Loss of sexual isolation in a hybridizing stickleback species pair

Abstract One approach to understand the importance of reproductive barriers to the speciation process is to study the breakdown of barriers between formerly distinct species. One reproductive barrier, sexual isolation, reduces gene flow between species through differences in mate preferences and mating signals and is likely important for species formation and maintenance. We measure sexual isolation in two limnetic-benthic threespine stickleback species pairs (Gasterosteus spp.). One species pair maintains strong reproductive isolation while the other species pair has recently collapsed into a hybrid swarm. We compare the strength of sexual isolation in the hybridizing pair to the currently isolated pair. We provide the first evidence that sexual isolation has been lost in the hybridizing pair and show furthermore that preferences females have for conspecific mates and the traits they use to distinguish conspecific and heterospecific males contribute to this loss. This work highlights the fragility of reproductive isolation between young species pairs and considers the role of sexual isolation in speciation [Current Zoology 59 (5): 591−603, 2013]

Types of Isolation: Premating, Postmating, Intrinsic, Extrinsic

For each of the 5 species pair systems, point estimates and strongest and weakest estimates of four types of isolation: premating, postmating, postmating intrinsic, and postmating extrinsic. Data presented in Figure 3

Loss of sexual isolation in a hybridizing stickleback species pair

One approach to understand the importance of reproductive barriers to the speciation process is to study the breakdown of barriers between formerly distinct species. One reproductive barrier, sexual isolation, reduces gene flow between species through differences in mate preferences and mating signals and is likely important for species formation and maintenance. We measure sexual isolation in two limnetic-benthic threespine stickleback species pairs (Gasterosteus spp.). One species pair maintains strong reproductive isolation while the other species pair has recently collapsed into a hybrid swarm. We compare the strength of sexual isolation in the hybridizing pair to the currently isolated pair. We provide the first evidence that sexual isolation has been lost in the hybridizing pair and show furthermore that preferences females have for conspecific mates and the traits they use to distinguish conspecific and heterospecific males contribute to this loss. This work highlights the fragility of reproductive isolation between young species pairs and considers the role of sexual isolation in speciation [Current Zoology 59 (5): 591-603, 2013]

Individual Barrier Asymmetries

For each of the 5 species pair systems, weighted mean or single estimates 95%CIs for individual barrier strengths for each species, which allowed us to test for asymmetries between species. Data presented in Figure 5

Data from: Evolution of reproductive isolation in stickleback fish

To understand how new species form and what causes their collapse, we examined how reproductive isolation evolves during the speciation process, considering species pairs with little to extensive divergence, including a recently collapsed pair. We estimated many reproductive barriers in each of five sets of stickleback fish species pairs using our own data and decades of previous work. We found that the types of barriers important early in the speciation process differ from those important late. Two premating barriers—habitat and sexual isolation—evolve early in divergence and remain two of the strongest barriers throughout speciation. Premating isolation evolves before postmating isolation, and extrinsic isolation is far stronger than intrinsic. Completing speciation, however, may require postmating intrinsic incompatibilities. Reverse speciation in one species pair was characterized by significant loss of sexual isolation. We present estimates of barrier strengths before and after collapse of a species pair; such detail regarding the loss of isolation has never before been documented. Additionally, despite significant asymmetries in individual barriers, which can limit speciation, total isolation was essentially symmetric between species. Our study provides important insight into the order of barrier evolution and the relative importance of isolating barriers during speciation and tests fundamental predictions of ecological speciation

Total Isolation Asymmetries

For each of the 5 species pair systems, point estimates and strongest and weakest estimates for total isolation of each species, which allowed us to test for asymmetries between species. Data presented in Figure 6

Total and Shared RI

For each of the 5 species pair systems, point estimates and strongest and weakest estimates of total reproductive isolation from all available barriers and total reproductive isolation from four shared barriers that were measured in all 5 systems. Data presented in Figure 1 and Table S3

Data from: Divergent sexual selection via male competition: ecology is key

Sexual selection and ecological differences are important drivers of speciation. Much research has focused on female choice, yet the role of male competition in ecological speciation has been understudied. Here, we test how mating habitats impact sexual selection and speciation through male competition. Using limnetic and benthic species of threespine stickleback fish, we find that different mating habitats select differently on male traits through male competition. In mixed habitat with both vegetated and open areas, selection favors two trait combinations of male body size and nuptial color: large with little color and small with lots of color. This matches what we see in reproductively isolated stickleback species, suggesting male competition could promote trait divergence and reproductive isolation. In contrast, when only open habitat exists selection favors one trait combination, large with lots of color, which would hinder trait divergence and reproductive isolation. Other behavioral mechanisms in male competition that might promote divergence, such as avoiding aggression with heterospecifics, are insufficient to maintain separate species. This work highlights the importance of mating habitats in male competition for both sexual selection and speciation

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