Given the inefficiency of sexual reproduction, its prevalence is somewhat surprising. In most sexually reproducing species, the sex ratio is 1:1 and therefore half the population (males) cannot produce offspring. And mating behaviors involve costs and risks. Costs include time and energy spent finding, attracting, and competing for a mate, as well as the “opportunity costs” of detracting from other activities such as feeding and caring for existing offspring. Risks include increased exposure to predation and the potential for physical damage. Despite these disadvantages, most eukaryotic organisms reproduce sexually. Thus it would seem that the production of genetic diversity is an evolutionary advantage that can overwhelm the cost of sex (see Key Concepts 20.2 and 20.4).
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In spermatogenesis, four haploid sperm are produced from each primary spermatocyte.
Oogenesis involves asymmetrical division of cytoplasm.
Specific recognition molecules mediate interactions between sperm and eggs in sea urchins.
Hermaphrodites can function both as male and as female.
Organisms in aquatic environments generally have external fertilization and engage in spawning to bring eggs and sperm together.
Mechanisms of internal fertilization are highly diverse.
Sexual reproduction requires the joining of two haploid sex cells to form a diploid individual. These haploid cells, or gametes, are produced through gametogenesis, a process that involves meiotic cell divisions. Two events in meiosis contribute to genetic diversity: crossing over between homologous chromosomes and the independent assortment of chromosomes (see Key Concepts 11.5 and 12.1). Mating behavior itself also contributes to genetic diversity. The genetic variation among the gametes of a single individual and the genetic variation between any two parents produce an enormous potential for genetic variation between any two offspring of a sexually reproducing pair of individuals.
Sexual reproduction in animals consists of three fundamental steps:
Gametogenesis: making gametes
Spawning or mating: bringing gametes together
Fertilization: fusing gametes
The process of gametogenesis is similar across sexually reproducing animal species. Processes of fertilization are also quite similar in widely different species. Therefore, while our discussion of gametogenesis will focus generally on mammals, and our discussion of fertilization will feature sea urchins, the facts would not be dramatically different were we to consider many other animal groups. Adaptations for spawning and mating, in contrast, show incredible anatomical, physiological, and behavioral diversity across species.