Download 1 What is Evolution? What causes evolution? What is natural

BIOL2007 Evolutionary Genetics
course website: http://ucl.ac.uk/~ucbhdjm/courses/
(searching for “BIOL2007 timetable” on Google is easier!)
What causes evolution?
What is Evolution?
a) Natural selection
b) Mutation
c) Genetic drift, or neutral,
random evolution
e) Migration, or gene flow
Darwin: “descent with modification”
A change in morphology, ecology, behaviour,
physiology
Change must be genetic
This lecture: simple examples of evolution by
natural selection
Modern, genetic definition:
“evolution is change in gene frequencies
between generations”
What is natural selection?
The peppered moth Biston betularia
“a consistent bias in survival or fertility between
genotypes within generations”
Selection often causes evolution, but may also
prevent evolution (e.g. stable polymorphism)
Evolution does not require selection (e.g. drift -important: > 95% of genome maybe "junk"!)
However, many interesting types of evolution
involve natural selection
Left: form typica (left, and
carbonaria (right) on lichen-covered
trunk in Dorset.
Right: on soot-covered tree near Birmingham
A flow diagram for evolution by ns
Selection against recessive allele
Selection AGAINST recessive allele (= selection FOR dominant allele)
Random mating
Suppose there is “viability selection” (i.e. survival affected) so that …
Offspring genotypes in
Hardy-Weinberg ratios
Natural
selection
Offspring after selection
So now you can write an
evolution computer program!
Numerical vs. analytical theory
Genotypes
AA
Aa
aa
Total
Relative fitness, W
1
1
1-s
-
Genotype frequencies
before selection
(Hardy-Weinberg law)
p2
2pq
q2
1
2pq
q2(1-s) <1
Rel. frequencies
p2
after selection
in this simple model, s is the “selection coefficient” (≈ fraction dying)
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BIOL2007 – SELECTION AND THE SINGLE GENE
SELECTION AGAINST RECESSIVE ALLELE
(EQUIVALENT TO SELECTION FOR DOMINANT ALLELE)
Suppose there is viability selection so that …
Genotypes
AA
Aa
aa
Total
Relative fitness, W
1
1
1-s
Frequencies before selection
(Hardy-Weinberg law)
p2
2pq
q2
1
Relative genotype frequencies
after selection
p2
2pq
q2(1-s)
≠1
Frequencies should sum to 1! Therefore, need to divide by
“mean fitness,” W = p 2 + 2 pq + q 2 (1 − s ) = 1 − sq 2
Genotype frequencies
after selection
p2
1 − sq 2
WHAT IS THE NEW FREQUENCY OF THE A
ALLELE
2 pq
1 − sq 2
q2 (1-s)
1 − sq 2
(p’)?
p’ = new frequency of AA + ½ new frequency of Aa
p' =
p 2 + 12 2 pq p 2 + pq p( p + q)
p2
1 2 pq
p
+
=
=
=
=
2
2
2
2
2
1 − sq
2 (1 − sq )
1 − sq
1 − sq
1 − sq
1 − sq 2
WHAT IS THE RATE OF EVOLUTION PER GENERATION? We need to know
the CHANGE OF GENE FREQUENCY, ∆p (obtained by subtracting old gene
frequency from the new gene frequency).
p
p − p(1 − sq 2 )
spq 2
∆p = p'- p =
−p=
=+
1 − sq 2
(1 − sq 2 )
1 − sq 2
This is the basic equation for all of evolution by natural selection!
1
The basic equation for evolution
Natural selection at a dominant gene
spq 2
∆ p = p '- p = +
≈ spq 2
1 − sq 2
(if s is small)
In words:
Dominance vs. recessives
We can now answer the question: How fast do populations respond
to natural selection?
Answer:
∆p =
spq 2
1 − sq 2
(p is frequency of A, q is freq. a)
2
If p is small, ~0.01 or less, q → 1; q → 1 : ∆p ≈
If p is large, so that q ≈ 0.01 or less,
sp
, i.e. RAPID
1− s
p → 1 : ∆p ≈
sq 2
, i.e. very SLOW
1
(q2 is a square of a very small number Ÿ is itself even smaller!)
The change in gene frequency per
generation is proportional to spq2
RESULT:
Selection for/against a DOMINANT gene at low frequency is RAPID (∝ p)
Selection for/against a RECESSIVE gene at low frequency is SLOW ((∝ q2)
…. many new single genes for resistance (melanism, insecticide resistance
and so on) are dominant!
The speed of evolution
(the rate of gene frequency change per unit time)
p
time (generations)
rare gene recessive
More generally …
Complications – many!
Overlapping generations
Many different kinds of selection
fertility selection
sexual selection
Dominance not complete
AA Aa
aa
1
1–hs 1–s
Non-random mating
inbreeding
mate choice
Multiple genes …
&c &c….
rare gene dominant
But the basic principle remains the same!
(from a programme written by a former B242 student,
Wei-Chung Liu, available from the B242 website)
Take-home points
Evolution to a geneticist: a change in gene frequencies.
Natural selection: a consistent bias favouring some genotypes over others.
Evolution can occur in the absence of natural selection,
via genetic drift or neutral evolution.
Natural selection can stabilize the status quo; zero evolution.
Further reading
FUTUYMA, DJ 2005. Evolution.
Chapter 12:270-280.
For readings on examples, see: Science Library: View BIOL2007 or B242
Teaching Collection by going to eUCLid; use Keyword, Basic Search,
All Fields: B242.
Evolution at a single dominant gene: rate can be predicted
If selected, dominant alleles evolve quickly when rare, slowly when
common; recessive alleles evolve slowly when rare, quickly when
common.
We can estimate selection coefficients (s), fitnesses (W=1-s)
and predict rates of evolution from data on survival or fecundity.
Mathematical theory makes evolution a predictive science
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ESTIMATING SELECTION
1) Change of gene frequencies per generation;
result of selection, estimate ∆p; e.g. peppered moth; JBS Haldane estimated s = 0.5.
2) Distortion of Hardy-Weinberg ratios - problems? see next lecture
3) Comparison of birth or death rates between individuals
W = RELATIVE fitness
MOST DIRECT METHOD
USING METHOD 3 TO ESTIMATE SELECTION IN PEPPERED MOTH
e.g. survival in a field experiment on the peppered moth
A) Central Birmingham
number
released
number
recaptured
proportion
recaptured
relative
fitness, W
typica
carbonaria
18
140
0.125
0.288
0.43
1.00
number
recaptured
67
32
proportion
recaptured
0.411
0.225
relative
fitness
1.82
1.00
144
486
B) Dorset wood
number
released
typica
163
carbonaria 142
(W, the other
way round)
1.00
2.30
SUMMARY OF FITNESSES:
typica
City
Wood
Wcc
0.43
1.82
carbonaria
WCc
1
1
WCC
1
1
(Wc = 1 - sc)
selection
coefficient against c
sc
+0.57
-0.82
HOW FAST WILL CARBONARIA INCREASE IN FREQUENCY in a city?
∆p = spq2/(1-sq2); suppose p = 0.5 to start with:
= 0.57 x 0.5 x 0.52 / (1 - 0.57x0.52) = 0.08, or 8% per generation.