Advantages of Sexual Reproduction

In organisms that reproduce sexually, all of the offspring are different. Each one contains a unique set of genetic information, half of it inherited from one parent and half from the other. Since individuals in the population vary, it is unlikely that a change in the environment would create problems for everyone. In fact, a change that reduces the survival rate of some might improve the survival rate of others. 

Populations of intertidal zone diatoms are enormous, and they can be found in every imaginable sunlit habitat. Diatoms grow attached to other plants or on the bodies of animals. In addition, some of the coastal species flourish on and in the sand and sediment or attached to surfaces and cavities of rocks. 

Diatoms living in the surf of the littoral zone are constantly on the move, migrating back and forth with the water. Offshore, they travel on air bubbles from the bottom of the shallow water to its surface. As a wave rolls to shore, diatoms ride it in and wash up on the beach. Here they secrete sticky mucus that helps them sink just below the sand s surface. As a wave rushes back to ocean, the diatoms wash out with it, back to their offshore position. From here, they float to the surface and reenter on another wave. 

Along both temperate and tropical shores, an unusual variety of diatom forms mucuslike sheaths, or tubes, that hold colonies of cells. These tubes resemble the long, thin threads of filamentous brown algae, but a close look under a microscope shows that the tubes hold large diatom cells. Unlike free-living diatoms, the cells of most tube-dwelling species are not covered with silicon frustules. 

One ciliate, Strombidium capitatum, is a heterotroph that feeds on single-celled organisms with chloroplasts. Instead of digesting the chloroplasts of its prey, S. capitatum keeps them alive and functional. In this way, this particular ciliate has two systems for getting food to prey on other organisms or to consume the products of photosynthesis. 

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Crow, J.F. (1994). Advantages of sexual reproduction. Developmental Genetics 15,205-213. 

Throughout the animal kingdom from protozoa to human beings sexual reproduction predominates. It is true that there are about 1000 species that reproduce asexually.242 243 Among them are 350 species of allfemale rotifers242 and even a species of tiny mites, all of which are haploid females.244 Nevertheless, sex seems to have conferred some advantage on most species. There are two thories that may explain this ... 

The Jerusalem artichoke can reproduce by two primary means. It can reproduce and colonize an area by the allocation of photosynthate and nutrients into both asexual (tubers and, to a lesser extent, rhizomes) and sexual (seed) reproductive organs. Flexibility in the amount of resources allocated between sexual and asexual means of reproduction confers a selective advantage in that conditions that inhibit or block sexual production (lack of pollen, herbivory of floral structures, undesirable weather) allow increased allocation to asexual reproduction. Artificially reduced allocation of resources to sexual reproduction, for example, results in a substantial increase in those allocated to asexual means. With flower bud removal, more (82 vs. 69) and larger (4.4 vs. 3.8 g) tubers were formed per plant than those with unlimited sexual reproduction (Wesdey, 1993). Total biomass was not altered, potentially indicating a relatively complete diversion of resources to asexual reproduction when sexual reproduction is blocked. From a reproductive standpoint, the risk of making it to the next season is high with sexual reproduction and relatively low with asexual reproduction. Increased investment in tubers increases the opportunity for sexual reproduction in the future. 

The life history of blue crabs is well known because of their abundance and economic value. A recent scholarly book, The Blue Crab, C. sapidus," edited by Victor Kennedy and Eugene Cronin, makes the interested reader up to date on what scientists know about blue crabs. Our research focus, and that of this chapter, is on sexual behavior of blue crabs, including pair bonding and mating, and the role of chemical communication in these behaviors. Because the pheromones of reproductive females are only released during a few days of their entire life, as described below, and in very small amounts, we use the advantages of the commercial fishery to obtain these chemicals in amounts sufficient to perform the requisite chemical analyses and behavioral bioassays. 

Chemical communication research is lacking in theoretical approaches (see, for example, Perrigo and Bronson, 1983 Sokolov et al., 1984). It should proceed within the context and theory of organic evolution, in which an active area of research has been sexual selection. Sexual selection depends on the advantage which certain individuals have over other individuals of the same sex and species in exclusive relation to reproduction, and it can affect an animal s glands for emitting odours (Darwin, 1871). Recent comments on sexual selection in mammals (Blaustein, 1981 Arnold and Houck, 1982) have stressed the possible role of scent communication without citing any comparative analyses of closely related species. 

Thus, some males could acquire a disproportionate share of matings at the expense of other males. For the highly rapid, directional selection (selection for an extreme) of male courtship traits to proceed, first a small percentage of discriminating females and males with an appropriate trait must be present in the population. Initially, such characters may confer an advantage in reproductive rather than courtship success (Thornhill, 1979). After that, sexual selection could proceed on its own to produce more extreme male scent dissemination structures and increasingly selective females. 

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