Preamble III. Population genetics and evolution
Alan Berryman
Tuesday, 12 May 2009 21:24 UTC
The effect of genetics on population behavior is a subject which I hoped to avoid, at least for the time being. I generally think that evolutionary processes have a significant effect on population dynamics on the long time scale and can be, more or less, forgotten for short time spans. Some populations do show genetic shifts on short time spans; for instance, color variations in larch budmoth (Baltensweiler and Fischlin 1980) and side-blotched lizard populations (Sinervo et al. 2000), but these do not give rise to evolution, as far as I know. My concern is to try and get a grip on ecological processes that determine dynamics on a relatively short time span (< 100 years) because that is the kind of data we have to deal with. Some of my colleagues, I know, think that genetic changes are responsible for the short-termed dynamics observed in nature (Witting 2000). I beg to disagree. Maybe I am wrong, but all the populations I have investigated (e.g., examples in my book and my publications; http://entomology.wsu.edu/Profiles/06BerrymanWeb/BerrymanOnlinePubs.htm) have data to support a non-evolutionary view on the time scale of observations (and this is all I can say). Thus, I open this section for further discussion of this interesting question. It is hoped that the remainder of the main internet discussion can continue without genetics for the immediate future. There is enough to worry about as it is. We can bring genetics into the picture after we have an outline of the ecological processes involved.
Baltensweiler, W. and Fischlin, A. 1980. The larch budmoth in the Alps. Pages 331-351 in Berryman, A. A. Dynamics of forest insect populations. Plenum Press, NY.
Berryman, A. A. 1999. Principles of population dynamics and their application. Stanley Thornes, UK.
Sinervo, B., Svensson, E. and Comendant, T. 2000. Density cycles and offspring quantity and quality game driven by natural selection. Nature 406: 985-988.
Witting, L. 2000. Population cycles caused by selection by density dependent competitive interactions. Bull. Math. Biol. 62: 1109–1136
Updated 30 May 2009 15:52 UTC
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Replies
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Hi Alan,
Thanks for putting up this topic! I’m beginning to think that separate time scales (ecological vs evolutionary) in population dynamics is an issue we should dispense with.
Genetic changes can be especially important in driving the dynamics of population densities in invasion fronts, co-evolving (natural enemy or co-operating) systems, responses to natural (or human induced) environmental changes, and/or managed systems (harvest-induced evolution1 is a topic that is becoming fashionable at the moment – although I think the scientific-political fervour to promote this idea far outweighs the evidence collected for it from natural or even lab populations so far).
The “eco-genetic” models required to investigate these dynamics are inherently more complex than equivalent ecological models. The adaptive dynamics framework still needs time to mature, but has the potential to help integrate ecological and evolutionary processes in a single framework
David Reznik has a lab focusing on rapid evolutionary processes (at least, phenotypic change) in Trinidadian guppies. Check out his publication page
I’ll try to post some more links when I have a little more time to search around.
1 Jørgensen et al. have a “Policy Forum” paper in Science discussing this issue, which highlights the paucity of genetic information to back up the claims – see the 2 supporting information tables, one of which lists 5 species from studies which demonstrate evolutionary changes caused by harvesting, the other lists many more studies which suggest evolutionary changes caused by harvesting. I think this subtle difference is crucial in terms of the weight of evidence collected so far. Follow this doi link for all the matertial.
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Most of the writing on genetic influences on population dynamics that occur over a relatively short time span (< 100 years) are, in my opinion, speculative. For instance, there is no concrete evidence that a change in throat color observed in lizards (Sinervo et al. 2000) has any dramatic effects on the reproductive qualities of individuals under controlled conditions (i.e., equal food supplies to all individuals). I presume, in fact, that the genetics would not change under these conditions. From this point of view, genetics are merely an expression of a change in density caused by ecological processes. I could therefore argue that the change in survival rates is due to ecological rather than genetic processes, that genetics are the result of population dynamics not the cause of it. Before genetics can be taken seriously as a meaningful determinant of population change it has to be shown that the genetic changes have a significant effect on the demographic rate of change of the population (Stevens and Sutherland 1999).
Sinervo, B., Svensson, E. and Comendant, T. 2000. Density cycles of an offspring quantity and quality game driven by natural selection. Nature 406: 985-988.
Stephens, P. A. and Sutherland, W. J. 1999. Consequences of the Allee effect for behaviour, ecology and conservation. Trends Ecol. Evol. 14: 401-405.
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I understand Alan’s reluctance but I think genetics cannot be avoided. This is because what we too readily assume about population processes (such as an intrinsic exponential growth when not modified by density dependence etc) has a genetic history, as Lars Witting has elegantly shown. Something similar applies to the process of body growth that has been studied so extensively and which is so closely linked to natural mortality rate and reproduction rate. Sidney Holt.
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Well, I have no major problem in to see genetic effects on population growth rates, it is totally ok that evolutionary forces changing genetics will influence survival or reproductive rates. There are examples of how adaptative changes to predation produces individuals that escape to predators growing faster, or having structures (spines in daphnia if I remember well), or the mentioned example of harvest adaptation in fishes. However, what I do not see is that evolutionary forces can represent a different or a particular principle of population dynamics itself. I think that genetic can determine changes in the maximum per capita growth rate, changes in the equilibrium density (by changing individual energy requirements) or even genetic responses can change the feedback structure of a population dynamics, selection can favor genetic variants more resistant to predators or individuals best adapted to escape to them, then a population cycling in a predator-prey dynamics can switch to a first order oscillation by evolutive changes. However, what I can not see clearly is that evolutive forces represent a particular principle by itself in population dynamic theory.
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