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Transcript
Processes of Evolution Chapter 12 Evidence • Biogeography • Comparative morphology • Geologic discoveries Biogeography • Size of the known world expanded enormously in 15th century • Accepted beliefs did not explain discovery of new organisms in previously unknown places Biogeography Comparative Morphology • Study of similarities and differences in body plans of major groups • Puzzling patterns: – Animals as different as whales and bats have similar bones in forelimbs – Some parts seem to have no function Comparative Morphology coccyx fossilized ankle bone ankle bone Comparative Morphology Comparative pelvic anatomy Geological Discoveries • Similar rock layers throughout world • Certain layers contain fossils • Deeper layers contain simpler fossils than shallow layers • Some fossils resemble known species 19th Century: New Theories • Scientists attempt to reconcile evidence of change with traditional belief in a single-creation event • Two examples – Georges Cuvier: multiple catastrophes – Jean Lamarck: inheritance of acquired characteristics The Theory of Uniformity • Lyell’s Principles of Geology • Earth shaped by subtle, repetitive processes of change • Challenged the view that Earth was only 6,000 years old Darwin’s Voyage • At age 22, Charles Darwin began a fiveyear, round-the-world voyage aboard the HMS Beagle • As ship’s naturalist, he collected and examined species that inhabited regions the ship visited Darwin’s Voyage Galapagos Islands The Galapagos Islands Fossil Evidence • Darwin found fossil Glyptodont • Proposed descent with modification Voyage of the Beagle EQUATOR Galapagos Islands Galapagos Islands Darwin Wolf • Volcanic islands far off coast of Ecuador Pinta Genovesa Marchena • All inhabitants are descended from species that arrived on islands from elsewhere Santiago Bartolomé Fernandia Seymour Baltra Rabida Pinzon Santa Cruz Santa Fe Tortuga San Cristobal Isabela Española Floreana Malthus: Struggle to Survive • Thomas Malthus, a clergyman and economist, wrote essay that Darwin read on his return to England • Argued that as population size increases, resources dwindle, the struggle to live intensifies, and conflict increases Galapagos Finches • Darwin observed finches with a variety of lifestyles and body forms • On his return, he learned that there were 13 species • He attempted to correlate variations in their traits with environmental challenges Galapagos Finches Galapagos Finches Finches of the Galapagos Islands Reproductive Capacity and Competition • All populations have the capacity to increase in numbers • No population can increase indefinitely • Eventually, individuals of a population end up competing for resources Variation in Populations • All individuals have the same genes that specify the same assortment of traits • Most genes occur in different forms (alleles), which produce different phenotypes • Some phenotypes compete better than others (fitness) Change over Time • Over time, alleles that produce the most successful phenotypes will increase in the population • Less successful alleles will become less common • Change leads to increased fitness – Increased adaptation to environment Natural Selection • Natural selection for various traits among individuals of a population affects which individuals survive and reproduce in each generation • Process results in adaptation to the environment (increases fitness) Alfred Wallace • Naturalist who arrived at the same conclusions Darwin did • Wrote to Darwin describing his views • Prompted Darwin to finally present his ideas in a formal paper Adaptation • Some heritable aspect of form, function, or behavior that improves the odds for surviving and reproducing • Environment specific • Outcome of natural selection Salt-Tolerant Tomatoes • An example of adaptation Adaptation to What? • Llamas live at high altitude and have hemoglobin with a high oxygen affinity • Is this an adaptation to altitude? Probably not • Llamas are related to camels, which live at low altitudes • Camels also have hemoglobin with high oxygen-binding capacity Common Ancestors • Llama and camel Adaptation to what? Populations Evolve • Biological evolution changes populations, not individuals • Traits in a population vary among individuals • Evolution: change in the frequency of traits • Main thing needed for evolution = diversity within the population The Gene Pool • All the genes in a population • Genetic resource that is shared (in theory) by all members of population Variation in Phenotype • Each gene in gene pool may have two or more alleles • Individuals inherit different allele combinations – leading to variation in phenotype • Offspring inherit genes, not phenotypes Variation in Populations What Determines Alleles in a New Individual? • Mutation • Crossing over at meiosis I • Independent assortment • Fertilization • Change in chromosome number or structure Genetic Equilibrium • Allele frequencies at a locus are not changing • Population is not evolving Five Conditions of Genetic Equilibrium • No mutation • Random mating • Gene doesn’t affect survival or reproduction • Large population • No immigration/emigration Microevolutionary Processes • Drive a population away from genetic equilibrium • Small-scale changes in allele frequencies brought about by – Natural selection – Gene flow – Genetic drift Gene Mutations • Infrequent but inevitable • Each gene has own mutation rate • Lethal mutations • Neutral mutations • Advantageous mutations Polymorphism Results of Natural Selection Three possible outcomes: • A shift in the range of values for a given trait in some direction • Stabilization of an existing range of values • Disruption of an existing range of values Number of individuals Directional Selection Range of values at time 2 Number of individuals Allele frequencies shift in consistent direction over time Number of individuals Range of values at time 1 Range of values at time 3 Directional selection Pinpointing the Target of Selection • Populations of rock pocket mice have fur that matches the rocks on which they live – Black basalt: dark fur – Tawny granite: light fur Pinpointing the Target of Selection • DNA comparisons show that the two populations differ in Mclr gene sequence Pesticide Resistance • Pesticides kill susceptible insects • Resistant insects survive and reproduce • If resistance has heritable basis, it becomes more common with each generation Rise of the Super Rats • Warfarin is a chemical pesticide used against rodents • When introduced the 1950s it was extremely effective • Where warfarin was used, rats became resistant Impacts, Issues Video Rise of the Super Rats Antibiotic Resistance • Antibiotics first came into use in the 1940s • Overuse has led to increase in resistant forms • Most susceptible cells died out, while resistant forms multiplied Number of individuals Range of values at time 1 Number of individuals Intermediate forms are favored and extremes are eliminated Range of values at time 2 Number of individuals Stabilizing Selection Range of values at time 3 Stabilizing selection Human Birth Weight 100 20 50 15 30 20 10 10 5 5 percent mortality percent of population 70 3 2 1 2 3 4 5 6 7 8 birth weight (pounds) 9 10 11 human newborns rate of death • Intermediate forms are selected against Number of individuals Range of values at time 1 Number of individuals • Happens when forms at both ends of the range of variation are favored Range of values at time 2 Number of individuals Disruptive Selection Range of values at time 3 Disruptive selection African Finches • Selection favors birds with very large or very small bill • Birds with intermediate-sized bill are less effective feeders African Finches lower bill 12 mm wide lower bill 15 mm wide Fig. 12-15, p.188 Selection for Bill Size Disruptive selection among African finches Sexual Selection • Selection favors certain secondary sexual characteristics • Through nonrandom mating, alleles for preferred traits increase • Leads to increased sexual dimorphism Sexual Selection in Birds Balanced Polymorphism • Polymorphism: “having many forms” • Occurs when two or more alleles are maintained at frequencies greater than 1 percent Sickle-Cell Trait: Heterozygote Advantage • Allele causes sickle-cell anemia when heterozygous HbS • Heterozygotes are more resistant to malaria than homozygotes Malaria case Sickle-cell trait less than 1 in 1,600 1 in 400–1,600 1 in 180–400 1 in 100–180 1 in 64–100 more than 1 in 64 Sickle Cell and Malaria Sickle-Cell Trait Distribution of sickle-cell trait Genetic Drift • Random change in allele frequencies brought about by chance • Effect is most pronounced in small populations • Sampling error: fewer times an event occurs, greater the variance in outcome Genetic Drift Simulation of genetic drift Bottleneck • A severe reduction in population size • Causes pronounced drift • Example – Elephant seal population hunted down to just 20 individuals – Population rebounded to 30,000 – Electrophoresis revealed there is now no allele variation at 24 genes Founder Effect • Effect of drift when a small number of individuals starts a new population • By chance, allele frequencies of founders may not be same as those in original population • Effect is pronounced on isolated islands or in voluntarily isolated human populations, as in religious isolates Founder Effect • Albatross carries seed to island phenotypes of mainland population phenotype of island population Fig. 12-19, p.191 Inbreeding • Nonrandom mating between related individuals • Leads to increased homozygosity • Can lower fitness when deleterious recessive alleles are expressed • Old Order Amish Gene Flow • Physical flow of alleles into a population • Tends to keep the gene pools of populations similar • Counters the differences that arise from mutation, natural selection, and genetic drift Gene Flow • Blue jay carries acorn between oak populations Table 12-1, p.192 Designer Dogs