Mechanisms of Population Change

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 © 2006 W.W. Norton & Company, Inc. DISCOVER BIOLOGY 3/e

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 © 2006 W.W. Norton & Company, Inc. DISCOVER BIOLOGY 3/e

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. © 2006 W.W. Norton & Company, Inc. DISCOVER BIOLOGY 3/e

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) © 2006 W.W. Norton & Company, Inc. DISCOVER BIOLOGY 3/e

Changing Populations Result in 3 major themes in biology: Adaptation of populations; better adapted to their environments Diversity of life Shared inherited characteristics between organisms © 2006 W.W. Norton & Company, Inc. DISCOVER BIOLOGY 3/e

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 © 2006 W.W. Norton & Company, Inc. DISCOVER BIOLOGY 3/e

Adaptations Are Not Perfect
Natural selection has limitations: Genetic constraints No variation, no adapting Multiple effects of genes in development Ecological trade-offs What is good for one thing may hurt in another © 2006 W.W. Norton & Company, Inc. DISCOVER BIOLOGY 3/e

What are Species? Members of a species generally look alike Bald eagles in Alaska (top) & Colorado (bottom) But not always Species –are reproductively isolated from other species, meaning that one can only reproduce within one’s own species © 2006 W.W. Norton & Company, Inc. DISCOVER BIOLOGY 3/e

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 © 2006 W.W. Norton & Company, Inc. DISCOVER BIOLOGY 3/e

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. © 2006 W.W. Norton & Company, Inc. DISCOVER BIOLOGY 3/e

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. © 2006 W.W. Norton & Company, Inc. DISCOVER BIOLOGY 3/e

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. © 2006 W.W. Norton & Company, Inc. DISCOVER BIOLOGY 3/e

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. © 2006 W.W. Norton & Company, Inc. DISCOVER BIOLOGY 3/e

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. © 2006 W.W. Norton & Company, Inc. DISCOVER BIOLOGY 3/e

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. © 2006 W.W. Norton & Company, Inc. DISCOVER BIOLOGY 3/e

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 © 2006 W.W. Norton & Company, Inc. DISCOVER BIOLOGY 3/e

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 © 2006 W.W. Norton & Company, Inc. DISCOVER BIOLOGY 3/e

Speciation The splitting of one species into two has never been seen in animals check back in a couple thousand years Walking sticks Apple maggots (worms) It has been seen in plants (2 species split into 3). © 2006 W.W. Norton & Company, Inc. DISCOVER BIOLOGY 3/e

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 © 2006 W.W. Norton & Company, Inc. DISCOVER BIOLOGY 3/e

Biological Evolution Only populations can evolve; individuals cannot (on molecular level, genes can evolve) Small scale change = microevolution Large scale change = macroevolution © 2006 W.W. Norton & Company, Inc. DISCOVER BIOLOGY 3/e

Impact and Review 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 Review of agents of change © 2006 W.W. Norton & Company, Inc. DISCOVER BIOLOGY 3/e

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