Abstract
The maintenance of reproductive isolation in the face of gene flow is a particularly contentious topic, but differences in reproductive behavior may provide the key to explaining this phenomenon. However, we do not yet fully understand how behavior contributes to maintaining species boundaries. How important are behavioral differences during reproduction? To what extent does assortative mating maintain reproductive isolation in recently diverged populations and how important are “magic traits”? Assortative mating can arise as a by‐product of accumulated differences between divergent populations as well as an adaptive response to contact between those populations, but this is often overlooked. Here we address these questions using recently described species pairs of three‐spined stickleback (Gasterosteus aculeatus), from two separate locations and a phenotypically intermediate allopatric population on the island of North Uist, Scottish Western Isles. We identified stark differences in the preferred nesting substrate and courtship behavior of species pair males. We showed that all males selectively court females of their own ecotype and all females prefer males of the same ecotype, regardless of whether they are from species pairs or allopatric populations. We also showed that mate choice does not appear to be driven by body size differences (a potential “magic trait”). By explicitly comparing the strength of these mating preferences between species pairs and single‐ecotype locations, we were able to show that present levels of assortative mating due to direct mate choice are likely a by‐product of other adaptations between ecotypes, and not subject to obvious selection in species pairs. Our results suggest that ecological divergence in mating characteristics, particularly nesting microhabitat may be more important than direct mate choice in maintaining reproductive isolation in stickleback species pairs.
Highlights
Behavior dictates the way in which an organism interacts with other members of the same species, with other living entities, and with its surrounding environment (Levitis et al, 2009)
Lackey and Boughman (2017) recently showed that divergence in habitat and mate choice is the two most important premating barriers during speciation in stickleback. They found that these two barriers evolve early and remain strong throughout the speciation process. We identified differences both in nesting microhabitat and in courtship behavior which strongly support the findings of Lackey and Boughman (2017), suggesting that these premating barriers may be important for maintaining reproductive isolation in lagoon – anadromous species pairs
Both in courtship rituals and preferred nesting substrate, which likely contribute to maintaining reproductive isolation in newly described saltwater stickleback species pairs
Summary
Behavior dictates the way in which an organism interacts with other members of the same species, with other living entities, and with its surrounding environment (Levitis et al, 2009). Assortative mating (the ability to recognize and choose to mate with conspecific individuals) is common between reproductively isolated stickleback ecotypes and has been documented in benthic–limnetic (Bay et al, 2017; Kozak et al, 2011), lake–stream (Andreou et al, 2017), lava–nitella (Olafsdottir et al, 2006), and anadromous–freshwater resident (Furin et al, 2012) species pairs This assortative mating can be driven by variation in factors directly involved in mating interactions such as mating behavior (Ishikawa & Mori, 2000), body size (McKinnon et al, 2012), nuptial coloration (McKinnon, 1995), and nest structure (Blouw & Hagen, 1990), or variation in spatial and temporal aspects of courtship that result in finescale segregation of phenotypes (Borzee et al, 2016; Hagen, 1967; Pegoraro et al, 2016; Snowberg & Bolnick, 2012). Our findings shed light on the role of behavior in adaptive divergence and in maintaining reproductive isolation in sympatry
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