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Mechanisms of Evolution Darwin Presents His Case Darwin returned to England in 1836. He did not present his findings to the scientific community until 25 years later Darwin Presents His Case While visiting the Galapagos Islands, Darwin had made observations of many different species that were found no where else on Earth. Darwin Presents His Case Each of these island species look very similar to species found on the mainland of South America. Yet, the island species were clearly different from the mainland species and different from each other. Darwin Presents His Case Darwin began filling notebooks with his ideas about species diversity and the process that would later be called evolution. Darwin was stunned by his findings which led to the delay in publishing his thoughts. All of Darwin’s ideas challenged the fundamental scientific beliefs of his day. Darwin Presents His Case In 1858, Darwin received a short essay from Alfred Russell Wallace, a fellow naturalist. This essay summarized thoughts on evolutionary change and gave Darwin the incentive to publish his own work. Darwin Presents His Case In 1859, Darwin published the results of his work, On the Origin of Species. Darwin Presents His Case In his book, he proposed a mechanism for evolution called natural selection. He presented evidence that evolution has been taking place for millions of years and continues in all living things. Darwin's work was considered as brilliant by many, while others strongly opposed his message. What did Darwin really say? What have we learned since Darwin? Evolution Darwin explained evolution as a change in the traits (alleles) of a population over time. Darwin also proposed that all organisms developed from previously existing organisms. Mechanisms of Evolution Organisms are a very stable system. There are many processes that keep the organism maintained and properly functioning (homeostasis). Likewise, populations tend to have stable proportions of traits unless one or more of the following processes acts on the variation contained in the population. Mechanisms of Evolution 1. Natural Selection: process by which individuals that are better suited for their environment survive and reproduce more successfully (survival of the fittest). Fitness: The ability of an individual to survive and reproduce in its specific environment. Fitness is the result of adaptations. Adaptations are inherited characteristics that increase an organisms chance for survival. The Process of Natural Selection Organisms in a population may vary: Each organism has unique DNA. This variation comes from mutations and crossingover during meiosis. These variations can be passed down to offspring. The Process of Natural Selection Organisms have the potential to reproduce quickly: Populations tend to grow exponentially. More offspring are produced than can survive. The Process of Natural Selection The ecosystem can only hold a limited number of organisms: Resources are limited; there is a carrying capacity for each species The Process of Natural Selection The organisms with the most favorable characteristics survive to reproduce: “Nature” selects the most fit organisms in a population for the particular environment. Favorable characteristics become more widespread in the population. Mechanisms of Evolution 2. Genetic Drift: Change in the traits in a population due to chance. a. Genetic drift can cause big losses of genetic variation for small populations. Mechanisms of Evolution Genetic Drift: Founder effect: A founder effect occurs when a new colony is started by a few members of the original population. This small population size means that the colony may have: reduced genetic variation from the original population. a non-random sample of the genes in the original population. Mechanisms of Evolution Genetic Drift: Genetic Bottle-Neck: Population bottlenecks occur when a population's size is reduced for at least one generation. Because genetic drift acts more quickly to reduce genetic variation in small populations, undergoing a bottleneck can reduce a population's genetic variation by a lot, even if the bottleneck doesn't last for very many generations. Mechanisms of Evolution 3. Gene Flow: migration of individuals from one population to another, introducing new traits. Mechanisms of Evolution 4. Non-Random Mating: the choosing of mates because of certain traits which results in amplification of those traits (alleles). Speciation Speciation: the formation of a new species A species is a group of organisms in a population that have the potential to interbreed. Speciation New species evolve do to a sub-population becoming geographically isolated from the rest of the population, then reproductively isolated through one of the barriers listed on the next slides. Speciation 1. Habitat Isolation: Two species that occupy different habitats within the same area may encounter each other rarely, if at all, even though they are not isolated by obvious physical barriers, such as mountain ranges. Speciation Habitat isolation: Two species of garter snakes occur in the same geographic areas, but one lives mainly in water while the other is primarily terrestrial Speciation 2. Temporal Isolation: species that breed during different times of day, different seasons, or different years cannot mix their gametes. Speciation Temporal Isolation: In North America, the geographic ranges of the eastern spotted skunk and the western spotted skunk overlap, but the western spotted skunk mates in late winter and the eastern spotted skunk mates in late summer. Speciation 3. Behavioral Isolation: courtship rituals that attract mates and other behaviors unique to a species are effective reproductive barriers, even between closely related species. Such behavioral rituals enable mate recognition—a way to identify potential mates of the same species. Speciation Behavioral Isolation: Blue-footed boobies, inhabitants of the Galapagos, mate only after a courtship display unique to their species. Part of the “script” calls for the male to high-step, a behavior that calls the female’s attention to his bright blue feet. Speciation 4. Mechanical Isolation: mating is attempted, but morphological differences prevent its successful completion. Speciation Mechanical Isolation: The shells of two species of snails spiral in different directions: Moving inward to the center, one spirals in a counter-clockwise direction, the other in a clockwise direction. As a result, the snails’ genital openings are not aligned, and mating cannot be completed Speciation 5. Gametic isolation- sperm of one species may not be able to fertilize the eggs of another species. For instance, sperm may not be able to survive in the reproductive tract of females of the other species, or biochemical mechanisms may prevent the sperm from penetrating the membrane surrounding the other species’ eggs. Speciation Gametic Isolation: Gametic isolation separates certain closely related species of aquatic animals, such as sea urchins. Sea urchins release their sperm into the surrounding water, where they fuse and form zygotes. Gametes of different species, such as the red and purple urchins shown here, are unable to fuse because proteins on the surfaces of the eggs and sperm cannot bind to each other Speciation 6. Hybrid Sterility: even if hybrids are vigorous, they may be sterile. If the chromosomes of the two parent species differ in number or structure, meiosis in the hybrids may fail to produce normal gametes. Since the infertile hybrids cannot produce offspring when they mate with either parent species, genes cannot flow freely between the species . Speciation Hybrid Sterility: The hybrid offspring of a donkey and a horse is a mule, which is robust but sterile. The Pace of Evolution Although Darwin and Wallace, the originators of the theory of evolution through natural selection, thought the pace of evolution was gradual (occurring over millions of years), many scientist think the pace varies. The Pace of Evolution Punctuated Equilibrium: This is an interpretation of the fossil record showing rapid speciation followed by long periods of equilibrium with little evolution.