Lecture 14 - Adaptation and the genetics thereof

Lecture 14 - Adaptation and the genetics thereof - 1...

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1 Lecture 14 - Adaptation and the Genetics Thereof I. Adaptation - Perhaps the most obvious fact in biology is that organisms are well- suited to their environments. Fish have fins, birds have wings, primates have grasping hands. Each of these structures is derived from the same homologous structure in development, and each has taken on a different form that obviously allows the organism to better survive in its environment. How well-suited is well suited? Pick an environment at random and pick an organism at random. Now compare the fitness of the random organism in the specific environment to that of any organism that lives in that env't. Arguably, the central mission of evolutionary biology is to explain adaptation. Specifically, why organisms have the traits they do and how they got them. Q: What is meant by "adaptive"? A trait is "adaptive". Roughly, means "confers higher fitness on its bearers than others that lack the trait". e.g., "beneficial mutation" "adaptive mutation" implies increase in fitness over the current optimum More specifically, a trait is an adaptation if it evolved in response to direct selection on that trait. Traits that are adaptations are adaptations to something, or for something. Wings are adaptations for flying. Did wings evolve in (insects, birds, bats) because of direct selection for flying? - The Adaptationist Program(me) - basically, the idea that every feature of an organism is optimized for maximum fitness in its current environment. Thus, we ask the question: what is (trait) an adaptation for? (or to) This line of reasoning leads to the question: how come there are no organisms that produce an infinite number of offspring? Surely that would maximize fitness. The obvious logical conclusion is that there must be tradeoffs - trait values that increase one component of fitness (e.g., fecundity) inevitably reduce another component of fitness (offspring survival). For a finite-sized organism to have an infinite number of offspring in a finite amount of time they would have to be infinitely small, which would reduce their chances of survival. - Gould and Lewontin offer a hierarchy of alternative explanations to what they call "immediate" adaptation Note that they (and any evolutionary biologist) do not doubt the primacy of natural selection. 1. Neutral evolution: neutral mutation + drift, including fixation of slightly deleterious alleles. For example, snail shell colors on hammocks in the Everglades.
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2 2. No adaptation, no selection on the trait in question, but rather a correlated response to selection on some other trait. E.g., size and age at maturity. 3. The decoupling of selection and adaptation i) Selection without adaptation - Lewontin offers the example of doubling of fecundity without a concomitant increase in offspring survival. Is this a logical conclusion? Has the trait (increased fecundity) not evolved in response to direct selection on the trait (selection for increased fecundity)? ii) Phenotypic plasticity
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Lecture 14 - Adaptation and the genetics thereof - 1...

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