The thrifty phenotype hypothesis assumes that over much of our evolutionary history the environment experienced by a fetus was highly predictive of the environment the individual would experience throughout life, metabolism is plastic, traits such storing fat in the abdomen are adaptive under certain circumstances. The hygiene hypothesis assumes that the environment experienced by many modern human societies is much different from the environment experience over most of our evolutionary history.The hypothesis that within-host selection favors higher virulence assumes that more rapidly replicating strains cause more damage to their hosts. Pathogens evolve rapidly because they reproduce rapidly and their mutation rates are high. In the mid-1990s, Svante Pääbo’s first analyses of Neanderthal mtDNA showed that Neanderthals and modern humans diverged before the ancestor of today’s humans had left Africa.The thrifty genotype hypothesis proposes that alleles causing diseases such as diabetes were advantageous at some point in our evolutionary history.Based on the history of marsupial colonization, extinction, and vicariance events, a phylogenetic tree of currently existing marsupials should show that South American marsupials are more closely related to North American marsupials than to Australian marsupials.Bird feathers are considered to be an exadaptation because they first evolved to serve a different function than they serve in modern birds.
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