BIOLOGY CONCEPTS & CONNECTIONS Fourth Edition Copyright © 2003 Pearson Education, Inc. publishing as Benjamin Cummings Neil A. Campbell Jane B. Reece Lawrence G. Mitchell Martha R. Taylor From PowerPoint ® Lectures for Biology: Concepts & Connections CHAPTER 13 How Populations Evolve Modules 13.4 – 13.12

Copyright © 2003 Pearson Education, Inc. publishing as Benjamin Cummings Darwin observed that –organisms produce more offspring than the environment can support –organisms vary in many characteristics –these variations can be inherited Natural Selection

Copyright © 2003 Pearson Education, Inc. publishing as Benjamin Cummings Natural Selection Darwin concluded that individuals best suited for a particular environment are more likely to survive and reproduce than those less well adapted Aka: survival of the fittest

Copyright © 2003 Pearson Education, Inc. publishing as Benjamin Cummings Darwin saw natural selection as the basic mechanism of evolution –As a result, the proportion of individuals with favorable characteristics increases –Populations gradually change in response to the environment –Phenotypes that are better reproduce more, eventually, better genotypes become more common.

Copyright © 2003 Pearson Education, Inc. publishing as Benjamin Cummings Darwin also saw that when humans choose organisms with specific characteristics as breeding stock, they are performing the role of the environment –This is called artificial selection –Example of artificial selection in plants: five vegetables derived from wild mustard Figure 13.4A

Copyright © 2003 Pearson Education, Inc. publishing as Benjamin Cummings –Example of artificial selection in animals: dog breeding Figure 13.4B German shepherdYorkshire terrier English springer spaniel Mini-dachshundGolden retriever Hundreds to thousands of years of breeding (artificial selection) Ancestral dog

Copyright © 2003 Pearson Education, Inc. publishing as Benjamin Cummings These five canine species evolved from a common ancestor through natural selection Figure 13.4C African wild dog CoyoteFoxWolfJackal Thousands to millions of years of natural selection Ancestral canine

Copyright © 2003 Pearson Education, Inc. publishing as Benjamin Cummings A species is a group of populations whose individuals can interbreed and produce fertile offspring –People (and animals) are more likely to choose mates locally Populations are the units of evolution Figure 13.6

Copyright © 2003 Pearson Education, Inc. publishing as Benjamin Cummings 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 New mutations are constantly being generated in a gene pool, by accident or as a response to environmental changes Microevolution

Copyright © 2003 Pearson Education, Inc. publishing as Benjamin Cummings What causes evolution? Genetic drift Bottleneck Effect Founder Effect Gene Flow Mutation

Copyright © 2003 Pearson Education, Inc. publishing as Benjamin Cummings Genetic drift is a change in a gene pool due to chance –Genetic drift can cause the bottleneck effect: an event that drastically reduces population size (fire, flood, earthquake) Genetic Drift Figure 13.11A Original population Bottlenecking event Surviving population

Copyright © 2003 Pearson Education, Inc. publishing as Benjamin Cummings Genetic drift… If a population is very diverse and something bad happens, at least a few individuals will survive. These individuals will then reproduce and the species will evolve, or change.

Copyright © 2003 Pearson Education, Inc. publishing as Benjamin Cummings –The founder effect is when some individuals leave a population and start living somewhere new. –Only a few people or animals leave, and the new population will be closely related to due lack of genetic diversity. Figure 13.11B, C

Copyright © 2003 Pearson Education, Inc. publishing as Benjamin Cummings Gene flow can change a gene pool due to the movement of genes into or out of a population (new organisms move in or leave) Mutation changes alleles, these are random changes in DNA that can create new proteins or new characteristics.

Copyright © 2003 Pearson Education, Inc. publishing as Benjamin Cummings Natural selection results in the accumulation of traits that adapt a population to its environment –If the environment should change, natural selection would favor traits adapted to the new conditions –Organisms with helpful traits survive and reproduce, and these traits are passed on to offspring. Natural Selection

Copyright © 2003 Pearson Education, Inc. publishing as Benjamin Cummings The Hardy-Weinberg equation shows that in a changing environment, evolution always happens. Hardy-Weinberg equilibrium states that the shuffling of genes during sexual reproduction does not alter the proportions of different alleles in a gene pool Populations are always evolving and not usually in equilibrium Hardy-Weinberg Equilibrium Figure 13.8A

Copyright © 2003 Pearson Education, Inc. publishing as Benjamin Cummings If all 5 of these conditions are met, evolution will not happen in nature: 1. The population is very large 2. The population is isolated 3. Mutations do not alter the gene pool 4. Mating is random 5. All individuals are equal in reproductive success ***This does not happen in nature! Five conditions are required for Hardy- Weinberg equilibrium

Copyright © 2003 Pearson Education, Inc. publishing as Benjamin Cummings The Equation (yes you have to do math) p 2 + 2pq + q 2 = 1 p + q = 1 p = frequency of the dominant allele in the population (A) q = frequency of the recessive allele in the population (a) p 2 = percentage of homozygous dominant individuals (AA) q 2 = percentage of homozygous recessive individuals (aa) 2pq = percentage of heterozygous individuals (Aa)

Copyright © 2003 Pearson Education, Inc. publishing as Benjamin Cummings Practice Problem: p 2 + 2pq + q 2 = 1 p + q = 1 You have sampled a population in which you know that the percentage of the homozygous recessive genotype (aa) is 36%. Using that 36%, calculate the following: The frequency of the "a" allele. The frequency of aa is 36%, which means that q 2 = If q 2 = 0.36, then q = 0.6. Since q equals the frequency of the a allele, then the frequency is 60% The frequency of the "A" allele. Since q = 0.6, and p + q = 1, then p = 0.4; the frequency of A is by definition equal to p, so the answer is 40%

Copyright © 2003 Pearson Education, Inc. publishing as Benjamin Cummings The frequencies of the genotypes "AA" and "Aa." The frequency of AA is equal to p 2, and the frequency of Aa is equal to 2pq. So, using the information above, the frequency of AA is 16% (i.e. p 2 is 0.4 x 0.4 = 0.16) and Aa is 48% (2pq = 2 x 0.4 x 0.6 = 0.48).

Copyright © 2003 Pearson Education, Inc. publishing as Benjamin Cummings Speciation Speciation: is the evolutionary process by which species arise.

Copyright © 2003 Pearson Education, Inc. publishing as Benjamin Cummings How new species form Stabilizing selection: average individuals are favored. Ex: average sized spiders Directional selection: one trait is more likely to reproduce than another. Ex: long beaked woodpeckers Disruptive selection: extreme traits are more likely to survive and reproduce. Ex: black moths

Copyright © 2003 Pearson Education, Inc. publishing as Benjamin Cummings How new species form Geographic isolation: physical barriers divide a population (Ex. Grand Canyon) Reproductive isolation: can no longer mate and produce fertile offspring (ex. CA salamanders)

Copyright © 2003 Pearson Education, Inc. publishing as Benjamin Cummings Gradualism vs. Punctuated How fast does evolution go? Gradualism says that it happens slowly and steadily over time Punctuated equilibrium says that it happens quickly in rapid bursts There are examples of both! Gradualism: dinosaurs into modern birds Punctuated: finches in the Galapagos and extreme weather (Grant study)

Copyright © 2003 Pearson Education, Inc. publishing as Benjamin Cummings Speciation 2 animals cannot produce viable, fertile offspring if they are from different species. This is because of either pre-zygotic barriers which prevent the animals from ever meeting and reproducing, or post-zygotic barriers that prevent a zygote from developing into a fertile adult.

Copyright © 2003 Pearson Education, Inc. publishing as Benjamin Cummings Pre-Zygotic Barriers 1. Habitat isolation: 2 species live in different habitats and don’t see each other 2. Temporal isolation: 2 species breed at different times of the day or year 3. Behavioral isolation: courtship rituals are species specific, and don’t attract other species 4. Mechanical isolation: the sexual pieces don’t fit together 5. Gametic isolation: even if sex occurs, the egg and sperm can’t meet

Copyright © 2003 Pearson Education, Inc. publishing as Benjamin Cummings Post-zygotic barriers 1. Reduced hybrid viability: hybrid babies die before reaching reproductive age 2. Reduced hybrid fertility: hybrid babies are sterile and can’t reproduce 3. Hybrid breakdown: first generation babies can breed, but their children are infertile (F2)

Copyright © 2003 Pearson Education, Inc. publishing as Benjamin Cummings Speciation There are a few exceptions, called hybrids: –Lion + tiger = liger (the grandchildren can’t reproduce) –Horse + donkey = mule (all sterile) –Polar bear + grizzly bear = pizzly (too soon to tell if they are fertile or not)

Copyright © 2003 Pearson Education, Inc. publishing as Benjamin Cummings The Results of Evolution Earth began 4.6 billion years ago. First, there were only bacteria cells. These evolved into eukaryotic multi celled simple animals. The first animals lived in the ocean and evolved into fish. Lungfish were able to survive on land and evolved into amphibians and reptiles. Reptiles evolved into birds and small mammals. Small rodent like mammals evolved into lemurs, which then evolved into apes, monkeys, and humans. Apes and monkeys are our close cousins, and we share a common ancestor.