Evolutionary origin of sex differences

The evolutionary origin of sex differences is best understood by examining the relative benefits of sexual, as compared to asexual, reproduction. Those forms of reproduction in which genes are not exchanged are considered asexual. Asexual reproduction may take place from already developed body parts (vegetative reproduction) or from special reproductive tissue. In either case, asexual reproduction results in the rapid production of numerous individuals genetically identical to their parents. Because asexual reproduction allows numerous offspring to be produced in a short time, it is favored in situations in which a species can gain an advantage by exploiting an abundant, but temporary, resource, such as a newly discovered cache of food. There is also a further advantage: The individual which finds and exploits a resource, if it can reproduce asexually, is assured that all its offspring will possess the same genotype as itself and will, thus, be equally able to exploit the same resource for as long as it lasts. Despite these advantages, asexual reproduction is much less common than sexual reproduction among animals. It is a temporary stage in many species, alternating with sexual reproduction. Asexual reproduction is far more common among microorganisms, such as bacteria.

Forms of Sexual Reproduction

Sexual reproduction may take many forms, but all involve the exchange of genes. Some algae, bacteria, protozoans, and other single-celled organisms exchange chromosomes without gametes in a process called conjugation. Most other forms of sexual reproduction use special sex cells called gametes, which exist in different “mating types.” Two gametes can combine only if their mating types are different. Some simple organisms, such as the one-celled green alga in the genus Chlamydomonas, have gametes that are indistinguishable in size or appearance—a condition known as isogamy. Most other organisms have gametes of unequal sizes—a condition called anisogamy. Selection often intensifies the differences between gametes, producing a small, motile sperm and a much larger, immobile egg laden with stored food (yolk).

Some sexually reproducing organisms have separate sexes—a condition called gonochorism. Individuals producing eggs are called females, while individuals producing sperm are called males. Since sperm are generally small and can be produced in great numbers, males tend to leave more offspring if they reproduce prolifically, indiscriminately, and often. Females, on the other hand, have fewer eggs to offer, and, in many species, they must also invest nutritional and behavioral energy into laying eggs and caring for the resultant offspring. Selection in these species favors females who choose their mates more carefully and take better care of their offspring.

The differing selective forces operating on the two sexes often give rise to sexual dimorphism, or differences in morphology between the sexes. Sexual dimorphism can also be reinforced by competition for reproductive success, a phenomenon first studied by Charles Darwin. Darwin called this type of competition sexual selection. It takes two basic forms—direct competition between members of the same sex and mate choices made by members of the opposite sex.

Direct male-male competition often takes such spectacular forms as rams or stags fighting in head-to-head combat. Similar fights also occur in many other species, including a variety of turtles, birds, mammals, fishes, and invertebrates. Many more species, however, engage in ritual fighting in which gestures and displays substitute for actual combat. Male baboons, for example, threaten each other in a variety of ways, including staring at each other, slapping the ground, jerking the head, or simply walking toward a rival.

Although male-male rivalry has attracted more attention in the past, female-female competition also occurs in many species. Now that more ethologists and sociobiologists are looking for evidence of such direct competition among females in the twenty-first century, it is being discovered that it is a fairly widespread occurrence, which had previously escaped notice only because so few scientists suspected its existence or were interested in looking for it. Some research asserts that females compete for mates in less risky ways than males, and rather than competing for a particular male mate, females compete indirectly for resources commonly provided by the male animal. Other studies show that female competition can be intense, even when there are more males than females in the group, likely because they are competing for quality genes to improve the viability of their offspring. Like males of some species, females may compete for dominance, nutrients, other resources, or territories. Female-female competition has been found among langur monkeys, golden lion marmosets, ring-tailed lemurs, spotted hyenas, ichneumon wasps, and several other species.

Sexual Selection

Sexual selection in mating is a selection in which reproductive success is determined, at least in part, by mate choice. No matter what form sexual selection takes, it results in greater reproductive success for those individuals chosen as mates. Those not chosen must try again and again if they are ever to succeed in leaving any offspring at all.

Sexual selection of this kind occurs in nearly all gonochoristic species. In some species, males will attract females by means of a visual display or by various sounds (also called calls or vocalizations). Females in such species will exercise choice by selecting among the available males based on their multimodal display. For example, male peacocks, lyrebirds, and birds of paradise will court females by showing off their elaborate tail feathers in bright, gaudy displays. In some species, like the pipefish, the females perform the display, and the males do the selecting. Other animals, like wild turkeys and crested auklets, engage in mutual courtship displays in which the female reciprocates the behavior.

Sometimes, the display will include an object, such as a nest constructed by one partner, as an attraction to its mate. Bowerbirds, for example, build elaborate nuptial bowers to attract their mates. These bowers, which contain a nest in the center, are sometimes adorned with attractive stones, flowers, and other brightly colored objects. In some species of animals, males and females will respond to one another by performing alternating steps; in this manner, each sex selects members of the other.

Many sexually reproducing organisms have male organs, which produce sperm, and female organs, which produce eggs—a condition known in the animal kingdom as hermaphroditism. Earthworms, nudibranch sea slugs, comb jellies, and many snails are simultaneous hermaphrodites, meaning both male and female organs are present at the same time. Simultaneous hermaphrodites may mate with other species or impregnate themselves. Other hermaphrodites often have their parts arranged so that self-fertilization is difficult or impossible. This may be because self-fertilization is much more likely to produce less viable offspring with more genetic mutations than would be conducive to the continuation of the species. One system that guarantees cross-fertilization is serial or sequential hermaphroditism. In this system, each individual develops the organs of one sex first, then changes into the opposite sex as it matures further. For example, clownfish are born male, while bank sea bass are born female.

Some sexually reproducing organisms have become secondarily asexual through a process called parthenogenesis, in which gametes (eggs) develop into new individuals without fertilization. In bees and wasps, males develop parthenogenetically from unfertilized eggs, while females (with twice the chromosome number) develop from fertilized eggs. While this is most common in microscopic rotifers or small insects like aphids and ants, scientists have documented some cases of other species producing parthenogenetically. A pair of critically endangered California condors reproduced this way.

The Cost of Sexual Reproduction

Sexually reproducing organisms experience a cost associated with the energy devoted to courtship behavior and to the growing of sexual parts. In addition, the act of courtship usually exposes an individual to a greater risk of predation, and the distractions of mating further increase this risk. In view of these costs, many evolutionists have wondered how sex ever evolved in the first place or why it is so widespread. Any adaptation so complex and so costly would long ago have disappeared if the organisms possessing it were at a selective disadvantage. The widespread occurrence of sex and numerous sexual systems shows that there must be some advantage to all the various forms of sexual reproduction, and that this advantage is sufficient to overcome the recognized advantages of asexual reproduction in terms of rapid proliferation with relatively low investment of energy.

The answer to this puzzle is based on the fact that asexually produced offspring are all genetically similar to the parent, while sexually produced offspring differ considerably from one another. Organisms exploiting a dependable habitat or food supply often leave more offspring if they produce numerous genetically similar offspring rapidly and asexually. On the other hand, organisms facing uncertain future conditions have a better chance of leaving more offspring if they reproduce sexually and, therefore, produce a more varied assortment of offspring, at least some of which might have the adaptations needed to survive in the uncertain future. An examination of species that are capable of reproducing, either way, confirms this hypothesis: Whenever favorable conditions are likely to persist, they reproduce rapidly and asexually. Faced with conditions of adversity or future uncertainty, however, these same species reproduce sexually. In species that alternate between sexually produced and asexually produced generations, the asexual phases typically occur during the seasons of assured abundance, while the sexual phases are more likely to occur at the onset of harsh or uncertain conditions. Sex, in other words, is a hedge against adversity and against an uncertain future.

Studying Sexual Reproduction

Most biologists who study reproduction are either ecologists, ethologists, or geneticists. Their methods include counting various kinds of offspring and measuring their genetic variability. Reproductive ecologists and ethologists also measure parental investment, or the amount of energy used by individuals of each type (and each sex) in courting their mates, in the production of gametes, and in caring for their young. Energy costs of this kind are generally measured by comparing the food consumption of individuals engaged in various types of activity using statistical methods of comparison among large numbers of observations.

The morphology of sex organs in various species is also studied by comparative anatomists and by specialists on particular taxonomic groups such as entomologists (who study insects), helminthologists (who study worms), malacologists (who study snails and other mollusks), and ichthyologists (who study fishes). In most hermaphroditic species, for example, the organs are so arranged as to make cross-fertilization easier and self-fertilization more difficult.

The above explanation of sexual reproduction as resulting from the greater variability among offspring facing an uncertain future is partially confirmed by studying species that can reproduce either sexually or asexually. Among these species, asexual reproduction is always favored in situations in which an individual discovers a resource (such as a habitat or a food source) too large to exploit by itself. These conditions favor individuals that can reproduce rapidly and asexually produce numerous individuals genetically similar to themselves, who then proceed to exploit the resource. Aphids, for example, produce one or several asexual generations during the spring and early summer, when plant food is abundant. In seasons or situations of great risk or uncertainty, however, the same species often reproduce sexually at a somewhat greater energetic cost, leaving a wider variety of offspring but a smaller total number. Under unpredictable conditions (such as those associated with wintering in a cold, temperate climate), the greater energetic costs of reproducing sexually are more than made up by the greater genetic and ecological variability among the offspring. Sexually reproducing individuals leave more offspring (on average) than asexual individuals under these conditions. Similarly, among hermaphroditic species, cross-fertilization results in more varied offspring than self-fertilization and, therefore, is favored under such conditions.

Testing Theories

The several reproductive methods studied by biologists provide a natural laboratory for the testing of several theories. Among these are theories concerned with genetic variability, natural selection, the evolution of sex, and the allocation of resources, including the theory of parental investment in the care of their offspring.

In terms of the two most general types of reproductive strategies, those species using a system called the r strategy (reproducing prolifically at small body size) may be sexual, asexual, or may alternate between these two methods of reproduction. On the other hand, species following the K strategy (reproducing in smaller numbers at larger body sizes and investing time and energy in parental care) are invariably sexually reproducing and often gonochoristic.

In addition to the theoretical considerations mentioned above, the study of alternative methods of reproduction gives us important insights into the reasons that our species, like other K strategists, is sexually reproducing and gonochoristic. In most species, sexual behavior is largely controlled by instincts, but learned behavior plays a major role among higher primates. Beyond what is necessary for copulation and childbirth, much of sex-specific behavior in humans is culturally defined and may differ from one society to another. This includes the norms of what behavior is appropriate (or inappropriate) for each sex and what personal qualities are considered masculine or feminine. Attempts to redefine sex roles will lead nowhere unless one is aware of both the biological and the social underpinnings of these roles.

Principal Terms

Anisogamy: Reproduction using gametes unequal in size or motility

Asexual Reproduction: Reproduction in which genes are not exchanged

Female: An organism that produces the larger of two different types of gametes

Gonochorism: Sexual reproduction in which each individual is either male or female, but never both

Hermaphroditism: Sexual reproduction in which both male and female reproductive organs are present in the same individual, either at the same time or at different times

Isogamy: Reproduction in which all gametes are equal in size and motility

Male: An organism that produces the smaller of two different types of gametes

Parthenogenesis: Asexual reproduction from unfertilized gametes, producing female offspring only

Sexual Dimorphism: Differences in morphology between males and females

Sexual Reproduction: Reproduction in which genes are exchanged between individuals

Sexual Selection: Selection for reproductive success brought about by the behavioral responses of the opposite sex

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