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32 33 sexUaL seLeCtion the beautiful things creatures find attractive Sexual reproduction introduces variation beautifully, which is why most complex species use it. Even nonsexual species occasionally employ some form of sexual reproduction to avoid stagnation. Species that reproduce asexually without meiosis or mitosis or by parthenogenesis without male fertilization do not produce the genetic variation necessary for effective natural selection. Males in some animal species hope to mate with as many females as they can, ensuring the wide dispersal of their DNA through their billions of sperm. Meanwhile, a female who has to devote significant energies to the survival of her offspring has fewer opportunites to pass on her DNA. Because she can only have a small number of offspring, it is generally in her interest to look for the best DNA available. As a result she is often choosier than a male, and can require proof of quality, or seduction. This can lead to amazing results. The classic example is the peacocks tail opposite. It is a totally cumbersome disadvantage to peacocks in every respect apart from its sexiness to peahens. The fittest peacock with the tidiest and most mesmerising display gets to pass on his genes. Female guppy fish find colourful males irresistible, especially those with prominent collars. So sexual selection gradually produces populations of colourful fish until the time that predators move in and easily target them. Natural selection leaves the duller ones. In stag beetles opposite the outsized and largely useless mandibles are the product of sexual selection by females. They are used by the males to fight over the females, but the males select themselves, the females mating with whomever wins.
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