Download Chapter 3 How natural selection works

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Influence of geological thinking on
By time of “The Beagle” voyage idea that
Earth was young was being challenged.
Opposition based on principle of
Idea that geological processes happening
today are the same as have operated in
the past.
Influence of geological thinking on
Uniformitarianism contrasted with
Catastrophism which proposed that
current geological formations had resulted
from catastrophic events (such as biblical
flood) which occurred on scale unknown
Uniformitarianism first proposed by James
Hutton and championed by Charles Lyell.
Influence of geological thinking on
Hutton and Lyell inferred Earth must be
very old based on measurements of rate
of ongoing rock forming processes (e.g.
deposition of mud and sand).
These developments in geology focused
Darwin on the potential importance of
gradual change in shaping structures.
Darwinian Natural Selection
Artificial Selection
Artificial Selection. Humans have
selectively bred for desirable traits in
domestic animals and plants for millenia.
Process has produced our crop plants,
garden plants, pets, and domestic
Darwin closely studied pigeon breeding.
Artificial Selection
Cauliflower, broccoli, kale, brussels
sprouts all descended from wild cabbage.
All these crops can be crossed and
produce fertile offspring.
Cauliflower: edible bit is the inflorescence
or flower stalk.
Artificial Selection
Cauliflower has large dense infloresence.
This results from mutant ‘loss of function’
alleles of two genes that affect flower
structure and infloresence density.
Artificial Selection
Early farmers choosing among their crops
selected those with largest infloresences.
Process has resulted in cauliflowers that
are homozygous for both loss of function
Evolution by Natural Selection
Darwin envisaged process similar to
artificial selection that had produced
organisms we see today. He called it
Natural Selection.
Evolution by Natural Selection
Darwin proposed evolution the inevitable
outcome of 4 postulates:
1. There is variation in populations.
Individuals within populations differ.
2. Variation is heritable.
Evolution by Natural Selection
3. In every generation some organisms
are more successful at surviving and
reproducing than other. Differential
reproductive success.
4. Survival and reproduction are not
random, but are related to variation among
individuals. Organisms with best
characteristics are ‘naturally selected.’
Evolution by Natural Selection
If 4 postulates are true then the population
will change from one generation to the
Evolution will occur.
Evolution by Natural Selection
Darwinian fitness: ability of an organism to
survive and reproduce in its environment.
Fitness measured relative to others of its
Evolution by Natural Selection
Adaptation is a characteristic or trait of an
organism that increases its fitness relative
to individuals that do not possess it.
Testing the postulates
Evolution of beak shape in Darwin’s
Peter and Rosemary Grant’s (and
colleagues) work on Medium Ground
Finches Geospiza fortis
On Daphne Major since 1973.
Evolution of beak shape in Darwin’s
Postulate 1. Is the population variable?
Finches vary in beak length, beak depth,
beak width, wing length and tail length.
Evolution of beak shape in Darwin’s
Postulate 2: Is variation among individuals
Variation can be a result of environmental
Heritability: proportion of the variation in a
trait in a population that is due to variation
in genes.
Evolution of beak shape in Darwin’s
Peter Boag compared average beak depth
of parents with that of their adult offspring.
Strong relationship between offspring and
parent beak depths.
FIG 3.7
Evolution of beak shape in Darwin’s
Postulate 3: Do individuals differ in their
success at survival and reproduction?
1977 drought 84% of G. fortis individuals
died, most from starvation. In two other
droughts 19% and 25% of the population
Evolution of beak shape in Darwin’s
Seed densities declined rapidly during
drought and the small soft seeds were
consumed first.
Average size and hardness of remaining
seeds increased over the course of the
FIG 3.8b
FIG 3.8A
Fig 3.8c
Evolution of beak shape in Darwin’s
Postulate 4: Are survival and reproduction
Do those who survive and reproduce have
different characteristics than those that
Evolution of beak shape in Darwin’s
As drought progressed small soft seeds
disappeared and large, hard Tribulus
seeds became a key food item.
Only birds with deep, narrow beaks could
open them.
Evolution of beak shape in Darwin’s
At end of the 1977 drought the average
survivor had a deeper beak than the
average non-survivor and also a larger
body size.
FIG 3.9
Did the population evolve?
Chicks hatched in 1978 had deeper beaks
on average than those hatched in 1976.
Population evolved.
Fig 3.10
Evolution of beak shape in Darwin’s
Variation in weather from year to year on
Daphne Major over 30 years has led to
variation in the traits that are favored by
Population has evolved over time.
Fig 3.11 A
Over the course of 30 years (1970 to 2000) beak size
evolved. Rose sharply during drought (red line) then
declined to pre-drought dimensions.
Fig 3.11 B
Over same 30-year period birds evolved more pointed
beaks and (next slide) significantly smaller body size.
Fig 3.11 C
The nature of Natural Selection
Many misconceptions about how selection
operates and evolution occurs.
Points to remember about natural
Natural selection acts on individuals,
but its effects accumulate in
Individual finches live or die during a
drought (the selection event).
But change occurs in the characteristics of
the population, not in individuals.
Natural selection acts on
individuals, but its effects
accumulate in populations
During drought individual finch’s beaks
did not change, but average beak
dimensions changed because more
small-beaked birds died than largebeaked birds.
Evolution causes changes in
allele frequencies
Evolution only occurs when traits have a
genetic basis.
If beak dimensions were environmentally
induced, no evolution could take place.
After drought, frequencies of phenotypes in
next generation might have been the same
as before.
Natural selection does not plan
Each generation is result of selection by
environmental conditions of the previous
Evolution always one generation behind
environmental changes.
New traits evolve even though
selection acts on existing
This occurs because:
1. mutation produces new alleles.
2. In sexually reproducing organisms
meiosis and fertilization recombine
existing alleles to produce new genotypes.
New traits evolve even though
selection acts on existing traits.
Artificial selection for oil content in corn.
After 60 generations oil levels were well
above starting values.
Fig 3.12
New traits evolve even though
selection acts on existing traits.
Natural selection can also modify existing
features over time for a new purpose e.g.
Panda’s thumb.
Trait used in novel way and eventually
developed into a new structure referred to
as a preadaptation. This does NOT
mean there is pre-planning by natural
Natural selection does not
produce ‘perfect’ solutions
Panda’s thumb not a perfect solution.
Natural selection does not
produce ‘perfect’ solutions
On Daphne Major during drought finches with
narrow beaks survived better than those with
wider bills.
At end of drought, however, selection for
deeper bills and bigger body size resulted in
wider beaks even through deeper narrower
beaks would have been a better solution.
Presumably same genes control all three
traits. And solution is not perfect.
Natural selection does not
produce ‘perfect’ solutions
Similarly, many characteristics of
organisms are the result of compromises
between different selection pressures.
The wings of various species of auk
(seabirds that dive and swim) are a
compromise between the need to “fly” (i.e.
swim) underwater and in the air, two very
different media.
Little Auk
Natural selection is nonrandom,
but not progressive
There is no “goal” of natural selection.
Evolution makes organisms better adapted
to their environments, but there is no trend
towards being more advanced.
E.g. Tapeworms have no digestive tract.
They are simpler than their ancestors.
Selection does not act for the
“good of the species”
Apparently altruistic acts (e.g. giving an
alarm call) are favored because they
enhance relatives’ survival.
Infanticide in lions benefits individual male
lions not the species as a whole.
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