Summary
The outcome of random cross‐pollination and incomplete self‐pollination in populations of two cytotypes not capable of hybridizing is analyzed in a simple model. The reproductive success of a cytotype is frequency‐dependent, the minority being at a disadvantage because it suffers a greater proportion of ineffectual pollinations. The reproductive handicap in a given generation leads to a greater handicap in the next, the minority cytotype being rapidly excluded from the population. Self‐pollination reduces the minority disadvantage and the tempo of the exclusion process as does the immigration of minority type pollen and seeds. It is concluded that unless two cytotypes have disparate ecological requirements, a minority disadvantage will cause the local extinction of one of the cytotypes, even though they may be equally fit in all respects.
Much has been written about the role of interspecific competition, disease, herbivory, and the loss of key mutualisms in the extinction of rare plant species. Interspecific hybridization rarely is considered among the biotic interactions that promote extinction. We show how hybridization may contribute to the demise of rare plant species through demographic swamping and genetic assimilation by an abundant congener. We contend that the growth of the hybrid subpopulation is the key to rare species assimilation, and we show how the production of hybrid seed, the fitness of hybrids, and pest pressure affect hybrid proliferation. We also discuss how habitat disturbance, unspecialized pollinators, and weak crossing barriers promote hybridization, and how the negative consequences of hybridization are unlikely to be compensated for by immigration from conspecific populations. We also illustrate stages in the demise of species in island floras. We suggest that hybridization is an increasing threat to rare species because ecological barriers are being disrupted by human activities.
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