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从档案:授粉,和查尔斯·达尔文思考吓的
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Flowers are the reproductive, seed-producing structures of angiosperms. Typically, flowers consist of sepals, petals, stamens, and carpels. Sepals and petals are the vegetative flower organs. Stamens and carpels are the reproductive organs.
Flowers must be pollinated to produce seeds. In angiosperms, pollination is the transfer of pollen from the anther of the stamen (the male structure) to the stigma of the carpel (the female structure). Flowers may be self-pollinated or...
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Natural selection is an evolutionary process in which individuals with survival-promoting traits reproduce at higher rates. These favorable traits become more common within a population or species. Naturally selected traits initially arise via random genetic mutations. In order for selection to occur, there must be variation within a population, the trait controlling the variation must be heritable, and there must be an evolutionary advantage for variation in the trait.
The Theory of Natural...
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Gene flow is the transfer of genes among populations, resulting from either the dispersal of gametes or from the migration of individuals.
This phenomenon plays a significant evolutionary role in all organisms, and depending on the rates of gene flow, the mechanism either induces genetic diversity or generates genetic homogeneity among populations. When the rate of gene flow is low, the introduction of new alleles into a population generates genetic diversity. On the other hand, a high rate of...
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Speciation is the evolutionary process resulting in the formation of new, distinct species—groups of reproductively isolated populations.
The genetics of speciation involves the different traits or isolating mechanisms preventing gene exchange, leading to reproductive isolation. Reproductive isolation can be due to reproductive barriers that have effects either before or after the formation of a zygote. Pre-zygotic mechanisms prevent fertilization from occurring, and post-zygotic...
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In a population that is not at Hardy-Weinberg equilibrium, the frequency of alleles changes over time. Therefore, any deviations from the five conditions of Hardy-Weinberg equilibrium can alter the genetic variation of a given population. Conditions that change the genetic variability of a population include mutations, natural selection, non-random mating, gene flow, and genetic drift (small population size).
Mechanisms of Genetic Variation
The original sources of genetic variation are...
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Most altruistic behavior—in which one animal helps another at a cost to themselves—occurs between relatives. Scientists think these altruistic behaviors evolved because they increase the inclusive fitness of the animal providing help.
Inclusive fitness is an individual’s ability to pass down their genes—both through their offspring and the offspring of close relatives with shared genes. If an animal helps a close relative improve their reproductive success, this...