Interspecific reproductive interactions are common when closely related species co-occur in sympatry. A growing body of work is revealing that such interactions can have large-scale ecological and evolutionary consequences. My research investigates how mating and fighting between species influences trait evolution and speciation using darters, a highly diverse group of North American stream fishes.

I study two wide-ranging groups of darters in the subgenus Oligocephalus: the orangethroat darter clade (Etheostoma:Ceasia) and the rainbow darter (Etheostoma caeruleum). The orangethroat darter clade consists of 15 recently diverged allopatric species, 13 of which occur sympatrically with the more distantly related rainbow darter. Orangethroat and rainbow darters exhibit similar male nuptial coloration, ecology, and behavior, and they hybridize at low levels in nature. I use behavioral assays and genomics to investigate which reproductive isolating barriers allow these species to co-occur syntopically with one another, and which factors have influenced the recent radiation of the orangethroat darter clade.

Click here to see a recording of my talk on behavioral isolation and character displacement in darters from Evolution 2017 .

Click here to see a recording of my talk on the genomic basis of postzygotic isolation in darters from the 2020 SMBE Fitch Award Symposium.

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The evolution of enhanced behavioral isolation in sympatry via male-driven reproductive and agonistic character displacement

My research has demonstrated that selection to avoid maladaptive interspecific mating and fighting between orangethroat and rainbow darters likely plays a large role in driving speciation. When orangethroat and rainbow darters occur in sympatry, males strongly prefer to mate with conspecific over heterospecific females, and bias their aggression towards conspecific over heterospecific males. However, no such preferences exist when these species occur in allopatry with respect to one another. Thus, these species exhibit behavioral patterns consistent with reproductive character displacement in male mating preferences and agonistic character displacement in male fighting biases (Moran et al. 2017 – Evolution; Moran & Fuller 2018 – Proc Roy Soc B). Surprisingly, female orangethroat and rainbow darters do not appear to exert mating preferences for conspecific males, regardless of sympatry or allopatry with one another. This may be attributable to the high cost of choosiness females incur via egg over-ripening (Moran et al. 2018 – Journal of Fish Biology), and/or the possibility that male-male competition precludes female choice.

In addition to strong behavioral prezygotic barriers, I have found that postzygotic barriers also prevent gene flow between orangethroat and rainbow darters (Moran et al. 2018 – Ecology and Evolution). This suggests that reproductive character displacement in male mating preferences has evolved in response to the fitness consequences associated with hybridization (i.e. reinforcement).

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Male F1 hybrids resulting from crosses between the orangethroat darter E. spectabile and the rainbow darter E. caeruleum exhibit color pattern intermediacy between both parental species (note caudal and anal fin coloration).

Secondary effects of reproductive and agonistic character displacement in sympatry can lead to divergence between allopatric lineages 

Character displacement between two sympatric species results in a shift in species recognition traits (see figure below). This sometimes has the secondary effect of causing mismatches in species recognition traits between allopatric lineages (e.g. between populations within a species, or between recently diverged species within a clade). In this manner, reproductive and agonistic character displacement between sympatric orangethroat and rainbow darters appears to have also contributed to divergence between species within the orangethroat darter clade. I found that recently diverged species within the orangethroat clade exhibit surprisingly high levels of behavioral isolation from one another, but only when they occur in sympatry with rainbow darters (Moran et al. 2017 – Evolution; Moran & Fuller 2018 – Current Zoology). Thus, reproductive and agonistic character displacement between orangethroat and rainbow darters has two effects: (1) directly promoting increased reproductive isolation between orangethroat and rainbow darters in sympatry, and (2) indirectly leading to reproductive isolation between allopatric species within the orangethroat darter clade.

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(A) Hypothetical ranges for Species 1 and Species 2. (B) Mating trait divergence is enhanced in sympatry between Species 1 and Species 2 due to reproductive character displacement (RCD), but populations of Species 2 respond to RCD in different phenotypic directions.

Mate choice copying in species with and without parental care

Individuals sometimes copy the mate choice of others to reduce the costs associated with choosing a mate. This phenomenon occurs in a variety of taxa, including birds, rats, deer, fish, insects, and humans, but has rarely been documented in both sexes within a species. Mate choice copying has been observed to occur more frequently in females than males. Traditionally, females have been thought to face a higher cost associated with choosing a mate compared to males because females invest more energy into producing larger gametes. However, in many species of fish, males invest heavily in reproduction by providing paternal care. Thus, investment in reproduction may be more equal between the sexes when males provide parental care. This has led to the prediction that when male parental care is present, both sexes should be more likely to mate choice copy.

For my Master’s thesis, I investigated mate choice copying in both sexes in two sympatric species of darters: Etheostoma flabellare, which exhibits paternal care, and Etheostoma zonale, which does not exhibit care. My objective was to determine whether the presence of mate choice copying is affected by differences in parental care between species while controlling to some extent for ecological and phylogenetic differences. Surprisingly, I found evidence of mate choice copying in both sexes of E. zonale (no parental care). I also found evidence of mate choice copying in male E. flabellare (male only parental care), but not in female E. flabellare (Moran et al. 2013). These results suggest that the presence of intense male-male competition in darters along with bright nuptial coloration in E. zonale and male care in E. flabellare may represent substantial costs to mating (Moran et al. 2014), leading to the observed pattern of mate choice copying in males of both species. Furthermore, in darter species in which males guard a nest and care for eggs, females prefer males that already have eggs in their nest (Moran 2012). Therefore, females in these species may gain more information about the quality of a male by directly observing his nest (and whether it contains eggs) than the presence of another female near his nest site.