Environmental Coupling of Selection and Heritability Limits Evolution
An article in the July issue of PLOS Biology details a study on environmental selection effects which came up with a rather interesting and almost counter-intuitive conclusion. The abstract:
There has recently been great interest in applying theoretical quantitative genetic models to empirical studies of evolution in wild populations. However, while classical models assume environmental constancy, most natural populations exist in variable environments. Here, we applied a novel analytical technique to a long-term study of birthweight in wild sheep and examined, for the first time, how variation in environmental quality simultaneously influences the strength of natural selection and the genetic basis of trait variability. In addition to demonstrating that selection and genetic variance vary dramatically across environments, our results show that environmental heterogeneity induces a negative correlation between these two parameters. Harsh environmental conditions were associated with strong selection for increased birthweight but low genetic variance, and vice versa. Consequently, the potential for microevolution in this population is constrained by either a lack of heritable variation (in poor environments) or by a reduced strength of selection (in good environments). More generally, environmental dependence of this nature may act to limit rates of evolution, maintain genetic variance, and favour phenotypic stasis in many natural systems. Assumptions of environmental constancy are likely to be violated in natural systems, and failure to acknowledge this may generate highly misleading expectations for phenotypic microevolution.
Of course, there’s nothing interesting or counter-intuitive about this:
Assumptions of environmental constancy are likely to be violated in natural systems, and failure to acknowledge this may generate highly misleading expectations for phenotypic microevolution.
That goes without saying: life in the field is more varying and harsh than life in, e.g., a beaker.
As for this
environmental dependence of this nature may act to limit rates of evolution, maintain genetic variance, and favour phenotypic stasis in many natural systems.
At least … there appears to be some evidence that this holds true for a breed of sheep phenotypes over short time periods in a very limited subset of “harsh environments.”
But we knew already that in most living organisms, harsh environments lead to stress which
tends to be mutagenic which, of course, increases the rate of adaptation aka “microevolution.”
For the record, I am continually amazed at how poorly most scientists write and how eager
they are to make their work sound more groundbreaking than it actually is. There are obvious reasons for these deficiencies but … I remain amazed.
Comment by Amy Lester — June 16, 2006 @ 3:07 am
Freawaru,
This highlights a catch-22: strong selection limits variation and therefore upward evolution, therefore the weaker the selection the better the chance for evolution in one sense! I’ve often pointed out the pro-selection view has fallen somewhat out of favor…
This has not been lost upon respected biologist Stanley Salthe. Salthe (who is not pro-ID) foresaw this problem based on Fisher’s fundamental theorem of Natural Selection. Fisher is one of the few Darwinists (along with Haldane) who are lauded by IDers because their work is scientifically and mathematically sensible.
Salthe’s take on the fundamental theorem of natural selection and Darwinism in general can be found in this wonderful article:
Critique of Natural Selection
I love population genetics. Have a great weekend, and I’ll see you all next week.
Salvador
Comment by Salvador T. Cordova — June 16, 2006 @ 5:01 pm
Very interesting data. Yes, when selective pressures are strong, variation will be reduced. As such natural evolutionary change may be slower than that obtained from data which do not assume variability. Interestingly enough, while selection may reduce variability in one area, this need not mean that genetic variation in other areas in reduced as well. Persistent harsh environments may cause genetic variation to be ‘depleted’. The study seems to still be based on environmental constancy. Environments when change will have the greatest potential for evolution since it can now reduce the genetic variability which is what evolution is all about. Continued lack of variation in the environment will deplete variation in the genetics, that seems self evident to me but perhaps it need not be.
Two populations separated in two different environments can evolve into distinct species. What the study has shown is that the parameters for evolution vary in different environments which by itself is quite fascinating and expected. Theoretical quantitative genetics is moving from ’static’ models into more realistic models. Next step is to look at environments while they change and determine the effects on evolutionary parameters.
Comment by PvM — June 16, 2006 @ 6:13 pm
Now we get to some interesting issues:
Stasis at the Phenotype level does not imply stasis at the genotype level. Check out Environmental Stress as an Evolutionary Force by ARY A. HOFFMANN, MIRIAM J. HERCUS in BioScience Volume 50, Issue 3 (March 2000) which addresses the effects of various mechanisms.
“periodically stressful conditions may influence evolutionary rates by generating and maintaining variability and by overcoming adaptation limits caused by gene flow, helping to explain diversification patterns in the fossil record”.
Personally I believe that the nature of evolvability and neutrality have been significantly underestimated especially since these mechanisms naturally explain stasis as well as rapid evolution.
Comment by PvM — June 16, 2006 @ 7:28 pm
This really contrasts the maturity of evolutionary biology with the scientific vacuity of ID.
IDers are stuck on “that flagellum thingy could not have evolved, it was designed, but we can’t say how or by who or what or when or why. Since we can’t, we won’t bother with research, but we will sell a lot of books filled with impressive mathematical formula based on false premises”.
Evolutionary biologists are down in the trenches, building complex mathematical models which help us quantify the possible rate of evolution under various scenarios, looking at real data, etc.
Comment by Don Baccus — June 16, 2006 @ 8:04 pm
PvM:
Actually, the key point in the paper is that the relationship between selection and (genetic) variation is not causal in this case, rather it is an outcome of both being dependent on environmnetal conditions. In a different system it could certainly be possible for evolutionary rates to be faster that those estimated when environmental variability is assumed. For example, in some invertebrates there is some evidence for a trend towards higher heritability in tougher environments. Assuming that selection is still stronger in harsh environments then this would be a positive relationship between selection and genetic variation which could accelerate evolutionary rates.
PvM:
Thats not necessarily true. Selection is not about fitness per se, it is about the relationship between fitness and phenotype.
PvM:
The study focused on microevolutionary changes, not macroevolutionary changes. It would be wrong to extrapolate any conclusions here to macroevolutionary processes like speciation which involve are proposed to involve additional processes (restriction of gene flow, etc) and where the environmental heterogeneity might more commonly be spatial rather than temporal.
Comment by Guts — June 28, 2006 @ 4:17 pm
LOL. The study was actualy pretty limited.
Comment by Guts — June 28, 2006 @ 5:20 pm