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Cecie Starr Christine Evers Lisa Starr www.cengage.com/biology/starr Chapter 17 Processes of Evolution (Sections 17.6 - 17.10) Albia Dugger • Miami Dade College 17.6 Stabilizing and Disruptive Selection • Stabilizing selection is a form of natural selection that maintains an intermediate phenotype • Disruptive selection favors forms of a trait at both ends of a range of variation Stabilizing Selection • Stabilizing selection is also called balancing selection because it tends to preserve the midrange phenotypes in a population • stabilizing selection • Mode of natural selection in which intermediate forms of a trait are favored over extremes Stabilizing Selection • Extreme forms of a trait are eliminated, and intermediates are favored • Red arrows indicate forms selected against; green, forms that are being favored Stabilizing Selection Fig. 17.8a, p. 264 Number of individuals in population Stabilizing Selection Time 1 Range of values for the trait Fig. 17.8a, p. 264 Stabilizing Selection Fig. 17.8b, p. 264 Stabilizing Selection Time 2 Fig. 17.8b, p. 264 Stabilizing Selection Fig. 17.8c, p. 264 Stabilizing Selection Time 3 Fig. 17.8c, p. 264 Time 1 Number of individuals in population Stabilizing Selection Range of values for the trait Time 2 Time 3 Stepped Art Fig. 17.8, p. 264 Animation: Stabilizing Selection Sociable Weavers • The body weight of sociable weavers (Philetairus socius) is subject to stabilizing selection • Body weight is a trade-off between risks of starvation and predation: Leaner birds do not store enough fat to avoid starvation, and predators select against birds of high body weight • Birds of intermediate weight have the selective advantage Stabilizing Selection: Sociable Weavers Disruptive Selection • Conditions that favor forms of a trait at both ends of a range of variation drive disruptive selection • disruptive selection • Mode of natural selection that favors forms of a trait at the extremes of a range of variation • Intermediate forms are selected against Disruptive Selection • Midrange forms are eliminated; extreme forms are maintained • Red arrows indicate forms selected against; green, forms that are being favored Disruptive Selection Fig. 17.10a, p. 265 Number of individuals in population Disruptive Selection Time 1 Range of values for the trait Fig. 17.10a, p. 265 Disruptive Selection Fig. 17.10b, p. 265 Disruptive Selection Time 2 Fig. 17.10b, p. 265 Disruptive Selection Fig. 17.10c, p. 265 Disruptive Selection Time 3 Fig. 17.10c, p. 265 Time 1 Number of individuals in population Disruptive Selection Range of values for the trait Time 2 Time 3 Stepped Art Fig. 17.10, p. 265 ANIMATION: Disruptive selection 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 African Seedcrackers • In black-bellied seedcrackers, dimorphism in bill size results from competition for two types of food in the dry season • Small-billed birds are better at opening soft seeds, but largebilled birds are better at cracking hard seeds • These conditions favor birds with bills that are either 12 millimeters wide or 15 to 20 millimeters wide. • Birds with bills of intermediate size are selected against African Seedcrackers ANIMATION: Disruptive selection among African finches 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 17.7 Fostering Diversity • Selection pressures that operate on natural populations are complex; an allele may be adaptive in one circumstance but harmful in another • Any mode of natural selection may maintain two or more alleles in a population Nonrandom Mating • With sexual selection, the most adaptive forms of a trait are those that help individuals defeat same-sex rivals for mates, or are the ones most attractive to the opposite sex • sexual selection • Mode of natural selection in which some individuals of a population out-reproduce others because they are better at securing mates Examples of Sexual Selection • Male elephant seals fight for sexual access to a cluster of females Examples of Sexual Selection • A male bird of paradise engages in flashy courtship display to catch the sexual interest of a female • Females are choosy; a male mates with any female that accepts him Examples of Sexual Selection • Stalk-eyed flies cluster on aerial roots to mate • Females prefer males with longer eyestalks • A male with very long eyestalks (top) has captured the interest of three females below Balanced Polymorphism • In an environment that favors heterozygotes (individuals with nonidentical alleles), any mode of natural selection may result in a balanced polymorphism • balanced polymorphism • Maintenance of two or more alleles for a trait at high frequency in a population as a result of natural selection against homozygotes Malaria and Sickle-Cell Anemia • A mutation in the normal beta globin chain of hemoglobin (HbA) causes sickle-cell anemia; individuals homozygous for the mutated HbS allele often die young • The HbS allele persists at high frequencies in tropical regions of Africa because HbA/HbS heterozygotes are more likely to survive than HbA/HbA homozygotes Distributions of Malaria and Sickle-Cell Anemia Malaria • Malaria is caused by Plasmodium infection carried by mosquitoes • A physician searches for mosquito larvae in Southeast Asia 17.8 Genetic Drift • Random change in allele frequencies, or genetic drift, can lead to loss of genetic diversity by causing alleles to become fixed, especially in small populations • genetic drift • Change in allele frequencies in a population due to chance alone • fixed • Refers to an allele for which all members of a population are homozygous Genetic Drift • The larger the population, the smaller the impact of random changes in allele frequencies • Example: Allele X occurs at a 10% frequency • In a population of 10, only one person carries the allele, and if that person dies, the allele is lost • In a population of 100, all 10 people who carry the allele would have to die for the allele to be lost Genetic Drift in Flour Beetles Genetic Drift in Flour Beetles Fig. 17.14a, p. 268 Genetic Drift in Flour Beetles Fig. 17.14b, p. 268 ANIMATION: 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 Bottlenecks • Genetic drift can be dramatic when a few individuals rebuild a population or start a new one • Example: Hunting reduced an elephant seal population to 20; the population is now homozygous at every gene; genetic drift after the bottleneck fixed all alleles in the population • bottleneck • Drastic reduction in population size so severe that it reduces genetic diversity Founder Effect • Bottlenecks also occur when a small group of individuals founds a new population – the new population’s genetic diversity may be quite reduced • founder effect • Change in allele frequencies that occurs when a small number of individuals establish a new population Inbreeding • Genetic drift is pronounced in inbreeding populations • Inbreeding lowers a population’s genetic diversity, so more individuals in the population are homozygous for recessive alleles with harmful effects • inbreeding • Nonrandom mating among close relatives Founder Effect and Inbreeding • Ellis-van Creveld syndrome: characterized by dwarfism, polydactyly, and heart defects • Caused by a recessive allele common in the Old Order Amish of Lancaster County, PA 17.9 Gene Flow • Individuals, along with their alleles, move into and out of populations • This gene flow stabilizes allele frequencies, so it counters the effects of mutation, natural selection, and genetic drift that tend to occur within a population • gene flow • The movement of alleles into and out of a population Gene Flow • Blue jays move acorns, and their alleles, among populations of oak trees that would otherwise be genetically isolated Key Concepts • Processes of Microevolution • Natural selection may maintain or shift the range of variation of a shared heritable trait in a population • Gene flow counters the evolutionary effects of mutation, natural selection, and genetic drift 17.10 Reproductive Isolation • Speciation differs in its details, but reproductive isolating mechanisms are always part of the process • speciation • One of several processes by which new species arise • reproductive isolation • Absence of gene flow between populations • Always part of speciation Reproductive Isolation Prevents Interbreeding • When gene flow does not occur between populations, different genetic changes accumulate in each • Reproductive isolation reinforces differences between diverging populations: • If pollination or mating cannot occur, or if zygotes cannot form, the isolation is prezygotic • If hybrids form but are unfit or infertile, the isolation is postzygotic Reproductive Isolation Prevents Interbreeding Reproductive Isolation Prevents Interbreeding 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). Fig. 17.17, p. 270 Reproductive Isolation Prevents Interbreeding 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 Fig. 17.17, p. 270 ANIMATION: 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 7 Mechanisms of Reproductive Isolation • Temporal isolation • Some populations can’t interbreed because the timing of their reproduction differs • Mechanical isolation • In some cases, the size or shape of an individual’s reproductive parts prevent it from mating with members of another population Mechanical Isolation in Sage Mechanical Isolation in Sage Fig. 17.18a, p. 271 Mechanical Isolation in Sage A Black sage is pollinated mainly by honeybees and other small insects. Fig. 17.18a, p. 271 Mechanical Isolation in Sage Fig. 17.18b, p. 271 Mechanical Isolation in Sage B The flowers of black sage are too delicate to support larger insects. Big insects access the nectar of small sage flowers only by piercing from the outside, as this carpenter bee is doing. When they do so, they avoid touching the flower’s reproductive parts. Fig. 17.18b, p. 271 Mechanical Isolation in Sage Fig. 17.18c, p. 271 Mechanical Isolation in Sage anthers stigma C The reproductive parts (anthers and stigma) of white sage flowers are too far away from the petals to be brushed by honeybees, so honeybees cannot pollinate this species. White sage is pollinated mainly by larger bees and hawkmoths, which brush the flower’s stigma and anthers as they pry apart the petals to access nectar. Fig. 17.18c, p. 271 7 Mechanisms of Reproductive Isolation (cont.) • Ecological isolation • Populations adapted to different microenvironments in the same region may be physically separated • Behavioral isolation • In animals, behavioral differences can stop gene flow between related species • Example: Males and females of some bird species engage in courtship displays before sex Behavioral Isolation in Albatrosses 7 Mechanisms of Reproductive Isolation (cont.) • Gamete incompatibility • Even if gametes of different species meet, they often have molecular incompatibilities that prevent them from fusing • Primary speciation route of animals that release freeswimming sperm in water 7 Mechanisms of Reproductive Isolation (cont.) • Hybrid inviability • If genetic incompatibilities disrupt development, a hybrid embryo may die, or hybrid offspring that survive may have reduced fitness (e.g. ligers) • Hybrid sterility • Some interspecies crosses produce robust but sterile offspring (e.g. mules) 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