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Introduction Darwin recognized that the young Galapagos Islands were a place for the genesis of new species. The central fact that crystallized this view was the many plants and animals that existed nowhere else. Evolutionary theory must also explain macroevolution , the origin of new taxonomic groups (new species, new genera, new families, new kingdoms). Speciation is the keystone process in the origination of diversity of higher taxon. The fossil record chronicles two patterns of speciation: anagenesis and cladogenesis. Anagenesis is the accumulation of changes associated with the transformation of one species into another. Cladogenesis , branching evolution, is the budding of one or more new species from a parent species. Cladogenesis promotes biological diversity by increasing the number of species. A. What Is a Species? Species is a Latin word meaning “kind” or “appearance.” Traditionally, morphological differences have been used to distinguish species. Today, differences in body function, biochemistry, behavior, and genetic makeup are also used to differentiate species. 1. The biological species concept emphasizes reproductive isolation In 1942 Ernst Mayr enunciated the biological species concept to address biological diversity. A species is a population or group of populations whose members have the potential to interbreed with each other in nature to produce viable, fertile offspring, but who cannot produce viable, fertile offspring with members of other species. A biological species is the largest set of populations in which genetic exchange is possible and that is genetically isolated from other populations. Species are based on interfertility, not physical similarity. For example, the eastern and western meadowlarks may have similar shapes and coloration, but differences in song help prevent interbreeding between the two species. In contrast, humans have considerable diversity, but we all belong to the same species because of our capacity to interbreed. 2. Prezygotic and postzygotic barriers isolate the gene pools of biological species No single barrier may be completely impenetrable to genetic exchange, but many species are genetically sequestered by multiple barriers. Typically, these barriers are intrinsic to the organisms, not simple geographic separation. Reproductive isolation prevents populations belonging to different species from interbreeding, even if their ranges overlap. Reproductive barriers can be categorized as prezygotic or postzygotic, depending on whether they function before or after the formation of zygotes. Prezygotic barriers
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