Chapter 5: Adaptationism

The Power of Natural Selection

When did the debate begin?

When did the debate begin?

How did Gould and Lewontin characterize adaptationism?

(1) An organism is atomized into "traits" and these traits are explained as structures optimally designed by natural selection for their functions.

(2) After the failure of part-by-part optimization, interaction is acknowledged via the dictum that an organism cannot optimize each part without imposing expenses on others.

How did Gould and Lewontin characterize adaptationism?

(1) If one adaptive argument fails, try another. (2) If one adaptive argument fails, assume that

another must exist; a weaker version of the first argument.

(3) In the absence of a good adaptive argument in the first place, attribute failure to imperfect understanding of where an organism lives and what it does.

(4) Emphasize immediate utility and exclude other attributes of form.

What is adaptationism?

(U) Natural selection played some role in the evolution of T in the lineage leading to X.

(I) Natural selection was an important cause of the evolution of T in the lineage leading to X.

(O) Natural selection was the only important cause of the evolution of T in the lineage leading to X.

Adaptationism: Most phenotypic traits in most populations can be explained by a model in which selection is described and nonselective processes are ignored.

What is adaptationism?

Some qualifications:– Adaptationists well realize that which phenotype is fittest

depends on the biological details.– Adaptationists might expect zebras to evolve from Slow to

Fast but will not expect them to evolve machine guns with which to counter lion attacks ( Krebs and Davies 1981). When adaptationists say that the fittest trait will evolve, they mean the fittest of the traits actually present in the population, not the fittest of all the traits we can imagine.

– No adaptationist holds that variation is limitlessly rich. This is why zebras have not evolved machine guns and why pigs don't fly.

How genetics can get in the way

Heterozygote has medium fitness. A allele goes to fixation.

How genetics can get in the way

Heterozygotes are superior. Stable polymorphism at p.

Sickle cell anemia

How genetics can get in the way

Heterozygote inferiority Either A or a goes to

fixation. Fitness is frequency dependent.

Is adaptationism testable?

Are factors that prevent the fittest trait from evolving (such as negative pleiotropy or heterozygote superiority) common or rare?

“According to critics and defenders alike, adaptationism seems to be an assumption rather than a hypothesis under test.”

“Whether adaptationism is testable is a quite separate question from how adaptationists behave.”

There is no single observation that could refute adaptationism. Adaptationism is tested in the long run because it is a research program.

The truth or falsity of adaptationism is determined after a lengthy chain of observations and experiments. How often do adaptationist models succeed in explaining specific phenomena? This is the crucial question.

Is adaptationism an a priori truth?

“Universal Darwinism” (Richard Dawkins) Lamarckism vs Natural Selection vs Pure

Chance (mutation, drift, etc) True test: natural selection (NS) vs

NS+M(utation) vs NS+GD vs NS+M+GD