Local adaptation within a hybrid species.

Ecological divergence among populations may be strongly influenced by their genetic background. For instance, genetic admixture through introgressive hybridization or hybrid speciation is likely to affect the genetic variation and evolvability of phenotypic traits. We studied geographic variation in two beak dimensions and three other phenotypic traits of the Italian sparrow (Passer italiae), a young hybrid species formed through interbreeding between house sparrows (P. domesticus) and Spanish sparrows (P. hispaniolensis). We found that beak morphology was strongly influenced by precipitation regimes and that it appeared to be the target of divergent selection within Italian sparrows. Interestingly, however, the degree of parental genetic contribution in the hybrid species had no effect on phenotypic beak variation. Moreover, beak height divergence may mediate genetic differentiation between populations, consistent with isolation-by-adaptation within this hybrid species. The study illustrates how hybrid species may be relatively unconstrained by their admixed genetic background, allowing them to adapt rapidly to environmental variation.

Single origin of human commensalism in the house sparrow.

The current, virtually worldwide distribution of the house sparrow (Passer domesticus) is a result of its commensal relationship with humans. It has been suggested that long before the advent of agriculture, an early glacial advance resulted in two disjunct ranges of ancestral house sparrows - one in the Middle East and another on the Indian subcontinent. Differentiation during this period of isolation resulted in two major groups of subspecies: the domesticus group and the indicus group. According to this hypothesis, commensalism with humans would have evolved independently in the two regions and at least twice. An alternative hypothesis is that morphological differences between the subspecies represent very recent differentiation, following expansions from a single source. To test between these hypotheses, we analysed genetic variation at the mitochondrial DNA control region and at three nuclear loci from several house sparrow populations in Europe, Asia and North Africa. No differentiation between the indicus and domesticus groups was found, supporting the single origin hypothesis. One of the subspecies in the indicus group, P. d. bactrianus, differs ecologically from other house sparrows in being migratory and in preferentially breeding in natural habitat. We suggest that bactrianus represents a relict population of the ancestral, noncommensal house sparrow. When agricultural societies developed in the Middle East about 10 000 years ago, a local house sparrow population of the bactrianus type adapted to the novel environment and eventually became a sedentary, human commensal. As agriculture and human civilizations expanded, house sparrows experienced a correlated and massive expansion in range and numbers. The pattern of genetic variation analysed here is consistent with this scenario.

Inferring local adaptation from QST-FST comparisons: neutral genetic and quantitative trait variation in European populations of great snipe.

We applied a phenotypic QST (PST) vs. FST approach to study spatial variation in selection among great snipe (Gallinago media) populations in two regions of northern Europe. Morphological divergence between regions was high despite low differentiation in selectively neutral genetic markers, whereas populations within regions showed very little neutral divergence and trait differentiation. QST > FST was robust against altering assumptions about the additive genetic proportions of variance components. The homogenizing effect of gene flow (or a short time available for neutral divergence) has apparently been effectively counterbalanced by differential natural selection, although one trait showed some evidence of being under uniform stabilizing selection. Neutral markers can hence be misleading for identifying evolutionary significant units, and adopting the PST-FST approach might therefore be valuable when common garden experiments is not an option. We discuss the statistical difficulties of documenting uniform selection as opposed to divergent selection, and the need for estimating measurement error. Instead of only comparing overall QST and FST values, we advocate the use of partial matrix permutation tests to analyse pairwise QST differences among populations, while statistically controlling for neutral differentiation.

Direct benefits and costs for hybridizing Ficedula flycatchers.

It is well understood that females may gain direct benefits from breeding with attractive males. However, the direct fitness effects of mate-choice are rarely considered with respect to mating between different species (hybridization), a field dominated by discussion of indirect costs of producing unfit hybrid offspring. Hybridizing females may also gain by the types of direct benefits that are important for intraspecific mate choice, and in addition may have access to certain benefits that are restricted to mating with males of an ecologically diverged sister-taxon. We investigate possible direct benefits and costs female Ficedula flycatchers gain from breeding with a heterospecific male, and demonstrate that hybridizing female collared flycatchers (F. albicollis) breed in territories that do not suffer the seasonal decline in habitat quality experienced by females breeding with conspecifics. We exclude the hypotheses that heterospecific males provide alternative food-types or assume a greater amount of the parental workload. In fact, the diets of the two species (F. albicollis and F. hypoleuca) were highly similar, suggesting possible interspecific competition over food resources in sympatry. We discuss the implications of direct fitness effects of hybridization, and why there has been such a disparity in the attention paid to such benefits and costs with regard to intraspecific and interspecific mate-choice.

Song similarity predicts hybridization in flycatchers.

Given that population divergence in sexual signals is an important prerequisite for reproductive isolation, a key prediction is that cases of signal convergence should lead to hybridization. However, empirical studies that quantitatively demonstrate links between phenotypic characters of individuals and their likelihood to hybridize are rare. Here we show that song convergence between sympatric pied (Ficedula hypoleuca) and collared flycatchers (F. albicollis) influence social and sexual interactions between the two species. In sympatry, the majority of male pied flycatchers (65%) include various parts of collared flycatcher song in their song repertoire (but not vice versa). Playback experiments on male interactions demonstrate that male collared flycatchers respond similarly to this 'mixed' song as to conspecific song. Long-term data on pairing patterns show that males singing a converged song attract females of the other species: female collared flycatchers only pair with male pied flycatchers if the males sing the mixed song type. From the perspective of a male pied flycatcher, singing a mixed song type is associated with 30% likelihood of hybridization. This result, combined with our estimates of the frequency of mixed singers, accurately predicts the observed occurrence of hybridization among male pied flycatchers in our study populations (20.45% of 484 pairs; predicted 19.5%). Our results support the suggestion that song functions as the most important prezygotic isolation mechanism in many birds.

Major histocompatibility complex variation and mate choice in a lekking bird, the great snipe (Gallinago media).

Genes of the major histocompatibility complex (MHC) play a major part in the activation of the vertebrate immune system. In addition, they also appear to function as cues for mate choice. In mammals especially, several kinds of MHC-dependent mate choice have been hypothesized and observed. These include choice of mates that share no or few alleles with the choosing individual, choice of mates with alleles that differ as much as possible from the choosing individual, choice of heterozygous mates, choice of certain genotypes and choice of rare alleles. We investigated these different aspects of mate choice in relation to MHC in a lekking bird species, the great snipe (Gallinago media). We found no evidence for MHC disassortative mating, no preference for males with many MHC alleles and no preference for rare alleles. However, we did find that some allelic lineages were more often found in males with mating success than in males without mating success. Females do not seem to use themselves as references for the MHC-dependent mate choice, rather they seem to prefer males with certain allele types. We speculate that these alleles may be linked to resistance to common parasites.

Male mate choice, sexual conflict and strategic allocation of copulations in a lekking bird.

The males of lekking species are not expected to be choosy about mating because a reduced reproductive rate due to lost mating opportunities should outweigh any benefits of male choice. Females have traditionally not been expected to be competitive in this system since their reproduction has usually been assumed to be unconstrained by male availability. Here we show that, in contrast to these predictions, males are choosy and females may be competitive in the lekking great snipe Gallinago media. Males preferred by many females often refused to copulate with and even chased away females that the male had already copulated with, whereas females seemed to compete for repeated copulations. We conclude that choosiness may sometimes pay for popular males in those lekking species where females copulate repeatedly. Apparently, evolutionary conflicts of interest between individuals may cause a richer repertoire of behavioural adaptations than, to our knowledge, hitherto realized.