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r
e s e a r c h.. f o c u s :
Postcopulatory
sexual selection
Because
females of most species mate with multiple males within a reproductive
cycle, intrasexual competition and intersexual choice can continue
after mating in the form of sperm competition and cryptic female
choice. Sperm competition has been well documented and is generally
considered to favor the production of many tiny sperm due to the
benefits of outnumbering sperm from rival males. However, the rapidly
divergent nature of sperm morphology, including the evolution of
relatively large sperm, suggests that sperm quality may also be
important in determining male fertilization success. Our comparative
investigations and experimental evolution studies in the laboratory
have demonstrated that the morphology of the female reproductive
tract determines how females bias paternity in favor of particular
sperm morphologies, and hence generates sexual selection on males.
This process underlies the
coevolution of sperm and certain dimensions of the female tract
observed in numerous taxa and may explain why sperm are so rapidly
divergent. In other words, giant sperm tails represent the cellular,
postcopulatory equivalent of peacocks tails, having evolved through
female sperm choice. 
Selected
Related Publications:
Miller,
G.T., Starmer, W. T. and S. Pitnick. 2003. Quantitative
genetic analysis of among-population variation in sperm and female
sperm-storage organ length in Drosophila mojavensis. Genetical
Research, 81: 213-220. [PDF]
Miller,
G.T. and S. Pitnick. 2003. Functional significance of seminal
receptacle length in Drosophila melanogaster. Journal
of Evolutionary Biology 16:114-126. [PDF]
Miller,
G.T. and S. Pitnick. 2002. Sperm-female co-evolution in
Drosophila. Science 269:1230-1233. [PDF]
Miller,
G.T., Starmer, W. T. and S. Pitnick. 2001. Quantitative
genetic analysis of the size of the primary sperm-storage organ
in Drosophila melanogaster. Heredity 86:25-32. [PDF]
Pitnick,
S., and W.D. Brown. 2000. Criteria for demonstrating female
sperm choice. Evolution 54:1052-1056. [PDF]
Pitnick,
S., T.A. Markow, and G.S. Spicer. 1999. Evolution of multiple
kinds of female sperm-storage organs in Drosophila. Evolution 53:1804-1822. [PDF]
Pitnick,
S., Spicer, G.S., and T.A. Markow. 1995. How long is a giant
sperm? Nature 375:109. [PDF]
Pitnick,
S., and T.A. Markow. 1994. Male gametic strategies: Sperm size,
testis size, and the allocation of ejaculate among successive mates
by the sperm-limited fly Drosophila pachea and its relatives.
The American Naturalist 143:785-819. [PDF]
Pitnick,
S., and T.A. Markow. 1994. Large-male advantages associated
with costs of sperm production in Drosophila hydei, a species
with giant sperm. Proceedings of the National Academy of Science,
USA 91:9277-9281. [PDF]
Pitnick,
S. 1993. Operational sex ratios and sperm limitation in populations
of Drosophila pachea. Behavioral Ecology and Sociobiology 33:383-391. [PDF]
Pitnick,
S. 1991. Male size influences mate fecundity and remating interval
in Drosophila melanogaster. Animal Behaviour 41: 735-745. [PDF]
For
more details and selected publications on the different aspects
of my research program, follow these links:
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