Correlational selection and the evolution of genomic architecture

Spatial Scale and Temporal Component of Selection in Side‐Blotched Lizards

Spatial variation in selection has long been recognized as promoting population divergence and in maintaining genetic polymorphisms, but selection at a fine spatial scale is seldom measured directly. We analyzed spatial and temporal variation in selective regimes on egg size using long-term population data of the side-blotched lizard (Uta stansburiana). Juvenile survival rates varied between years at a small spatial scale that was reflected as a strong interaction between the local neighborhood level and year. Spatially and temporally variable selection acted jointly on egg mass, which presumably would facilitate the maintenance of high additive genetic variance for this trait. Local selection gradients calculated at the neighborhood level were significantly correlated with the annual global selection gradients calculated at the metapopulation level. However, there was substantial variance in these local selective regimes, which suggests that strong local selection could go undetected if the analysis was limited to the global level. We also investigated the degree of spatial synchronization among outcrop in local selection gradients. The degree of synchrony was higher among later-clutch hatchlings than among first-clutch hatchlings, and we suggest that more intense density- and frequency-dependent selection on egg size later in the season is responsible for this effect.

What have two decades of laboratory life-history evolution studies on Drosophila melanogaster taught us?

A series of laboratory selection experiments on Drosophila melanogaster over the past two decades has provided insights into the specifics of life-history tradeoffs in the species and greatly refined our understanding of how ecology and genetics interact in life-history evolution. Much of what has been learnt from these studies about the subtlety of the microevolutionary process also has significant implications for experimental design and inference in organismal biology beyond life-history evolution, as well as for studies of evolution in the wild. Here we review work on the ecology and evolution of life-histories in laboratory populations of D. melanogaster, emphasizing how environmental effects on life-history-related traits can influence evolutionary change. We discuss life-history tradeoffs - many unexpected - revealed by selection experiments, and also highlight recent work that underscores the importance to life-history evolution of cross-generation and cross-life-stage effects and interactions, sexual antagonism and sexual dimorphism, population dynamics, and the possible role of biological clocks in timing life-history events. Finally, we discuss some of the limitations of typical selection experiments, and how these limitations might be transcended in the future by a combination of more elaborate and realistic selection experiments, developmental evolutionary biology, and the emerging discipline of phenomics.

Variable selection in Platanthera bifolia (Orchidaceae): phenotypic selection differed between sex functions in a drought year

We estimated selection on three morphological characters in the hermaphroditic, hawkmoth-pollinated orchid Platanthera bifolia and explored selection surfaces through male and female function. The work was carried out in northern Sweden during two flowering seasons (1994 and 1995) in one natural population and one season (1995) in another natural population. Fitness was estimated as number of pollinia removed (male function) and number of fruits produced (female function). We detected directional selection towards larger inflorescence size (flower number) through both sex functions in both populations in 1995. In 1994, with an unusually dry growing season, 78% of the individuals failed to set any fruit, and there was selection for larger inflorescences only through male function. In this year, there was selection towards longer flower spurs, which could be a direct or indirect effect of spurs being shortened by drought. The results demonstrate that selection patterns may vary temporally and spatially, and that the 'male function hypothesis' may be applicable as female function is more resource dependent than male function.

Extreme polymorphism in a Y-linked sexually selected trait

Males of the livebearing fish, Poecilia parae, exhibit one of the most complex polymorphisms known to occur within populations, whereas females are monomorphic. We describe five distinct male colour morphs and an associated size dimorphism, and demonstrate through pedigree analysis that the locus or loci controlling the male colour polymorphism is linked to the Y-chromosome. Field surveys from 1999 to 2002 of nine populations in Guyana and Suriname, South America, indicate that some morphs are consistently abundant and others are rare, implying that the colour polymorphism has important fitness consequences. By rearing offspring of field-inseminated females, we showed that the common morph is also the most successful morph in terms of reproduction. However, dichotomous choice tests show that two rare morphs are preferred by females over the common morph. These results suggest that alternative male mating strategies, sperm competition, overt male-male competition, or other processes are overriding female preferences in these populations. Furthermore, Y-linkage of the colour polymorphism in P. parae supports the hypothesis that heterogametic sex chromosomes harbour sexually antagonistic traits beneficial to the heterogametic sex.

Behavioral drive versus behavioral inertia in evolution: a null model approach

Some biologists embrace the classical view that changes in behavior inevitably initiate or drive evolutionary changes in other traits, yet others note that behavior sometimes inhibits evolutionary changes. Here we develop a null model that quantifies the impact of regulatory behaviors (specifically, thermoregulatory behaviors) on body temperature and on performance of ectotherms. We apply the model to data on a lizard (Anolis cristatellus) and show that thermoregulatory behaviors likely inhibit selection for evolutionary shifts in thermal physiology with altitude. Because behavioral adjustments are commonly used by ectotherms to regulate physiological performance, regulatory behaviors should generally constrain rather than drive evolution, a phenomenon we call the "Bogert effect." We briefly review a few other examples that contradict the classical view of behavior as the inevitable driving force in evolution. Overall, our analysis and brief review challenge the classical view that behavior is invariably the driving force in evolution, and instead our work supports the alternative view that behavior has diverse--and sometimes conflicting--effects on the directions and rates at which other traits evolve.