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What is Evolution?

BIOL2007 Evolutionary Geneticscourse website: http://ucl.ac.uk/~ucbhdjm/courses/

(searching for “BIOL2007 timetable” on Google is easier!)

Darwin: “descent with modification”

A change in morphology, ecology, behaviour, physiology

Change must be genetic

Modern, genetic definition:“evolution is change in gene frequencies between generations”

a) Natural selectionb) Mutationc) Genetic drift, or neutral,

random evolutione) Migration, or gene flow

This lecture: simple examples of evolution by natural selection

What causes evolution?

What is natural selection?

“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

The peppered moth Biston betularia

Left: form typica (left, and carbonaria (right) on lichen-coveredtrunk in Dorset.

Right: on soot-covered tree near Birmingham

A flow diagram for evolution by ns

Random mating

Offspring genotypes in Hardy-Weinberg ratios

Offspring after selection

Natural selection

So now you can write anevolution computer program!

Numerical vs. analytical theory

Selection against recessive alleleSelection AGAINST recessive allele (= selection FOR dominant allele)

Suppose there is “viability selection” (i.e. survival affected) so that …

Genotypes AA Aa aa Total

Relative fitness, W 1 1 1-s -

Genotype frequenciesbefore selection p2 2pq q2 1(Hardy-Weinberg law)

Rel. frequencies p2 2pq q2(1-s) <1after selection

in this simple model, s is the “selection coefficient” (≈ fraction dying)

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 p2 2pq q2 1 (Hardy-Weinberg law) Relative genotype frequencies p2 2pq q2(1-s) ≠1 after selection

Frequencies should sum to 1! Therefore, need to divide by “mean fitness,” 222 1)1(2 sqsqpqpW −=−++=

Genotype frequencies

after selection 2

2

1 sqp

− 21

2sqpq

− 2

2

1 sqq (1-s)

− 1

WHAT IS THE NEW FREQUENCY OF THE A ALLELE (p’)? p’ = new frequency of AA + ½ new frequency of Aa

( ) 222

2

2212

22

2

11)(

112

12

21

1 '

sqp

sqqpp

sqpqp

sqpqp

sqpq

sqpp

−=

−+=

−+=

−+=

−+

−=

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).

2

2

2

2

2 1)1()1(

1 '-

sqspq

sqsqppp

sqpppp

−+=

−−−=−

−==∆

This is the basic equation for all of evolution by natural selection!

2

The basic equation for evolution

Natural selection at a dominant gene

22

2

1 '- spq

sqspqppp ≈−

+==∆

(if s is small)

In words:

The change in gene frequency per generation is proportional to spq2

Dominance vs. recessivesWe can now answer the question: How fast do populations respond

to natural selection?

Answer: (p is frequency of A, q is freq. a)

If p is small, ~0.01 or less, , i.e. RAPID

If p is large, so that q ≈ 0.01 or less, , i.e. very SLOW

(q2 is a square of a very small number � is itself even smaller!)

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!

2

2

1

sqspqp−

=∆

ssppqq−

≈∆→→1 :1;1 2

1 :1

2sqpp ≈∆→

The speed of evolution

p

(the rate of gene frequency change per unit time)

time (generations)

rare gene recessive rare gene dominant

(from a programme written by a former B242 student, Wei-Chung Liu, available from the B242 website)

More generally …Complications – many!

Many different kinds of selection- fertility selection- sexual selection

Non-random mating- inbreeding- mate choice

Overlapping generations

Dominance not completeAA Aa aa1 1–hs 1–s

Multiple genes …

&c &c….

But the basic principle remains the same!

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.

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

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.

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 number proportion relative (W, the other released recaptured recaptured fitness, W way round)

typica 144 18 0.125 0.43 1.00 carbonaria 486 140 0.288 1.00 2.30 B) Dorset wood

number number proportion relative released recaptured recaptured fitness

typica 163 67 0.411 1.82 carbonaria 142 32 0.225 1.00 SUMMARY OF FITNESSES:

(Wc = 1 - sc) typica carbonaria selection

coefficient against c Wcc WCc WCC sc

City 0.43 1 1 +0.57 Wood 1.82 1 1 -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.