Download Adaptation and Evolution

General Ecology
Adaptation and Evolution
cont: Population Genetics
Evolution in Populations
Evolution is often a process of
adaptation.
 Adaptation is not possible w/o genetic
variation.
 Organisms present a phenotype to the
environment (selective regime). The
phenotype is an expression of the
genotype.

Evolution in Populations

The phenotypic variation we see can be
a consequence of a number of things:
– Genotypic or genetic variation.
– Environmental variation.
– Error
Basic Population Genetics
We have defined evolution as a change
in allele frequencies over time.
 The sum of all genes in a population is
the gene pool.
 We characterize the gene pool be
measuring allele frequencies.

Popuation Genetics
In pea plants, there are red flowers and
white flowers. Flower color is controlled
by a single gene with 2 alleles.
 R is dominant and r is recessive.
 RR and Rr individuals produce red
flowers while rr individuals produce
white flowers.

Population Genetics
‘RR’ is homozygous dominant
 ‘Rr’ is heterozygous
 ‘rr’ is homozygous recessive.
 If there are 100 individuals in a
population, there are 200 flower color
alleles.
 The number of those alleles that are ‘R’
is the allele frequency of ‘R’, and 1
minus this is the allele frequency of ‘r’.

Population Genetics
If we note that the frequency of R has
changed from .20 to .30 in 1 generation,
then evolution has occurred.
 Imagine we have 30 RR individuals, 20
Rr individuals, and 50 rr individuals.
 Let p be the frequency of R and 1-p, or
q, be the frequency of r. Note p + q = 1.

Population Genetics

For our example,
2
(
30
)

20
p

0
.
4
200
Population Genetics

Also,
2
(
50
)

20
q

0
.
6
200
Population Genetics
Note that p+q=1.
 Now, imagine that the individuals in this
population mate panmictically.
 What is the probability that a R allele
will combine with a R allele?
 This is simply p2.
 This is also the expected frequency of
RR individuals in the next generation.

Population Genetics
The probability of rr will be q2, and the
probability of Rr is 2pq.
 Note: there are 2 ways of getting Rr.
You can get Rr or rR.
 Note also that p2+2pq+q2 = 1
 Finally, note that this is (p+q)2

Population Genetics

What happens? If we repeat this for
many generations, we find that the
system reaches equilibrium: a point at
which allele frequencies no longer
change. This is called Hardy-Weinberg
equilibrium. In other words, allele
frequencies will not change unless
something happens.
Population Genetics

The system is dependent on a number
of assumptions:
– Panmictic breeding
– Equal survival and reproduction of
individuals.
– The population is closed.
– No mutation.
Neither of these is likely to be true.
 Thus, evolution is inevitable.

Mechanisms of Evolution

What can disrupt HW equilibrium?
– Forms of selection already discussed.
– Genetic drift.
• Here, changes in allele frequencies can occur
by chance alone. This is a small population
size phenomenon. Effective population size is
critical.
– Gene flow
• This can wash out changes in allele
frequencies, introduce new alleles, or promote
change.
Mechnisms of Evolution
Modeling natural selection
 Imagine the homozygous recessive
genotype does not survive as well as
the heterozygote or homozygous
dominant. We can denote this with a
selction coefficient ‘s’.
 If the fitness of the homozygous
dominant and heterozygote are 1, then
the fitness of the homozygous recessive
is 1-s.

Mechanisms of Evolution
We can use algebra to model changes
in allele frequency. Let Dp = change in
frequency of p from original generation,
to generation after selection has
occurred. So, Dp = p – p’
 We want to compute the new frequency
of p, p’. We do this just as before, but
know that some alleles were removed
by selection.

Mechanisms of Evolution
The portion removed by selection is
q2 * their selective disadvantage ‘s’.
Thus, the number of alleles in the next
generation is 1-sq2,
 since (p2+2pq+q2)-sq2.
 Also, the total number of alleles is
2(1-sq2)

Mechanism of Evolution
2( p )  2 pq
p(1)
p' 

2
2
2(1  sq )
1  sq
p
p' 
2
1  sq
p
p  p' p 

p
2
1  sq
2
Mechanism of Evolution

Note that the rate of change in allele
frequency is a consequence of the
strength of selection (s) and the initial
allele frequency (p and q).
Finally

We often characterize selection in a
number of ways:
Density
independent
Density
dependent
Frequency
independent
Hard Seln Density
Depn
Frequency
dependent
Freq.
Depn
Soft Seln
Finally
Convergent evolution
 ESS: Evolutionary stable strategy.
 Wrights adaptive landscape.
