Natural Hybridization and Evolution

Natural Hybridization and Evolution

Natural Hybridization and Evolution

Natural Hybridization and Evolution

Synopsis

Natural Hybridation and Evolution includes data from numerous sources that support the paradigm of natural hybridization as an important evolutionary process. The review of these data results in a challenge of the dogma that is the explicit or implicit framework used by a large proportion of evolutionary biologists - that the process of natural hybridization is maladaptive and it is because it represents a violation of divergent evolution. In contrast, this book presents evidence of a significant role for natural hybridization in furthering adaptive evolution and evolutionary diversification in both plants and animals.

Excerpt

The role of hybridization in evolution has been one of the most controversial topics in the whole field of evolutionary study. (Stebbins, 1963)

Several terms used in this book need clear definitions. These include hybridization, hybrid, and hybrid zone. Harrison (1993) has discussed the various definitions of the term hybridization that have been used in scientific literature. Each of these relates to levels of divergence between the individuals that undergo reproduction. the extremes of these definitions are crosses between genetically distinct individuals, and between individuals from different species (Harrison, 1993). the former is frequently used by plant and animal breeders and the latter by evolutionary biologists. Harrison's (1990) definition includes crosses between "individuals from two populations, or groups of populations, which are distinguishable on the basis of one or more heritable characters." I have adopted this definition with the following modifications. First, natural hybridization involves matings that occur in a natural setting -- this excludes cases of experimental hybridizations. Second, I will consider those crosses that are successful in producing some viable F progeny that possess some level of fertility. This latter restriction reflects my desire to focus on the potential ongoing evolutionary effects from hybrid generations past the initial F . However, it is important to point out that almost all cases of natural hybridization result in at least a few viable individuals with some measure of fertility (e.g., Grant, 1963). Thus, when an author states that inviable or infertile offspring are produced, it is usually meant that the offspring are fewer in number or are less fertile relative to progeny from crosses between genetically more similar individuals. the reduction in levels of viability and fertility has led most authors to discount natural hybridization as an evolutionarily important process. This conclusion ignores the importance of rare events in evolution and is contradicted by actual cases where unlikely hybrid events have led to diversification (Arnold and Hodges, 1995a). the validity of the arguments presented in this book do not depend on hybrids being relatively common in nature. However, it is also apparent that natural hybridization does not usually lead to 100% inviability or infertility.

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