SEX

the sum of features by which members of species can be divided into two groupsmale and femalethat complement each other reproductively. Sex, sexuality, and reproduction are all closely woven into the fabric of living things. All relate to the propagation of the race and the survival of the species. Yet there can be sex without sexuality, and reproduction need not be sexual, although for most forms of life sexual reproduction is essential for both propagation and long-term survival. the sum of features by which members of a species can be divided into two groupsmale and femalethat complement each other reproductively. Sex is not essential for reproduction. Many lower organisms, including most bacteria protozoa, can reproduce without sex. Even among the higher plantsall of which can reproduce sexuallyvegetative (asexual) reproduction is common (e.g., by means of tubers, runners, and bulbs). In asexual reproduction, each offspring is identical to its single parent, except for differences arising from mutations in the genetic material. In sexual reproduction, in which each offspring receives a combination of genes from two parents, each offspring is genetically unique. Thus, populations of organisms that reproduce sexually display far more genetic diversity than do populations of organisms that reproduce asexually. The advantage of such genetic diversity is in adjusting to environmental change. The greater its genetic diversity, the more likely it is that a population will include some members that carry the genetic combinations needed to adjust to a changed environment. Evolution has strongly favoured the development of sexual reproduction. Beginning with mechanisms for ensuring occasional genetic recombination in primitive microorganisms, sex has undergone an evolution of its own throughout the whole history of life on Earth. Elaborate patterns of behaviour, complex structures, and delicate physiological equilibriums have all been developed under the influence of natural selection, ensuring the continued existence of species with an abundant supply of new combinations of genes in each generation. Although structural differences between male and female are not always apparent, as in many lower organisms, functional differences can always be distinguished. In the majority of species above the level of complexity of viruses and bacteria, a new individual is brought into being by the cooperation of a male and female organism, each of which contributes a special cell, a gamete, or sex cell, toward that creation; the male donates a spermatozoon, or sperm; the female, an ovum, or egg. The affinity that sperm and egg have for each other may result in their fusion (fertilization). The single-celled product, a zygote, carries its own blueprint for development derived from each of its parents. Sexuality is not always an eitheror determination. Organisms exist that produce both sperm and eggs; these, called hermaphrodites, occur naturally in many groups of lower animals. Among fungi and protozoa, sex is often found in more than two genetic forms, which are referred to as mating types. In higher animals and plants, intersexes, gynandromorphs, and sexual mosaics are not uncommon; the occurrence of such intergraded forms reflects the range of possibilities that may proceed from variable action of genetic endowment and hormonal influence. In sexual reproduction every individual receives two sets of chromosomesone set from each of two parentsand passes on mixtures of these chromosomes to its offspring. Thus, in each generation the genetic material is recombined in new ways. Actually, the shuffling of the hereditary determinants, or genes, is even more thorough, because during the formation of gametes (meiosis) a process of crossing over and exchange takes place between the two sets of parental chromosomes. This leads to the formation of gametes whose single set of chromosomes is actually a composite of the parental organism's maternal and paternal genes. The number of genes is so great that in a sexually reproducing population of higher organisms no two individuals (aside from those, such as identical twins, that result from a single zygote) are precisely alike. Each individual has a unique combination of genes that will not recur in populations of even many trillions of individuals. The uniqueness of the individual is thus a consequence of sexual reproduction. The significance of sex can be fully appreciated when one compares it to the phenomenon of parthenogenesis, in which the egg regularly develops into an embryo without having been fertilized. The great biological diversity characteristic of sexually reproducing species is thus not found in parthenogenetic forms. In the great majority of bisexual organisms some kind of genetic mechanism of sex determination is operative, causing certain zygotes to develop into males, others into females. The mechanism that controls the direction of development consists in most cases of a special pair of sex chromosomes, present in all the cells of the body. In one sex the two sex chromosomes are alike and are known as X chromosomes. In the other sex, there is a pair consisting of an X chromosome and a chromosome that differs from it genetically, known as a Y chromosome. In humans and most (but not all) groups of animals, the female will have two X chromosomes and the male will have an X and a Y chromosome. This pattern affects the development of the organism by controlling cellular metabolism and stimulating the production of hormones that trigger the development of sexual glands or organs. An excess or lack of hormones during embryological development may cause an individual to develop the superficial appearance of one sex while retaining the genetic constitution of the other sex. The mere existence of eggs, sperm, and accessory reproductive organs does not ensure fertilization. There must also be behavioural tendencies and reactions through which the male approaches and inseminates the female or her eggs. The sexual behaviour of all species has a common functionto secure the fertilization of the egg. Species exhibit an enormous range of behaviours to carry out this objective. Additional reading General works Adrian Forsyth, A Natural History of Sex (1986, reissued 1993), treats the role of sex in the natural world. The origins of sex and genetic recombination are considered in John Maynard Smith, The Evolution of Sex (1978); and Lynn Margulis and Dorion Sagan, Origins of Sex: Three Billion Years of Genetic Recombination (1986), and Mystery Dance: On the Evolution of Human Sexuality (1991); and the benefits derived by all species from genetic recombination are presented in James L. Gould and Carol Grant Gould, Sexual Selection (1989). Animals and plants George C. Williams, Adaptation and Natural Selection (1966, reissued 1974), offers an illuminating and thoughtful discussion of animal sexual reproduction in relation to the processes of biological evolution and adaptation. Courtship and mating are addressed in Margaret Bastock, Courtship: An Ethological Study (1967); J.H. Prince, The Universal Urge: Courtship and Mating Among Animals (1972); Robert L. Smith (ed.), Sperm Competition and the Evolution of Animal Mating Systems (1984); and Mark Jerome Walters, The Dance of Life (1988; also published as Courtship in the Animal Kingdom, 1989). The active role of the females in mate selection is investigated in Patrick Bateson (ed.), Mate Choice (1983); Evelyn Shaw and Joan Darling, Female Strategies (1985); Bettyann Kevles, Females of the Species: Sex and Survival in the Animal Kingdom (1986); and Mary Batten, Sexual Strategies: How Females Choose Their Mates (1992).The sexual systems, mate choices, and pollination strategies of plants are detailed in Mary F. Willson, Plant Reproductive Ecology (1983); Bastiaan Meeuse and Sean Morris, The Sex Life of Flowers (1984); and Jon Lovett Doust and Lesley Lovett Doust (eds.), Plant Reproductive Ecology: Patterns and Strategies (1988). Further works treating reproduction in animals and plants may be found in the bibliography to the article reproduction.