Evolutionary science explains the huge diversity of present and past life forms. Heritable variation between particular characteristics (traits) of individuals causes differential reproductive success (fitness), leading to the accumulation of beneficial variations (adaptations) in subsequent generations. Changes in genotype (caused by mutations or recombination) are the basis of heritable variation. For selection to act, those changes must cause differences in the phenotype. Selection acts on phenotypic characteristics influencing survival and reproduction (natural selection) or ability to obtain a mate (sexual selection). Random genetic drift can influence the evolution of a species, particularly in the presence of founder effects and population bottlenecks. There are constraints on evolutionary possibilities, including those imposed by limits on variation and by the evolutionary history of a lineage. Not all the characteristics of an organism need have an adaptive explanation, and many adaptive arguments must remain hypothetical rather than proven.
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