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The history of evolutionary theory is the story of the application of the scientific method to the questions of the origin and nature of living organisms, as scientists learned to gradually give up their presuppositions and look to nature herself for the answers. Adhering faithfully to the spirit of the scientific method, Charles Darwin was able to synthesize his observations and thoughts with those of many others to formulate a theory that laid the groundwork for our modern understanding of biological evolution. Basic genetics, as first outlined by Mendel, gives us an explanation as to the origin of phenotypic variation and to the nature of inheritance. The evolution of organisms is based on the concepts of ecology and the adaptation of species to their habitats. Because environments are always changing, it stands to reason that changes in species' adaptations can account for evolutionary change. The basic process that brings this about is natural selection, which maintains a species' adaptive relationship with its environment and, if there is sufficient variation, alters a species' adaptations in response to changed environmental circumstances. Change also occurs within a species' gene pool. Mutations supply new genetic variation. Gene flow and genetic drift affect the distribution of genetic variation within a species. Thus, not only do environments change over time, so do species themselves—all this constantly providing natural selection with new and variable sets of relationships between species and environments. When a portion of a species is isolated from the rest of the species, the stage is set for speciation, the evolution of a new species. If the isolated portion accumulates enough genetic and therefore physical differences over time, it may become reproductively isolated from the original species; that is, it may no longer be able to produce offspring with members of the original group. A new species has evolved. The diversity of life on earth—the result of countless speciation events—can be depicted as a huge, incredibly complex bush. The main stem represents the single origin of life, but it then begins branching, producing millions upon millions of twigs, each standing for a new species, a new natural experiment in adaptation. |