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What is Evolution?
Produces biological diversity
- DNA sequence variation
- Bacteria
- Flowering plants
- Sexual selection in birds
- Human civilization
Evolutionary Genetics: mechanisms
Science: understanding; predictions
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”
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
Evolution, a fact?
You don’t have to believe in evolution to take
this course, but you do have to know the
arguments to get a good grade!
Evolution is a fact, and it’s hard to ignore
… but, theory and fact: not so different
Science: prediction
According to Karl Popper: science is falsifiable.
Falsehoods disprovable; truth more difficult!
Religion: truth is by faith. Very different.
Selection and the single gene
“Quantitative traits” (e.g. size, behaviour):
usually multiple loci
Single-locus traits:
great examples of evolution by natural selection
Many single-locus traits are involved in
resistance to stress (often 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.)
Pesticide resistance (mosquitoes, insects, weeds, fungi,
warfarin resistance in rats)
• Antibiotic resistance in bacteria
We used to do this for tutorial; there are many references on
reserve, still; see eUCLid
The peppered moth Biston betularia
Left: form typica (left, and
carbonaria (right) on lichen-covered
trunk in Dorset.
Right: on soot-covered tree near Birmingham
How does evolution by natural
selection work?
Evolution by natural selection is an inevitable,
mathematical process.
The frequency of a particular allele will change, and its
rate of change will depend mathematically on the
advantage (or relative fitness) of that allele.
Mathematical evolutionary theory is useful. For
example, given information about natural selection,
how rapidly will evolution occur?
The answers help us understand antibiotic resistance,
or pest resistance, for instance.
Evolution is a predictive science! Useful, as well as fun!
A flow diagram for
Random mating
Offspring genotypes in
Hardy-Weinberg ratios
Offspring after selection
So now you can write an
evolution computer program!
Numerical vs. analytical theory
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
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 1998. Evolutionary Biology. Chapters 12 and 13
(pp. 371-381).
References on natural selection at single genes for resistance (see web)
Science Lbrary: View B242 Teaching Collection by going to eUCLid; use
Keyword, Basic Search, All Fields: B242