Evolution of Sexual Reproduction
M.Tevfik Dorak, M.D., Ph.D.
Asexual reproduction is still used by some organisms but in general failed to pass the test of natural selection. Sexual reproduction is the favored way of reproducing for many organisms. In sexual reproduction, new combinations of genes can be assembled on the same chromosomes through recombination. Independent assortment during meiosis, which changes combinations of chromosomes, generates endless genetic diversity. This variation enables a species to overcome novel environmental changes by fast adaptive change. In asexual reproduction, however, natural selection has to wait for some sort of mutation or change due to drift to take place, to act on. Sexual reproduction can also put two beneficial mutations together (although there is always a possibility to break a favorable combination too), or eliminate a deleterious one. The phenomenon that a favorable combination of genes may be broken during meiosis is called the 'cost of recombination'. Overall, groups reproducing sexually can evolve more quickly than those do not, because the combination of beneficial mutations will occur more quickly and deleterious mutations will accumulate more slowly. This is why in eukaryotic multicellular life forms sexual reproduction is a rule. Even in bacteria, which reproduce asexually, there are mechanisms allowing gene transfers between organisms (see conjugation in Viral and Bacterial Genetics). It is believed that sexual reproduction evolved as early as 2.5 to 3.5 billion years ago (Bernstein H et al., Am Nat 1981). Sexual reproduction is beneficial particularly when environment changes (including changes in the number and genetic constitution of parasites), and when offspring are dispersed widely to end up in different places from their parents. This is why aphids produce winged offspring when reproduced sexually, and wingless ones when reproduced asexually. Similarly, common grass grows asexually (locally) but produces seeds by sexual reproduction to travel away, so that in the new environments, there will be diversity for best adaptation. A host genotype successful against parasites may not be so in the next generation as the rate of evolution in parasites is so fast. The only way in which an animal makes sure that its descendants will be able to deal with different parasites is to reproduce sexually. This is currently the most widely favored theory on the evolution of sex. An alternative theory suggests that it may have evolved as an adaptation for competition with other species.
The evidence suggests that no major group of organisms (except one group of invertebrate rotifers) has evolved and diversified over long periods using parthenogenesis (development of eggs without fertilization 'virgin birth' as in aphids or Daphnia (water flea), or apomixis in plants) alone. When, however, the cost of sex (this is the cost of males, mating and recombination) outweighs the benefits of sex, a species may switch back to asexual reproduction. This is what is believed to have happened to dandelions. Interestingly, some species have adapted to have both modes of reproduction depending on the environmental conditions (see the link for the Encyclopaedia Britannica article at the end). Aphids reproduce parthenogenetically in the spring and summer when food supply is plenty. This enables them to reproduce very rapidly (and produce wingless offspring). When they face the long winter with no food around, they switch to sexual reproduction and deposit their fertilized large eggs on plants (alternation of generations). These eggs start a new generation of asexually reproducing (winged) females next spring. Similarly, plants may also have alternation in generations with different modes of reproduction. In some plants, one generation consists of diploid plants (sporophytes). Some of their cells undergo meiosis and produce haploid spores. These spores develop directly into haploid plants (gametophytes). Haploid gametophytes produce gametes by mitosis which will fuse to form a diploid zygote that develops into a diploid sporophyte plant. Most algae, fern, mosses and some vascular plants go through these separate phases in their life cycle.
The two important features of sexual reproduction are meiosis (production of haploid gametes to fuse with a different haploid gamete and crossing-over during this process) and syngamy (fusion of two haploid gametes to produce a diploid zygote). These features result in production of offspring significantly varied and rather different from the parents. In asexual reproduction, however, the parent produces progeny that are exact genetic replicas of themselves. Despite the expectation that sex should produce greater genetic diversity, asexual species may have very high levels of heterozygosity for unknown reasons. Asexual reproduction is generally favored when a species lives in a stable environment. The exception of sexual reproduction resulting in haploid offspring is what is seen in social insects such as wasps and bees. In the situation called 'haplodiploidy', male offspring are haploid as they result from unfertilized eggs, and females are diploid -the product of fertilization of an egg by a sperm-. This creates a situation that sisters are genetically 75% identical to each other as they all get the same haploid set from their fathers and either haploid set from their mothers. This has important implications in altruism (see Hamilton's rule in the introduction).
One consequence of sexual reproduction is anisogamy (the existence of two kinds of gamete, eggs and sperm) which is the basis of morphological and behavioral differences between the sexes (see sexual selection).
Modes of reproduction
Self-fertilizing hermaphroditism [simultaneous or sequential]
Sex with polyembryony
Inbreeding: Despite sexual reproduction, inbreeding lowers offspring fitness. This is called inbreeding depression. This occurs because of the presence of deleterious recessive mutations in populations. Inbreeding depression is due to homozygous offspring expressing deleterious alleles. In humans, for example, it is estimated that each individual caries three to five recessive lethal genes. In humans, >40% of progeny born to full sib matings either die before reproductive age or suffer severe disabilities (May RM. When to be incestuous. Nature 1979;279:192-4; see also other references). One theoretically possible advantage of inbreeding is that it reduces the cost of recombination. Mechanisms evolved to prevent inbreeding include differential dispersal by two sexes, similarly for plants, differential seed dispersal by the two sexes and the self-incompatibility (SI) system, and MHC-disassortative mating preferences (Penn DJ & Potts WK, 1999). The most extreme form of inbreeding, self-fertilization, however, still occurs in many plants. This usually occurs in monoecious plants in which both male and female flowers exist on the same plant. A mechanism evolved to prevent inbreeding in plants is dioecious plants which have only one kind of flower (male or female) on each plant. Other mechanisms have evolved to prevent self-fertilization in monoecious plants: differences in flowering times of male and female flowers, in hermaphrodites with perfect flowers, male and female parts of the flower are physically separated, and the self-incompatibility system which reduces the viability of self-fertilized embryos.
Self-incompatibility, is a genetically controlled mechanism to prevent inbreeding in plants. The SI loci prevent matings with self and also reduce matings with close kin; when pollen and maternal tissue share an incompatibility allele, fertilization is prevented, thereby enforcing disassortative matings (Haring V, 1990). The secreted glycoproteins encoded by the SI loci are envisaged to interact with a pollen component to cause arrest of pollen tube growth. This finding that some plants avoid inbreeding through disassortative mating preferences controlled by a highly polymorphic self-incompatibility system provides a striking precedent for the evolution of MHC-disassortative mating to avoid inbreeding in vertebrates (Jordan WC & Bruford MW, 1998a & 1988b; Penn DJ & Potts WK, 1999). There seems to be no mechanism to counteract outbreeding depression.
Inbreeding leads to an increase in homozygosity at all loci because the breeding pairs are initially genetically more similar to one other than would be the case if a pair of individuals had been taken at random from the population. Inbreeding distributes genes from the heterozygous to homozygous state. Thus, homozygosity increases without any change in allele frequencies.
Although inbreeding may be harmful to a species, sometimes outbreeding may also cause depression. It appears that there is an optimum amount of inbreeding and outbreeding. Very close relatives (including self) and totally unrelated individuals are unsuitable as mates, but medium to close relatives should be preferred. In plants, fertilization by donors of plants about 10m distant from them generally yields higher numbers of seeds than self-fertilization, fertilization with near-neighbors, or fertilization with plants further away than 10m.
Sexual reproduction in plants: The advantages of cross-pollination are such that plants have evolved elaborate mechanisms to prevent self-pollination and to have their pollen carried to distant plants. Many plants avoid self-pollination by producing chemicals (products of the SI system) that prevent pollen from growing on the stigma of the same flower, or from developing pollen tubes in the style. Other plants, such as date palms, some orchard trees, and stinging nettles, have become dioecious, producing only male (staminate) flowers on some plants and female (pistillate) flowers on others. Some plants are dichogamous - that is, the pistil ripens before or after the stigma in the same flower becomes receptive. See also Plant Genetics.
Advantages of sex: slower rate of reproduction but faster evolution, lower extinction rates, fast removal of deleterious mutations and better adaptation to host-parasite arms race.
Disadvantage of sex: cost of recombination, cost of mating (competition, no mating, wrong mating), cost of males.
Asexual reproduction: generates genetically identical progeny, conservation of harmful mutations (but also favorable genotypes), new genotypes are generated only through mutation.
Please update your bookmark: http://www.dorak.info/evolution/sreprod.html
The abstract of a review on Advantages of Sexual Reproduction
A full-text book by H-R Gregorius on Mating Systems
Matt Ridley's book: The Red Queen: Sex and the Evolution of Human Nature
Encyclopedia Britannica article on Sexual Reproduction (subscribers only)
M.Tevfik Dorak, MD, PhD
Last updated 9 January 2007