PROJECTS
What drives the evolution of life histories in guppies?​
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The question here is the importance of density-dependent evolution in nature. Density-dependent selection has a long history, having attracted attention from population geneticists and ecologists alike. Yet its importance is nature as a recurrent, widespread driver of evolution is not so clear.
Operationally, we - David Reznick, Ron Bassar, Tim Coulson, and I - are asking whether density-dependent selection has driven directional evolution of life histories in the Trinidadian guppy (Poecilia reticulata). This work builds on a body of work developed by Caryl Haskins, John Endler, David Reznick, and their students and colleagues. Guppies in low predation locations mature later and larger, live longer, have lower reproductive efforts, and make fewer, larger offspring than guppies in high predation locations. The alleviation of predation cannot directly drive these evolutionary differences. The other, obvious potential driver is the difference in population density between low- and high-predation locations, which is itself a direct effect of the different predation regimes. Our approach to assessing the importance of density-dependent selection has been to perform experiments in natural pools, artificial streams, and in the laboratory, using these results to parameterize mathematical models that make predictions about the long-term effects of higher population. We are testing those predictions with experimental transplants of guppies from a high- to four low-predation locations, an experiment that is now, after ten years, the longest-running experimental study of evolution in nature.
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Check out the guppy website & papers here, here & here to learn more.
Why are melanic male mosquitofish so rare?
One of the biggest of big questions in ecological genetics is why there is so much apparently adaptive genetic variation within natural populations? When natural selection acts on alleles, the allele with the greatest contribution to fitness ought to rise in relative frequency to near-fixation. Yet genetic variation for traits that appear to be under selection is ubiquitous in incidence and often appreciable in magnitude. What maintains this variation?
This is a difficult problem for two reasons. First, it requires identifying the ecological genesis of the selection on phenotypes. Second, it requires understanding the effect of selection at individual genes. As it happens, the eastern mosquitofish, Gambusia holbrooki, offers an opportunity to study this question, albeit with an interesting twist. Many populations of this mosquitofish have a sex-linked gene that causes males to develop eumelanin spots after they mature. The spots expand as the fish ages and many males become entirely black. Melanic males can be found in many populations (although not all). Biologists have long noticed that melanic male mosquitofish are hardier, more aggressive, and less subject to predation than their silver-colored colleagues. Yet melanic males are reliably rare: quantitative surveys show that their abundance, when they are present, ranges only from about 1% of males to, at most, 10% of males.
There are two paradoxes here. First, why are melanic males so rare when they occur? Second, why are they so reliably rare across a broad geographic area in a wide variety of habitats with a wide diversity of co-occurrence competitors and predators?
My student Lisa Horth introduced this problem to me and I continue to pursue it with my colleague Kim Hughes, postdoc Zach Culumber, and graduate students Brittany Kraft and Eve Humphrey. Our studies at present are focused on the behavioral interactions among melanic males, silver males, and females. These interactions create so-called indirect genetic effects that act in a manner that could maintain the genetic variation that we see. Our approach has included observations of social structure in natural populations (including underwater videos of fish shoals), small-scale field experiments, laboratory studies, and mesocosm experiments. Read more here, here & here
How do guppies and Hart’s killifish co-exist?
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The big question here is how two species coexist when the nature of their interaction would seem, in theory, to make coexistence difficult. Intraguild predation describes a two-species interaction in which individuals of each species compete in one stage of their life cycle (or in one range of body sizes) but prey upon each other in another. Theory says that coexistence is unlikely in such a system but, paradoxically, there appear to be many examples of such coexistence.
Guppies and Hart’s killifish coexist in low predation locations in headwater streams across the mountains of the Northern Range of Trinidad. Each species preys upon smaller individuals of the other and larger individuals appear to compete for food. We know that guppies affect killifish; in the experimental introduction of guppies mentioned above, the first response of killifish was a steady decrease in population density, compared to control reaches without guppies. This result suggests that the two species somehow co-evolve after their first encounter toward coexistence - but how?
Our approach is to use small scale laboratory experiments to quantify how the interactions between individuals of each species changes with their relative sizes and how that changes with different combinations of populations: killifish and guppies with and without a history of co-occurring together. We are also doing manipulations in natural pools to quantify the effect of one species on another. We’ll use the results in mathematical models that will predict the long-term effects of introducing guppies into reaches that had previously had killifish but no guppies. The introduction experiment described above will offer us a test of these predictions. See more information here, here & here
Social interactions and the maintenance of alternative male life histories in sailfin mollies
Phenotypic variation is a prerequisite for natural selection, but when selection favors a particular phenotype, variation can be quickly eroded. Herein lies a central paradox of evolutionary biology: how is variation maintained in the face of selection on adaptive traits? Alternative life histories are a particularly striking example of variation in adaptive traits. Social interactions are implicated as a crucial factor in the maintenance of phenotypic variation in some species because the fitness consequences of alternative life history phenotypes are typically mediated by social interactions, and development of alternative phenotypes can be regulated by social factors.
This project is testing the hypothesis that variation in the social environment is critical to maintaining alternative life history strategies in sailfin mollies, a species that demonstrates dramatic variation in male size and maturation rate within and among populations. We are also assessing whether life history variation is due in part to “predictive adaptive response” or “socially-cued anticipatory plasticity”; i.e. that juveniles use social cues to alter their development in order to match their phenotype at maturation to the social environment they will encounter as adults. We are using experiments, field studies, and theoretical models to test these hypotheses, and this project is a collaborative effort between multiple research groups at FSU.
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To read more about the ideas behind this project, see here, here & here
Does the evolution of individual traits drive patterns in macroevolution?
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Some clades are more diverse than others. By deploying analyses of molecular evolution, biologists have found that some of the variation in diversity has been driven by different rates of speciation. Of course, this begs the question of why some clades diversify more rapidly than others? In some situations, there may be simply more opportunity for diversification because of variation in geography or diversity of habitats. But in other situations, it may be that the evolution of a key trait is responsible for a subsequent acceleration of diversification.
This is a big question: Can microevolutionary processes explain macroevolutionary patterns? I am collaborating with David Reznick, Gil Rosenthal, and Mark Springer to address it with the diversity in the family Poeciliidae. Specifically, we are investigating whether the evolution of placentation in these fish affects the rate of speciation in clades with placentas. Why placentas, of all traits? With placentas comes a dramatic change in how embryos are provisioned, from pre-fertilization to post-fertilization. With post-fertilization provisioning comes the potential for embryos to influence their own provisioning, thus provoking parent-offspring conflict and a host of cascading processes that have long been thought to affect the rate of speciation.
This is also a “young” project in the sense that we have just received funding for it. It is based on our collective prior work on Poeciliid phylogenies, molecular evolution, and reproductive biology.
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To read more about the ideas behind this project, see here, here, here, here, here & here
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How important is social crowding for the reproduction of Least Killifish?
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Population regulation occurs through the depensatory effects of increased density on mortality and the compensatory effects of increased density on fertility. The compensatory effects on fertility emerge through two avenues: the effects of increased density on per capita food levels (more individuals, all else equal, means less food for everyone) and the stress produced by the increased crowding at higher densities. Oddly enough, we know how important lower per capita food levels can be and that there is often reproductive suppression at higher densities, but the relative strength of these is not at all well understood. Ecologists usually think about food limitation and behavioral biologists often think about social stress and reproductive suppression.
This is a big question: how important is each of these processes in depressing fertility at higher densities?
This is a “young” project in that the experimental work my students and I have initiated to answer this question is not yet published. It is, like all of my work, a collaborative effort, in this case with Erica Crespi. We are using the Least Killifish to address this question because, in my prior work on population regulation and density-dependent evolution in this species, my colleagues and students and I were able to classify a dozen or so populations in terms of their ecological and evolutionary histories of density regime and regulation. Our experiments exploit this knowledge: might the relative importance of food limitation and social stress be different in populations with different long-term histories of numerical density?
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To read more about the background for this project, see here, here & here
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