Download E3_Selection_2011 Part 1

The Nature of Selection
E3: Lecture 2
Darwin’s Voyages
•
The voyage of the Beagle:
– At the age of 22, Darwin set out on a trip around
the world aboard the HMS Beagle.
“What fine opportunities for geology and studying
the infinite host of living beings: Is this not a
prospect to keep up the most flagging spirit?"
– During this five year voyage, Darwin collected
many specimens and studied geological
formations.
– Contributions to Darwin’s later ideas:
• Reading Lyell’s Principles of Geology
• Discovery of fossils similar to known species
• Observation of geographical patterns in groups of
species
The ensuing intellectual voyage:
– Further contributions to Darwin’s theory:
• Discovery that some of his collected “variants” were
actually different species
• Reading Malthus’ Principle of Population
Malthusian Principle
population
amount
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intense
comp.
time
food
supply
The Origin
• What was Darwin trying to explain?
• The collection of living forms on this planet is
diverse:
– There is variation in form within species.
– There is variation in form between species.
– There are hierarchical connections across species.
• Darwin postulated that the variation within
species was the raw material for variation
between species:
rat
“I look at varieties which are in any degree distinct and
permanent, as steps leading to more strongly marked
and more permanent varieties; and at these latter, as
leading to sub-species, and to species.”
wallaby
kangaroo
mouse
• In many cases, organisms are well-adapted to
their environment.
• Darwin proposed a mechanistic process by
which a lineage could change over time,
accounting for adaptation and the origin of
new species:
– Evolution by natural selection.
hummingbird
The Nature of Selection
•
•
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Evolution by natural selection occurs
when:
1. There is variation in phenotypic traits
among individuals within a population.
2. At least some of the variation is
heritable.
3. There is variation in the ability to
survive and reproduce (fitness), which
is correlated to phenotype.
OR… Natural selection occurs when
there is heritable phenotypic variation
correlated with fitness
Darwin’s postulates offer evolutionary
biologists a set of conditions to explore.
–
•
Do natural populations satisfy all three
postulates? If so, why?
Darwin’s postulates spell out criteria for
animal and plant breeders.
phenotype
average fitness
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1
0
Natural Selection
Lecture Outline
• Artificial selection
• Case studies of natural selection
• Dr. Pangloss and some caveats
• Summary
Natural Selection
Lecture Outline
• Artificial selection
• Case studies of natural selection
• Dr. Pangloss and some caveats
• Summary
Darwin’s Analogy
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Breeding explicitly involves a realization of
Darwin’s third postulate:
–
•
pouter
By allowing animals and plants with desired
phenotypes to breed and those with undesired
phenotypes not to breed …there will be variation
in the ability to survive and reproduce (fitness),
which is correlated to phenotype.
Darwin himself was a pigeon breeder and he
spends several pages in his first chapter
discussing the morphological and behavioral
pecularities of the different breeds.
•
All pigeons were known to be (1) descendents
of the rock pigeon and (2) inter-fertile.
•
Darwin claimed an ornithologist “would certainly
[rank these breeds] as well-defined species.”
•
Here Darwin is building his case for the origin
of new species through natural selection
through an analogy with the origin of new
breeds through artificial selection:
“I felt fully as much difficulty in believing that since
they had been domesticated they had all proceeded
from a common parent, as any naturalist could in
coming to a similar conclusion in regard to the many
species of finches, or other groups of birds, in nature.”
tumbler
rock
fantail
trumpeter
The Birmingham roller
Domestication as Selection Experiments
•
Human agriculture and domestication involve a
series of selection experiments.
•
Maize, which accounts for over 20% of human
nutrition, was domesticated from the Mexican
grass teosinte 9000 years ago.
•
A Mediterranean plant is ancestor of a set of
vegetables (cabbage, cauliflower, broccoli, kale,
etc.) via selection over 7000 years.
•
All breeds of dogs derive from the gray wolf via
domestication 10,000-100,000 years ago.
GO
UW!
teosinte
maize
QuickTime™ and a
decompressor
are needed to see this picture.
The Fox Farm Experiment
Clearly Darwin’s first (variability) and third (fitness differences) postulates hold.
Question: How would you establish Darwin’s
second postulate (heritability) experimentally?
Natural Selection
Lecture Outline
• Artificial selection
• Case studies of natural selection
• Dr. Pangloss and some caveats
• Summary
Classic Study of Natural Selection
•
During his voyage aboard the Beagle,
Darwin visited an archipelago of volcanic
islands 600 miles off the west coast of
South America: the Galápagos.
•
These islands are home to a set of
interesting organisms as well as
interesting research.
•
Darwin collected finches from some
of the islands, finding out later that
these belonged to different species.
•
Coming full circle, Rosemary and
Peter Grant have studied natural
selection in Galápagos finch
populations under natural settings.
•
Here, we focus on beak evolution in
the medium ground finch.