THE EVOLUTION OF POPULATIONS

A population is a group of individuals of the same species living in the same place at the same time Evolution is the change in heritable traits in a population over generations Populations may be isolated from one another (with little interbreeding), or individuals within populations may interbreed Student Misconceptions and Concerns 1. Students often suggest that individuals evolve. As this chapter section clarifies, populations are the smallest units that can evolve. Individuals do not have genetic diversity from which to select. However, individuals can change during their lifetime in response to the environment. Muscles can grow larger through use. However, these individual changes are not passed on to the next generation (after all, boys have been circumcised for thousands of years, but are still born with a foreskin). 2. Another misperception is that evolution results from need. Challenge your students to explain how need and want have nothing to do with evolution (because neither need nor want can generate genetic variation!) 3. Challenge your students to explain how extinction is predicted by an understanding of natural selection. (Genetic diversity is generated by random processes, but the variety generated may not be sufficient for the survival of a population or species, leading to its extinction.) Teaching Tips 1. Try to find good local examples of populations. If you are near a seashore, the many invertebrate populations (starfish, sea urchin, and kelp) might be ideal. Further inland, you might find somewhat isolated populations of fish and continuous and clumped populations of squirrels, separated by vast fields of corn, wheat, or soybeans! Bring the subject home with local examples. Copyright © 2009 Pearson Education, Inc.

A gene pool is the total collection of genes in a population at any one time Microevolution is a change in the relative frequencies of alleles in a gene pool over time Student Misconceptions and Concerns 1. Students often suggest that individuals evolve. As this chapter section clarifies, populations are the smallest units that can evolve. Individuals do not have genetic diversity from which to select. However, individuals can change during their lifetime in response to the environment. Muscles can grow larger through use. However, these individual changes are not passed on to the next generation (after all, boys have been circumcised for thousands of years, but are still born with a foreskin). 2. Another misperception is that evolution results from need. Challenge your students to explain how need and want have nothing to do with evolution (because neither need nor want can generate genetic variation!) 3. Challenge your students to explain how extinction is predicted by an understanding of natural selection. (Genetic diversity is generated by random processes, but the variety generated may not be sufficient for the survival of a population or species, leading to its extinction.) Teaching Tips 1. Try to find good local examples of populations. If you are near a seashore, the many invertebrate populations (starfish, sea urchin, and kelp) might be ideal. Further inland, you might find somewhat isolated populations of fish and continuous and clumped populations of squirrels, separated by vast fields of corn, wheat, or soybeans! Bring the subject home with local examples. Copyright © 2009 Pearson Education, Inc.

The modern synthesis connects Darwin’s theory with population genetics Population genetics studies how populations change genetically over time The modern synthesis connects Darwin’s theory with population genetics Student Misconceptions and Concerns 1. Students often suggest that individuals evolve. As this chapter section clarifies, populations are the smallest units that can evolve. Individuals do not have genetic diversity from which to select. However, individuals can change during their lifetime in response to the environment. Muscles can grow larger through use. However, these individual changes are not passed on to the next generation (after all, boys have been circumcised for thousands of years, but are still born with a foreskin). 2. Another misperception is that evolution results from need. Challenge your students to explain how need and want have nothing to do with evolution (because neither need nor want can generate genetic variation!) 3. Challenge your students to explain how extinction is predicted by an understanding of natural selection. (Genetic diversity is generated by random processes, but the variety generated may not be sufficient for the survival of a population or species, leading to its extinction.) Teaching Tips 1. Try to find good local examples of populations. If you are near a seashore, the many invertebrate populations (starfish, sea urchin, and kelp) might be ideal. Further inland, you might find somewhat isolated populations of fish and continuous and clumped populations of squirrels, separated by vast fields of corn, wheat, or soybeans! Bring the subject home with local examples. Copyright © 2009 Pearson Education, Inc.

Hardy Weinberg Equations The Hardy Weinberg principle states that allele and genotype frequencies within a sexually reproducing, diploid population will remain in equilibrium unless outside forces act to change those frequencies Hardy Weinberg Equations p2 + 2pq + q2 = 1 p + q = 1

p q pp pq qq 25% 50% 25% 25% 25% 25% 25% p2 + 2pq + q2 = 1 Hardy Weinberg Equation p2 + 2pq + q2 = 1 25% 50% 25% p q pp pq qq 25% 25% 25% 25%

Hardy Weinberg Equation p2 + 2pq + q2 = 1 50% 25% 25% p2 2pq q2

Gene Pool Hardy Weinberg Equation p + q = 1 p q p q p q p p q q q q p

No gene flow between populations No mutations Random mating The Hardy-Weinberg equation can be used to test whether a population is evolving For a population to remain in Hardy-Weinberg equilibrium for a specific trait, it must satisfy five conditions: Very large population No gene flow between populations No mutations Random mating No natural selection Ask the class: Are these conditions ever met in a real population? Go through them one by one, asking students for specific examples of populations that meet (or fail to meet) these conditions with respect to a specific trait. Student Misconceptions and Concerns 1. Students often suggest that individuals evolve. As this chapter section clarifies, populations are the smallest units that can evolve. Individuals do not have genetic diversity from which to select. However, individuals can change during their lifetime in response to the environment. Muscles can grow larger through use. However, these individual changes are not passed on to the next generation (after all, boys have been circumcised for thousands of years, but are still born with a foreskin). 2. Another misperception is that evolution results from need. Challenge your students to explain how need and want have nothing to do with evolution (because neither need nor want can generate genetic variation!) 3. Challenge your students to explain how extinction is predicted by an understanding of natural selection. (Genetic diversity is generated by random processes, but the variety generated may not be sufficient for the survival of a population or species, leading to its extinction.) Teaching Tips 1. No doubt about it, the Hardy-Weinberg equation is problematic for some students. Students should create a quick reference key to the definitions of the elements. Consider some practice problems varying the value of p and q. Much of the remainder of the chapter builds upon deviations from a Hardy-Weinberg equilibrium. 2. Heterozygotes can form in two ways, the recessive from mom, the dominant from dad . . . or the reverse. This should serve to remind students that the 2pq portion of the equation represents the heterozygotes. Copyright © 2009 Pearson Education, Inc.

So: 37% 26% 37% 50% 25% 25% p2 2pq q2 p2 2pq q2 Small population Gene flow between populations Mutations Selective mating Natural selection What would change the frequency of Genotypes! 37% 26% 37% 50% 25% 25% p2 2pq q2 p2 2pq q2

Thumb crossing: In a relaxed interlocking of fingers, left thumb over right results from having 1 or 2 copies of the dominant version of the gene. People with 2 recessives place right thumb over left. Cross Left Thumb Over Right Cross Right Thumb Over Left

p2 + 2pq + q2 = 1 p + q = 1 CHARACTERISTIC – Thumb Crossing Population size ________ Number with recessive phenotype ________ 3. Freq. of recessive pheno. (q2) (#2/#1) ______ 4. Freq. of q in pop. (square root of q2) ______ 5. Freq. of p in pop. (1.0 – q ) _______ Freq. of homozygous dominant in population (p2) _________ Frequency of heterozygote’s in population (2pq) ________ 8. Frequency of homozygous recessive in population (q2) _________   p2 + 2pq + q2 = 1 p + q = 1

Ear lobes: Recessives have attached ear lobes Ear lobes: Recessives have attached ear lobes. People with a dominant version of the gene have detached ear lobes.