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BIOL2007 – SELECTION AND THE SINGLE GENE 

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WHAT IS EVOLUTION?

 

Evolution produces the diversity of life.  For example:

 
- DNA sequence variation
- Bacteria
- Flowering plants
- Sexual selection in birds
- Human civilization

Evolutionary Genetics more concerned with mechanisms  than history.  Science is about understanding; we should be able to use science to make predictions.

DEFINITION OF EVOLUTION

To Darwin, evolution was: "descent with modification".

Evolution can be a change in morphology, ecology, behaviour or physiology.  Underlying all these changes must be genetic change, if these changes are evolutionary. A 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?
 

natural selection is a consistent bias in survival or fertility between genotypes within generations

Selection may be for VIABILITY (survival) vs. FERTILITY (number of offspring).

Natural selection often causes evolution, but natural selection may prevent evolutionary change (e.g. stable polymorphism; next lecture), and evolutionary change does not require natural selection (e.g. neutral evolution, or genetic drift).  Potentially important: over 90% of your genome may be evolving neutrally -- our DNA may be literally mostly "junk"!

Some of the most interesting parts of evolution, of course, involve natural selection.

Evolution is a fact (or is it?); for a couple of the many other philosophical questions about evolution, click here.

SELECTION AT A SINGLE LOCUS

Many "quantitative traits" like size and behaviour patterns are inherited at multiple loci. However, simpler single-locus traits provide ideal examples of evolution by natural selection.  Many single-locus traits seem to have evolved because of stress, especially recent stress imposed by humans.  Selection has led to the evolution of resistance to these challenges.  Some examples:

• Industrial melanism in moths llike the peppered moth (resistance predation following urban pollution of bark where the moths rest)
    Recently, some doubt has been cast on this example.  The doubt expressed by evolutionary biologists
    in this classic example of natural selection has cheered up creationists, who are having a field day.  But
    in my view there isn't a shred of evidence against the classic story of peppered moth melanism.

• Heavy metal tolerance in plants growing in mine tailings
• Malaria resistance in humans (sickle-cell haemoglobin, etc.)
• Pesticide resistance (mosquitoes, crop pests, warfarin resistance in rats)
• Antibiotic resistance in bacteria

Many examples from the above have been left for you on reserve in the Science Lbrary: View 2007 Teaching Collection by going to eUCLid; use Keyword, Basic Search, All Fields: 2007.

How does evolution by natural selection work?  Evolution by natural selection is an inevitable, mathematical process.  The frequency of a particular allele will change; its rate of change will depend mathematically on the advantage (or relative fitness) of that allele.

MATHS!  EEK!  This doesn't sound very pleasant, and indeed the maths can get very complicated (though not in 2007).  However, much of the excitement in modern evolutionary theory is about theory. Even though we don't expect you to get to grips with all of it, we will try to give you an outline.

We give tutorials, as well as lectures, and practice doing the problems to help you understand.  After doing these examples you will be able to do exam problems easily.

Mathematical evolutionary theory is useful.  For example, given information about natural selection, how rapidly will evolution occur?  The answers help us to take precautions about antibiotic resistance, or pest resistance, for instance.  Evolution is a predictive science!

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SELECTION AGAINST RECESSIVE ALLELE (Equivalent to selection FOR dominant allele)

Evolution is controlled by the strength of selection, or, to put it another way, the fitnesses of different genotypes. The actual fitness is the number of offspring that an individual has during its lifetime.  However, in evolution we are interested  whether there is a tendency for one form to replace another, not in the actual numbers of individuals or actual fitnesses.  We are more interested in the relative fitness.

We can represent selection against a genotype by some fractional amount, call this s, which is the selection coefficient.  The relative fitness can be represented as 1 - s, compared with 1 for a different genotype.

Genotypes	AA	Aa	aa	Total
Relative fitness, W	1	1	1-s
From the Hardy-Weinberg law:
Frequencies before selection	p2	2pq	q2	=(p+q)2 = 1
Frequencies after selection	p2.1	2pq.1	q2.(1-s)	But, total now < 1 !!
= 1 - sq2
So ... new genotype frequencies				1

What is the new gene frequency, p’ ?
p’ = new frequency of AA + 0.5 x new frequency of Aa

What is the RATE of evolution per generation? We need to know:

the CHANGE OF GENE FREQUENCY,  (obtained by subtracting old gene frequency from the new gene frequency).

This doesn't look like much fun, but equations like these ARE THE BASIS OF ALL EVOLUTION BY NATURAL SELECTION!!

We now have a basic equation for the effect of natural selection on the change in gene frequency, or evolution.  Arranging this as a flow diagram:-

So now you could easily write a computer programme for evolution by natural selection!

Complications – There are many!  For instance:

1) Many different kinds of selection:

• fertility selection
• viability selection

• inbreeding
• mate choice, sexual selection

4) Dominance not complete; use intermediate value of h for dominance;
 

if h=0, then allele A is dominant (genotypes AA and Aa have similar fitnesses);
if h=1, then A is recessive (Aa and aa have similar fitnesses);
if h=0.5, then A is exactly "co-dominant", or with intermediate fitness.

      5) Multiple genes, and so on...

But in all these different cases, the basic principle remains the same!  Simple models like the one we just studied are the basis of all models in population genetics.

So, we can now answer the question:

Answer: 
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,
              because q² is the square of a very small number, and so is itself even smaller!

SO:

Selection for/against a DOMINANT allele at low frequency is RAPID (~p)
Selection for/against a RECESSIVE allele at low frequency is SLOW (~q²)

THREE POSSIBLE METHODS:

1) Estimate from change of gene frequencies per generation ; the result of selection. Example: peppered moth melanic frequency (CC and Cc, see below) increased from 0.01 in 1848 to 95% in 1898; JBS Haldane estimated s = 0.20 agains the nonmelanic recessive, c.

2) Distortion of Hardy-Weinberg ratios - problems? see next lecture.

3) Comparison of birth or death rates between individuals (W = relative measure of fitness of the different genotypes). This is the MOST DIRECT METHOD

USING METHOD (3) TO ESTIMATE SELECTION IN PEPPERED MOTH
e.g. survival in a field experiment on Kettlewell's experiments on the peppered moth

A) Central Birmingham

	number  released	number recaptured	proportion  recaptured	relative fitness	W relative  to typica
typica	144	18	0.125	0.43	1.00
carbonaria	486	140	0.288	1.00	2.30

B) Dorset wood

	number  released	number recaptured	proportion  recaptured	relative fitness	W relative  to typica
typica	163	67	0.411	1.82	1.00
carbonaria	142	32	0.225	1.00	0.55

SUMMARY OF FITNESSES (Note, the fitness of cc genotypes, W_cc = 1 - s_c):

	typica	carbonaria	carbonaria	selection  coefficient  against typica
	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 (where s_c = 0.57)?

  = spq²/(1-sq²); suppose p = 0.5 to start with:

  = 0.57 x 0.5 x 0.5² / (1 - 0.57x0.5²) = 0.08; carbonaria will increase by 8% per generation.

• Evolution is the change in genetic constitution of a population over generations.  To a geneticist: a change in gene frequencies.
• Natural selection is 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 maintain the status quo, it doesn't necessarily lead to evolution.
• Evolution at a single dominant gene occurs at a predictable rate, .
• Under selection, dominant alleles will evolve quickly when rare, and slowly when common, whereas recessive alleles evolve slowly when rare, and quickly when common.
• We can estimate selection coefficients (s), fitnesses and predict rates of evolution from data on survival or fecundity.  Mathematical theory makes evolution a predictive science.

FUTUYMA, DJ 1998. Evolutionary Biology.  Chapters 12 and 13 (pp. 371-381).
References on natural selection at single genes for resistance
Science Lbrary: View 2007 Teaching Collection by going to eUCLid; use Keyword, Basic Search, All Fields: 2007