What sexual selection is; the two general strategies that living things might us

Question

What sexual selection is; the two general strategies that living things might use to achieve reproductive success. How the relative numbers of male and female individuals affect the reproductive strategy of each sex, and the characteristics of the two different strategies. Where and how sexual selection might occur in different kinds of organisms.

      Demonstrating sexual selection. Two things that need to be done experimentally to show that sexual selection is occurring, and how they are done.

Explanation / Answer

Sexual selection is a "special case" of natural selection. Sexual selection acts on an organism's ability to obtain or successfully copulate with a mate. Selection makes many organisms go to extreme lengths for sex: peacocks (top left) maintain elaborate tails, elephant seals (top right) fight over territories, fruit flies perform dances etc. Sexual selection is often powerful enough to produce features that are harmful to the individual's survival. For example, extravagant and colorful tail feathers or fins are likely to attract predators as well as interested members of the opposite sex. Sexual selection is essential to pass on our genes to next generation.

The reproductive success of an organism is measured by the number of offspring left behind, and their quality or probable fitness. Sexual selection usually works in two ways, although in some cases we do see sex role reversals:

Male competition- Males compete for access to females, the amount of time spent mating with females, and even whose sperm gets to fertilize her eggs.

Female choice- Females choose which males to mate with, how long to mate, and even whose sperm will fertilize her eggs. Some females can eject sperm from an undesirable mate.

According to Fisher and Haldane's theorems to account for varying degrees of inbreeding and effects of population density in terms of fitness- organisms must adapt to their environment to maximize their fitness, and environments vary in stability. In unstable environments, the best strategy would be to produce large numbers of offspring, many of which will die but a few are likely to survive. In stable environments, the better strategy is to have fewer offspring, but invest more in each so that offspring survival chances are increased. Thus, r-selected species are referred as short-lived, reproduce rapidly, take advantage of open niches, and are prone to boom or bust populations depending on the vagaries of the environment. K-selection refers to species that are longer-lived, reproduce slower, and are more immune to environmental swings. Compared with r-strategists, K-strategists are larger, the energy to produce one offspring is high, few offspring are produced, life expectancy is long, individuals can reproduce multiple times, sexual maturity is slow to arrive, and survival of offspring should be fairly high—with most offspring living a full-maximum lifespan.

How the relative no. of male and female individuals affect reproductive strategies of each sex can be understand by Bateman's principle. It postulates that variance among females in mating success is low, whereas variance among males in mating success is high. This stems from the fact that one mating in females should be enough to fertilize all their eggs whereas in males reproductive success is based on the number of times they have mated. In other words, nearly all females in a population mate and have offspring, but relatively few males mate successfully. Those males that do mate tend to mate with many females-thus a few males have very high reproductive output, but many males have little or no reproductive output (Bateman 1948). This leads to the prediction that sexual selection should act more strongly on males, leading to greater elaboration of behavior and structures used in attracting mates in males than in females.

Figure :- Bateman’s principle. These figures illustrate Bateman’s principle — after one mating, female mating frequency increases and relative fitness remains constant, as the sperm from one mating is adequate to fertilize all the female’s eggs. In males, as mating frequency increases relative fitness also increases proportionally.

Criticisms of Bateman's theory focus on the generality of the predictions. Contrary to the predictions of Bateman's principle, there are several possible advantages to female multiple matings. The female cichlid fish Pseudotropheus spiliopterus mates with any male they meet because they have a high risk of getting predated and a small population. This often leads to multiple matings by a single female (Kellogg et al.1998). Mating with any male that is seen ensures that these cichlids have a chance at producing offspring. The female Malawi blue cichlid has a high population but still participates in multiple matings. In this case multiple matings occur to avoid inbreeding and increase genetic diversity among the offspring (Kellogg et al. 1998). Additionally, multiple matings by females may increase the likelihood that they will find a compatible mate, one that is not sterile, or even help prevent infanticide.

Different Types of Mating Systems & Sexual Selection -:

Monogamy - Social monogamy is the behavioral pairing of a single male with a single female. It is most common in birds and rare in other animals. Theoretically, individuals in monogamous pairs will both contribute to the defense and parental care of offspring. Choosing an inappropriate mate could have a high fitness cost.

Polygyny - is the association of one male with multiple females. This mating system is found in a few birds and insects, but is most common in mammals. Polygyny is a strategy used by males to increase their reproductive fitness.

Resource Defense Polygyny - groups of females are attracted to a resource — males then compete for territorial possession of the resource, and, by extension, mating priority with females at the resource (Beletsky 1994).

Harems -Another common type of polygyny is membership in a harem, a defended group of females associated with one male. Females may initially associate in a harem for group defense, or they may be herded together by a male. Males compete for control of the groups. Harems typically exhibit a dominance hierarchy among the females in the group.

Leks - is an aggregation of males that are each seeking to attract a mate. Within a lek, males typically perform sexual displays. Unlike most other mating systems, leks are not associated with resources. Aggregations of males may be near particularly attractive females or in areas where females are likely to travel (Lank et al. 1995, Aspbury & Gibson 2004). It is thought that males form leks because they attract more females than do isolated males. Attracting more females is a strategy used by males to help increase their reproductive success.

Polyandry - is a group with one female and many males. Polyandry is a reproductive strategy that helps a female ensure reproductive success by providing her with multiple mating options.

Resource Defense Polyandry -In the Spotted Sandpiper, females control resources, which in turn controls male mating associations (Oring et al. 1994).

Cooperative Polyandry -The Galapagos hawk exhibits cooperative polyandry. In this case all males in the group copulate with the female and all participate in brood provisioning (Fabborg et al. 1995).

Polygynandry - Some mating systems have looser male-female bonds within groups. In polygynandrous groups, multiple females and males mate with each other, and males may care for the broods of several females. Chimpanzees and bonobos rely on this strategy — it allows groups of males and females to live together and spend less time being concerned with mate competition. Polygynandry may be advantageous from the female's perspective because it causes paternity confusion, which decreases infanticide and allows her to have multiple males care for her brood (Hrdy 1981, 2000).

Promiscuity - there are no pair bonds, and males and females, although sometimes choosy, often seem to mate randomly. As it is typically more advantageous for one or both sexes to pick their mate, promiscuity may occur in species for which the environment is unpredictable (Birkhead 2000, Burton 2002).

Sperm Competition -Although sperm competition is not a type of mating system per se, it is a form of male-male competition that plays an important role in mating systems. If more than one male mates with a female in a short time period, competition can occur after the males have released their sperm.

Related Questions

Navigate

• Browse (All)
• Browse W
• Subjects

Integrity-first tutoring: explanations and feedback only — we do not complete graded work. Learn more.