Evolutionfields of Study

Anatomy, evolutionary science, reproduction science

Summary

In asexual reproduction, genetic material is not exchanged; offspring are genetically identical to the parent. Sexual reproduction involves the exchange of genes. Natural selection favors asexual reproduction in the exploitation of dependable resources, but selection favors sexual reproduction whenever the future is uncertain.

Principal Terms

Basic Principles

The evolutionary origin of sex differences can be understood only 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 fromspecial reproductive tissue. In either case, however, 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 that finds a resource that it can effectively exploit, 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 of them involve the exchange of genes. Some algae and protozoans 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 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 female, while individuals producing sperm are called male. 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 in the laying of eggs and the care of 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, 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. Female-female competition has been found among langur monkeys, golden lion-marmosets, ichneumon wasps, and several other species.

Sexual Selection

Sexual selection in mating is selection in which reproductive success is determined at least in part by mate choice. No matter what form sexual selection may take, it results in greater reproductive success for those individuals chosen as mates, while 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. For example, male peacocks, lyre-birds, and birds of paradise will court females by showing off their elaborate tail feathers in bright gaudy displays. In other species, the females perform the display and the males do the selecting.

Sometimes, the display will include an object such as a nest constructed by one partner as an attraction to its mate. Bowerbirds, for example, construct elaborate nuptial bowers as a means of attracting 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 both male organs which produce sperm and female organs which produce eggs, a condition known as hermaphroditism. Earthworms and many snails are simultaneous hermaphrodites, meaning that both male and female organs are present at the same time. Hermaphrodites often have their parts so arranged that self-fertilization is difficult or impossible. One system that guarantees cross-fertilization is serial hermaphroditism. In this system, each individual develops the organs of one sex first, then changes into the opposite sex as it matures further.

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.

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 of 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. Examination of those 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 biologistswhostudy 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 the courting of 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 ich- thyologists (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 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 the average) than asexual individuals under these conditions. Similarly, among hermaphroditic species, cross-fertilization results in more varied offspring than self-fertilization, and is therefore 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 re- sources, 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 either sexual or 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 size and investing time and energy in parental care) are invariably sexually reproducing and most often gonochoristic as well.

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 in 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. All attempts to redefine sex roles will lead nowhere, unless one is aware of both the biological and the social underpinnings of these roles.

—Eli C. Minkoff

See also: Asexual reproduction; Copulation; Gene flow; Genetics; Natural selection; Reproductive systems of female mammals; Reproductive systems of male mammals; Sexual development.

Further Reading