In hybrid zones in which two divergent taxa come into secondary contact and interbreed, selection can maintain phenotypic diversity despite widespread genetic introgression. Red‐breasted (
We studied hybridization between the Black-crested and Tufted titmouse across two geographically distinct transects that differ in the timing of secondary contact by hundreds to thousands of years. We found that hybridization patterns correspond to localized hybrid swarms and that the titmouse hybrid zone is likely slowly expanding over time, a product of short post-natal dispersal distances coupled with weak or absent selection against admixture. We show the southern part of the hybrid zone located in Texas is four times wider than the northern region of hybridization in Oklahoma, which is likely due to geographic differences in hybrid zone age. Despite differences in width, most individuals in both transects are advanced-generation hybrids and backcrosses, suggesting geographically consistent hybridization dynamics. We documented a strong correlation between genotypes and plumage index, suggesting that hybridization has not yet resulted in the decoupling of plumage and genome-wide ancestry as observed in some other avian hybrid zones. Although our results suggest the ongoing expansion of the hybrid zone, the rate of expansion appears to be slow, on the scale of tens of meters a year, and it will likely take hundreds of thousands to millions of years before homogenization of the parental populations. While we did not find support for partial reproductive isolation in the hybrid zone itself, there is the possibility that ecological or sexual selection limits introgression into allopatric regions. Broadly, the results of our study highlight the value of multiple, geographically distant, transects across a hybrid zone for assessing the evolutionary dynamics of hybridizing lineages.
more » « less- NSF-PAR ID:
- 10414531
- Publisher / Repository:
- Oxford University Press
- Date Published:
- Journal Name:
- Ornithology
- Volume:
- 140
- Issue:
- 3
- ISSN:
- 0004-8038
- Format(s):
- Medium: X
- Sponsoring Org:
- National Science Foundation
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Abstract Sphyrapicus ruber ) and red‐naped (S. nuchalis ) sapsuckers meet and hybridize along a narrow contact zone that stretches from northern California to southern British Columbia. We found strong evidence for changes in the structure of this hybrid zone across time, with significant temporal shifts in allele frequencies and in the proportions of parental phenotypes across the landscape. In addition to these shifts, we found that differences in plumage predict genetic differences (R 2 = 0.80), suggesting that plumage is a useful proxy for assessing ancestry. We also found a significant bimodal distribution of hybrids across the contact zone, suggesting that premating barriers may be driving reproductive isolation, perhaps as a result of assortative mating based on plumage differences. However, despite evidence of selection and strong patterns of population structure between parental samples, we found only weak patterns of genetic divergence. Using museum specimens and genomic data, this study of sapsuckers provides insight into the ways in which phenotypic and genetic structure have changed over a 40‐year period, as well as insight into the mechanisms that may contribute to the maintenance of the hybrid zone over time. -
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Abstract As hybrid zones exhibit selective patterns of gene flow between otherwise distinct lineages, they can be especially valuable for informing processes of microevolution and speciation. The bumble bee,
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Woodhouse’s Scrub-Jay (Aphelocoma woodhouseii) comprises 7 subspecies, ranging from the Rocky Mountains to southern Mexico. We quantified the phenotype of specimens throughout Mexico and found support for significant phenotypic differences between “Sumichrast’s group” in southern Mexico (A. w. sumichrasti and A. w. remota) and the 2 subspecies in northern Mexico, or “Woodhouse’s group” (A. w. grisea and A. w. cyanotis). Despite significant differentiation in body size and mantle color, we found no clear geographic boundary between the groups, suggesting either a geographic cline or hybridization upon secondary contact. We tested for selection against hybridization by fitting models to geographic clines for both body size and back color, and found support for a stable contact zone centered near Mexico City, with selection against intermediate back color. Based on these results, we infer that Sumichrast’s and Woodhouse’s groups diverged during a period of geographic and genetic isolation. The phenotypic introgression between Sumichrast’s and Woodhouse’s groups near Mexico City likely represents a case of recent secondary contact, with selection against hybridization maintaining a geographically stable contact zone.more » « less
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null (Ed.)Abstract Background Exploring hybrid zone dynamics at different spatial scales allows for better understanding of local factors that influence hybrid zone structure. In this study, we tested hypotheses about drivers of introgression at two spatial scales within the Saltmarsh Sparrow ( Ammospiza caudacuta ) and Nelson’s Sparrow ( A. nelsoni ) hybrid zone. Specifically, we evaluated the influence of neutral demographic processes (relative species abundance), natural selection (exogenous environmental factors and genetic incompatibilities), and sexual selection (assortative mating) in this mosaic hybrid zone. By intensively sampling adults (n = 218) and chicks (n = 326) at two geographically proximate locations in the center of the hybrid zone, we determined patterns of introgression on a fine scale across sites of differing habitat. We made broadscale comparisons of patterns from the center with those of prior studies in the southern edge of the hybrid zone. Results A panel of fixed SNPs (135) identified from ddRAD sequencing was used to calculate a hybrid index and determine genotypic composition/admixture level of the populations. Another panel of polymorphic SNPs (589) was used to assign paternity and reconstruct mating pairs to test for sexual selection. On a broad-scale, patterns of introgression were not explained by random mating within marshes. We found high rates of back-crossing and similarly low rates of recent-generation (F1/F2) hybrids in the center and south of the zone. Offspring genotypic proportions did not meet those expected from random mating within the parental genotypic distribution. Additionally, we observed half as many F1/F2 hybrid female adults than nestlings, while respective male groups showed no difference, in support of Haldane’s Rule. The observed proportion of interspecific mating was lower than expected when accounting for mate availability, indicating assortative mating was limiting widespread hybridization. On a fine spatial scale, we found variation in the relative influence of neutral and selective forces between inland and coastal habitats, with the smaller, inland marsh influenced primarily by neutral demographic processes, and the expansive, coastal marsh experiencing higher selective pressures in the form of natural (exogenous and endogenous) and sexual selection. Conclusions Multiple drivers of introgression, including neutral and selective pressures (exogenous, endogenous, and sexual selection), are structuring this hybrid zone, and their relative influence is site and context-dependent.more » « less
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Abstract Phenotypic differentiation plays an important role in the formation and maintenance of reproductive barriers. In some cases, variation in a few key aspects of phenotype can promote and maintain divergence; hence, the identification of these traits and their associations with patterns of genomic divergence is crucial for understanding the patterns and processes of population differentiation. We studied hybridization between the
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