Transcript What is Natural Selection?

What is Natural Selection?
Elliott Sober
Philosophy Department
UW-Madison
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topics
1. Natural selection and artificial selection
2. Natural selection and the struggle for existence
3. Fitness and actual reproductive success
4. Natural selection as variation in fitness
5.Natural selection ≠ evolution by natural selection
6. Natural Selection and Drift
7. Natural selection and side-effects
8. “Adaptation” and “Adaptive”
9. Natural selection and sexual selection
10. Group selection
11. Can altruism evolve by natural selection?
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1. Natural selection and artificial selection
• the obvious difference
• Is variation necessary for artificial selection?
• Is variation necessary for natural selection?
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1. Natural selection and artificial selection
• the obvious difference
• Is variation necessary for artificial selection?
• Is variation necessary for natural selection?
• Darwin became unhappy with the label “natural selection” and
thought that “the survival of the fittest” would be better.
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2. Natural selection and the struggle for existence
Darwin’s debt to Thomas Malthus’s (1798) Essay on the Principle of
Population:
Population increases geometrically.
Food increase arithmetically.
So, eventually there will be starvation.
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2. Natural selection and the struggle for existence
“I should premise that I use the term Struggle for Existence in a large
and metaphorical sense, including dependence of one being on
another, and including (which is more important) not only the life of
the individual, but success in leaving progeny. Two canine animals in a
time of dearth, may be truly said to struggle with each other which
shall get food and live. But a plant on the edge of a desert is said to
struggle for life against the drought …” (Darwin 1859, ch. 3)
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2. Natural selection and the struggle for existence
Carrying capacity
(n individuals)
A
B
A organisms have 3 babies per generation; B organisms have 2.
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3. fitness and actual reproductive success – how are they related?
• The tautology problem:
Q: “Why were X individuals more reproductively successful than Y individuals?”
A: “Because X individuals were fitter than Y individuals.”
• What is a “tautology”?
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3. fitness and actual reproductive success – how are they related?
• The tautology problem:
Q: “Why were X individuals more reproductively successful than Y individuals?”
A: “Because X individuals were fitter than Y individuals.”
• What is a “tautology”? A tautology is a statement that is true by
definition: “Bachelors are unmarried,” “either it is raining now or it is
not raining now.”
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3. fitness and actual reproductive success – how are they related?
• The tautology problem:
Q: “Why were X individuals more reproductively successful than Y individuals?”
A: “Because X individuals were fitter than Y individuals.”
Suppose fitness means actual reproductively success. This is the
definition: X is fitter than Y if and only if X is more reproductively
successful than Y.
Why does this definition create a problem?
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3. fitness and actual reproductive success – how are they related?
• The tautology problem:
Q: “Why were X individuals more reproductively successful than Y individuals?”
A: “Because X individuals were fitter than Y individuals.”
Suppose fitness means actual reproductively success. This is the
definition: X is fitter than Y if and only if X is more reproductively
successful than Y.
Is this statement a tautology: “X was more reproductively successful
than Y because X was fitter than Y”?
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3. fitness and actual reproductive success – how are they related?
• The tautology problem:
Q: “Why were X individuals more reproductively successful than Y individuals?”
A: “Because X individuals were fitter than Y individuals.”
• What does “fitter” mean? What does “fitter” mean?
Solution: Don’t define fitness as actual reproductive success. Define it
as a probability of reproductive success.
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It might be true that X was more reproductively successful than
Y because X was fitter than Y, but this need not be so.
What’s the other possibility?
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It might be true that X was more reproductively successful than
Y because X was fitter than Y, but this need not be so.
What’s the other possibility?
It could be that X and Y were equally fit, and that X happened to be
more reproductively successful than Y by chance.
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An analogy
We have two 10 Coins labelled X and 10 Coins labelled Y and toss each of
them 5 times. Suppose we get 28 heads among the X coins and 25 heads
among the Y coins. Why did this happen?
Here are two hypotheses:
H1: X coins have a higher probability of landing heads than Y coins do.
H2: X coins and Y coins have the same probability of landing heads.
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4. Natural selection as variation in fitness
A definition: A population of organisms experiences natural selection
at time t if and only if the organisms in the population differ in fitness
at time t.
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4. Natural selection as variation in fitness
A definition: A population of organisms experiences natural selection
at time t if and only if the organisms in the population differ in fitness
at time t.
The breeder’s equation:
Response to selection = heritability x strength of selection
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5.Natural selection ≠ evolution by natural selection
• Natural selection can occur without evolution.
• Evolution can occur without natural selection.
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6. Natural Selection and Drift
When two traits have the same fitness value, trait frequencies can
evolve, but this won’t be because of natural selection. Rather, the
change is due to drift.
When there is variation in fitness in a finite population, it is possible
that the fitter trait fails to increase in frequency. This is due to drift.
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Variation in fitness versus no variation in fitness in a finite population
Trait A is fitter than trait B
0
100
% A in the Population
Traits A and B are equally fit
0
100
% A in the Population
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Selection and Population Size
If trait A is fitter than trait B and the traits are heritable, the probability
that A will increase in frequency goes up (and approaches 1) as
population size goes up.
In the limit (= infinite population size), trait A must increase in
frequency. Geneticists often describe this by saying that selection is
“deterministic.”
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Selection and population size
Population size?
Fitness
differences?
A fitter than B
A,B equal in fitness
finite
infinite
Selection + drift
Pure selection
Pure drift
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7. Natural selection and side-effects
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In this toy, a ball’s size, not its color, influence how far the ball descends when
the toy is shaken.
If traits X and Y are correlated, where X is advantageous and Y is neutral,
selection can cause both X and Y to evolve.
There is selection for X, but no selection for Y.
biological examples that are similar
• red blood and hemoglobin
• Male nipples and female nipples
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8. “Adaptation” and “Adaptive”
• Trait T is now an adaptation in a population if and only if T evolved in
the lineage leading to that population because there was selection for
trait T.
• Trait T is now adaptive in a population if and only if T now helps
organisms to survive and reproduce.
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8. “Adaptation” and “Adaptive”
• Trait T is now an adaptation in a population if and only if T evolved in
the lineage leading to that population because there was selection for
trait T.
• Trait T is now adaptive in a population if and only if T now helps
organisms at time to survive and reproduce.
Don’t confuse the current utility of a trait
with the reason the trait evolved!
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examples
• Skull sutures in mammals now facilitate live birth, but …
• The forelimbs of turtles now help them to dig nests in the sand, but …
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9. Sexual and natural selection – how are they related?
Darwin says:
“Sexual Selection ... depends, not on a struggle for existence, but on a
struggle between males for possession of the females; the result is not
death to the unsuccessful competitor, but few or no offspring. Sexual
selection is therefore less rigorous than natural selection.”
(Origin of Species, p88).”
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10. Group selection
A population of organisms experiences individual selection
if and only if some of the individuals are fitter than others.
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10. Group selection
A population of organisms experiences individual selection
if and only if some of the individuals are fitter than others.
A metapopulation of groups experiences group selection
if and only if some of the groups are fitter than others.
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10. Group selection
A metapopulation of groups experiences group selection
if and only if some of the groups are fitter than others.
Why did Darwin introduce the hypothesis of group selection?
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one example: the honeybee’s barbed stinger
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11. Can altruism evolve by natural selection?
Altruists, by definition, are less fit than selfish individuals in the same group.
For example, consider a metapopulation of groups of size 2.
In each group there are two individuals (call them X and Y).
Here are their fitnesses:
Y is altruistic
Y is selfish
X is altruistic
3,3
1,4
X is selfish
4,1
2,2
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11. Can altruism evolve by natural selection?
Altruists, by definition, are less fit than selfish individuals in the same group.
Y is altruistic
Y is selfish
X is altruistic
3,3
1,4
X is selfish
4,1
2,2
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11. Can altruism evolve by natural selection?
Altruists, by definition, are less fit than selfish individuals in the same group.
If altruists always live with selfish individuals, which trait is fitter?
Fitness of altruists = 1; fitness of selfish individuals = 4
Y is altruistic
Y is selfish
X is altruistic
3,3
1,4
X is selfish
4,1
2,2
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11. Can altruism evolve by natural selection?
Altruists, by definition, are less fit than selfish individuals in the same group.
If like always lives with like, which trait is fitter?
Y is altruistic
Y is selfish
X is altruistic
3,3
1,4
X is selfish
4,1
2,2
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11. Can altruism evolve by natural selection?
Altruists, by definition, are less fit than selfish individuals in the same group.
If like always lives with like, which trait is fitter?
Fitness of altruists = 3; fitness of selfish individuals = 2
Y is altruistic
Y is selfish
X is altruistic
3,3
1,4
X is selfish
4,1
2,2
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11. Can altruism evolve by natural selection?
Altruists, by definition, are less fit than selfish individuals in the same group.
The definition doesn’t settle whether altruists are on average fitter than selfish
individuals. What it takes for altruism to be fitter is for similar individuals to tend to
live together.
Y is altruistic
Y is selfish
X is altruistic
3,3
1,4
X is selfish
4,1
2,2
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