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Whale Evolution: Fossil Record of Evolution Modern toothed whales Rodhocetus kasrani reduced hind limbs could not walk; swam with up-down motion like modern whales Pakicetus attocki lived on land; skull had whale characteristics Ambulocetus natans walked on land …like sea lions swam by flexing & paddling … like otters IV. Comparative Anatomy Organisms can be grouped (classified) by unique anatomical characteristics Similar anatomy is found in organisms with greatly divergent functions 1 Similar anatomy is found in organisms with greatly divergent functions Homologous structures ! structures that may have different appearances and functions, yet all derived from a common ancestor Evolution helps us understand patterns in the diversity of life Vestigial structures are Evolutionary relicts. They present a strong argument for common ancestry. Vestigial structures ! No apparent purpose ! Resemble similar functional structures in other, closely related species ! also human appendix 2 V. Embryology and Comparative Development Embryological stages pig cow rabbit human All vertebrate embryos look similar early in their development evidence of common ancestry ! similar developmental “instructions” in DNA V. Embryology and Comparative Development Comparative Development Reveals Descent from a Common Ancestor 3 VI. Genetic Analysis Modern technology reveals relatedness among diverse organisms Essentially all organisms have DNA as genetic material With very few exceptions, all organisms use the same genetic code Similarities in genes and proteins exist in predictable ways (based on morphological similarities) Molecular Record – Independent test ! Distantly related organisms are expected to accumulate a greater number of evolutionary differences than closely related species. 4 VII. Biogeographical Evidence Islands often have unique (endemic) species Species on islands are most similar to those on nearest continent (or nearest island) Species of Gallotia lizards on the Canary Islands I. Populations, Genes, & Evolution • Evolution is a property of populations - NOT individuals • Genes and environment interact to determine traits (P = G + E) • Genes: – functional segments of DNA – located on chromosomes • Alleles – alternative forms of genes • Genotype – combination of an individual’s alleles Chromosomes 5 I. Populations, Genes, & Evolution • Gene pool: All the possible alleles for a particular gene (or all the genes) within a given population EVOLUTION = Changes in: • allele frequencies (% of total) • in a population’s gene pool • over time EVOLUTION = Descent with Modification Charles Darwin, 1859: On the Origin of the Species Charles Darwin proposed Natural Selection as the mechanism of evolution 6 Darwin proposed natural selection as the mechanism of evolution: – Darwin observed that organisms • • Produce more offspring than the environment can support Vary in many characteristics that can be inherited – Darwin reasoned that natural selection • Results in favored traits being represented more and more and unfavored ones less and less in ensuing generations of organisms – Darwin found convincing evidence for his ideas in the results of artificial selection • The selective breeding of domesticated plants and animals II. Hardy-Weinberg Principle Under certain conditions: • allele and genotype frequencies in a population: – will remain constant over time – No evolution ! equilibrium population (This is a hypothetical population!) How Many Natural Populations are in Genetic Equilibrium? VERY FEW, if ANY! So why use Hardy-Weinberg? Provides a useful starting point for studying the mechanisms of Evolution by establishing a baseline to compare change 7 II. Hardy-Weinberg Principle Hardy-Weinberg Equilibrium Under several assumptions, evolution will not occur i.e., Allele frequencies will not change 1. no mutation 2. no gene flow (no immigration/emigration) 3. large population size (no genetic drift) 4. random mating 5. no natural selection If these assumptions are met, sexual reproduction alone will not change allele frequencies in the population III. What Causes Evolutionary Change? (1) Mutation – changes in DNA sequence – ultimate source of new alleles – mutations are fairly rare occurrences (2) Gene flow (migration between populations) – populations exchange genetic material – can change allele frequencies by altering the gene pool 8 (3) Genetic drift in small populations: Northern Two cases elephant seal: ~ 20 animals ~1900, now A. Population bottlenecks • populations reduced to small # • then recover • but lose genetic variation • reduced capacity to evolve ~130,000 animals (3) Genetic drift in small populations: Two cases A. Population bottlenecks • populations reduced to small # • then recover • but lose genetic variation • reduced capacity to evolve B. Founder effect: • population “founded” by only a few individuals • founders not likely to have all the alleles present in the source population • rare alleles can be a significant fraction of the allele pool • leads to lowered genetic diversity 9 Agents of Evolutionary Change, or the Mechanisms of Evolution. (All of these can change allelic frequencies!) (4) Nonrandom mating • assortative mating When phenotypically similar individuals mate. Increases the proportions of homozygotes. • • • dominant males females can be picky inbreeding is a type of nonrandom mating (5) Selection • Not all genotypes are equal A. Artificial Selection: the breeder selects for the desired characteristics B. Natural Selection: environmental conditions determine which individuals in a population produce more offspring ! Those that are more fit will leave more progeny IV. Natural Selection A. Important points (1) Natural selection does not cause genetic changes in individuals (2) Change in allele frequency occurs in populations (3) Fitness "! Reproductive Success = survival to reproductive maturity, passing on genes, > # of offspring (4) Evolutionary changes are not goal oriented mutations are random environments/selective forces constantly change>> (5) Of the 5 agents of Evolutionary Change, ONLY selection produces adaptive evolutionary change Example: Penicillin Resistant Staphylococcus bacteria • Antibiotic 1st used in WWII • As enters body - kills majority of bacteria • A few survive - have rare allele, not susceptible • They reproduce • A few bacterial generations later… • Frequency of rare “resistant” allele !100% 10 C. Components of Fitness (1) Competition: • scarce resources favor the best-adapted individuals (2) Predator/Prey Interactions • one organism (the predator) eats another (the prey) – coevolution of predator and prey (3) Symbiosis • leads to the most intricate coevolutionary adaptations (4) Sexual selection (access to sexual partners) ! Favors traits that help an organism attract a mate ! Usually, all females get to breed, but only some males ! Result is sexual dimorphism 2 mechanisms: Male-male combat (large structures for fighting; large male size) Female choice (males have attractive structures to lure females) ***Under both scenarios – females get “good genes” for offspring 11 C. Components of Fitness (cont.) (5) Kin Selection • Actions of an individual increase survival and reproductive success of relatives • Who have some of the same alleles! Prairie Dogs Spadefoot toad tadpoles V. Forms of Selection (Natural or Artificial) (1) Directional selection • shifts traits in specific direction • favors individuals at one end of trait distribution range • (ex: Giraffe neck length) (2) Stabilizing selection • favors average individuals • selects against individuals with extreme values • due to opposing pressures (ex: human birth weights) (3) Disruptive selection • favors individuals at both ends of the distribution of a trait • selects against average individuals • adapts individuals within a population to different habitats: • (ex: African seed cracker finches) 12 Favors one extreme Favors average Favors both extremes V. Forms of Selection: What type of Selection? 100 70 50 30 20 15 10 10 5 7 5 3 2 Percent infant mortality Percent of births in population 20 2 3 4 5 6 7 8 9 10 Birth weight in pounds 13 Adaptation and Natural Selection: Peppered Moths and Industrialized Melanism ! Until the mid-nineteenth century, peppered moths, Biston betularia, had predominately light-colored wings. ! Subsequently, dark individuals became predominant ! Industrial smog helped turn lichens on tree trunks dark. ! Contrasting colors between trunk color and moth color led to differential predation by birds. Natural Selection and Adaptation Peppered Moths 14 Peppered Moths and Industrialized Melanism ! The second half of the twentieth century saw widespread implementation of pollution controls, thus trends have reversed and light colored moths again are the dominate form. VI. Adaptation (cont.) Camouflage – a result of natural selection Flower mantid Leaf mantid in Costa Rica 15 VII. Constraints on Evolutionary Perfection ! Historical constraints • Evolution tinkers with existing structures ! Adaptations are compromises • Seal flippers on land and water ! Chance and selection interact • No prediction of future conditions ! Selection can edit only existing variation • Mutation is random 16