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What Homologies tell us… • Similarities in structure and chemistry provide powerful evidence that all living things evolved from a common ancestor • Darwin Concluded: – Living organisms evolved through gradual modifications of earlier forms descent with modification What Similarities tell us… • Two types of evolution can account for homologous AND analogous structures – Convergent evolution – Divergent evolution What Similarities tell us… • Divergent evolution – two species evolve from a common ancestor (speciation) – They share similarities in anatomy, biochemistry, and embryology due to common ancestry – Explains homologous structures What Similarities tell us… • Convergent – two species apparently becoming more similar – Two species have adapted in similar ways to similar environmental conditions – NOT due to common ancestry – Explains analogous structures Convergent Evolution • Ocotillo from California and allauidi from Madagascar have evolved similar mechanisms for protecting themselves Convergent Evolution • Adaptive radiation of anoles has occurred on the islands of the Greater Antilles in a convergent fashion. On each island, different species of the lizards have adapted to living in different parts of trees, in strikingly similar ways. Convergent Evolution Convergent Evolution Diversity of Life • Fitness: – Physical traits and behaviors that enable organisms to survive and reproduce in their environment arises from adaptation. • Adaptation allows species to be better suited to their environment and therefore can survive and reproduce. Evolution on Different Scales • Microevolution – generation-togeneration change in a population’s allele frequencies • Macroevolution – origin of new taxonomic groups; speciation 4 Driving Forces behind Evol. 1. Mutation – Any change in the original DNA – ONLY ultimate source of variation in a population 2. Gene Flow – Movement of genes either into or out of a population – Migration – Immigration (add alleles) and Emigration (subtract alleles) 4 Driving Forces behind Evol. 3. Genetic Drift – Change in the allele frequency in a small population by chance alone • • Bottleneck Effect Founder Effect 4 Driving Forces behind Evol. 3. Genetic Drift – Bottleneck Effect: population undergoes a high mortality rate; genetic variation decreases dramatically – Ex: Cheetahs Genetic Drift: Bottleneck Effect 4 Driving Forces behind Evol. 3. Genetic Drift – Founder Effect: few individuals leave a large population to start their own; gene pool is very limited – Ex: polydactyly in PA Amish Genetic Drift: Founder Effect Genetic Drift: Founder Effect 4 Driving Forces behind Evol. 4. Selection – Natural – differential success in the reproduction of different phenotypes resulting from the interaction of organisms with their environment • Nature does the selecting 4 Driving Forces behind Evol. 4. Selection (Natural) – Resistance – overuse of insecticides and antibiotics have bred resistant species of bugs and germs 4 Driving Forces behind Evol. 4. Selection – Artificial – breeding of domesticated plants and animals • Humans intentionally do the selecting • Cabbage, cauliflower, Brussels sprouts, kale, kohlrabi and broccoli have a common ancestor in one species of wild mustard 4 Driving Forces behind Evol. • Problems with artificial selection – not enough genetic variation 4 Driving Forces behind Evol. 4. Selection (Sexual) – Intrasexual selection – selection within the same sex (competition, usually between males – – Competition, usually between males Exaggerated anatomy Bighorn Sheep Rocky Mountain Elk Five-horned Rhinoceros Beetles Stagbeetles 4 Driving Forces behind Evol. 4. Selection (Sexual) – Intersexual selection – one sex selects mate based on phenotypes – Exaggerated anatomy • Selection can influence populations in three major ways: – Directional Sel. – Stabilizing Sel. – Disruptive (diversifying) Sel. Directional Selection • Environment selects against one phenotypic extreme, allowing the other to become more prevalent Disruptive Selection • Environment selects against intermediate phenotype, allowing both extremes to become more prevalent Stabilizing Selection • Environment selects against two extreme phenotypes, allowing the intermediates to become more prevalent 1. Key Points Natural selection does not cause genetic changes in individuals. 2. Natural selection acts on individuals; evolution occurs in populations. 3. Evolution is a change in the allele frequencies of a population, owing to unequal success at reproduction among organisms bearing different alleles. 4. Evolutionary changes are not “good” nor “progressive” in any absolute Evolutionary Theory • Foundation on which the rest of the biological science is built. Collection of carefully reasoned and tested hypotheses about how evolutionary change occurs. Speciation • What is a species? –Biological definition: a group of closely related organisms (population) that can interbreed to produce fertile, viable offspring Speciation • Why can’t/don’t populations interbreed? –Prezygotic barriers –Postzygotic barriers Prezygotic Barriers • Ecological (habitat) isolation – pops live in different habitats and do not meet – Parasites generally don’t transfer hosts • Temporal isolation – active or fertile at different times – Flowering plants pollinate on different days or different times of the day Prezygotic Barriers • Behavioral isolation – differences in activities – Mating calls or actions are different Prezygotic Barriers • Mechanical isolation – mating organs do not fit or match – Enough said • Gametic isolation – gametes cannot combine – Sperm destroyed in “different” vaginal cavity – Sperm and egg don’t fuse due to different membrane proteins Postzygotic Barriers • Hybrid inviability – hybrid zygotes fail to develop or reach sexual maturity • Hybrid infertility – hybrids fail to produce functional gametes Summary • 2 or more mechanisms may occur at once • Ex: Bufo americanus and Bufo fowleri are ecologically, temporally, and behaviorally isolated • Bufo americanus breeds in early spring in small, shallow puddles or nearby dry creeks • Bufo fowleri breeds in late spring in large pools and streams Limitations of Biological Species Concept • How do you classify organisms that: – have the potential to interbreed, but do not do so in nature? – do not reproduce sexually? – exist only as fossils? • Alternative species concepts (ecological, pluralistic, morphological, genealogical) help address limitations Modes of Speciation • Allopatric (Greek, allos = other; Latin, patria = homeland) • Speciation due to geographic separation – Barrier stops gene flow between populations – Evolutionary change acts independently on each pop to establish reproductive barriers • Mitochondrial DNA analysis has shown that certain tamarin monkey pops (those separated by wide rivers) are diverging toward speciation Where the Amazon is very wide, tamarins on one side are brown, but on the other side are white. Where the Amazon is narrow, tamarins of both colors are found on either side Allopatric Speciation • Birds can move freely across the gorge of the Grand Canyon; squirrels cannot A. leucurus A. harrisi • Two species arose when their original pop was disrupted by the A. harrisi A. leucurus Allopatric Speciation • If not given enough time, speciation will not occur • Also, even if they do come back together, they need to interbreed to be the same species Allopatric Speciation • Figure 24.11 • Adaptive Radiation: evolution of many diversely-adapted species from a common ancestor • Ex: Hawaiian archipelago Sympatric Speciation • Sympatric (Greek, sym = together; Latin, patria = homeland) • Speciation occurs in populations that share a habitat • Results from: – Ecological isolation – Polyploidy (number of sets of chromosomes increases) Sympatric Speciation • Polyploidy (number of sets of chromosomes increases) • A result of accidents in meiosis Will Speciation Occur? • p+q=1 • p2 + 2pq + q2 = 1 • Will speciation occur? You tell me! • Hardy-Weinberg PPT 1 • Hardy-Weinberg PPT 2 Evolutionary Time Scales • Evolution can take a long time or can occur relatively quickly – Gradualism – Punctuated Equilibrium Evolutionary Time Scales • Gradualism – big evolutionary changes are the result of many small ones over a long period of time Evolutionary Time Scales • Punctuated Equilibrium – speciation occurs fairly rapidly then remain constant Evolutionary Novelties • Unique and highly specialized organs seem to complicated to have been naturally selected • Ex: eyes are really just photoreceptors; some are more developed, but all do the basic function: receive light Evolutionary Novelties Evo-devo • Evolutionary development • A field of interdisciplinary research that examines how slight genetic divergences can become magnified into major morphological differences between species Evo-devo • By blocking expression of one gene, researchers forced a chicken’s foot to develop to resemble a duck’s foot • Two embryos from the same animal Evo-devo • • • • Left, a normal chicken leg will develop Right, a normal duck leg will develop… from a chicken embryo Chicken leg: scaled with 4 digits Duck leg: smooth and webbed Duck legs, due to one genetic evolutionary difference, help ducks do many things chickens cannot,