What is Evolution?

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What is Evolution?
BIOL2007 Evolutionary Genetics course website
http//ucl.ac.uk/ucbhdjm/courses/ (searching
for "biol2007" on Google is easier!)

• 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
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DNA sequence variation
200 bp of the 18,000 bp of aligned mitochondrial
DNA of great apes
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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

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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
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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 gt 95 of genome maybe "junk"!)
• However, many interesting types of evolution
  involve natural selection

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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)

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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,
  Huntingtons 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

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Cyclodiene (dieldrin, aldrin, endosulfan, ?-BHC)
resistance GABA-gated chloride channel
insensitivity
Insecticide resistance
Ala302 ?Ser
.In all these species 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
creates increasing problems in agriculture and
disease control (e.g. malaria)
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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!
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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!

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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!!)
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Selection against recessive allele
Evolutionists study changes in gene
frequency. Wed like to know How fast is
evolution by natural selection?

• 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 pq1, therefore (pq)2 p22pqq21
• Genotype frequencies
• (Hardy-Weinberg law) p2 2pq q2 1
• Relative frequencies p2.1 2pq.1 q2(1-s) lt 1
• after selection

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Selection against recessive contd.
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Selection against recessive contd.
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Selection against recessive contd.
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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
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The basic equation for evolution
Natural selection at a dominant gene
(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
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Dominance vs. recessives

• 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. 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?
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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)
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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
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Estimating selection

• 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

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Estimating selection in peppered moth

• Survival in field experiments on the peppered
  moth
•  
• Central Birmingham
• number number percent relative WC- the
  other
• released recaptured recaptured fitness,
  Wcc way round
•  
• typica, cc 144 18 12.5 0.43 1.00
• carbonaria, Cc CC 486 140 28.8 1.00
  2.30
•  
• B) Dorset wood
• number number percent relative
• released recaptured recaptured fitness
• typica, cc 163 67 41.1 1.82
• carbonaria, Cc CC 142 32 22.5 1.00
• SUMMARY OF FITNESSES
  (note W 1 - s)
• typica carbonaria selection
  coefficient against carbonaria
• Wcc WCc WCC scc

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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.
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More generally

• Complications many!
•  
• Many different kinds of selection
• -        fertility selection
• -        sexual selection
• Non-random mating
• -        inbreeding
• -        mate choice
•  

Overlapping generations Dominance not
complete AA Aa aa 1 1hs 1s Multiple genes
c c.
But the basic principle remains the same!
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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 its 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!

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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

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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)