Sex
| ADAPTIVE ADVANTAGES OF SEX
| | SEXUAL CHARACTERISTICS | | SEX HORMONES
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I INTRODUCTION
Sex, physical and behavioral difference that distinguishes individual organisms according to their functions in the reproductive process. Through this difference, termed male and female, a species can constantly reshuffle its genetic information, thereby creating genetically different offspring, some of which may be better adapted to changing environments.
Sex occurs at all levels of biological organization, with the exception of viruses. At the lowest level, bacteria conjugate and a length of the single chromosome is passed from the male, or donor cell, to the female, or recipient cell. At more advanced levels, multicellular individuals have specialized organs (gonads) that produce specialized sex cells (gametes). Upon fertilization, genetic information is transferred from the small, motile spermatozoa (male gametes) to the much larger ova (female gametes). Many organisms, including most plants, many protozoans and invertebrates, and some fishes, have both male and female gonads and are called hermaphroditic (see Hermaphroditism). Hermaphroditic organisms, however, are rarely self-fertilizing; the male and female reproductive organs ripen at different times-times that also coincide with those of other individuals, thereby ensuring cross-fertilization.
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II ADAPTIVE ADVANTAGES OF SEX
Many organisms also reproduce asexually (the parent multiplying without prior union), as in the case of bacteria and protozoans, which divide through mitosis (see Cell) into separate individuals. Plants and hydras reproduce asexually by budding. Many other organisms-including plants, the water fleas Daphnia, and some wasps-reproduce by parthenogenesis, in which the unfertilized egg develops into an adult. Such asexual reproduction has the advantage of colonizing great populations of a species in a very short period; indeed, Daphnia and wasps switch from sexual reproduction to parthenogenesis to populate ponds and their nests within the short warm season. Such populations, however, are made up of genetic replicas of the parent, and should some adversity occur in the environment, the entire population or species risks extermination.
Although sexual reproduction is slower and less direct, it holds the greater advantage of creating a wide diversity of individuals, each of a slightly different genetic makeup. During meiosis, or the formation of sex cells, the double (diploid) set of chromosomes, which appears in every cell of the adult, is randomly reduced to a single (haploid) set in the gamete. When this single set is united with that of the other gamete in fertilization, the genes are again shuffled, so that no offspring is an exact copy of the parent. If little or no change occurs in the environment, the offspring most resembling the parents will be most able to adapt and procreate; if a major change occurs in the environment, some of the offspring that deviated from the parents may be favored. The role of sex in perpetually reassorting parental genes is a fundamental mechanism of natural selection and probably became well established long before multicellular organisms first appeared.
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III SEXUAL CHARACTERISTICS
In animals, the sex of an individual is generally determined at the time of fertilization by the sperm cell. If a sperm cell carrying an X chromosome fertilizes the egg, the offspring will be female (XX); if a sperm cell carrying a Y chromosome fertilizes the egg, the offspring will be male (XY). See Genetics. The term primary sexual characteristics denotes the kind of gamete the gonad produces: The ovary produces egg cells in the female, and the testis produces sperm cells in the male. The term secondary sexual characteristics denotes all other sexual distinctions that play indirect roles in uniting sperm and eggs. Secondary sexual characteristics include everything from the specialized male and female features of the genital tract, to the brilliant plumage of male birds or facial hair of humans, to behavioral features such as courtship.
Generally, the more highly evolved the species, the more elaborate are its secondary sexual characteristics. At the time the eggs of starfish ripen, the male merely releases great quantities of sperm cells into the ocean water, and a tiny but sufficient number of them find and penetrate distant eggs. Frogs and toads are drawn to mates by calls, and they spawn in water; the male makes cloacal contact with the female and releases sperm externally, simultaneously with the eggs. Terrestrial animals, especially mammals, which do not have an environment of water through which sperm can propel itself, rely on herding and flocking, courtship, competition among males, and more specialized genitalia including an erectile penis, Fallopian tubes, and a uterus in which eggs are fertilized and develop internally.
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IV SEX HORMONES
In mammals, the hormones that influence sexual differentiation and development are androgens (mainly testosterone), which stimulate later development of the ovary. In the sexually undifferentiated embryo, testosterone stimulates the development of the Wolffian duct system, the forerunner of the male genital tract. Later, testosterone, along with gonadotrophins released by the pituitary gland, stimulates spermatogenesis. The Müllerian duct system, the forerunner of the female genital tract in the female embryo, probably differentiates spontaneously without hormonal stimulus. After female sex is well defined, estradiol, produced in the ovaries and the placenta, plays a major role in the development and the functioning of the female reproductive tract.
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