Since most of the living things we encounter in our daily lives, from our pets to the flowers in our gardens, undergo some form of sexual reproduction, it is easy to think that this is simply the way things are done. However, not all organisms sexually reproduce. Parthenogenesis, literally "virgin origin", is a common reproductive strategy. It is mostly seen in small organisms, such as insects and bacteria. Such asexual reproduction results in clone-like offspring that are genetically identical to the parent. In species that reproduce asexually, evolution results only from mutation of parental genetic material.
In many ways asexual reproduction is actually a better reproductive strategy than sexual reproduction. Sex can be quite costly. In fact, it is doubly costly, since both parents suffer the cost for producing only one offspring. While sexual reproduction allows for the mixing of the parental genes to create genetically diverse offspring, it can result in the loss of favorable genes, which decreases fitness. Meiosis and syngamy, the cellular processes necessary for sexual reproduction, take longer than does mitosis, the process utilized in asexual reproduction. In terms of behavior, courtship and mating are risky, leaving the involved parties vulnerable to communicable disease and predation. The probability of finding a mate depends on the density of individuals of the species, whereas asexual reproduction does not require finding a partner.
So why might organisms choose sexual reproduction? Sexual reproduction, although costly, has several advantages. In competitive situations, sexual reproduction is a better strategy because sexually reproducing species evolve at a faster rate, giving the sexual reproducers an advantage over asexually reproducing competitors. Sexual reproduction also allows natural selection to throw out unfavorable genes. Recombination facilitates DNA repair, elimination of deleterious mutations, and the acquisition of favorable homozygous mutations. G.C. Williams' "lottery theory" claims that parthenogenesis is akin to buying 100 lottery tickets with the same number. All the tickets can win, but chances are that all will lose. Sexual reproduction is like buying 100 tickets with different numbers. Only one number can win, but there is a better chance of having a winning ticket. Sexual reproduction does not produce as many individuals as parthenogenesis, but there is a better chance of at least a few individuals surviving.
The environment in which a species evolved effects the type of reproduction it carries out. Sexual reproduction often occurs in stable, but highly competitive environments while asexual reproduction occurs more often in less competitive environments. For instance, asexual reproduction occurs more often in freshwater environments where competition is lower than in marine environments. Early apphid colonizers of a habitat may reproduce asexually, to increase population density. When density becomes high, increasing competition, individuals may start to reproduce sexually. At extreme altitudes and latitudes, asexual reproduction is more common because the environments are unstable.
A 1:1 sex ratio of males to females is an evolutionarily stable strategy. If there were more females than males, males would have the reproductive advantage, and their genes would be passed down at a higher rate. All offspring should receive equal parental investment since they share the same relatedness to their parents. In humans, 20% more males are conceived than females, but only 6% more males are born than females. By age 15, the ratio is 1:1. In other words, males have a higher mortality rate, and this has been evolutionarily compensated for by increasing the conception rate of males. In some social species, there is a correlation between the mother's dominance rank and the sex of her offspring. High-ranking females are more likely to birth males while low ranking females are more likely to birth females. High-ranking females will likely attract high quality males, and so her male offspring will have a good chance of being dominant and having his own reproductive advantage. Low ranking males have a much higher mortality rate than low ranking females, and so if the mother is low-ranking in the dominance hierarchy, her female offspring have a better chance at survival than her lower quality male offspring.
In 1871, Charles Darwin added sexual selection to his mechanisms of evolution. Intrasexual selection and intersexual selection can result in differential reproduction, and, hence, evolution. In general, females are choosy when it comes to selecting a mate, while males are desperate. This is due to the differential investment made by males and females. Females put great effort into their young while males often play no part in rearing their offspring and simply try to impregnate as many females as possible. A female therefore puts more energy into passing on her genes, and so must be more careful that her young will be fit. In situations when males do put effort into raising offspring, monogamy often results.
One of the major factors that determine the type of mating system a population will have is the operational sex ratio (OSR), the ratio of sexually available females to sexually available males. This ratio is usually male biased, which is why most species are polygamous, in which either sex shares multiple mates. Polygyny, where males mate with multiple females, is the most frequent form of polygamy. However, some species are polyandrous, where females mate with multiple males. When both sexes of a species mate several times with different individuals, there is a mixture of polygyny and polyandry. These species are said to be promiscuous.
The most frequently observed monogamy is behavioral monogamy. The male helps to raise young and guard his mate. This increases his own fitness by raising the successful survivorship of his offspring. This case is most frequently seen where the OSR is close to 1:1. Males who desert their mates will have a hard time finding other females because they will most likely already be taken. Even among so-called monogamous species, monogamy is often not strictly followed. Extra-pair copulations are extremely common. DNA fingerprinting has revealed that some offspring were not sired by their "father".
In situations where more males are ready to mate than females, we have the typical, polygamous case. Females are choosy and males are desperate to mate as often as possible. The mating choices of choosy females are influenced by four main benefits: direct benefits, the health of their mate, good genes, and exploitation of female preferences.
In mating with a particular male, some females may receive direct benefits, such as good territories. This is known as resource defense polygyny, where males fight for territory early in the breeding season, and females are attracted to males based on their territory. Another direct benefit comes in the form of nuptial gifts. The male katydid, for example, produces a spermatophore, which is basically a ball of sperm with highly proteinaceous material. The protein boost is very valuable to females when food is scarce, and they will even compete for matings to win this nuptial gift.
The second benefit a female receives by being choosy is phenotypically healthy mates. A brightly colored male suffers a cost, and by the honesty principle (see Honesty Principle ) he is probably a high quality male. It is also easier to see parasites on a bright background, and so a female can avoid catching a disease from her mate.
If the female does not receive a direct benefit, she will probably strongly be influenced by the desire for "good genes". The genotypic quality of the male is not necessarily apparent, but the phenotype provides a good clue. In barn swallows, for instance, there is a strongly negative relationship between a male's tail length and the number of parasites present on his offspring. This suggests a male with a long tail is more resistant to parasites than males with short tails. Once again, recall the honesty principle , which tells us that low quality males cannot afford to display costly traits.
It is also possible that the female receives no benefit by choosing a particular mate, but rather that he is appealing to her preferences. For instance, stalk eyed flies have eyes on stalks that are very far removed from their head. This conveys no advantage to the male, but the females prefer to mate with long-stalked males. Begun as a sort of genetic whim, the traits soon become correlated and entrenched. If a female who prefers long stalks mates with a long-stalked male, their sons are likely to be long stalked due to the father's genes, and the females are likely to prefer to mate with long-stalked males due to the mother's genes. Those females will likely mate with long-stalked males, and are even more likely to have long-stalked sons because she carries the long-stalk genes from her father as well. Another possibility is that males are exploiting pre-existing, noncorrelated female sensory biases. For instance, some male water mites mimic the vibrations of prey, which attracts females and increases male mating success.