Monday, November 22, 2010

Evolutionary causes and consequences of plasticity

Changes in phenotype directly induced by the environment, called phenotypic plasticity, can have strong evolutionary consequences. Recent papers from the Hendry lab have examined plasticity-evolution relationships (Crispo et al., 2010, EER 12: 47-66; Thibert-Plante & Hendry, in press, JEB).

Crispo et al. conducted a meta-analysis to determine whether phenotypic plasticity generally tends to evolve as a response to human-induced changes to the environment. They examined 20 studies in which plasticity was estimated between populations that were under the influence of anthropogenic stressors, and between closely related populations that had not. They found that it many cases, plasticity had evolved in response to anthropogenic disturbance, when compared to their non-disturbed counterparts. The direction of this change, however, varied greatly among taxa and trait types. For example, invertebrates often showed the evolution of increased plasticity in life history traits and decreased plasticity in morphology, whereas plants showed no trends in plasticity evolution. The authors therefore conclude that plasticity and its evolution might be important for adaptation, but that it should be examined on a case-by-case basis, rather than making general statements about whether increased or decreased plasticity is likely to evolve as an adaptive strategy. The full article can be found at http://www.evolutionary-ecology.com/abstracts/v12/n01/ffar2517.pdf.

Thibert-Plante and Hendry looked at the consequences of phenotypic plasticity on ecological speciation. The consequences of plasticity for ecological speciation depend on the timing of dispersal relative to the expression of the plasticity. On the one hand, if plasticity is expressed early in development, before any dispersal, the individuals dispersing to a different environment will have reduced fitness, relative to the case of no plasticity. On the other hand, if plasticity is expressed after dispersal, the fitness cost of dispersing can be greatly reduced or even completely removed. Those facts are of great importance in the context of ecological speciation, where we study the rise and fall of barriers to gene flow among populations. More details can be found at http://onlinelibrary.wiley.com/doi/10.1111/j.1420-9101.2010.02169.x/abstract.

Xavier Thibert-Plante (FQRNT and NIMBioS postdoctoral fellow, Knoxville)

Erika Crispo (NSERC postdoctoral fellow in Royal Ontario Museum, Toronto)

* The authors have no competing interest, apart from space in high profile scientific journals.

Thursday, November 18, 2010

Constraints on ecological speciation?

“Constraints on speciation suggested by comparing lake-stream stickleback divergence across two continents” – just appeared in Molecular Ecology (19:4963-4978). This paper tests whether the striking phenotypic (foraging morphology) and neutral genetic (microsatellite) divergence that characterizes incipient speciation across lake-stream transitions in Canadian (Vancouver Island) stickleback fish can also be found in some very young (150 years or less) European lake-stream pairs. The main findings are that, first, morphological divergence is generally much lower in the European population pairs, and there are striking overall phenotypic differences between the continents. Although alternative explanations are possible, it seems likely that there are some genetic constraints. Alleles that serve in adaptive lake-stream divergence (e.g. in body shape) in Canada appear to be absent in Europe. Limited time for morphological adaptation might also play a role. Second, there is only trivial divergence in microsatellite frequencies in the European pairs, contrary to the very high divergence found in some Canadian watersheds. This result suggests high gene flow and no evolved reproductive barriers in the young European systems. However, individual-based simulations tailored to these systems reveal that, even if reproductive barriers were absolute, it would be unlikely to see them with neutral markers. Those markers just evolve too slowly. It is thus possible that despite weak morphological divergence, reproductive isolation in Europe pairs could be strong, and hence we are simply dealing with the earliest stages of speciation. This could be possible if isolation is mediated by other traits not studied (e.g. behavior). Follow-up work is examining genome-wide divergence in these lake-stream pairs. This post was contributed by Daniel Berner.