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Chapter 17 Processes of Evolution Sections 7-12 Albia Dugger • Miami Dade College 17.7 Fostering Diversity • Individuals may be selective agents for their own species • Any mode of natural selection may maintain two or more alleles in a population • An allele may be adaptive in one circumstance but harmful in another Nonrandom Mating • Competition for mates is a selective pressure • With sexual selection, some version of a trait gives an individual an advantage over others in attracting mates • Distinct male and female phenotypes (sexual dimorphism) is one outcome of sexual selection Sexual Selection: Elephant Seals Sexual Selection: Birds of Paradise Sexual Selection: Stalk-Eyed Flies Balanced Polymorphism • Balanced polymorphism • A state in which natural selection maintains two or more alleles at relatively high frequencies • Occurs when environmental conditions favor heterozygotes • Example: Sickle cell anemia and malaria • Mosquitoes transmit the parasitic protist that causes malaria, Plasmodium, to human hosts • HbA/HbS heterozygotes survive malaria more often than people who make only normal hemoglobin Searching for Mosquito Larvae in Southeast Asia Sickle Cell Anemia and Malaria Take-Home Message: How does natural selection maintain diversity? • With sexual selection, a trait is adaptive if it gives an individual an advantage in securing mates • Sexual selection reinforces phenotypical differences between males and females, and sometimes gives rise to exaggerated traits • Environmental pressures that favor heterozygotes can lead to a balanced polymorphism 18.7 Genetic Drift and Gene Flow • Especially in small populations, random changes in allele frequencies can lead to a loss of genetic diversity • Individuals, along with their alleles, move into and out of populations • This flow of alleles counters genetic change that tends to occur within a population Genetic Drift • Genetic drift • A random change in allele frequencies over time • Can lead to a loss of genetic diversity, especially in small populations • When all individuals of a population are homozygous for an allele, that allele is fixed Genetic Drift in a Small Population Genetic Drift in a Larger Population Bottlenecks • Bottleneck • A drastic reduction in population size brought about by severe pressure • After a bottleneck, genetic drift is pronounced when a few individuals rebuild a population • Example: Northern elephant seals The Founder Effect • Founder effect • Genetic drift is pronounced when a few individuals start a new population • Inbreeding • Breeding or mating between close relatives who share a large number of alleles • Example: Old Order Amish in Lancaster County, Pennsylvania (Ellis-van Creveld syndrome) Ellis-van Creveld Syndrome Gene Flow • Gene flow • Physical movement of alleles caused by individuals moving into and away from populations • Tends to counter the evolutionary effects of mutation, natural selection, and genetic drift on a population • Example: Movement of acorns by blue jays allows gene flow between oak populations Gene Flow Between Oak Populations Take-Home Message: How does a population’s genetic diversity become reduced? • Genetic drift, or random change in allele frequencies, can reduce a population’s genetic diversity; its effect is greatest in small populations, such as one that endures a bottleneck • Gene flow is the physical movement of alleles into and out of a population; it tends to counter the evolutionary effects of mutation, natural selection, and genetic drift ANIMATED FIGURE: Simulation of genetic drift To play movie you must be in Slide Show Mode PC Users: Please wait for content to load, then click to play Mac Users: CLICK HERE 17.9 Reproductive Isolation • Speciation differs in its details, but reproductive isolating mechanisms are always part of the process • Speciation • Evolutionary process by which new species form • Reproductive isolating mechanisms are always part of the process • Reproductive isolation • The end of gene exchange between populations • Beginning of speciation Reproductive Isolating Mechanisms • Reproductive isolating mechanisms prevent interbreeding among species • Heritable aspects of body form, function, or behavior that arise as populations diverge • Prezygotic isolating mechanisms prevent pollination or mating • Postzygotic isolating mechanisms result in weak or infertile hybrids Prezygotic Isolating Mechanisms • With temporal isolation populations can’t interbreed because the timing of their reproduction differs • With mechanical isolation, the size or shape of an individual’s reproductive parts prevent it from mating with members of another population Figure 17-17a p285 Figure 17-17b p285 anthers stigma Figure 17-17c p285 Prezygotic Isolating Mechanisms (cont.) • Populations adapted to different microenvironments in the same region may be ecologically isolated • In animals, behavioral differences can stop gene flow between related species (behavioral isolation) • In gamete incompatibility, gametes of different species meet but have molecular incompatibilities that prevent a zygote from forming Behavioral Isolation in Jumping Spiders Postzygotic Isolation Mechanisms • Hybrid inviability • Extra or missing genes, or incompatible gene products • Offspring may be inviable, or have reduced fitness (ligers, tigons) • Hybrid sterility • Some interspecies crosses produce robust but sterile offspring (e.g. mules) • Fertile offspring may have lower fitness with successive generations Different species form and . . . Prezygotic reproductive isolation Individuals reproduce at different times (temporal isolation). Physical incompatibilities prevent individuals from interbreeding (mechanical isolation). Individuals live in different places so they never meet up for sex (ecological isolation). Individuals ignore or do not get the required cues for sex (behavioral isolation). Mating occurs and . . . No fertilization occurs (gamete incompatibility). Zygotes form and . . . Interbreeding is successful Postzygotic reproductive isolation Hybrid embryos die early, or new individuals die before they can reproduce (hybrid inviability). Hybrid individuals or their offspring do not make functional gametes (hybrid sterility). Stepped Art Figure 17-16 p284 ANIMATED FIGURE: Reproductive isolating mechanisms To play movie you must be in Slide Show Mode PC Users: Please wait for content to load, then click to play Mac Users: CLICK HERE Take-Home Message: How do species attain and maintain separate identities? • Speciation is an evolutionary process by which new species form; it varies in its details and duration • Reproductive isolation, which occurs by one of several mechanisms, is always a part of speciation ANIMATION: Temporal isolation among cicadas To play movie you must be in Slide Show Mode PC Users: Please wait for content to load, then click to play Mac Users: CLICK HERE 17.10 Allopatric Speciation • In allopatric speciation a physical barrier arises and ends gene flow between populations • Genetic divergence results in speciation • Example: Geographic isolation of Atlantic and Pacific species caused by the formation of the Isthmus of Panama Allopatric Speciation in Snapping Shrimp Allopatric Speciation in Archipelagos • Winds or ocean currents carry a few individuals of mainland species to remote, isolated islands chains (archipelagos) such as Hawaii • Habitats and selection pressures that differ within and between the islands foster divergences that result in allopatric speciation • Example: Hawaiian honeycreepers and thousands of other species of finches are unique to the Hawaiian archipelago Allopatric Speciation on an Archipelago Take-Home Message: What happens after a physical barrier arises between populations? • A physical barrier that intervenes between populations or subpopulations of a species prevents gene flow among them • As gene flow ends, genetic divergences give rise to new species • This process is allopatric speciation ANIMATED FIGURE: Allopatric speciation on an archipelago To play movie you must be in Slide Show Mode PC Users: Please wait for content to load, then click to play Mac Users: CLICK HERE 17.11 Other Speciation Models • In sympatric speciation and parapatric speciation, populations speciate even without a physical barrier that blocks gene flow Sympatric Speciation • In sympatric speciation, new species form within a home range of an existing species, in the absence of a physical barrier • Sympatric speciation can occur in a single generation when the chromosome number multiplies (polyploidy) • Example: Common bread wheat originated after related species hybridized, then the chromosome number of the hybrid offspring doubled Sympatric Speciation in Wheat Aegilops Triticum (wild goatgrass, unknown (hybrid) species) Triticum urartu (wild einkorn) 14 AA X 14 BB 14 AB Triticum turgidum (emmer) 28 AABB Aegilops tauschii (goatgrass) X 14 DD Triticum aestivum (bread wheat) 42 AABBDD Sympatric Speciation in Cichlids • Sympatric speciation can also occur with no change in chromosome number • Example: More than 500 species of cichlid speciated in the shallow waters of Lake Victoria – they vary in color and in patterning depending on differences in light color and water clarity in different parts of the lake (reproductive isolation) Sympatric Speciation in Cichlids Sympatric Speciation in Warblers • A chain of populations of greenish warblers encircles the Tibetan plateau central Asia (a ring species) • Gene flow occurs continuously all around the chain, but the two populations at the ends of the chain are different species • Individuals of these two populations overlap in range, but do not interbreed because they do not recognize one another’s songs (behavioral isolation) Sympatric Speciation in Warblers Parapatric Speciation • In parapatric speciation, populations in contact along a common border evolve into distinct species • Hybrids in the contact zone are less fit than individuals on either side • Example: Two species of velvet walking worm in Tasmania can interbreed, but they only do so in a tiny area where their habitats overlap – hybrid offspring are sterile A Giant velvet walking worm, Tasmanipatus barretti Figure 17-23a p289 b Blind velvet walking worm, T. anophthalmus Figure 17-23b p289 T. barretti hybrid zone T. anophthalmus C The habitats of the worms overlap in a hybrid zone on the island of Tasmania. Figure 17-23c p289 Take-Home Message: Can speciation occur without a physical barrier? • By a sympatric speciation model, new species arise from a population even in the absence of a physical barrier • By a parapatric speciation model, populations maintaining contact along a common border evolve into distinct species Table 17-2 p292 ANIMATED FIGURE: Sympatric speciation in wheat To play movie you must be in Slide Show Mode PC Users: Please wait for content to load, then click to play Mac Users: CLICK HERE ANIMATION: Models of speciation To play movie you must be in Slide Show Mode PC Users: Please wait for content to load, then click to play Mac Users: CLICK HERE 17.12 Macroevolution • Macroevolution includes large-scale patterns of change such as one species giving rise to multiple species, the origin of major groups, and major extinction events • Examples: • Flowering plants evolved from seed plants, • Animals with four legs (tetrapods) evolved from fish • Birds evolved from dinosaurs Exaptation and Stasis • Exaptation (preadaptation) • Some complex traits in modern species held different adaptive value in ancestral lineages (e.g. feathers in birds and dinosaurs) • Stasis • A lineage exists for millions of years with little or no change (e.g. coelacanth) Fossil Coelacanth Living Coelacanth Mass Extinctions • Extinction • The irrevocable loss of a species from Earth • Mass extinctions • Extinctions of many lineages, followed by adaptive radiations • Five catastrophic events in which the majority of species on Earth disappeared Adaptive Radiation • Adaptive radiation • A burst of speciation that occurs when a lineage encounters a new set of niches • Key innovation • A structural or functional adaptation that allows individuals to exploit their habitat in a new way Coevolution • Two species in close ecological contact act as agents of selection on each other (coevolution) • Predator and prey • Host and parasite • Pollinator and flower • Over time, the two species may come to depend on each other Coevolved Species: Myrmica sabuleti and Maculinea arion. • After hatching, the larvae (caterpillars) of the large blue butterfly (Maculinea arion), feed on wild thyme flowers and then drop to the ground • An ant (Myrmica sabuleti) strokes the caterpillar, eats the honey the caterpillar exudes, and takes the caterpillar back to the ant nest • The caterpillar lives in the nest and feeds on ant larvae until it metamorphoses into a butterfly Coevolved Species: Myrmica sabuleti and Maculinea arion. Maculinea arion Myrmica sabuleti Evolutionary Theory • Evolutionary biologists try to explain how all species are related by descent from common ancestors • Genetic change is the basis of evolution, but many biologists disagree about how it occurs Take-Home Message: What is macroevolution? • Macroevolution comprises large-scale patterns of evolutionary change such as adaptive radiation, the origin of major groups, and loss through extinction Video: Pesticides: Will More Restrictions Help or Hinder?