What is Evolution? Produces biological diversity - DNA sequence variation - Bacteria - Flowering plants - Sexual selection in birds - Human species BIOL2007 Evolutionary Gene course website: http://ucl.ac.uk/~ucbhdjm/cours (searching for "biol2007" on Google is easi Futuyma 2005: 14 copies in science library Barton 2007 6 copies Freeman & Herron 2004-7 18 copies
BIOL2007 Evolutionary Genetics course website: http://ucl.ac.uk/~ucbhdjm/courses/ (searching for "biol2007" on Google is easier!). What is Evolution?. Produces biological diversity - DNA sequence variation - Bacteria - Flowering plants - Sexual selection in birds - PowerPoint PPT Presentation
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What is Evolution?Produces biological diversity - DNA sequence variation - Bacteria - Flowering plants - Sexual selection in birds - Human species
Evolutionists study changes in gene frequency….We’d like to know: How fast is evolution by natural selection?
Selection against recessive contd.
Selection against recessive contd.
Selection against recessive contd.
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
The basic equation for evolution
Natural selection at a dominant gene
22
2
1 '- spq
sq
spqppp
(if s is small)
(p is the frequency of the dominant allele)
In words:
The change in gene frequency per generation is proportional to spq2
Dominance vs. recessivesHow 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. SLOW
(q2 is a square of a very small number is itself even smaller!) RESULT:Selection for/against a DOMINANT allele at low frequency is RAPID ( p)Selection for/against a RECESSIVE allele at low frequency is SLOW (( q2)
…. many new single genes for resistance (melanism, insecticide resistance and so on) are dominant! Why?
2
2
1
sq
spqp
sppqq :1;1 2
2 :1 sqpp
The speed of evolution
p
(the rate of gene frequency change per unit time)
time (generations)
advantageous recessive advantageous dominant
(from a programme written by a former BIOL2007 student, Wei-Chung Liu, available from the BIOL2007 website)
The peppered moth Biston betularia
Left: form typica (left, and carbonaria (right) on lichen-coveredtrunk in my parents’ garden in Kent
Right: on soot-covered tree near Birmingham in the 1960s
Estimating selection
1)Change of gene frequencies per generation (e.g. peppered moth in 19th C; Haldane estimated s 0.5)
2) Deviation from Hardy-Weinberg ratios (next lecture)
3) Direct comparison of birth or death rates
We will use this method here using survival data in the peppered moth
Estimating selection in peppered moth
Survival in field experiments on the peppered moth A) Central Birmingham
number number percent relative WC- the “otherreleased recaptured recaptured fitness, Wcc way round”
typica, cc 144 18 12.5% 0.43 1.00carbonaria, Cc &CC 486 140 28.8% 1.00 2.30 B) Dorset wood
number number percent relativereleased recaptured recaptured fitness
typica, cc 163 67 41.1% 1.82carbonaria, Cc & CC 142 32 22.5% 1.00
SUMMARY OF FITNESSES: (note: W = 1 - s)
typica carbonaria selection coefficient against carbonaria
The speed of evolution by nat. sel.HOW FAST would carbonaria increase in frequency in a 1950s 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.
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!
Evolution, a fact?
You can be a creationist and still take this course, but you do have to learn evolutionary biology to get a good grade!
Evolution is a fact, and it’s hard to ignore… but, theory and fact: not so differentScience: prediction, rather than “absolute truth”Religion: truth, belief is by faith. Very different.Karl Popper: science is falsifiable. Falsehoods
disprovable; scientific truth cannot be proved!
Take-home points
Evolution to a geneticist: a change in gene frequencies.
Natural selection: a consistent bias favouring some genotypes.
Evolution can occur in the absence of natural selection.
Natural selection can stabilize the status quo; zero evolution.
Evolution occurs at predictable rates. 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. Chapters 9 (p. 195), 11 (all) and 12 (pp. 270-285).
FREEMAN, S, and HERRON, JC 2004. Evolutionary analysis. 3rd Ed. Chapters 3 and 5.
References on natural selection :
Science Library: View BIOL2007 Teaching Collection by going to eUCLid; use Keyword, Basic Search, All Fields: BIOL2007 or B242 (old number)