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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.
Question: What are the problems with
this approach?
•
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.
Checking the Postulates (Continued)
Question: What are your predictions
about how the drought of 1977
affected the finches?
•
Third Postulate: The average beak depth
increased after the drought as the
survivors had deeper beaks.
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.
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).
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?
Securing Postulate 3
•
Vignieri (a UW alumnus!) and her
colleagues performed an experiment
to gather evidence for Darwin’s third
postulate.
Sacha Vignieri
•
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 noncryptic color was attacked
significantly more often.
•
These authors have
provided strong evidence
that coat color has evolved
by natural selection.
Joanna Larson
Hopi Hoekstra
Selection in Our Backyard?
•
Brooks Miner
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.
Pond below treeline
 High organic input
 Low transperency
 Low UVR exposure
Pond above treeline
 Low organic input
 High transperency
 High UVR exposure
Daphnia
melanica