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BIOLOGY CONCEPTS & CONNECTIONS Fourth Edition Neil A. Campbell • Jane B. Reece • Lawrence G. Mitchell • Martha R. Taylor CHAPTER 13 How Populations Evolve Modules 13.4 – 13.12 From PowerPoint® Lectures for Biology: Concepts & Connections Copyright © 2003 Pearson Education, Inc. publishing as Benjamin Cummings Natural Selection • Darwin observed that – organisms produce more offspring than the environment can support – organisms vary in many characteristics – these variations can be inherited 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 German shepherd Yorkshire terrier English springer spaniel Mini-dachshund Golden retriever Hundreds to thousands of years of breeding (artificial selection) Ancestral dog Copyright © 2003 Pearson Education, Inc. publishing as Benjamin Cummings Figure 13.4B • These five canine species evolved from a common ancestor through natural selection African wild dog Coyote Fox Wolf Jackal Thousands to millions of years of natural selection Ancestral canine Figure 13.4C Copyright © 2003 Pearson Education, Inc. publishing as Benjamin Cummings 13.6 Populations are the units of evolution • 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. Figure 13.6 Copyright © 2003 Pearson Education, Inc. publishing as Benjamin Cummings 13.7 Microevolution • 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. 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 • 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) Original population Bottlenecking event Surviving population Figure 13.11A 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 • 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. Copyright © 2003 Pearson Education, Inc. publishing as Benjamin Cummings 13.8 Hardy-Weinberg Equilibrium • 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. Figure 13.8A Copyright © 2003 Pearson Education, Inc. publishing as Benjamin Cummings 13.10 Five conditions are required for HardyWeinberg equilibrium • 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! Copyright © 2003 Pearson Education, Inc. publishing as Benjamin Cummings The Equation (yes you have to do math) • p2 + 2pq + q2 = 1 • p+q=1 • p = frequency of the dominant allele in the population (A) q = frequency of the recessive allele in the population (a) p2 = percentage of homozygous dominant individuals (AA) q2 = percentage of homozygous recessive individuals (aa) 2pq = percentage of heterozygous individuals (Aa) Copyright © 2003 Pearson Education, Inc. publishing as Benjamin Cummings Practice Problem: p2 + 2pq + q2 = 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 q2 = 0.36. If q2 = 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 p2, and the frequency of Aa is equal to 2pq. So, using the information above, the frequency of AA is 16% (i.e. p2 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. Copyright © 2003 Pearson Education, Inc. publishing as Benjamin Cummings