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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.
Evolutionary relationships in the biosphere
DNA sequence data has allowed estimation of the points of divergence between the lineages of the organisms, including plants, fungi, animals and prokaryotes, extant today. The common precursor to all forms of life lived about 3 billion years ago, and the earliest fossil evidence of bacteria is dated to about 3.5 billion years ago – the age of the earliest stromatolites, which are still formed by some cyanobacteria. The organisation of the prokaryotic kingdoms is complex and uncertain, probably reflecting horizontal transfer of genes between emergent species. Two major groupings are recognised: the Archaea (which includes many anaerobic organisms which live in extreme environments) and the Bacteria which includes the bacteria we are traditionally familiar with. The first vertebrates evolved some 525 million years ago, whereas it was only about 640 million years ago that the first multicellular organisms appeared. Dinosaurs (and their descendants, birds) along with other classes of reptile diverged from the lineage which was to lead to mammals some 350 million years ago and the large dinosaurs emerged between 250 and 300 million years ago. Some 70-80 million years ago all species of extant primate shared a common ancestor and placental mammals including marsupials shared a common ancestor as recently as about 110 million years ago, whereas monotremes shared a common ancestor with birds, reptiles, some 300 million years ago.