Curtis M. Lively
Abstracts of Selected Publications

ABSTRACT
Many species exhibit colour polymorphisms associated with alternative male reproductive strategies, including territorial males and 'sneaker males' that behave and look like females. The prevalence of multiple morphs is a challenge to evolutionary theory because a single strategy should prevail unless morphs have exactly equal fitness or a fitness advantage when rare. We report here the application of an evolutionary stable strategy model to a three-morph mating system in the side-blotched lizard. Using parameter estimates from field data, the model predicted oscillations in morph frequency, and the frequencies of the three male morphs were found to oscillate over a six-year period in the field. The fitnesses of each morph relative to other morphs were non-transitive in that each morph could invade another morph when rare, but was itself invadable by another morph when common. Concordance between frequency-dependent selection and the among-year changes in morph fitnesses suggest that male interactions drive a dynamic 'rock-paper-scissors' game.

ABSTRACT
The mutation accumulation hypothesis predicts that sex functions to reduce the population mutational load, while the Red Queen hypothesis holds that sex is adaptive as a defense against coevolving pathogens. We used computer simulations to examine the combined and separate effects of selection against deleterious mutations and host-parasite coevolution on the spread of a clone into an outcrossing sexual population. The results suggest that the two processes operating simultaneously may select for sex independent of the exact shape of the function that maps mutation number onto host fitness.
© 1998 The Society for the Study of Evolution. All rights reserved.

ABSTRACT
In theory, parasites can create time-lagged, frequency-dependent selection in their hosts, resulting in oscillatory gene-frequency dynamics in both the host and the parasite (the Red Queen hypothesis). However, oscillatory dynamics have not been observed in natural populations. In the present study, we evaluated the dynamics of asexual clones of a New Zealand snail, Potamopyrgus antipodarum, and its trematode parasites over a five-year period. During the summer of each year, we determined host-clone frequencies in random samples of the snail to track genetic changes in the snail population. Similarly, we monitored changes in the parasite population, focusing on the dominant parasite, Microphallus sp., by calculating the frequency of clones in t samples of infected individuals from the same collections. We then compared these results to the results of a computer model that was designed to examine clone frequency dynamics for various levels of parasite virulence. Consistent with these simulations and with ideas regarding dynamic coevolution, parasites responded to common clones in a time-lagged fashion. Finally, in a laboratory experiment, we found that clones that had been rare during the previous five years were significantly less infectible by Microphallus when compared to the common clones. Taken together, these results confirm that rare host genotypes are more likely to escape infection by parasites; they also show that hose-parasite interactions produce, in a natural population, some of the dynamics anticipated by the Red Queen hypothesis.
© 1998 The Society for the Study of Evolution. All rights reserved.

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ABSTRACT |   FULL TEXT
The evolution of parasite virulence is thought to involve a trade-off between parasite reproductive rate and the effect of increasing the number of propagules on host survivorship. Such a rate trade-off should lead to selection for an intermediate level of within-host reproduction ([represented by lambda symbol]). Here I consider the effects of parasite propagule number on selection affecting [lambda symbol] when (i) the effect of each propagule is independent of propagule number, and (ii) when the effect of each propagule changes as a function of propagule number. Virulence evolves in these models as a correlated response to selectino on I. If each propagule has the same effect (s) as in all previous propagule, the surviorship of infected hosts is reduced by more than 60% at equilibrium, independent of the value of s. If, instead, each propagule has a more negative effect on host survivorship than previous propagules, host survivorship at equilibirium is expected to increase as the effect becomes more pronounced. These results are directly parallel to results derived for population mean fitness at mutation-selection balance; and they suggest that high virulence should be associated with parasites for which the effect of adding propagules either remains constant or diminishes with propagule number.   Full text of article (PDF)

ABSTRACT
The purpose of this study was to determine how two morphs of the acorn barnacle, Chthamalus anisopoma, coexist on rocky intertidal shores in the northern Gulf of California. The test of one of these forms (here called 'typical') has the conical, volcano shape which is characteristic of acorn barnacles while the test of the atypical form (here called 'bent') grows bent-over so that the plane of the aperture's rim is perpendicular to the substrate. I tested the hypotheses that bents are more resistant than typicals to: (1) desiccation during low tides and (2) attack by a carnivorous snail (Acanthina angelica) involving the use of a labial spine. These two hypotheses (which were suggested from analysis of the distribution patterns of the two morphs) were tested in conjunction with experiments designed to determine whether the bent form is genetically controlled or environmentally induced. The results indicated that the bent-over morph is a developmental response to the presence of A. angelica and that it is more resistant than the typical form to specialized predation by this gastropod. I also tested the hypotheses that: (1) bents are inferior competitors for primary rock space, and (2) the bent-over morphology places constraints on growth and reproduction. I found no evidence to suggest that bents are inferior competitors for space. They were, however, found to grow more slowly than typicals and to brood fewer eggs per unit body size. In summary, the bent-over form of C. anisopoma is a conditional response to the presence of a predator and both the conditional strategy and the dimorphism appear to be maintained by a trade-off between resistance to predation and the ability to convert resources into offspring.

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