Natural selection recognizes only one currency: successful offspring. Yet even though all living organisms have presumably been selected to maximize their own lifetime reproductive success, they vary greatly in exact modes of reproduction. Some, such as most annual plants, a multitude of insects, and certain fish like the Pacific salmon, reproduce only once during their entire lifetime. These 'big-bang' or semelparous reproducers typically exert a tremendous effort in this one and only opportunity to reproduce - in fact their exceedingly high investment in reproduction may contribute substantially to their own demise.
Many other organisms, including perennial plants and most vertebrates, do not engage in such suicidal bouts of reproduction but reproduce again and again during their lifetime. Such organisms are iteroparous (repeated parenthood). Even within organisms that use either the big-bang or the iteroparous tactic, individuals and species differ greatly in numbers of progeny produced. Annual seed set of different species of trees ranges from a few hundred or a few thousand in many oaks (which produce relatively large seeds - acorns) to literally millions in redwood trees. Seed production may vary greatly even among individual plants of the same species grown under different environmental conditions; an individual poppy produces as few as four seeds under stress conditions, but as many as a third of a million seeds when grown under conditions of high fertility. Fecundity is equally variable among fish; the large ocean sunfish, Mola mola, is perhaps the most fecund of all vertebrates with a clutch of 200 million tiny eggs. A female codfish also lays millions of relatively tiny eggs. Most elasmobranchs (sharks, skates, and rays), however, produce considerably fewer but much larger offspring. Variability of clutch and/or litter size is not nearly so great among other classes of vertebrates, but it is still significant. Among lizards, for example, clutch size varies from a fixed clutch of one in some geckos and Anolis to as many as 40 in certain horned lizards (Phrynosoma) and large Ctenosaurus and Iguana. Timing of reproduction also varies considerably among organisms.
Due to the finite chance of death, earlier reproduction is always advantageous, all else being equal. Nevertheless, many organisms postpone reproduction. The century plant, an Agave, devotes years to vegetative growth before suddenly sending up its inflorescence (some related monocots bloom much sooner). Delayed reproduction also occurs in most perennial plants, many fish such as salmon, a few insects like cicadas, some lizards, and many mammals and birds, especially among large seabirds.
High fecundity early in life is often correlated with decreased fertility later on, an excellent example of the principle of allocation. When early fecundity is lower, plots of fecundity versus age are flatter. Innumerable other examples of the diversity of existing reproductive tactics could be listed. Clearly, natural selection has shaped observed reproductive tactics, with each presumably corresponding in some way to a local optimum that maximizes an individual's lifetime reproductive success in its particular environment.
In different strains of white leghorn domestic chickens, fecundity drops off faster with age in birds that lay many eggs early in life, as might be anticipated from the principle of allocation.
Population biologists would like to understand factors that influence evolution of various modes of reproduction.
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