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Transcript
Speciation and Extinction Allopatric Speciation (with spatial separation) Sympatric Speciation (no spatial separation Allopatric speciation • Involves a physical separation • Separation leads to different selective pressure Allopatric separation speciation • This is like going away, then coming back and finding everyone has changed • (except it is over generations of time) • If directional selection is large enough the organisms will not be able to breed Sympatric Speciation • Populations are not physically or geographically isolated from each other; they live in the same place. – Gene flow between the two populations is interrupted through mechanisms other than simply being physically separated Disruptive selection, eliminate offspring of “mixed” breeds, and bring about sympatric speciation Sympatric, Ecological Speciation • Organisms may live quite near each other, but in different “micro” portions of the habitat Spadefood toads; species are connected to different soil types • Organisms live near each other, but select different “micro” portions of the habitat Temporal Isolation • Species mate at different times • Plants often have different flowering seasons • Closely related fireflies often mate at different times of the night Behavioral Isolation •Movie link • Differences in courtship behavior, or in chemical signals or vocalizations • Differences in color morphology Mechanical isolation • Sexual organs of closely related sympatric species are incompatible Gamete Mortality • Pre-zygotic mechanism • Note: Gametes are specialized, and usually perform well in the reproductive tract of the opposite sex • Pollen transferred to the stigma of another species will not result in fertilization Post Zygotic Isolation • The zygote may die shortly after fertilization • Low hybrid viability • Hybrid sterility • Low hybrid fitness Hybrid Sterility • a mule is the offspring of a male donkey and a female horse • Donkeys have 62 chromosomes, • Horses have 64. • Mules have 63 chromosomes which cannot evenly divide. Host-Race • A population of a species that becomes reproductively isolated from other populations due to adaptation to a specific host • This “gallmaker” seems to be undergoing ecological speciation • Populations have adapted to two goldenrod species S. gigantean. Female Goldenrod Gallmaker Polyploidization can produce instant sympatric speciation Hybridization • Members of one species mate with members of another species. • Hybrids may be as fit, or more fit than the parental species Non-geographical isolation Pre-mating Pre zygotic Post zygotic ecological gamete incompatibility hybrid inviability behavioral sperm/pollen specificity hybrid sterility mechanical hybrid fitness Nature abhors a vacuum • There have been 5 documented periods of extinction • Each of these has been followed by major bursts of speciation • KT Asteroid Species Radiations • Burgess shale • From time to time, the world has experienced a wide scale proliferation and radiation of life • Essentially all the major body plans evolved in the Cambrian “explosion”, documented in the Burgess shale The geological history of Earth • Geological evolution (full video) • Sixty-five million years ago: The Cretaceous period and Extinction Gradualism • Phyletic gradualism : The assumption that “missing links are due to failure to find an intermediate fossil Punctuated Equilibrium • Large populations are relatively static • Splinter small populations may evolve quickly – and leave no evidence • Small isolated populations may appear abruptly in the fossil record Retail Extinction • Parallel evolution: “repeats” are more likely explanations for the similarity of ichthyosaurs & dolphins • Extinction really does happen Balance of Life • There exists a balance between the process of speciation and extinction