On the environmental zero-point problem in microevolutionary climate response predictions

It is well documented that populations adapt to climate change by means of phenotypic plasticity, but few reports on adaptation by means of genetically based microevolution caused by selection. Disentanglement of these separate effects requires that the environmental zero-point is defined, and this should not be done arbitrarily. Together with parameter values, the zero-point can be estimated from environmental, phenotypic and fitness data. A prediction error method for this purpose is described, with the feasibility shown by simulations. An estimated environmental zero-point may have large errors, especially for small populations, but may still be a better choice than use of an initial environmental value in a recorded time series, or the mean value, which is often used. Another alternative may be to use the mean value of a past and stationary stochastic environment, which the population is judged to have been fully adapted to, in the sense that the mean fitness was at a global maximum. An exception is here cases with constant phenotypic plasticity, where the microevolutionary change per generation follows directly from phenotypic and environmental data, independent of the chosen environmental zero-point.

The stagnation paradox: the ever-improving but (more or less) stationary population fitness

Fisher's fundamental theorem states that natural selection improves mean fitness. Fitness, in turn, is often equated with population growth. This leads to an absurd prediction that life evolves to ever-faster growth rates, yet no one seriously claims generally slower population growth rates in the Triassic compared with the present day. I review here, using non-technical language, how fitness can improve yet stay constant (stagnation paradox), and why an unambiguous measure of population fitness does not exist. Subfields use different terminology for aspects of the paradox, referring to stasis, cryptic evolution or the difficulty of choosing an appropriate fitness measure; known resolutions likewise use diverse terms from environmental feedback to density dependence and ‘evolutionary environmental deterioration’. The paradox vanishes when these concepts are understood, and adaptation can lead to declining reproductive output of a population when individuals can improve their fitness by exploiting conspecifics. This is particularly readily observable when males participate in a zero-sum game over paternity and population output depends more strongly on female than male fitness. Even so, the jury is still out regarding the effect of sexual conflict on population fitness. Finally, life-history theory and genetic studies of microevolutionary change could pay more attention to each other.

Microevolutionary change in viscerocranial bones under congeneric sympatry in the Lake Tanganyikan cichlid genus Tropheus

The endemic Lake Tanganyika cichlid genus Tropheus lives at rocky shores all around the lake and comprises six species which are subdivided into about 120 morphologically similar but color-wise distinct populations. Typically, they live without a second Tropheus species, but there are some regions where two or even three sister species live in sympatry. We previously showed that there are morphological differences concerning head shape, eye size and insertion of fins among populations living alone compared to those living in sympatry with a second Tropheus. This study goes one step further to test if sympatry affects the shape of viscerocranial bones. By means of geometric morphometrics, we compare the shape of four bones among thirteen Tropheus populations, some of which in sympatry and some living alone. We quantify patterns of shape variation and estimate morphological disparity among the four bony elements in the study species and populations. We found consistent differences in the shape of one bony element among non-sympatric and sympatric populations, besides an extensive variation in the shape of viscerocranial bones within and among species. Furthermore, sexual dimorphism in Tropheus is clearly evident in the viscerocranial bones analyzed. We suggest that the relatively subtle morphological signal in sympatric vs. non-sympatric Tropheus populations is owed to the fact that the depth segregation does not yet represent a full shift in the trophic niche, albeit our data confirm that differences in ecologically relevant traits, such as bones of the preorbital region, play an important role in the process of niche separation and in the context of explosive diversification of cichlid fishes.

Multiple Mutualist Effects generate synergistic selection and strengthen fitness alignment in a tripartite interaction between legumes, rhizobia, and mycorrhizal fungi

Nearly all organisms interact with multiple mutualists, and complementarity within these complex interactions can result in synergistic fitness effects. However, it remains largely untested how multiple mutualists impact eco-evolutionary dynamics. We tested how multiple microbial mutualists-- N-fixing bacteria and mycorrrhizal fungi-- affected selection and heritability in their shared host plant (Medicago truncatula), as well as fitness alignment between partners. Our results demonstrate for the first time that multispecies mutualisms synergistically affect selection and heritability of host traits and enhance fitness alignment between mutualists. Specifically, we found that multiple mutualists doubled the strength of selection on a plant architectural trait, resulted in 2-3-fold higher heritability of reproductive success, and more than doubled the strength of fitness alignment between N-fixing bacteria and plants. Taken together, these findings show that synergism generated by multiple mutualisms extends to key components of microevolutionary change, and emphasizes the importance of multiple mutualist effects in understanding evolutionary trajectories.

Using genomic prediction to detect microevolutionary change of a quantitative trait

Detecting microevolutionary responses to natural selection by observing temporal changes in individual breeding values is challenging. The collection of suitable datasets can take many years and disentangling the contributions of the environment and genetics to phenotypic change is not trivial. Furthermore, pedigree-based methods of obtaining individual breeding values have known biases. Here, we apply a genomic prediction approach to estimate breeding values of adult weight in a 35-year dataset of Soay sheep (Ovis aries). During the study period adult body weight decreased, but the underlying genetic component of body weight increased, at a rate that is unlikely to be attributable to genetic drift. Thus cryptic microevolution of greater adult body weight has probably occurred. Using genomic prediction to study microevolution in wild populations can remove the requirement for pedigree data, potentially opening up new study systems for similar research.

The Growth Rate Hypothesis as a predictive framework for microevolutionary adaptation to selection for high population growth: an experimental test under phosphorus rich and phosphorus poor conditions

The growth rate hypothesis, a central concept of Ecological Stoichiometry, explains the frequently observed positive association between somatic growth rate and somatic phosphorus content (Psom) in organisms across a broad range of taxa. Here, we explore its potential in predicting intraspecific microevolutionary adaptation. For this, we subjected zooplankton populations to selection for fast population growth (PGR) in either a P-rich (HP) or P-poor (LP) food environment. With common garden transplant experiments we demonstrate evolution in HP populations towards increased PGR concomitant with an increase in Psom. In contrast we show that LP populations evolved higher PGR independently of Psom. We conclude that the GRH hypothesis has considerable value for predicting microevolutionary change, but that its application may be contingent on stoichiometric context. Our results highlight the potential of cryptic evolution in determining the performance response of field populations to elemental limitation of their food resources.

Urban Sexual Selection

Human-modified habitats can present both challenges and opportunities for wild animals. Changes in the environment caused by urbanization can affect who survives and reproduces in wild animal populations. Accordingly, we can expect that changes in sexual selection pressures may occur in response to urbanization. Changes in sexually selected traits like bird song and colouration have been one of the main thrusts of urban ecology in recent decades. However, studies to date have focused on describing changes in sexual phenotypes in response to urban environmental change, and knowledge about genetic/microevolutionary change is lacking. Also, while some signalling modalities have been well studied and linked to human activities (e.g., changes in auditory signals in response to anthropogenic noise), others have received comparatively less attention in this context (e.g., effects of air pollution on chemical signalling). In addition, the focus has been mainly on the signal sender, instead of the signal receiver, thereby missing an important side of sexual selection. This chapter reviews the evidence that sexual selection pressures and sexually selected traits have been impacted by urban environments, with attention to the potential for rapid adaptive and plastic shifts in traits of signallers and receivers. It explores the possibilities that urbanization causes evolutionary change and speciation in wild animal populations through sexual selection. Finally, it provides new ideas for future studies to explore these questions and especially the evolution of female preferences in urban environments.

The measurement of species selection on evolving characters

Many processes can contribute to macroevolutionary change. This fact is the source of the wide variety of macroevolutionary change across time and taxa as well as the bane of pale-obiological research trying to understand how macroevolution works. Here, I present a general framework for understanding the variety of macroevolutionary phenomena. Based on Price’s theorem, this framework provides a simple quantitative means to understand (1) the macroevolutionary processes that are possible and (2) the way those processes interact with each other. The major qualitative features of macroevolution depend first on the number of processes that co-occur and then on the magnitudes and evolutionary directions of those processes. Species selection, the major macroevolutionary process, consists of patterns of differential rates of speciation and extinction. Its macroevolutionary efficacy depends on the presences of sufficient microevolutionary change. Conversely, microevolutionary change is limited in power by the independent evolution of species, and species selection acting across populations of species can amplify or suppress microevolution. Non-trends may result if species selection sufficiently neutralizes microevolution and may yield stable macroevolutionary patterns over many millions of years.