What does natural selection “act on”?
E.g., the evolution of pepper spiciness…
What does natural selection “act on”?
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the gene
the individual
the population or group
the species
The gene…
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.
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).