What is Evolution?Produces biological diversity - DNA sequence variation - Bacteria - Flowering plants - Sexual selection in birds - Human species

BIOL2007 Evolutionary Geneticscourse website: http://ucl.ac.uk/~ucbhdjm/courses/

(searching for "biol2007" on Google is easier!)

Futuyma 2005:14 copies in science library

Barton 20076 copies

Freeman & Herron 2004-718 copies

DNA sequence variation

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

Evolution: definitionDarwin: “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: mechanismsScience: understanding; predictions

a) Natural selectionb) Mutationc) Genetic drift, or neutral,

random evolutione) Migration, or gene flow

This lecture: simple examples of evolution by natural selection

What causes evolution?

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 sizeusually 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

Ala302 Ser

Cyclodiene (dieldrin, aldrin, endosulfan, -BHC) resistance:

GABA-gated chloride channel insensitivity

….In all these species:Drosophila melanogasterDrosophila simulansPeach potato aphid Myzus persicaeCoffee-berry borer Hypothenemus hampeiHousefly Musca domesticaCockroach Blatella germanicaWhitefly Bemisia tabaciFlour beetle Tribolium castaneum

Insecticide resistance

… creates increasing problems in agriculture and disease control (e.g. malaria)

Cystic fibrosis in humans (a recessive):

… a large diversity of “loss-of-function” alleles

Incidence: 1/2500 of births. So q = (1/2500) = 0.02.About 2-3 of you in this room will carry the allele ... hmm!

F508

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!!)

Selection against recessive alleleSelection 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 p2.1 2pq.1 q2(1-s) < 1after selection

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

Wcc WCc WCC sccCity 0.43 1 1 +0.57Wood 1.82 1 1 -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! 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)