Evolutionary and plastic rescue in multitrophic model communities

Under changing environmental conditions, intraspecific variation can potentially rescue populations from extinction. There are two principal sources of variation that may ultimately lead to population rescue: genetic diversity and phenotypic plasticity. We compared the potential for evolutionary rescue (through genetic diversity) and plastic rescue (through phenotypic plasticity) by analysing their differential ability to produce dynamical stability and persistence in model food webs. We also evaluated how rescue is affected by the trophic location of variation. We tested the following hypotheses: (i) plastic communities are more likely to exhibit stability and persistence than communities in which genetic diversity provides the same range of traits. (ii) Variation at the lowest trophic level promotes stability and persistence more than variation at higher levels. (iii) Communities with variation at two levels have greater probabilities of stability and persistence than communities with variation at only one level. We found that (i) plasticity promotes stability and persistence more than genetic diversity; (ii) variation at the second highest trophic level promotes stability and persistence more than variation at the autotroph level; and (iii) more than variation at two trophic levels. Our study shows that proper evaluation of the rescue potential of intraspecific variation critically depends on its origin and trophic location.     

PERSPECTIVE: GENETIC ASSIMILATION AND A POSSIBLE EVOLUTIONARY PARADOX: CAN MACROEVOLUTION SOMETIMES BE SO FAST AS TO PASS US BY?

Abstract.— The idea of genetic assimilation, that environmentally induced phenotypes may become genetically fixed and no longer require the original environmental stimulus, has had varied success through time in evolutionary biology research. Proposed by Waddington in the 1940s, it became an area of active empirical research mostly thanks to the efforts of its inventor and his collaborators. It was then attacked as of minor importance during the \"hardening\" of the neo-Darwinian synthesis and was relegated to a secondary role for decades. Recently, several papers have appeared, mostly independently of each other, to explore the likelihood of genetic assimilation as a biological phenomenon and its potential importance to our understanding of evolution. In this article we briefly trace the history of the concept and then discuss theoretical models that have newly employed genetic assimilation in a variety of contexts. We propose a typical scenario of evolution of genetic assimilation via an intermediate stage of phenotypic plasticity and present potential examples of the same. We also discuss a conceptual map of current and future lines of research aimed at exploring the actual relevance of genetic assimilation for evolutionary biology.     

Evolution of growth by genetic accommodation in Icelandic freshwater stickleback

Classical Darwinian adaptation to a change in environment can ensue when selection favours beneficial genetic variation. How plastic trait responses to new conditions affect this process depends on how plasticity reveals to selection the influence of genotype on phenotype. Genetic accommodation theory predicts that evolutionary rate may sharply increase when a new environment induces plastic responses and selects on sufficient genetic variation in those responses to produce an immediate evolutionary response, but natural examples are rare. In Iceland, marine threespine stickleback that have colonized freshwater habitats have evolved more rapid individual growth. Heritable variation in growth is greater for marine full-siblings reared at low versus high salinity, and genetic variation exists in plastic growth responses to low salinity. In fish from recently founded freshwater populations reared at low salinity, the plastic response was strongly correlated with growth. Plasticity and growth were not correlated in full-siblings reared at high salinity nor in marine fish at either salinity. In well-adapted lake populations, rapid growth evolved jointly with stronger plastic responses to low salinity and the persistence of strong plastic responses indicates that growth is not genetically assimilated. Thus, beneficial plastic growth responses to low salinity have both guided and evolved along with rapid growth as stickleback adapted to freshwater.     

Camouflaged or tanned: plasticity in freshwater snail pigmentation

By having phenotypically plastic traits, many organisms optimize their fitness in response to fluctuating threats. Freshwater snails with translucent shells, e.g. snails from the Radix genus, differ considerably in their mantle pigmentation patterns, with snails from the same water body ranging from being completely dark pigmented to having only a few dark patterns. These pigmentation differences have previously been suggested to be genetically fixed, but we propose that this polymorphism is owing to phenotypic plasticity in response to a fluctuating environment. Hence, we here aimed to assess whether common stressors, including ultraviolet radiation (UVR) and predation, induce a plastic response in mantle pigmentation patterns of Radix balthica. We show, in contrast to previous studies, that snails are plastic in their expression of mantle pigmentation in response to changes in UVR and predator threats, i.e. differences among populations are not genetically fixed. When exposed to cues from visually hunting fish, R. balthica increased the proportion of their dark pigmentation, suggesting a crypsis strategy. Snails increased their pigmentation even further in response to UVR, but this also led to a reduction in pattern complexity. Furthermore, when exposed to UVR and fish simultaneously, snails responded in the same way as in the UVR treatment, suggesting a trade-off between photoprotection and crypsis.     

Response of 19 cultivars of soybeans to ultraviolet-B irradiance

Nineteen soybean cultivars were grown for four weeks in controlled environmental chambers with artificial daylight supplemented by five UV-B irradiance regimes to determine the range of growth and development responses of seedlings. Data from nine plant characteristics were assessed: leaf area, dry weight of leaves, stems and roots, total plant dry weight, height, ratio of roots to shoots and leaf area to weight and rating of leaves for damage. Significant differences were observed in the responses noted. Stunting, leaf chlorosis and loss of apical dominance were three general symptoms apparent on all cultivars which received UV-B irradiance. Varying degrees of reduced leaf area and dry weight of the plants and altered ratios of weights of leaves per unit area and weight of roots to shoots were also found. It was concluded that different soybean cultivars demonstrate a marked difference in sensitivity to UV-B radiation under the artificial conditions of controlled environmental growth chambers and this may indicate a genetic basis for variability in sensitivity of soybean cultivars to this waveband. However, the sensitivity to UV-B radiation was increased by the lower than normal photon fluence of photosynthetically active radiation (225 μE m⁻² s⁻¹).     

Does rapid evolution matter? Measuring the rate of contemporary evolution and its impacts on ecological dynamics

Rapid contemporary evolution due to natural selection is common in the wild, but it remains uncertain whether its effects are an essential component of community and ecosystem structure and function. Previously we showed how to partition change in a population, community or ecosystem property into contributions from environmental and trait change, when trait change is entirely caused by evolution (Hairston et al. 2005). However, when substantial non-heritable trait change occurs (e.g. due to phenotypic plasticity or change in population structure) that approach can mis-estimate both contributions. Here, we demonstrate how to disentangle ecological impacts of evolution vs. non-heritable trait change by combining our previous approach with the Price Equation. This yields a three-way partitioning into effects of evolution, non-heritable phenotypic change and environment. We extend the approach to cases where ecological consequences of trait change are mediated through interspecific interactions. We analyse empirical examples involving fish, birds and zooplankton, finding that the proportional contribution of rapid evolution varies widely (even among different ecological properties affected by the same trait), and that rapid evolution can be important when it acts to oppose and mitigate phenotypic effects of environmental change. Paradoxically, rapid evolution may be most important when it is least evident.