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Transcript
Maintaining Variation Sexual selection Balanced polymorphism Sexual Selection Most species have distinctively male and female phenotypes: ____________________ One of the sexes is often larger, more colorful, more aggressive Sexual selection is based on any trait that gives the individual a competitive edge in: _______________ ___________________________________________ Females are usually the agents of selection when they pick their mates Courtship varies partially due to the varying parental involvement of males and females Sexual Selection Figure 17.12, page 274 Balanced Polymorphism With _______________________________ , two or more alleles of a gene persist at high frequencies in a population. Balancing selection occurs when conditions favor heterozygotes Hb gene: codes for hemoglobin (oxygentransporting protein in the blood) HbA : _______________ HbS : ________________ Causes: ___________________________ Balanced Polymorphism Mosquitoes transmit Plasmodium, parasite which causes malaria Humans that are homozygous for sicklecell anemia develop the disease and die at an early age However, individuals with alleles for both normal (HbA) and sickle-cell (HbS) have the greatest chances of surviving malaria Balanced Polymorphism Distribution of malaria cases in 1920’s b) Distribution of people who carry the sickle-cell allele Figure 17.13, page 275 a) Genetic Drift: Chance Changes Genetic drift: the random fluctuation in allele frequencies over time, due to chance occurrences alone More significant in small populations ___________________ helps explain the difference Increases the chance of any given allele becoming more or less prevalent when the number of individuals in a population is small _______________ means that one kind of allele remains at a specified locus in a population Occurs when all are homozygous for one allele Genetic Drift Effect of population size on genetic drift Figure 17.14, page 276 Bottlenecks and the Founder Effect Genetic drift is most pronounced in small or inbred populations _________________ : some stressful situation greatly reduces the size of a population, leaving a few (typical or atypical) individuals to reestablish the population _________________________: a few individuals (carrying genes that may or may not be typical of the whole population) leave the original population to establish a new one Genes of those individuals are the only ones that get passed on Founder Effect Figure 17.15, page 277 Inbred Populations Inbreeding: nonrandom mating among closely related individuals Tends to increase the homozygous condition, thus leading to lower fitness and survival rates Human populations that remain isolated and inbred may concentrate disease-causing alleles Old Order Amish in Lancaster County, PA Gene Flow Genes move with individuals when they move out or into a population ______________: individuals leave ______________: individuals move in Gene flow: movement of alleles among populations Helps keep populations of same species _________ Physical flow and resultant shuffling tends to minimize genetic variation between populations Species and Speciation How can we define a species? Morphological species concept: Species: Latin word for ________ – its simplest interpretation Attempting to determine whether similar yet different animals are the same species by appearance (phenotype) is not reliable due to the subtle variations that are displayed Biological Species Concept The biological species concept uses ______________ to define relatedness of species Developed by biologist Ernst Myer “Species are groups of interbreeding natural populations that are reproductively isolated from other such groups” As good as it is, this definition does not work so well for organisms that are non-sexually reproducing and those only known from fossils Only applies locally Speciation New species arise by the evolutionary process of ______________, which begins as gene flow ends between populations Difficult to classify a new species – how can you define the exact moment which made it significantly different than predecessors? Reproductive Isolation Individuals of a sexually reproducing species can produce fertile offspring, but are _________________ ________________ Reproductive isolating ________________ begin as two populations diverge from one another: Reproductive isolation occurs when gene flow between populations stops Heritable features of body form, function, or behavior that prevent interbreeding between genetically divergent populations Divergences may lead to new species Reproductive Isolation Prezygotic: prevent successful pollination or mating (______________ the zygote) Temporal Mechanical Behavioral Ecological Gamete incompatibility Postzygotic: result in offspring which are weak or infertile hybrids (___________the zygote) Prezygotic Mechanisms Prezygotic isolating mechanisms take effect before or during fertilization ______________ isolation: different groups may not be reproductively mature at the same season, month, or year (for example, periodical cicadas) _______________ isolation: Two populations are mechanically isolated when differences in reproductive organs prevent successful interbreeding Example: floral arrangements in sage plants discriminate between different bee pollination Prezygotic Mechanisms _____________ isolation: patterns of courtship may be altered to the extent that sexual union is not achieved ______________ isolation: potential mates may be in the same general area, but not in the same habitat where they are likely to meet Ex: albatross courtship rituals Ex: different species of manzanita shrubs live at different altitudes and habitats __________________: incompatibilities between egg and sperm prevent fertilization Ex: signals to pollen grains to begin growing toward egg Postzygotic Mechanisms ______________ or ______________ Sometimes fertilization does occur between different species, but the hybrid embryo is weak and dies In some instances, the hybrids are vigorous but sterile Example: a mule, produced by a male donkey and a female horse Reproductive Isolating Mechanisms Figure 17.17, pg. 278 Allopatric Speciation Genetic changes leading to a new species usually begin with physical separation _____________ speciation: refers to the “different lands” occupied by two species In this model, a _________________separates two populations, ending the gene flow between them Most common way new species form Reproductive isolating mechanisms evolve in the genetically diverging populations and will result in complete speciation when the two species can no longer interbreed Allopatric Speciation The pace of geographic isolation can vary depending upon the organism’s means of travel Isolation may be sudden, such as an earthquake that results in the separation of species Glaciers may produce conditions that yield separate species over vast periods of time Archipelagos An _________________ is an island chain some distance away from a continent Example: the islands of ________ , which formed less than 1 million years ago The islands’ isolation is enough to favor divergence, but not enough to stop the occasional colonizers While the islands of Hawaii make up just 0.01 percent of the world’s land mass, 40 percent of the 1,450 species of Drosophila species arose there
