A hybrid zone revisited: molecular and morphological analysis of the maintenance, movement, and evolution of a Great Plains avian (Cardinalidae: Pheuc

R. D. METTLER and G. M. SPELLMAN

KEYWORDS
Great Plains • hybrid zone • mitochondrial DNA • nuclear loci • Pheucticus • tension zone

ABSTRACT

Black-headed grosbeaks (Pheucticus melanocephalus) and rose-breasted grosbeaks (Pheucticus ludovicianus) are passerine bird species known to hybridize in the Great Plains of North America. Both extrinsic (environmental) and intrinsic factors (pre- and postzygotic reproductive isolation) have been credited for the generation and maintenance of the grosbeak hybrid zone, but little is known about the genetic characteristics of this hybrid zone. To investigate the stability and extent of the grosbeak hybrid zone, we constructed clines from both molecular sequence data (mtDNA, three autosomal intron loci, and one Z-linked locus) and morphological data (morphometric analyses and hybrid index scores) to determined zone centre and width. Hybrid zone centre and width were also determined for samples collected across the zone 40 years ago from morphological data. The present and past clines were compared and provided support for stability in hybrid zone location and width, and the evolutionary implications of this are discussed. Three models of hybrid zone maintenance were investigated to consider the influence of intrinsic and extrinsic factors on this zone. Our results suggest low hybrid frequencies, a stable zone location and narrow width, and reduced hybrid fitness over the past 40 years best categorize the grosbeak hybrid zone as a tension zone.

http://www3.interscience.wiley.com/journal/122505418/abstract

Morphology, performance, fitness:

Functional insight into a post-Pleistocene radiation of mosquitofish

R. Brian Langerhans*
Biology Letters 2009;5 488-491

Bahamas mosquitofish (Gambusia hubbsi) colonized blue holes during the past approximately 15 000 years and exhibit relatively larger caudal regions in blue holes that contain piscivorous fish. It is hypothesized that larger caudal regions enhance fast-start escape performance and thus reflect an adaptation for avoiding predation. Here I test this hypothesis using a three-pronged, experimental approach. First, G. hubbsi from blue holes with predators were found to possess both greater fast-start performance and greater survivorship in the presence of predatory fish. Second, using individual-level data to investigate the morphology–performance–fitness pathway, I found that (i) fish with larger caudal regions produced higher fast-start performance and (ii) fish with higher fast-start performance enjoyed greater survivorship in the presence of fish predators—trends consistently observed across both predator regimes. Finally, I found that morphological divergence between predator regimes at least partially reflects genetic differentiation, as differences were retained in fish raised in a common laboratory environment. These results suggest that natural selection favours increased fast-start performance in the presence of piscivorous fish, consequently driving the evolution of larger caudal regions. Combined with previous work, this provides functional insight into body shape divergence and ecological speciation among Bahamian blue holes.

PNAS special issue on Darwin

Check out the June 16 issue of PNAS: In the Light of Evolution III: Two Centuries of Darwin Sackler Colloquium. Lots of good stuff here, including Nick and Adam's paper on sexual selection, genomics of ecological speciation, etc. Looking forward to more posts as everyone comes back to higher bandwidth!

Ecological context shapes hybridization dynamics

Molecular Ecology (Volume 18 Issue 10, Pages 2077 - 2079)

PERSPECTIVE
Ecological context shapes hybridization dynamics

C. ALEX BUERKLE

ABSTRACT

Gene exchange among oak species (Quercus) in Europe is known to be pervasive and to complicate population genetic studies of this species complex. A study in this issue of Molecular Ecology adds geographical and stand-level resolution to the patterns of genetic variation among four species and documents the relatively high frequency of hybrids (10.7–30.5% of trees in a population, including hybrids between all pairs of species; Lepais et al. 2009). In addition, the authors show that the relative abundance of parental species affects the genetic composition of hybrids and shifts the average direction of introgression. Variation in the relative abundance of parental species is one example of how the ecological context of hybridization can influence the dynamics and outcome of contact between species and represents an opportunity to investigate the components of reproductive isolation between species. This research raises several questions about the dynamics of hybridization in this well-studied species complex, and highlights methodological and conceptual issues associated with contemporary research on hybridization.

The impact of environmental heterogeneity on genetic architecture in a wild population of Soay sheep

Matthew R. Robinson, AJ Wilson, JG Pilkington TH Clutton-Brock, JM Pemberton and LEB Kruuk.
Genetics 181: 1639-1648

Finally a paper in Genetics that I can understand, and it presents an intriguing idea. It caught my eye, because a friend of mine recently asked me if I knew of any examples of sexual dimorphism driven by natural selection. I couldn't think of any, at that time, and also was wondering how that could work. Well... this is perhaps how. From the giant database of the Soay sheep on that cold and windswept island of St Kilda. Be aware of complicated stats...


This work demonstrates that environmental conditions experienced by individuals can shape their development and affect the stability of genetic associations. The implication of this observation is that the environmental response may influence the evolution of traits in the wild. Here, we examined how the genetic architecture of a suite of sexually dimorphic traits changed as a function of environmental conditions in an unmanaged population of Soay sheep (Ovis aries) on the island of Hirta, St. Kilda, northwest Scotland. We examined the stability of phenotypic, genetic, and environmental (residual) covariance in males during the first year of life between horn length, body weight, and parasite load in environments of different quality. We then examined the same covariance structures across environments within and between the adult sexes. We found significant genotype-by-environment interactions for lamb male body weight and parasite load, leading to a change in the genetic correlation among environments. Horn length was genetically correlated with body weight in males but not females and the genetic correlation among traits within and between the sexes was dependent upon the environmental conditions experienced during adulthood. Genetic correlations were smaller in more favorable environmental conditions, suggesting that in good environments, loci are expressed that have sex-specific effects. The reduction in genetic correlation between the sexes may allow independent evolutionary trajectories for each sex. This study demonstrates that the genetic architecture of traits is not stable under temporally varying environments and highlights the fact that evolutionary processes may depend largely upon ecological conditions.

Alternative mating strategies may favour the persistence of a genetically based colour polymorphism in a pentamorphic fish

Jorge L. Hurtado-Gonzales and J. Albert C. Uy


Males of the pentamorphic fish, Poecilia parae, show a striking colour polymorphism that ranges from drab coloration resembling females to brilliant black and red stripes. We tested the hypothesis that the observed polymorphism may persist because each morph uses a unique but equally successful mating strategy. We quantified the mating behaviour, testes investment and sperm morphometrics of each morph. We found that the smallest morph (immaculata), which resembles drab females, used a ‘sneaker’ tactic (i.e. forgoing courtship and mostly ‘stealing’ copulations) as a mating strategy. Immaculata males also had larger testes, and produced larger ejaculates and sperm with longer flagella, which should provide advantages during sperm competition. The largest morph (parae) and the coloured blue, red and yellow morphs used a combination of courtship and sneaker tactics. However, they achieved greater copulation success as a result of courtships. The testes investment of parae and yellow morphs were similar but larger than those of the blue and red males. Our results suggest that male morphs of the pentamorphic Poecilia parae use unique mating strategies that exploit different aspects of pre- and postcopulatory sexual selection, which, in turn, may help maintain their relatively stable frequencies in the wild.

Keywords: alternative mating strategy; frequency-dependent selection; genetic polymorphism; Poecilia parae; sperm competition

link from campus

Sex chromosome evolution

Accelerated Adaptive Evolution on a Newly Formed X Chromosome Bachtrog D, Jensen JD, Zhang Z PLoS Biology Vol. 7, No. 4, e82 doi:10.1371/journal.pbio.1000082

I'm finally getting back on this after my computer crash. Where is everybody?