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Assortative flocking in crossbills and implications for ecological speciation.

ABSTRACT: How reproductive isolation is related to divergent natural selection is a central question in speciation. Here, we focus on several ecologically specialized taxa or 'call types' of red crossbills (Loxia curvirostra complex), one of the few groups of birds providing much evidence for ecological speciation. Call types differ in bill sizes and feeding capabilities, and also differ in vocalizations, such that contact calls provide information on crossbill phenotype. We found that two call types of red crossbills were more likely to approach playbacks of their own call type than those of heterotypics, and that their propensity to approach heterotypics decreased with increasing divergence in bill size. Although call similarity also decreased with increasing divergence in bill size, comparisons of responses to familiar versus unfamiliar call types indicate that the decrease in the propensity to approach heterotypics with increasing divergence in bill size was a learned response, and not a by-product of calls diverging pleiotropically as bill size diverged. Because crossbills choose mates while in flocks, assortative flocking could lead indirectly to assortative mating as a by-product. These patterns of association therefore provide a mechanism by which increasing divergent selection can lead to increasing reproductive isolation.

SUBMITTER: Smith JW

PROVIDER: S-EPMC3441086 | biostudies-literature | 2012 Oct

REPOSITORIES: biostudies-literature

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Assortative flocking in crossbills and implications for ecological speciation.

Smith Julie W JW Sjoberg Stephanie M SM Mueller Matthew C MC Benkman Craig W CW

Proceedings. Biological sciences 20120822 1745

How reproductive isolation is related to divergent natural selection is a central question in speciation. Here, we focus on several ecologically specialized taxa or 'call types' of red crossbills (Loxia curvirostra complex), one of the few groups of birds providing much evidence for ecological speciation. Call types differ in bill sizes and feeding capabilities, and also differ in vocalizations, such that contact calls provide information on crossbill phenotype. We found that two call types of r ...[more]

PMID: 22915674

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Project description:Cultural evolution may accelerate population divergence and speciation, though most support for this hypothesis is restricted to scenarios of allopatric speciation driven by random cultural drift. By contrast, the role of cultural evolution in non-allopatric speciation (i.e. speciation with gene flow) has received much less attention. One clade in which cultural evolution may have figured prominently in speciation with gene flow includes the conifer-seed-eating finches in the red crossbill (Loxia curvirostra) complex. Here we focus on Cassia crossbills (Loxia sinesciuris; an ecotype recently split taxonomically from red crossbills) that learn social contact calls from their parents. Previous work found that individuals modify their calls throughout life such that they become increasingly divergent from a closely related, sympatric red crossbill ecotype. This open-ended modification of calls could lead to character displacement if it causes population-level divergence in call structure that, in turn, reduces (maladaptive) heterospecific flocking. Heterospecific flocking is maladaptive because crossbills use public information from flockmates to assess resource quality, and feeding rates are depressed when flockmates differ in their ability to exploit a shared resource (i.e. when flockmates are heterospecifics). We confirm the predictions of character displacement by documenting substantial population-level divergence in Cassia crossbill call structure over just two decades and by using field experiments to demonstrate that Cassia and red crossbills differentially respond to these evolved differences in call structure, reducing heterospecific flock formation. Moreover, because crossbills choose mates from within flocks, a reduction in heterospecific flocking should increase assortative mating and may have been critical for speciation of Cassia crossbills in the face of ongoing gene flow in as few as 5000 years. Our results provide evidence for a largely neglected yet potentially widespread mechanism by which reproductive isolation can evolve between sympatric lineages as a byproduct of adaptive cultural evolution.

Project description:Speciation among members of the Anopheles gambiae complex is thought to be promoted by disruptive selection and ecological divergence acting on sets of adaptation genes protected from recombination by polymorphic paracentric chromosomal inversions. However, shared chromosomal polymorphisms between the M and S molecular forms of An. gambiae and insufficient information about their relationship with ecological divergence challenge this view. We used Geographic Information Systems, Ecological Niche Factor Analysis, and Bayesian multilocus genetic clustering to explore the nature and extent of ecological and chromosomal differentiation of M and S across all the biogeographic domains of Cameroon in Central Africa, in order to understand the role of chromosomal arrangements in ecological specialisation within and among molecular forms.Species distribution modelling with presence-only data revealed differences in the ecological niche of both molecular forms and the sibling species, An. arabiensis. The fundamental environmental envelope of the two molecular forms, however, overlapped to a large extent in the rainforest, where they occurred in sympatry. The S form had the greatest niche breadth of all three taxa, whereas An. arabiensis and the M form had the smallest niche overlap. Correspondence analysis of M and S karyotypes confirmed that molecular forms shared similar combinations of chromosomal inversion arrangements in response to the eco-climatic gradient defining the main biogeographic domains occurring across Cameroon. Savanna karyotypes of M and S, however, segregated along the smaller-scale environmental gradient defined by the second ordination axis. Population structure analysis identified three chromosomal clusters, each containing a mixture of M and S specimens. In both M and S, alternative karyotypes were segregating in contrasted environments, in agreement with a strong ecological adaptive value of chromosomal inversions.Our data suggest that inversions on the second chromosome of An. gambiae are not causal to the evolution of reproductive isolation between the M and S forms. Rather, they are involved in ecological specialization to a similar extent in both genetic backgrounds, and most probably predated lineage splitting between molecular forms. However, because chromosome-2 inversions promote ecological divergence, resulting in spatial and/or temporal isolation between ecotypes, they might favour mutations in other ecologically significant genes to accumulate in unlinked chromosomal regions. When such mutations occur in portions of the genome where recombination is suppressed, such as the pericentromeric regions known as speciation islands in An. gambiae, they would contribute further to the development of reproductive isolation.

Project description:In bacteria, physiological change may be effected by a single gene acquisition, producing ecological differentiation without genetic isolation. Natural selection acting on such differences can reduce the frequency of genotypes that arise from recombination at these loci. However, gene acquisition can only account for recombination interference in the fraction of the genome that is tightly linked to the integration site. To identify additional loci that contribute to adaptive differences, we examined orthologous genes in species of Enterobacteriaceae to identify significant differences in the degree of codon selection. Significance was assessed using the Adaptive Codon Enrichment metric, which accounts for the variation in codon usage bias that is expected to arise from mutation and drift; large differences in codon usage bias were identified in more genes than would be expected to arise from stochastic processes alone. Genes in the same operon showed parallel differences in codon usage bias, suggesting that changes in the overall levels of gene expression led to changes in the degree of adaptive codon usage. Most significant differences between orthologous operons were found among those involved with specific environmental adaptations, whereas "housekeeping" genes rarely showed significant changes. When considered together, the loci experiencing significant changes in codon selection outnumber potentially adaptive gene acquisition events. The identity of genes under strong codon selection seems to be influenced by the habitat from which the bacteria were isolated. We propose a two-stage model for how adaptation to different selective regimes can drive bacterial speciation. Initially, gene acquisitions catalyze rapid ecological differentiation, which modifies the utilization of genes, thereby changing the strength of codon selection on them. Alleles develop fitness variation by substitution, producing recombination interference at these loci in addition to those flanking acquired genes, allowing sequences to diverge across the entire genome and establishing genetic isolation (i.e., protection from frequent homologous recombination).

Dataset Information

Assortative flocking in crossbills and implications for ecological speciation.

Publications

Assortative flocking in crossbills and implications for ecological speciation.

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