In the Light of Evolution Volume III: Two Centuries of Darwin (2009) / Chapter Skim
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12 Postcopulatory Sexual Selection: Darwin's Omission and Its Consequences--William G. Eberhard
12 Postcopulatory Sexual Selection: Darwin's Omission and Its Consequences--William G. Eberhard
Pages 243-262
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From page 243... ...
These include the hyperdiverse and paradoxically elaborate morphology of both sperm and male genitalia, the equally puzzling and elaborate morphology of nongenitalic male structures that are specialized to grasp and stimulate females, powerful manipulative effects of substances in male semen on female reproductive physiology, paradoxical male courtship behavior that occurs after copula tion has already begun, variability in parental investments, and the puzzlingly complex and diverse interactions between sperm and female products that surround animal eggs and between male gametophytes and female tissues in flowering plants. Many bizarre traits are involved, including male genitalia that are designed to explode or fall apart during copulation leaving behind parts within the female, male genitalia that "sing" during copulation, potent seminal products that invade the female's body cavity and her nervous system to influence her behavior, and a virtual Kama Sutra of courtship behavior performed after rather smithsonian Tropical research institute and escuela de Biologia, Universidad de Costa rica, san José, Costa rica.
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From page 244... ...
he discussed at length how competition between males for sexual access to females leads to sexual selection (Darwin, 1871/1959) , but failed to realize that sexual selection [sperm competition and cryptic female choice (CFC)
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From page 245... ...
, this male will stand to sire more offspring and win out over the others. Appreciation that female biases can also have postcopulatory effects on male reproductive success, and thus exercise the postcopulatory equivalent of female choice among males, lagged behind (Thornhill, 1983; eberhard, 1985)
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From page 246... ...
invest more in caring for offspring of male genitalia; the diverse morphology of sperm and the chemical constitution of male seminal products and their striking physiological effects on female reproductive processes; the common, but paradoxical, male courtship behavior that occurs after copulation has already begun; and the otherwise puzzling complexity and diversity of interactions between sperm and eggs and between the pollen tube and the female tissue through which it grows in plants. MECHANISMS OF POSTCOPULATORY SEXUAL SELECTION Direct Male-Male Interactions (Sperm Competition)
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From page 247... ...
. in fact, some studies have claimed to demonstrate sperm competition without having eliminated alternative possibilities such as female choice (below)
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From page 248... ...
. The likelihood that natural selection will favor female mechanisms to trigger her reproductive processes on the basis of whether she has mated makes the subsequent evolution of sexual selection via female-imposed biases particularly likely to occur.
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From page 249... ...
The logical, direct way to test the crucial difference between CFC and sAC theories would be to measure the direct and indirect benefits and losses that a female derives from cooperating or failing to cooperate with males. such data have been obtained in the laboratory with respect to the effects of male seminal products on female reproductive physiology in Drosophila melanogaster (Chapman et al., 1995; rice, 1996)
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From page 250... ...
Male genitalia, like peacock trains and other sexually selected traits, are often very complex in form and frequently differ sharply even among closely related species. Taxonomists working on many different animal groups with internal insemination have found that the shapes of male genitalia are often more useful in distinguishing closely related species than the forms of most other body parts.
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From page 251... ...
, which proposed that natural selection favored differences in genitalia to prevent cross-specific fertilizations; only the genital key of a conspecific male could fit into the lock of the female's genitalia. This idea explained both the baroque aspects of male genitalia and their rapid evolutionary divergence.
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From page 252... ...
Both the CFC and the sAC hypotheses attempt to explain the frequency of rapid divergent evolution of male genitalia, and the abundant data on genitalia permit strong tests. one test contrasts genital evolution in groups in which sAC would seem a priori much more likely to occur with that in others in which it is less likely.
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From page 253... ...
checked for the defensive morphological female coevolution that is predicted by sAC in species with rapidly evolving male genitalia or other nongenital structures that are specialized to contact females in sexual contexts (see next section)
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From page 254... ...
27p result in decreased reproductive cooperation from the female; further experiments showed that the changes in female behavior were caused by changes in stimula tion of her wing, as expected if the male legs have evolved under sexual selection by female choice. (scale lines in A–C = 1 mm, 0.1 mm, and 0.1 mm, respectively; width photo bottom right = 0.46 mm.)
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From page 255... ...
none of the male copulatory courtship behavior patterns observed to date involve direct manipulation of the female reproductive organs in a way that would suggest sperm competition, and in only a very few species is physical coercion of the female even feasible, so this courtship did not evolve under sperm competition or coercive sAC. As expected if it is under sexual selection, male copulatory courtship consistently differs among closely related species (eberhard, 1991, 1994, 1996)
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From page 256... ...
; the functional effects of longer or shorter female structures are not clear, however, nor are the selective pressures that result in changes in these traits. swimming speed is one frequently mentioned sperm competition mechanism that may exercise selection on sperm morphology.
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From page 257... ...
. Interactions Between Egg and Sperm Molecules recent summaries reveal that the genes coding for molecules involved in fertilization in mammals and marine invertebrates such as sea urchins and abalone show a general evolutionary pattern strikingly similar to the patterns seen in male genital morphology: in essentially all steps of animal fertilization where the molecular interactions between sperm and egg proteins have been studied, there is evidence for rapid divergence of
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From page 258... ...
Females could respond by making it more difficult for sperm to enter, and the resulting coevolutionary race could result in rapid divergent evolution of both sperm and egg molecules. A CFC explanation is that females are under selection to favor sperm cells with particularly effective designs, to obtain sons with these same designs; increased female selectivity could result in competition among males to evolve even more effective designs, resulting in rapid coevolution between male and female molecules.
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From page 259... ...
This predicted biogeographic pattern occurs in sea urchins: the 3 genera with rapidly diverging bindin molecules contain species that live in sympatry with congeners; in contrast, species in 3 other genera that do not show rapid divergence in bindin do not have overlapping geographic ranges (Zigler and lessios, 2003a)
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From page 260... ...
. The consistent finding that a female's offspring show greater vigor when the competition among pollen grains from a single donor plant is more intense indicates a payoff for female selectivity that is compatible with CFC explanations (and contrary to those of sAC)
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From page 261... ...
There are reasons to suppose that sexual selection has been important in many cases, but it is possible that no single explanation accounts for all cases or rapid divergence. selection for species isolation mechanisms may have had an important role in the evolution of abalone lysins, but species isolation has probably been of little importance in producing the widespread rapid divergent evolution of male genitalia or mammal sperm and egg proteins.
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