population. founder effect- a situation in which allele frequencies change as a result of the migration of a small subgroup of a population founder effect- a situation in which allele frequencies change as a result of the migration of a small subgroup of a population

Genetic Drift In small populations, individuals that carry a particular allele may have more descendants than other individuals. Over time, a series of chance occurrences of this type can cause an allele to become more common in a population. This model demonstrates how two small groups from a large, diverse population could produce new populations that differ from the original group.

Hardy-Weinberg Principle States that the allele frequency in a population will remain constant unless one or more factors cause those frequencies to change States that the allele frequency in a population will remain constant unless one or more factors cause those frequencies to change genetic equilibrium- situation in which allele frequencies remain constant genetic equilibrium- situation in which allele frequencies remain constant

Hardy-Weinberg cont… Five conditions: Five conditions: Random mating Random mating Population must be very large Population must be very large No movement into or out of the population No movement into or out of the population No mutations No mutations No natural selection No natural selection If the conditions are not met, the genetic equilibrium will be disrupted, and the population will evolve If the conditions are not met, the genetic equilibrium will be disrupted, and the population will evolve

Conditions Random mating Random mating Ensures that each individual has an equal chance of passing on its alleles to offspring Ensures that each individual has an equal chance of passing on its alleles to offspring Large population Large population Genetic drift has less effect on large populations Genetic drift has less effect on large populations No movement No movement Population’s gene pool must be kept together and kept separate from the gene pools of other populations Population’s gene pool must be kept together and kept separate from the gene pools of other populations No mutations No mutations If genes mutate from one form into another, new alleles may be introduced into the population, and allele frequencies will change If genes mutate from one form into another, new alleles may be introduced into the population, and allele frequencies will change No natural selection No natural selection All genotypes in the population must have equal probabilities of survival and reproduction All genotypes in the population must have equal probabilities of survival and reproduction

16.3 The Process of Speciation Speciation- formation of new species Speciation- formation of new species As new species evolve, populations become reproductively isolated from each other As new species evolve, populations become reproductively isolated from each other Reproductive isolation- occurrence in which the members of two populations cannot interbreed and produce fertile offspring Reproductive isolation- occurrence in which the members of two populations cannot interbreed and produce fertile offspring Populations have separate gene pools Populations have separate gene pools

Reproductive Isolation Behavioral Isolation- occurs when two populations are capable of interbreeding but have differences in courtship rituals or other reproductive strategies that involve behavior Behavioral Isolation- occurs when two populations are capable of interbreeding but have differences in courtship rituals or other reproductive strategies that involve behavior Bird songs Bird songs Geographic Isolation- two populations are separated by geographic barriers Geographic Isolation- two populations are separated by geographic barriers i.e. rivers, mountains, or bodies of water i.e. rivers, mountains, or bodies of water Kaibab and Albert squirrels (Colorado River) Kaibab and Albert squirrels (Colorado River) May not isolate all species (bird populations) May not isolate all species (bird populations) Temporal Isolation- two or more species reproduce at different times Temporal Isolation- two or more species reproduce at different times Orchid pollen release Orchid pollen release