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Transcript
Mechanisms of Population Change Populations may change through two major genetic forces: Natural Selection (includes artificial seltn) Genetic Drift Two other forces that change a population: Mutations Gene flow Not much change, mutation is too rare & gene flow tends to equalize gene frequencies Natural Selection Occurs because: Organisms with favorable traits have a better chance at survival The longer an organism lives, the more offspring it may produce More of the next generation will have the favorable trait than the previous generation Natural Selection Individuals of one phenotype are favored Peppered moths Genetic Drift Differences in reproduction or survival between organisms due to random chance Environmental disturbances can cause genetic drift: Hurricanes or Volcanic eruptions Clearing land for development or Over-hunting Can be from random events seeds from white flowers are blown to gravel, some seeds from red flowers blown to good soil. Genetic Drift How Populations Change Mutation creates genetic variation (genes) Gene flow, genetic drift and natural selection act on this genetic variation to change allele frequencies in populations Gene flow tends to equalize allele frequencies between 2 pops. Genetic drift and natural selection tend to diverge allele frequencies between 2 pops. (populations tend to diverge) Changing Populations Adaptation of populations; better adapted to their environments Diversity of life Shared inherited characteristics between organisms Adaptations Adaptation –a characteristic that improves the survival of a population or individual Adaptation An improvement in a population over time, changing a population to better fit its environment Only the result of natural selection acting on genetic variation, no other force adapts a population Environments change and new genetic variation arises, so adaptation never reaches perfection Adaptation: Matches organisms to environment Can be complex Accomplishes specific functions Adaptations Are Not Perfect Natural selection has limitations: Genetic constraints Multiple effects of genes in development Ecological trade-offs No variation, no adapting What is good for one thing may hurt in another What are Species? Members of a species generally look alike Bald eagles in Alaska But not always (top) & Colorado (bottom) Species –are reproductively isolated from other species, meaning that one can only reproduce within one’s own species Walking Sticks Adapting to match their environment (two mating pairs) Striped form is well-hidden among the needle-like leaves w/ stripes, solid form among solid leaves Walking Sticks The striped form of the species prefers needle-like leaves of the chemise (sheh-mēz) bush. The solid-green form prefers the solid green leaves of the wild lilac. The two forms are more likely to survive on their preferred plant. Predators (birds) more likely to catch them if they are not on their preferred plant. Walking Sticks Is a complex adaptation, more than coloration. Lilac and chemise bush leaves have different pulp, fiber, chemical contents, and plant toxins. The solid-green walking sticks have enzymes to better digest the leaves of the wild lilac. The striped form has enzymes adapted to better digest the leaves of the leaves of the chemise bush. Walking Sticks “Hybrid offspring” of a cross between solid and striped parents are less fit than offspring of a single form mating. Do not blend as well on either Lilac or chemise bush leaves; predators more likely to eat them. Do not digest either plant as well as single forms. Walking Sticks Because “Hybrid offspring” (a solid & striped cross) are less fit than “single form” offspring, the population is adapting in one other way: Individuals prefer to mate with others of the same color pattern. Walking Sticks Population is diverging with respect to alleles for: coloration, digestion, dealing with plant toxins, mating behavior. Natural selection is better adapting each form to their preferred plant. W/ time, disruptive selection will further diverge the two forms & further decrease their interbreeding. When the two forms loose the ability to interbreed, they will become two different species. Speciation When one species splits to form two species Results from reproductive isolation Often, it is a secondary consequence of changing populations Occasionally, is a direct consequence of a diverging population The hybrid is less fit, walking sticks Rates of Speciation differ in different organisms Walking sticks, have a reason to evolve reproductive isolation quickly, hybrids are less fit speciation may occur in a couple thousand years. European and N. American sycamore trees do not have a reason to evolve reproductive isolation they were separated geographically 20 mya & do not normally interbreed speciation has not yet occurred Speciation The splitting of one species into two has never been seen in animals check It back in a couple thousand years Walking sticks Apple maggots (worms) has been seen in plants (2 species split into 3). Biological Evolution Is a change in the genetic characteristics of a population over time If any allele frequency in a population changes w/ time, the population is evolving Most people think of evolution as Darwin’s speciation, but this is not correct Biological Evolution Only populations can evolve; individuals cannot (on molecular level, genes can evolve) Small scale change = microevolution Large scale change = macroevolution Biological Evolution Impact of Evolutionary Thought Understanding evolution has impacted technology: Development and use of pesticides Antibiotics and the development of drug resistance Doonesbury cartoon? I have assigned the readings on Evolution, but I have only lightly covered this topic