Evolution theory explains how organisms have changed over time.
Scientific understanding requires both facts and theories that can explain those facts in a coherent manner. Evolution, in this context, is both a fact and a theory. It is an incontrovertible fact that organisms have changed, or evolved, during the history of life on Earth. And biologists have identified and investigated mechanisms that can explain the major patterns of change.
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There are four major patterns of change.
Patterns in Nature
The field of evolutionary biology seeks to provide explanations for four conspicuous patterns that are manifest in nature. The first three concern living species, whereas the fourth relates to fossils.
Genes are linked to how organisms look and behave.
• Genetic variation. There is tremendous genetic diversity within almost all species, including humans. No two individuals have the same DNA sequence, with the exception of identical twins or clones. This genetic variation contributes to phenotypic variation - that is, diversity in the outward appearance and behavior of individuals of the same species.
Organisms must adapt to their environment to survive.
• Adaptation. Living organisms have morphological, biochemical, and behavioral features that make them well adapted for life in the environments in which they are usually found. For example, consider the hollow bones and feathers of birds that enable them to fly, or the cryptic coloration that allows many organisms to hide from their predators. These features may give the superficial appearance that organisms were designed by a creator (or engineer) to live in a particular environment. Evolutionary biology has demonstrated that adaptations arise through selection acting on genetic variation.