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20 The Evolution of Sex Determination In 1871, Charles Darwin published his long awaited Descent of Man and Selection in Relation to Sex.1 Darwin considered sexual evolution as a lesser form of natural selection and he gave it the term sexual selection. This distinction made sense because natural selection operated on all aspects of survival, while sexual selection acted mostly on the differencesbetweenthetwosexes .Adultmalehumansaretaller,heavier,more muscular,andtheirbodyfatandmusclesaredistributeddifferentlyfrom females.Femaleshavewiderhipsthanshoulders;maleshavethereverse. The secondary sexual differences in breast enlargement and facial hair mayplayaroleinsexualattractiveness.Theymayalsoaltertherelationof infants to their parents, making the mother-child bond more important for survival. In many animals, this sexual dimorphism between males and females is even more pronounced. The peacock is elaborate in color, size, and distribution compared to the peahen. Male deer have large antlers that they use for display as well as for combat with other males in theirrivalryforaccesstofemales.Femaledeerlackthoseantlers.Darwin felt that sexual reproduction gives an advantage to a species by fostering what was later called “hybrid vigor”—relatively specialized breeds of animals are less likely to survive in the wild than mongrels or hybrids of those strains that are closer to their ancestral wild type. August Weismann endorsed Darwin’s thesis.2 He argued that variations are likely to be more numerous in sexually reproducing species than in asexual forms. Thomas Hunt Morgan and his students followed that reasoning in their work with fruit flies. Advantages for sexually reproducing species include an opportunity to rapidly make numerous combinations of genetic variations in the population, many of them from hidden recessive traits or from partially dominant traits. Most of The Evolution of Sex Determination 147 these expressed traits are eliminated, but the beneficial combinations will survive if there are dramatic changes in the environment. The chromosome theory of heredity and the theory of the gene combined to put sexual selection on a genetic basis. The discovery of hormones in the early-twentiethcenturysuggestedaphysiologicalbasisforsexualdimorphism . Sex hormones and their ratios in males and females were shown to play major roles in male and female anatomical differences. Population genetics provided models for establishing sexual selection and the spread of sex-determining genes. More importantly, Darwin’s theory showed why sexual reproduction leads to a rapid deployment of traits to meet environmental crises that would threaten a species. In establishing the evolutionary history of the genetics of sex determination , biologists considered the problem in separate stages (Figure 20.1). First was the evolution of two mating types, a phenomenon that wouldbesolvedatthesingle-celllevel,wherestrainsdifferingbyasingle gene could lead to conjugation and an exchange of nuclei for mutual fertilization or the fusion of two cells.3 Paramecia used the nucleusswapping approach, and most algae and fungi used the fusion of two cells.Ineithercase,therehadtobealongperiodoftimefortheevolution of meiosis, in which haploid cells formed diploid cells and the diploid cells produced a recombination of genetic traits through chromosome pairing, crossing over, and reduction division. The fusion of two cells also established an alternation of generations. Cell populations could be haploidordiploid.Inmulticellularorganismslikeliverwortsandmosses, thosetwogenerationswereindividualorganisms,eachmulticellularand each with a different morphology.4 The second phase was the development of functional gametes. The earliest would be a physiological difference between two cells of the same size. These are called isogametic cells. Cells that differ in size, one serving as a larger cell where food was abundant and the other specializing as a motile small unit that could drift to encounter a suitable cell to fertilize, extend the range of the population. Such smaller cells meeting larger cells constitute anisogametes. The smaller would evolve into sperm as they shifted from water-current-dependent motion to their own propulsion with a motile tail. The evolution of sperm is thus tied to mobility. The evolution of the egg is tied to the storage of food. More Isogamous Mating 2N N N N + N N N + + 2N 2N N N N N N N N N N N Anisogamous Mating N N 2N mitosis 2N + 2N reduct on div sion equation division N N N N N N N N + N N N N Figure 20.1. A, In isogamous mating, the cells are the same size and differ chemically as haploid mating types (shown with white or black nuclei). At fertilization, the diploid cells (shown in gray) multiply by mitosis. At an appropriate time, some of these diploid cells undergo meiosis and produce the two mating types shown with black or white nuclei. B, In anisogamous mating, the two gametes are different in size. The male gamete is shown with a black...


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