UBC Theses and Dissertations
Adaptive challenges : fitness-valley crossing and evolutionary rescue Osmond, Matthew M
One of the most striking features of the natural world is the fit of an organism to its surroundings. Much of this fit, i.e., adaptation, arises from evolution by natural selection. Adaptation is thus often thought to be a sure thing; eventually a beneficial allele will arise and/or increase in frequency, ad infinitum. But sometimes adaptation is more challenging and evolution by natural selection is not a sure thing. This thesis deals with one such type of adaptive challenge, adaptation that requires the prolonged persistence of genotypes that are expected to be declining in number. The first example is fitness-valley crossing, where adaptation is the result of multiple components that are each selected against when alone but are beneficial in combination. Chapter 2 extends the mathematical framework describing such situations to include biased transmission of traits from one generation to the next. The analysis shows that meiotic drive, uniparental inheritance, and cultural inheritance can greatly facilitate peak shifts across a valley of low fitness. Chapters 3-5 deal with a second example, evolutionary rescue, where declining populations are rescued from extinction by rapid adaptation. Two of the mathematical models analyze how species interactions (predation) and alternative selective surfaces (fitness functions), respectively, affect the ability of a focal population to adapt and persist in a gradually changing world. They find that predators can counterintuitively help prey persist (e.g., through an 'evolutionary hydra effect') and that weakening selection (i.e., antagonistic epistasis) can produce unexpected extinctions ('evolutionary tipping points'). The final model explores evolutionary rescue following an abrupt environmental change on a fitness landscape. The analysis shows that rescue can be more likely by two mutations than one and that the number of mutations that rescue takes leaves a signature in the distribution of fitness effects among survivors.
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