When does sexual selection actually drive speciation?
Sexual selection can drive speciation when it creates strong prezygotic barriers—meaning individuals from different populations stop mating with each other. A 2021 review found that sexual selection alone rarely causes full reproductive isolation unless it is paired with ecological divergence (natural selection) [3]. For instance, in the selfing plant Capsella rubella and its outcrossing ancestor C. grandiflora, differences in sexual selection intensity (male competitiveness vs. self-fertilization) create a strong barrier: outcrosser pollen rarely fertilizes selfing flowers, and selfers rapidly self-pollinate, reducing hybridization rates to near zero in sympatric populations [4]. However, this barrier is directional—selfers are more likely to be pollinated by outcrossers than vice versa—and postzygotic barriers (hybrid inviability) still depend on natural selection [4].
Experimental evolution in fruit flies (Drosophila pseudoobscura) provides direct evidence that sexual selection leaves genomic signatures similar to those seen during speciation. After 160+ generations of either polyandry (elevated sexual selection) or monogamy (no sexual selection), researchers found that divergence was concentrated in 'islands' on the X chromosome containing genes for mating behaviors, with lower genetic diversity and signs of selective sweeps [5]. This shows sexual selection can drive rapid genomic divergence, but the authors note that outcomes vary across genomic regions and that natural selection and demography also play roles [5].
How often do natural and sexual selection conflict?
More often than not, natural and sexual selection align rather than conflict, which means they can work together to speed up speciation. A 2025 study on cricket cuticular hydrocarbons (CHCs)—waxy compounds that both prevent desiccation (drying out) and attract mates—found that male attractiveness and desiccation resistance were both maximized on a high-carbohydrate, low-protein diet [1][2]. Although the optimal diets differed slightly (the angle between the two fitness peaks was small), both traits were associated with the same CHC blend: moderate abundance of shorter-chain alkenes rather than a single long-chain alkane [1][2]. Partial correlations even suggested that CHCs contribute to attractiveness indirectly through their role in survival [1][2]. The authors conclude that this alignment facilitates rapid adaptation to new environments and promotes speciation [1][2].
However, alignment is not guaranteed. The same cricket study found that the exact nutritional optima for attractiveness and desiccation resistance were significantly different, meaning a trade-off exists—a male cannot be maximally attractive and maximally drought-resistant at the same time [1][2]. This small conflict could slow divergence if ecological conditions push in opposite directions. The 2021 review also notes that sexual selection can sometimes oppose natural selection, for example when a flashy ornament attracts both mates and predators, but such cases are less common than alignment [3].
What are the caveats and limitations?
The biggest caveat is that most studies focus on prezygotic isolation (mating barriers), but speciation requires postzygotic isolation (hybrid inviability or sterility) as well. The plant study found that postzygotic barriers in Capsella are likely driven by differences in parental conflict intensity—a form of natural selection—not sexual selection [4]. So even when sexual selection creates strong mating barriers, natural selection is still needed to complete speciation.
Another limitation is that experimental evolution studies, like the fruit fly experiment, use simplified lab conditions that may not reflect natural complexity. After 160 generations, the genomic divergence patterns were similar to those seen between wild species, but the authors caution that different genomic regions responded differently, and that neutral processes and demography could mimic selection signatures [5]. Finally, the cricket study only examined one species under controlled lab diets, so the alignment between natural and sexual selection may not hold across all environments or species [1][2].
Sources used in this answer
Nutritional geometry provides insight into the dual roles of natural and sexual selection in insect cuticular hydrocarbon evolution
In crickets, male attractiveness and desiccation resistance both peak on high-carbohydrate diets and are linked to the same cuticular hydrocarbon blend, suggesting natural and sexual selection are broadly aligned [1].
Data from: Nutritional geometry provides insight into the dual roles of natural and sexual selection in insect cuticular hydrocarbon evolution
Same as paper 1—this is a data repository version confirming that the optimal diets for attractiveness and desiccation resistance differ slightly but the angle between them is small [2].
Speciation by sexual selection: 20 years of progress
A 2021 review concludes that sexual selection alone rarely causes reproductive isolation without ecological divergence, but it can accelerate speciation when combined with natural selection [3].
Sexual selection drives the speciation of lineages with contrasting mating systems.
In Capsella plants, differences in sexual selection intensity between selfing and outcrossing lineages create strong prezygotic isolation, but postzygotic barriers still depend on natural selection [4].
Experimental evolution supports signatures of sexual selection in genomic divergence
Experimental evolution in fruit flies shows that elevated sexual selection produces genomic divergence patterns (X-chromosome islands, selective sweeps) similar to those seen in natural speciation, but outcomes vary across genomic regions [5].