1. Evolution in Large Populations I: Natural Selection & Adaptation
Chapter 6
Evolution in Large Populations I:
Natural Selection & Adaptation
Species have to cope with a plethora of
Environmental changes.
Red Queen Hypothesis -- Adaptations in
competitors, parasites, and pests are so common
that species have to continually evolve to avoid
falling behind competing organisms.

2. There are limits to physiological adaptations.
If environmental changes are greater than any
individual can cope with then the species
becomes extinct.
Evolutionary change through natural selection is
an alternative means (vs. physiological adaptation)
for adjusting to environmental change.
This is Adaptive Evolution!

3. Natural Selection -- differential reproduction &
survival of different genotypes.
When adaptive evolutionary changes occur over
long periods of time, they may allow a population
to cope with conditions more extreme than any
individual could originally tolerate.
Adaptive evolution is observed when large
genetically variable populations are subjected to
altered biotic or physical environments.

4. Conservation Importance of Adaptive Evolution
Preservation of ability of species to evolve in
response to new environments.
Loss of adaptive evolutionary potential in small
populations.
Most endangered species exist on the periphery of
their historic range so they must adapt to what
was previously marginal habitat.

5. Genetic adaptation to captivity and its deleterious
effects on reintroductions.
Adaptation of translocated populations to their
new environment.

6. Conservation biology is concerned with preserving
species as dynamic entities that can evolve to
cope with environmental change.
Retaining the ability to evolve requires the
preservation of genetic diversity.
Consequently, we must understand the factors
that influence the evolution of natural populations.

7. An evolving population is a complex system
influenced by mutation, migration, selection, and
chance operating within the context of the
breeding system.
To understand this complexity, we use modelling
with no factors, one factor, two factors etc.
In its simplest form, evolution involves a change
in gene frequency and its importance can be
summarized as:

8. Mutation is the source of all genetic diversity
but is a weak evolutionary force over the
short-term.
Selection is the only force causing adaptive
evolutionary change.
Migration reduces differences between populations
generated by mutations, selection, and chance.

9. Chance effects in small populations lead to loss
of genetic diversity and reduced adaptive
evolutionary potential.
Fragmentation and reduced migration lead to
random differentiation among subpopulations
derived from the same original source population.

10. Selection arises because different genotypes have
different rates of reproduction and survival
(reproductive fitness) and such selection changes
allele frequencies.
Selection operates at all stages of life-cycle.
In animals this involves mating ability and fertility
of males and females, fertilizing ability of sperm,
number of offspring per female, survival or
offspring to reproductive age and longevity.

11. The most intensive selection that can apply against
a recessive allele is when all homozygotes die
(lethal).
For example, all individuals homozygous for
Chondrodystrophic dwarfism (dwdw) in endangered
California condors die around time of hatching.
Modeling impact of selection against
Chondrodystrophy in California condors.

12. Genotype
++ +dw dwdw total
Zygotic p2 2pq q2 1.0
Frequency
Relative (lethal)
Fitness
After p2X1 2pqX1 q2X0 1 -q2
Selection
Adjusted p2
(1-q2)
2pq

13. The frequency of the dw allele in the next
Generation (q1) is:
q1 = q/(1 + q)
The change in frequency (q) = -q2/(1 + q)
Thus, the lethal allele always declines in frequency.
Importance: it becomes progressively harder to
reduce the frequency of the deleterious recessive
allele as its frequency declines!

14. In conservation genetics we are concerned with
both selection against deleterious mutations and
selection favoring alleles that improve the ability
of a population to adapt to changing environments.

15. Prior to industrial revolution, its peppered wings
provided camouflage as it rested on lichen-covered
tree trunks.
Sulfur pollution killed most lichen and soot darkened
Previously rare dark variants (melanics) were now
better camouflaged.
Melanic form was first reported in 1848 but by
1900 they represented 99% of all moths in this
part of England.

16. Simple model for this type of selection:
Beginning frequencies of melanic (M) and typical (m)
Alleles with frequencies of p and q, respectively.
Assumptions:
Large random mating population
no migration
no mutation
selection occurs on adults but before
reproduction
mm individuals have a relative fitness of 1 - s
where s is the selection coefficient.

17. MM Mm mm total
Zygotic
Freq. p2 2pq p2 1.0
Relative
Fitness
After
Selection p2 2pq q2(1 - s) 1 - sq2
Adjusted
Freq. p2
(1 - sq2)
2pq
(q2 - sq2)

18. Frequency of M after selection (p1):
Change of M (p):
Since the sign of p is positive, melanic allele
increases in frequency.
Rate of increase depends upon the selection
coefficient (s) and allele frequencies. p
(1 - sq2)
(spq2)

19. 1848, frequency of M = p = 0.005 and typicals had
Only 70% survival of melanics (s = 0.3) then:
p1 = p/(1 - sq2) = 0.005/(1 - (0.3 X )] =
Change in frequency (p) =
p1 - p = =

20. Models of 4 different degrees of dominance are
given in Figure 6.5
In each case, the selection coefficient (s)
represents the reduction in relative fitness of the
genotype compared to that in the most fit
genotype (Fitness = 1.0).
Values of s range from 0 to 1.

21. Additive Case -- heterozygote has a fitness
intermediate between the two homozygotes.
Completely Dominant Case -- heterozygote has a
fitness equal to the A1A1 homozygote.
Partial Dominance Case -- heterozygote has a
fitness nearer one of the homozygotes than the
other with its position on the scale depending on
the value of h.
Overdominant Case -- heterozygote has higher
fitness than either homozygote.

22. The length of time it takes for an allele frequency
to change by a given amount of selection depends
upon the intensity of selection and on the mode
of inheritance.
For a recessive lethal allele we can determine the
number of generations to change an allele freq.
from q0 to qt as:
t = 1/qt - 1/q0.

23. Directional Selection: Captive populations are likely
to show adaptation to captivity.
Typically results in decreased
fitness when returned to the
wild.
Fitness
Freq. Before
Selection
Freq. After
Phenotype

24. Stabilizing Selection: Favors intermediate phenotype.
Expected to reduce genetic
variation.
May cause phenotypic stabilizing
selection, leading to retention
of genetic diversity.
Fitness
Freq. Before
Selection
Freq. After
Phenotype

25. Disruptive Selection: Favors both phenotypic extremes
May lead to increased variation
In future.
In fragmented habitats this leads
To adaptation in each local
Environment.
Speciation is possible.
Fitness
Freq. Before
Selection
Freq. After
Phenotype