Multiplication of Species
Multiplication of Species
Multiplication of species through time is a logical corollary to Darwin’s theory of common descent. A branch point on the evolutionary tree means that an ancestral species has split into two different ones. Darwin’s theory postulates that genetic variation present within a species, especially variation that occurs between geographically separated populations, provides the material from which new species are produced. Because evolution is a branching process, the total number of species produced by evolution increases through time, although most of these species eventually become extinct. A major challenge for evolutionists is to discover the process by which an ancestral species “branches” to form two or more descendant species.
Before we explore the multiplication of species, we must decide what we mean by “species.” As explained in Classification and Phylogeny of Animals, no consensus exists regarding definition of species. Most biologists agree, however, that important criteria for recognizing species include (1) descent of all members from a common ancestral population, (2) reproductive compatibility (ability to interbreed) within and reproductive incompatibility between species, and (3) maintenance within species of genotypic and phenotypic cohesion (lack of abrupt differences among populations in allelic frequencies [see the following text] and organismal characteristics). The criterion of reproductive compatibility has received the greatest attention in studies of species formation, also called speciation.
Biological features that prevent different species from interbreeding are called reproductive barriers. The primary problem of speciation is to discover how two initially compatible populations evolve reproductive barriers that cause them to become distinct, separately evolving lineages. How do populations diverge from each other in their reproductive properties while maintaining complete reproductive compatibility within each population?
Reproductive barriers between populations usually evolve gradually. Evolution of reproductive barriers requires that diverging populations must be kept physically separate for long periods of time. If diverging populations reunite before reproductive barriers are completely formed, interbreeding occurs between the populations and they merge. Speciation by gradual divergence in animals may require extraordinarily long periods of time, perhaps 10,000 to 100,000 years or more. Geographical isolation followed by gradual divergence is the most effective way for reproductive barriers to evolve, and many evolutionists consider geographical separation a prerequisite for branching speciation.
Multiplication of species through time is a logical corollary to Darwin’s theory of common descent. A branch point on the evolutionary tree means that an ancestral species has split into two different ones. Darwin’s theory postulates that genetic variation present within a species, especially variation that occurs between geographically separated populations, provides the material from which new species are produced. Because evolution is a branching process, the total number of species produced by evolution increases through time, although most of these species eventually become extinct. A major challenge for evolutionists is to discover the process by which an ancestral species “branches” to form two or more descendant species.
Before we explore the multiplication of species, we must decide what we mean by “species.” As explained in Classification and Phylogeny of Animals, no consensus exists regarding definition of species. Most biologists agree, however, that important criteria for recognizing species include (1) descent of all members from a common ancestral population, (2) reproductive compatibility (ability to interbreed) within and reproductive incompatibility between species, and (3) maintenance within species of genotypic and phenotypic cohesion (lack of abrupt differences among populations in allelic frequencies [see the following text] and organismal characteristics). The criterion of reproductive compatibility has received the greatest attention in studies of species formation, also called speciation.
Biological features that prevent different species from interbreeding are called reproductive barriers. The primary problem of speciation is to discover how two initially compatible populations evolve reproductive barriers that cause them to become distinct, separately evolving lineages. How do populations diverge from each other in their reproductive properties while maintaining complete reproductive compatibility within each population?
Reproductive barriers between populations usually evolve gradually. Evolution of reproductive barriers requires that diverging populations must be kept physically separate for long periods of time. If diverging populations reunite before reproductive barriers are completely formed, interbreeding occurs between the populations and they merge. Speciation by gradual divergence in animals may require extraordinarily long periods of time, perhaps 10,000 to 100,000 years or more. Geographical isolation followed by gradual divergence is the most effective way for reproductive barriers to evolve, and many evolutionists consider geographical separation a prerequisite for branching speciation.
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