Natural Selection

Natural Selection is based on reproductive success (fitness)…

• Absolute fitness (think how hard that is to measure for genotypes)

• Relative fitness• Relative fitness depends on other genotypes

• Fitness values may change when the environment changes

Normal and sickle hemoglobin alleles

AA and AS SS

Which genotype has the highest fitness in our environment?

AA

Which genotype has the highest fitness in malaria environment?

AS

Malaria parasitesdestroy RBC’s

To predict what will happen in this environment, need to know relative fitness:• When malaria is present:

– Relative fitness AS = 1.00– Relative fitness AA = 0.91—reduced by malaria– Relative fitness SS = 0.11—reduced by sickle-cell anemia

• What IF the environment changes?– What will happen to relative fitness if malaria becomes more

virulent (or more resistant to available treatments)?– What will happen to relative fitness if malaria is eradicated?

Another hemoglobin allele you might not have heard of, “C”

Hemoglobin c allele frequency indicated by red

What happens to fitness if C allele is present? (AC, SC, and CC genotypes)

•Assignment

Selection pressure: Intense vs. weakDepends on fitness differentials

“Winning” with Natural Selection

“Live Long and Prosper”

Strategy 1:Survive long enough to reproduce Provide well for your offspringStraight “Natural Selection”

Medium ground finches on Daphne Major

Rosemary and Peter GrantField work since 1973 (half of each year)

A variable population

Variation is heritable

Drought:Seeds rarerSeeds harder

Many medium ground finches died!

Avg. beak depth increased

Note: the babies born later had bigger beaks… (due to differential reproduction in their parents’ generation)