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Evolutionary relationships in the biosphere

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Evolutionary theory: key points

  • Evolutionary science explains how the huge diversity of present and past life forms arose.
  • Hereditable 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 an individual’s 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 phenotypics characteristics influencing survival and reproduction (natural selection) or ability to obtain a mate (sexual selection).
  • It is a fundamental principle of evolutionary medicine that selection acts to optimize reproductive success, not necessarily the health or longevity of an individual.
  • Random genetic drift can influence the evolution of a species, particularly in the presence of founder effects and population bottlenecks.
  • While evolution does not have a purpose or a direction 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.
  • Many adaptive arguments, no matter how plausible, must remain hypothetical rather than proven. Evolutionary thinking should avoid the trap of teleology.


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