It has been argued that the resolution of Darwin's first dilemma lies in the fact that out-crossing sexual reproduction has an intrinsic cost of rarity. The cost of rarity arises as follows. If, on a resource gradient, a large number of separate species evolve, each exquisitely adapted to a very narrow band on that gradient, each species will, of necessity, consist of very few members. Finding a mate under these circumstances may present difficulties when many of the individuals in the neighborhood belong to other species. Under these circumstances, if any species' population size happens, by chance, to increase (at the expense of one or other of its neighboring species, if the environment is saturated), this will immediately make it easier for its members to find sexual partners. The members of the neighboring species, whose population sizes have decreased, experience greater difficulty in finding mates, and therefore form pairs less frequently than the larger species. This has a snowball effect, with large species growing at the expense of the smaller, rarer species, eventually driving them to extinction. Eventually, only a few species remain, each distinctly different from the other. Rarity not only imposes the risk of failure to find a mate, but it may also incur indirect costs, such as the resources expended or risks taken to seek out a partner at low population densities.
it has been argued that the resolution of darwin's first dilemma lies in the fact that out-crossing sexual reproduction has an intrinsic cost of rarity the cost of rarity arises as follows if on a resource gradient a large number of separate species evolve each exquisitely adapted to a very narrow band on that gradient each species will of necessity consist of very few members finding a mate under these circumstances may present difficulties when many of the individuals in the neighborhood belong to other species under these circumstances if any species' population size happens by chance to increase (at the expense of one or other of its neighboring species if the environment is saturated) this will immediately make it easier for its members to find sexual partners the members of the neighboring species whose population sizes have decreased experience greater difficulty in finding mates and therefore form pairs less frequently than the larger species this has a snowball effect with large species growing at the expense of the smaller rarer species eventually driving them to extinction eventually only a few species remain each distinctly different from the other rarity not only imposes the risk of failure to find a mate but it may also incur indirect costs such as the resources expended or risks taken to seek out a partner at low population densities