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Populations are the units of evolution Figure 13.6 Copyright © 2003 Pearson Education, Inc. publishing as Benjamin Cummings 1. What is evolving? gene pool, microevolution 2. Four agents of evolution 3. Types of natural selection • Evolution happens when populations of organisms with inherited variations are exposed to environmental factors that favor the reproductive success of some individuals over others Figure 1.6C Populations are the units of evolution • A population is a group of interbreeding individuals • A species is a group of populations whose individuals can interbreed and produce fertile offspring Figure 13.6 Copyright © 2003 Pearson Education, Inc. publishing as Benjamin Cummings What is evolving? • gene pool = 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 Copyright © 2003 Pearson Education, Inc. publishing as Benjamin Cummings Four agents of microevolution 1. Mutation changes alleles 2. Genetic drift = random changes in allele frequency Bottleneck Founder effect Genetic drift - effects of population size: LARGE POPULATION = 10,000 1,000 allele frequency = 10,000 = 10% 50% of population survives, including 450 allele carriers 450 allele frequency = 5,000 = 9% little change in allele frequency (no alleles lost) SMALL POPULATION = 10 allele frequency = 1 10 = 10% 50% of population survives, with no allele carrier among them allele frequency = 0 5 = 0% dramatic change in allele frequency (potential to lose one allele) Bottleneck effect Founder effect Figure 13.11B, C Copyright © 2003 Pearson Education, Inc. publishing as Benjamin Cummings 3. Gene flow can change a gene pool due to the movement of genes into or out of a population ex. Migration 4. Natural selection leads to differential reproductive success Nonrandom mating changes genotype frequency but not allele frequency. Copyright © 2003 Pearson Education, Inc. publishing as Benjamin Cummings • Natural selection - results in the accumulation of traits that adapt a population to its environment - the only agent of evolution that results in adaptation. Copyright © 2003 Pearson Education, Inc. publishing as Benjamin Cummings What are sources of genetic variation? • Recombination of genes in sexual reproduction • Mutation can create new alleles, new genes. - homeobox genes can create major changes Copyright © 2003 Pearson Education, Inc. publishing as Benjamin Cummings What is an organism’s evolutionary fitness? • An individual’s Darwinian fitness is the contribution it makes to the gene pool of the next generation relative to the contribution made by other individuals; i.e., number of progeny • Production of fertile offspring is the only score that counts in natural selection Copyright © 2003 Pearson Education, Inc. publishing as Benjamin Cummings There are three general outcomes of natural selection Frequency of individuals Original population Phenotypes (fur color) Original population Evolved population Stabilizing selection Directional selection Diversifying selection Figure 13.19 Copyright © 2003 Pearson Education, Inc. publishing as Benjamin Cummings 80 beak depth 1976 Number of individuals 60 40 Average beak depth, 1976 20 Average beak depth, 1978 1978 0 5 6 7 8 9 10 11 12 13 Beak depth (mm) Shift of average beak depth during drought 14 20 70 Infant deaths 60 Infant births 15 50 Percent of infant deaths Percent of births in 10 population 40 30 20 5 10 0 2 3 4 5 6 7 8 Birth weight in pounds 9 10 11 0 Why doesn’t natural selection eliminate all genetic variation in populations? • Natural selection tends to reduce variability in populations. Mechanisms which counteract: – The diploid condition preserves variation by “hiding” recessive alleles (Bb) – Balanced polymorphism (2+ phenotypes stable in population) may result from: 1. heterozygote advantage Aa > aa and AA 2. frequency-dependent selection 3. variation of environment for a population Copyright © 2003 Pearson Education, Inc. publishing as Benjamin Cummings • Many populations exhibit polymorphism and geographic variation Figure 13.13 Copyright © 2003 Pearson Education, Inc. publishing as Benjamin Cummings Not all genetic variation may be subject to natural selection • Some variations may be neutral, providing no apparent advantage or disadvantage – Example: human fingerprint patterns Figure 13.16 Copyright © 2003 Pearson Education, Inc. publishing as Benjamin Cummings Endangered species often have reduced variation • Low genetic variability may reduce their capacity to survive as humans continue to alter the environment – cheetah populations have extreme genetic uniformity Figure 13.17 Copyright © 2003 Pearson Education, Inc. publishing as Benjamin Cummings Why do male and female animals differ in appearance? • Sexual selection leads to the evolution of secondary sexual characteristics • Sexual selection may produce sexual dimorphism Figure 13.20A, B Copyright © 2003 Pearson Education, Inc. publishing as Benjamin Cummings Natural selection cannot fashion perfect organisms • This is due to: – historical constraints – adaptive compromises – chance events – availability of variations Copyright © 2003 Pearson Education, Inc. publishing as Benjamin Cummings • The excessive use of antibiotics is leading to the evolution of antibiotic-resistant bacteria – Example: Mycobacterium tuberculosis Figure 13.22 Copyright © 2003 Pearson Education, Inc. publishing as Benjamin Cummings What is a species? • appearance alone does not always define a species – Example: eastern and western meadowlarks Figure 14.1A Copyright © 2003 Pearson Education, Inc. publishing as Benjamin Cummings What is a species? • Naturally interbreeding populations - potentially interbreeding - reproductively isolated from other species What about asexually reproducing organisms? Extinct species? Shy species? Copyright © 2003 Pearson Education, Inc. publishing as Benjamin Cummings MECHANISMS OF SPECIATION When does speciation occur? • When geographically isolated, species evolution may occur – gene pool then changes to cause reproductive isolation = allopatric speciation Figure 14.3 Copyright © 2003 Pearson Education, Inc. publishing as Benjamin Cummings • A ring species may illustrate the process of speciation 1 OREGON POPULATION Sierra Nevada COASTAL POPULATIONS Yelloweyed Yellowblotched 2 Gap in ring Monterey INLAND POPULATIONS Largeblotched 3 Figure 14.1C Copyright © 2003 Pearson Education, Inc. publishing as Benjamin Cummings Reproductive barriers between species • Habitat - different locations • Timing - mating, flowering • Behavioral - mating rituals, no attraction • Mechanical - structural differences • Gametic - fail to unite • Hybrid weak or infertile Copyright © 2003 Pearson Education, Inc. publishing as Benjamin Cummings • Hybrid sterility is one type of postzygotic barrier – A horse and a donkey may produce a hybrid offspring, a mule – Mules are sterile Figure 14.2C Copyright © 2003 Pearson Education, Inc. publishing as Benjamin Cummings Sympatric speciation • No geographical isolation • Mutation creates reproductive isolation • Polyploidization • Hybridization Copyright © 2003 Pearson Education, Inc. publishing as Benjamin Cummings When does speciation occur? • Specialists - Galapagos finches • Generalists - horseshoe crabs, cockroaches • New environments - ecological niche Copyright © 2003 Pearson Education, Inc. publishing as Benjamin Cummings • Adaptive radiation on an island chain - specialization for different niches 1 A Species A from mainland 2 B B 3 B C B 4 C C D C C Copyright © 2003 Pearson Education, Inc. publishing as Benjamin Cummings D 5 Figure 14.4B Cactus ground finch Medium ground finch Large ground finch Small Large cactus ground finch ground finch Small tree finch Vegetarian finch Medium tree finch Large tree finch Woodpecker finch Mangrove finch Green Gray warbler finch warbler finch Sharp-beaked ground finch Seed eaters Cactus flower eaters Ground finches Bud eaters Insect eaters Tree finches Warbler finches Common ancestor from South America mainland Figure 15.9 Continental drift has played a major role in macroevolution • Continental drift is the slow, steady movement of Earth’s crustal plates on the hot mantle Eurasian Plate North American Plate African Plate Pacific Plate Nazca Plate South American Plate Split developing Indo-Australian Plate Antarctic Plate Edge of one plate being pushed over edge of neighboring plate (zones of violent geologic events) Copyright © 2003 Pearson Education, Inc. publishing as Benjamin Cummings Figure 15.3A CENOZOIC • influenced the distribution of organisms Eurasia Africa Copyright © 2003 Pearson Education, Inc. publishing as Benjamin Cummings MESOZOIC Antarctica PALEOZOIC – Separation of continents caused the isolation and diversification of organisms Millions of years ago – Continental mergers triggered extinctions India South America Laurasia Figure 15.3B Speciation - how much change is needed? • Gradual vs. jerky • Evidence: – Fossil record – Genetic differences between species – Homeotic genes Copyright © 2003 Pearson Education, Inc. publishing as Benjamin Cummings • homeotic genes control body development • Single mutation can result in major differences in body structure Fly chromosomes Mouse chromosomes Fruit fly embryo (10 hours) Mouse embryo (12 days) Adult fruit fly Adult mouse Figure 11.14 Copyright © 2003 Pearson Education, Inc. publishing as Benjamin Cummings