1. Evolution of Populations

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

3. Variation and Gene Pools  Genetic variation is studied in populations. A population is a group of individuals of the same species that interbreed. 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. 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. 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. Relative frequency is often expressed as a percentage.

4. Gene Pools:

5. Allele Frequency: Gene Pool for Fur Color in Mice: Gene Pool for Fur Color in Mice:

6. Genetic Drift: –A random change in allele frequency is called genetic drift –In small populations, individuals that carry a particular allele may leave more descendants than other individuals do, just by chance. –Over time, a series of chance occurrences of this type can cause an allele to become common in a population.

8. The Founder Effect Genetic drift may occur when a small group of individuals colonizes a new habitat. 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. Individuals may carry alleles in different relative frequencies than did the larger population from which they came. The new population will be genetically different from the parent population. The new population will be genetically different from the parent population.

9. Genetic Drift Genetic Drift

10. Microevolution: Evolution as Genetic Change Natural selection affects which individuals survive and reproduce and which do not. Natural selection affects which individuals survive and reproduce and which do not. If an individual dies without reproducing, it does not contribute its alleles to the population’s gene pool. If an individual dies without reproducing, it does not contribute its alleles to the population’s gene pool. If an individual produces many offspring, its alleles stay in the gene pool and may increase in frequency. If an individual produces many offspring, its alleles stay in the gene pool and may increase in frequency.

11. Sources of Genetic Variation 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. Sources of Genetic Variation: Sources of Genetic Variation: –mutations –genetic shuffling that results from sexual reproduction.

12. Mutations: Any change in a sequence of DNAAny change in a sequence of DNA Occur because of mistakes in DNA replication or as a result of radiation or chemicals in the environmentOccur because of mistakes in DNA replication or as a result of radiation or chemicals in the environment Do not always affect an organisms phenotypeDo not always affect an organisms phenotype

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

14. Gene Shuffling:

15. Single-Gene and Polygenic Traits Many traits are controlled by two or more genes and are called polygenic traits. Many traits are controlled by two or more genes and are called polygenic traits. One polygenic trait can have many possible genotypes and phenotypes. One polygenic trait can have many possible genotypes and phenotypes. Height in humans is a polygenic trait. Height in humans is a polygenic trait. A bell-shaped curve is typical of polygenic traits. A bell-shaped curve is typical of polygenic traits. A bell-shaped curve is also called normal distribution. A bell-shaped curve is also called normal distribution.

16. Natural Selection on Polygenic Traits 3 categories: 3 categories: Directional: favors one extreme Directional: favors one extreme Stabilizing: favors the middle Stabilizing: favors the middle Disruptive: favors both extremes Disruptive: favors both extremes

17. Types of Natural Selection

18. What type of selection?

19. Genetic Equilibrium A population is in genetic equilibrium if allele frequencies are not changing from one generation to the next A population is in genetic equilibrium if allele frequencies are not changing from one generation to the next According to the Hardy-Weinberg theory, a population is in genetic equilibrium if the following conditions are met simultaneously: According to the Hardy-Weinberg theory, a population is in genetic equilibrium if the following conditions are met simultaneously: –Large population size –Random mating –No mutations –No migration –No natural selection

20. Divergent v. Convergent Evolution Divergent One species gives rise to many species One species gives rise to many species Also known as adaptive radiation Also known as adaptive radiation Many species with common ancestor Many species with common ancestor Many homologous structures Many homologous structures Convergent Similar looking species that do not have a common ancestor Similar looking species that do not have a common ancestor Similar behavior and appearance due to environmental similarities Similar behavior and appearance due to environmental similarities Many analogous structures Many analogous structures

21. Convergent Evolution

22. Coevolution The evolution of one species is directly influenced by the evolution of another

23. Punctuated Equilibrium Slow background evolution (stasis) is interrupted by rapid bursts of change Slow background evolution (stasis) is interrupted by rapid bursts of change Rapid bursts of change usually occur after a mass extinction Rapid bursts of change usually occur after a mass extinction

24. Speciation: Speciation is the formation of new species. Speciation is the formation of new species. A species is a group of organisms that breed with one another and produce fertile offspring. A species is a group of organisms that breed with one another and produce fertile offspring. The gene pools of two populations must become separated for them to become new species The gene pools of two populations must become separated for them to become new species When the members of two populations cannot interbreed and produce fertile offspring, reproductive isolation has occurred and speciation will result. When the members of two populations cannot interbreed and produce fertile offspring, reproductive isolation has occurred and speciation will result.

25. Types of Reproductive Isolation Behavioral Isolation – Different mating rituals prevent reproduction Behavioral Isolation – Different mating rituals prevent reproduction Geographic Isolation – barriers such as rivers or mountains prevent reproduction Geographic Isolation – barriers such as rivers or mountains prevent reproduction Temporal Isolation – different mating times (seasonal, nocturnal v. diurnal) prevent reproduction Temporal Isolation – different mating times (seasonal, nocturnal v. diurnal) prevent reproduction

26. Speciation in Darwin's Finches founding of a new population founding of a new population geographic isolation geographic isolation changes in new population's gene pool changes in new population's gene pool reproductive isolation reproductive isolation ecological competition ecological competition

27. STEP 1: Founders Arrive A few finches, “species A”, travel from S. America to one of the Galápagos Islands. There, they survive and reproduce.

28. STEP 2: Geographic Isolation Some birds from species A cross to a second island. The two populations no longer share a gene pool.

29. STEP 3: 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.

30. STEP 4: Reproductive Isolation If population B birds cross back to the first island, they will not mate with birds from population A. 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. Populations A and B are separate species.

31. STEP 5: 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. 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. A new species—C—may evolve.

32. Continued Evolution This process of isolation, genetic change, and reproductive isolation probably repeated itself often across the entire Galápagos island chain. This process of isolation, genetic change, and reproductive isolation probably repeated itself often across the entire Galápagos island chain.

33. Post-Darwin Evolutionary Studies Scientific evidence supports the theory that living species descended with modification from common ancestors that lived in the ancient past. Scientific evidence supports the theory that living species descended with modification from common ancestors that lived in the ancient past. Scientists predict that as new fossils are found, they will continue to expand our understanding of how species evolved. Scientists predict that as new fossils are found, they will continue to expand our understanding of how species evolved. As our knowledge of DNA and genomes grows we are better able to understand relationships between species. As our knowledge of DNA and genomes grows we are better able to understand relationships between species.