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Obligate parthenogenesis is the process in which organisms exclusively reproduce through asexual means. [39] Many species have been shown to transition to obligate parthenogenesis over evolutionary time. Well documented transitions to obligate parthenogenesis have been found in numerous metazoan taxa, albeit through highly diverse mechanisms.
Sexual selection in birds. Male greater frigatebird displaying. Sexual selection in birds concerns how birds have evolved a variety of mating behaviors, with the peacock tail being perhaps the most famous example of sexual selection and the Fisherian runaway. Commonly occurring sexual dimorphisms such as size and color differences are ...
Asexual reproduction in liverworts: a caducous phylloid germinating. Asexual reproduction is a type of reproduction that does not involve the fusion of gametes or change in the number of chromosomes. The offspring that arise by asexual reproduction from either unicellular or multicellular organisms inherit the full set of genes of their single ...
Parthenogenesis in squamates. Parthenogenesis is a mode of asexual reproduction in which offspring are produced by females without the genetic contribution of a male. Among all the sexual vertebrates, the only examples of true parthenogenesis, in which all-female populations reproduce without the involvement of males, are found in squamate ...
Sexual selection is a mode of natural selection in which some individuals out-reproduce others of a population because they are better at securing mates for sexual reproduction. [16] [failed verification] [17] It has been described as "a powerful evolutionary force that does not exist in asexual populations". [18]
Trematoda is a class of flatworms known as flukes or trematodes. They are obligate internal parasites with a complex life cycle requiring at least two hosts. The intermediate host, in which asexual reproduction occurs, is usually a snail. The definitive host, where the flukes sexually reproduce, is a vertebrate.
One species of seastar, Ophidiaster granifer, reproduces asexually by parthenogenesis. [67] In certain other asterozoans, adults reproduce asexually until they mature, then reproduce sexually. In most of these species, asexual reproduction is by transverse fission with the disc splitting in two. Both the lost disc area and the missing arms ...
On this it may alternate between sexual and asexual generations (holocyclic) or alternatively, all young may be produced by parthenogenesis, eggs never being laid (anholocyclic). Some species can have both holocyclic and anholocyclic populations under different circumstances but no known aphid species reproduce solely by sexual means. [50]