Hybridization within genera occurs more frequently in avian families and subfamilies where there is considerable male parental investment, less frequently in families with moderate levels of male parental investment, and rarely in lineages where males contribute only genetic material to their offspring. In addition, genera that show considerable male parental investment are typically less speciose than genera where there is reduced male parental investment. Species showing high levels of male parental investment, however, typically have more subspecies, indicating that local adaptation evolves in these groups, but reproductive isolation does not evolve. Some hybrid matings appear to involve females of one species showing an apparent preference for mating with larger or dominant males, even if these males are heterospecific (e.g., black and mallard ducks). Similar patterns occur in fishes, amphibians, and mammals, the other three vertebrate lineages that show extensive parental care. Hybridization in birds may be an evolutionary mechanism that allows increased genetic diversity and adaptability under changing environmental conditions, particularly environments disturbed by humans. It is also possible that new forms (incipient species) may arise through hybridization that are better adapted to disturbed environments than either parental species.
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