What does natural selection “act on”?E.g., the evolution of pepper spiciness…

What does natural selection “act on”?

• the gene• the individual• the population or group• the species

They are in you and me; they created us, body and mind; and their preservation is the ultimate rationale for our existence. They have come a long way, those replicators. Now they go by the name of genes, and we are their survival machines.

The gene…

Kin selection and inclusive fitness ideas are centered on the gene as a unit of natural selection

The group?

Females of many species lay fewer eggs during periods of high population density. Has selection favored “restraint” in breeding?

The main problem with group selection as a major mechanism for altruism…

Group selection may not work well because “cheaters always win”

Group selection favoring “restrained” reproduction in parasites? Maybe.Or kin selection may be a better explanation (parasites reside with relatives)

The species…

Some lineages are much more speciose than others. Why?

Phenotypes and behaviors are the things most “visible” to natural selection

Selection can act on phenotypes in 3 ways:

Directional Selection on Phenotypes (Medium ground finches)

Stabilizing Selection on Phenotypes(Gall-making fly)

Parasitoid wasps kill fly larvae inside small galls

Birds kill fly larvae inside large galls

Selection can act on phenotypes in 3 ways:

Disruptive Selection on Phenotypes(Black-bellied seed cracker)

Small-billed birds specialize on small seeds and survive

Large-billed birds specialize on large seeds and survive

Disruptive selection is probably rare

What kind of selection would you expect there to be on mammalian birth weight? (and why?)What about litter size?

Directional?Stabilizing?Disruptive?

Human survival by birth weight

The graph is based on infants born in London from 1935 to 1946. From Karn and Penrose (1951).