Abstract
A major conundrum in evolution is that, despite natural selection, polymorphism is still omnipresent in nature: Numerous species exhibit multiple morphs, namely several abundant values of an important trait. Polymorphism is particularly prevalent in asymmetric traits, which are beneficial to their carrier in disruptive competitive interference but at the same time bear disadvantages in other aspects, such as greater mortality or lower fecundity. Here we focus on asymmetric traits in which a better competitor disperses fewer offspring in the absence of competition. We report a general pattern in which polymorphic populations emerge when disruptive selection increases: The stronger the selection, the greater the number of morphs that evolve. This pattern is general and is insensitive to the form of the fitness function. The pattern is somewhat counterintuitive since directional selection is excepted to sharpen the trait distribution and thereby reduce its diversity (but note that similar patterns were suggested in studies that demonstrated increased biodiversity as local selection increases in ecological communities). We explain the underlying mechanism in which stronger selection drives the population towards more competitive values of the trait, which in turn reduces the population density, thereby enabling lesser competitors to stably persist with reduced need to directly compete. Thus, we believe that the pattern is more general and may apply to asymmetric traits more broadly. This robust pattern suggests a comparative, unified explanation to a variety of polymorphic traits in nature.
Highlights
Individuals in a population of a given species often exhibit very different trait values, owing either to genetic variability or to purely phenotypic plasticity even when their genes associated with the trait are identical
We studied the evolution of continuous asymmetric traits that entail a tradeoff between better ability to compete locally with neighboring population and better ability to disperse offspring
We demonstrated the following general pattern: The steady state distribution of traits values comprises (1) a single dominant trait value when selection is weak, (2) increasing number of dominant values with increased selection, and (3) a continuous spectrum of coexisting trait values at extreme selection levels
Summary
Individuals in a population of a given species often exhibit very different trait values, owing either to genetic variability or to purely phenotypic plasticity even when their genes associated with the trait are identical. Seedlings initiated from larger seeds may have competitive advantage over those initiated from smaller seeds, but smaller seeds are cheaper to produce and favorable in the absence of competition; this may lead to coexistence between small and large seeds [17] For another example, hornless male beetles may sneak around and mate with females without encountering the stronger, horned males, which allows them to persist at a low abundance [24]. We use a variant of our model to show that the same mechanism may apply to cooperative traits that decrease the reproductive potential of their carrier but increases the reproductive potential of their neighbors This idea is supported by previous studies [17, 21, 25, 26], each focused on a special case and demonstrates how the number of coexisting species increases with a certain environmental parameter that is equivalent to the strength of local disruptive selection. Our study puts these observations in a general framework and suggests an underlying mechanism that may lead to the emergence of polymorphism
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