1. Checking the Postulates
First Postulate: By simply collecting and measuring birds, the Grants demonstrated that there is variation for a number of morphological traits, including beak depth.
Question: How would you
establish heritability?
Second Postulate: The beak depth of offspring correlates with the average beak depth of their parents.
More recent work, including natural situations involving “cross-fostering” (males raising chicks that are not their offspring) and genetic work suggests that beak depth is heritable.
Question: What are the problems with
this approach? 1

2. Checking the Postulates (Continued)
Third Postulate: The average beak depth increased after the drought as the survivors had deeper beaks.
Question: What are your predictions
about how the drought of 1977
affected the finches? 2

3. Beak Evolution Via Natural Selection
All of Darwin’s postulates have been confirmed for the phenotype of beak depth in the medium ground finch.
Given that these constitute sufficient conditions for evolution by natural selection– the population of finches should evolve in the predicted direction.
Indeed, the Grants and their colleagues found that beak depth in hatched chicks did increase across the drought of 1977. 3

4. A Mouse’s Tale: Selection for Coat Color
In the early 1900’s, naturalists proposed that the dorsal coat color of field mice was the product of natural selection.
Hopi
Hoekstra
Hopi Hoekstra and her colleagues have studied selection in Peromyscus polionotus, oldfield mice of the southeast US.
Most mice live on dark soils and have a relatively dark dorsal coat.
On the beaches in the Gulf states, the mice have lighter coats.
Postulate 1: Mice do differ in dorsal coat color (e.g., between mainland and beach mice). 4

5. A Mammoth Discovery: The Genetic Basis
The data presented for heritability of beak size in the finches was correlative– in general, evolutionary biologists strive to discover the genetic basis of phenotypes.
Through a series of breeding and genetic mapping studies, Hoekstra and colleagues identified a gene, Mc1r (the melanocortin-1 receptor) that affected dorsal coat color.
The product of this gene signals for dark eumelanin to be produced when activated.
Postulate 2: Coat color is heritable. Beach and mainland mice differed by a single mutation in this gene; the “mainland protein” responded to activator, whereas the “beach protein” did not.
The exact same mutation has been found in Mc1r sequences from 14,000 year old DNA extracted from mammoth bones!
Mainland
Mc1r
beach Mc1r
blonde wooly
mammoths? 5

6. Securing Postulate 3
Vignieri (a UW alumnus!) and her colleagues performed an experiment to gather evidence for Darwin’s third postulate.
Sacha Vignieri
Joanna Larson
Hopi Hoekstra
They made plasticine models of mice and painted them light (“beach colored”) or dark (“mainland colored”).
They placed models in both mainland and beach habitat.
They found that the non-cryptic color was attacked significantly more often.
These authors have provided strong evidence that coat color has evolved by natural selection. 6

7. Selection in Our Backyard?
Brooks Miner, former graduate student in UW Biology, studied adaptation in Daphnia melanica in the Seven Lakes Basin in the Olympic Mountains.
Lakes differ in how much ultraviolet radiation (UVR) permeates (depending on the dissolved organic material).
Because UVR can cause detrimental mutations, it is reasonable to ask if Daphnia have adapted to deal with it.
Brooks Miner
Daphnia
melanica
Pond below treeline
 High organic input
 Low transperency
 Low UVR exposure
Pond above treeline
 Low organic input
 High transperency
 High UVR exposure 7