Transcript What is Evolution?

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
- Human species
Futuyma 2005:
14 copies in
science library
Barton 2007
6 copies
Freeman & Herron
2004-7
18 copies
DNA sequence variation
200 bp of the 18,000 bp of aligned
mitochondrial DNA of great apes
Evolution: definition
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”
Evolutionary Genetics: mechanisms
Science: understanding; predictions
What causes evolution?
a) Natural selection
b) Mutation
c) Genetic drift, or neutral,
random evolution
e) Migration, or gene flow
This lecture: simple examples of evolution by
natural selection
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
Selection and the single gene
“Quantitative traits”
e.g. behaviour, IQ, beak size
usually multiple loci
versus … Single-locus traits
Evolution by natural selection can occur in both
Many single-locus traits are involved in
resistance to stress (often caused by humans)
Examples of single-gene traits
• Industrial melanism in moths (resistance to urban
pollution)
• Heavy metal tolerance in plants growing in mine tailings
• Malaria resistance in humans (sickle-cell haemoglobin, etc.)
• Drug/antibiotic resistance in bacteria, protozoan parasites
• Human genetic diseases like cystic fibrosis, Huntington’s
disease etc.
• Pesticide resistance (mosquitoes, insects, weeds, fungi,
warfarin resistance in rats)
We used to do an essay on this for tutorial; there are many references
on reserve, still; see eUCLid
Cyclodiene (dieldrin,
aldrin, endosulfan,
-BHC) resistance:
Insecticide resistance
GABA-gated chloride
channel insensitivity
Ala302
Ser ….In all these species:
… creates increasing problems
in agriculture and disease
control (e.g. malaria)
Drosophila melanogaster
Drosophila simulans
Peach potato aphid Myzus persicae
Coffee-berry borer Hypothenemus hampei
Housefly Musca domestica
Cockroach Blatella germanica
Whitefly Bemisia tabaci
Flour beetle Tribolium castaneum
Cystic fibrosis in humans (a recessive):
… a large diversity of “loss-of-function” alleles
F508
Incidence: 1/2500 of births. So q = (1/2500) = 0.02.
About 2-3 of you in this room will carry the allele ... hmm!
How does evolution by natural
selection work?
Evolution by natural selection is an inevitable,
mathematical process
The frequency of an allele will change, and its rate of
change depends on relative fitness.
Mathematical evolutionary theory helps us understand.
For example, given information about fitness,
how fast is evolution?
Useful: help us understand antibiotic resistance, or pest
resistance, for instance
Evolution is a predictive science! Useful, as well as fun!
Differences between ecology and
evolution
Ecologists: dynamics of numbers of individuals (or
species); generally ignore genetic variation.
Evolutionists: changes within populations,
& how might lead to speciation and macroevolution;
ignore numbers of individuals.
Ecology has Lotka-Volterra competition equations:
concerned with numbers of individuals
(…………v. difficult to solve!!)
Evolutionists study changes in gene frequency….
We’d like to know: How fast is evolution by natural selection?
Selection against recessive allele
Selection AGAINST recessive allele (= selection FOR dominant allele)
Genotypes
AA
Aa
aa
Total
Relative fitness, W
1
1
1-s
-
in this simple model, s is the “selection coefficient” ( fraction dying)
[NB: p+q=1, therefore (p+q)2 = p2+2pq+q2=1]
Genotype frequencies
(Hardy-Weinberg law)
p2
2pq
q2
1
Relative frequencies
after selection
p2.1
2pq.1
q2(1-s)
<1
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
Natural
selection
Offspring after selection
So now you can write an
evolution computer program!
Numerical vs. analytical theory
The basic equation for evolution
Natural selection at a dominant gene
2
spq
2
p  p'- p  
 spq
2
1  sq
(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. recessives
How fast do populations respond to natural selection?
Answer:
spq 2
p 
1  sq 2
(p is frequency of A, q is freq. a)
If p is small, ~0.01 or less, q  1; q 2  1 : p  sp, i.e. RAPID
If p is large, so that q  0.01 or less,
p  1 : p  sq 2 , 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?
The speed of evolution
(the rate of gene frequency change per unit time)
p
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-covered
trunk 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
released
typica, cc
144
carbonaria, Cc &CC 486
number percent
relative
recaptured recaptured fitness, Wcc
WC- the “other
way round”
18
140
1.00
2.30
12.5%
28.8%
0.43
1.00
B) Dorset wood
number
released
typica, cc
163
carbonaria, Cc & CC 142
number percent
relative
recaptured recaptured fitness
67
41.1%
1.82
32
22.5%
1.00
SUMMARY OF FITNESSES:
City
Wood
typica
Wcc
0.43
1.82
(note: W = 1 - s)
carbonaria
WCc
WCC
1
1
1
1
selection coefficient against carbonaria
scc
+0.57
-0.82
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!
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….
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 different
Science: 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)