Genitcal Theory of Natural Selection
By Julie Sobolewski and Angela Rose
Terms:
Allele - one of several forms of the same gene
Allele Frequency - the proportion of gene copies in a population that are at a
given allele
Evolution - origin of genetic variation by mutation or recombination, followed by
changes in the frequencies of alleles and genotypes
Fitness - the success of an entity in reproducing; average contribution of an allele
or genotype to he next generation or to succeeding generations
Natural Selection - differential reproductive success
Reproductive Success - average lifetime contribution of individuals of a particular
genotype to a population after one or more generations
(generally includes average number of offspring produced
and average number that survive)
How Natural Selection Differs from Evolution:
-evolution is a two step process the origin of variation followed by changes in the
frequencies of alleles.
-natural selection can only have an evolutionary effect when phenotypes differ in
genotype
- a feature can only evolve through natural selection if it has a positive
contribution to the reproduction or survival of individuals that bear it.
Beginnings of the Gentical Theory of Natural Selection:
In 1930 R. A. Fisher published his work, The Genetical Theory of Natural
Selection.
-Population fitness
- The consequences of natural selection depend on four things
- the relationship between phenotype and fitness
- the relationship between phenotype and genotype
- a relationship between fitness and genotype
-whether or not evolutionary change occurs
Modes of Selection (Figure 12.1):
Directional - when disadvantageous alleles are replaced by more advantageous
alleles.
-Coral Snakes
Stabilizing- if an intermediate phenotype is fittest
-Heterozygote advantage, each generation produces all three genotypes are
produced which results in a stable equilibrium.
- Sickle cell
Diversifying - if two or more phenotypes are fitter than the intermediates between
them
-Multiple-niche polymorphism
-Black-bellied seedcracker
Advantageous Allele Frequency:
Δp = (½ spq) ÷ (1 – sq)
-A character state with even a minuscule advantage will be fixed by natural
selection
-Advantageous allele may initially be fairly common but is likely to be rare if it is
a newly arisen mutation or if it was disadvantageous before an environmental
change
Food for Thought:
-Do you think coloration rather than inconspicuousness is advantageous?
Why or why not? (Figure 12.4)
Deleterious Allele Frequency:
up = (spq2) ÷ w
solve for q
q = (u ÷ s) ½
-Deleterious alleles can be hidden by heterozygous states but they can persist
because they are repeatedly reintroduced either by mutation or by gene flow from
other population in which they are favored by different environment
-Going to move towards a stable equilibrium
Inverse frequency-dependent selection:
- the rarer a phenotype is, the greater its fitness
Sex allocation:
-defined as the distribution of resources to male vs. female
reproductive function
-Male to female ratio = 1:1
-Evolutionarily Stable Strategy (ESS) = stable equilibrium
Food for Thought:
-How could global warming affect sex allocation?
-Are there any species where sex allocation my not apply as strongly, if at
all?
Inverse frequency-dependent selection does not just apply to sex allocation:
-cichlids in Lake Tanganyika (dextral vs. sinistral)
-Figure 12.16
How Strong is Natural Selection?
-Peppered Moths used as example for very strong selection.
-Kettlewell’s results- when moths were released in polluted area typical had lower
survival rate. When released in unpolluted area typical had
higher survival rate.
-Well’s and Hooper claimed experiment and results were false because of
methods used.
-Further research used similar methods obtained some controversial results.
Food for Thought:
-Was Kettlewell right? If not would this weaken the case for evolution by
natural selection?
References:
1) Askew, R. R., Cook, L. M., Bishop, J. A. (1971). Atmospheric Pollution
and Melanic Moths in Manchester and Its Environs. The Journal of
Applied Ecology, 8 (1), 247-256.
2) Brakefield, Paul M., Liebert, Tony G. (2000). Evolutionary Dynamics of
Declining Melanism in the Peppered Moth in the Netherlands.
Proceedings: Biological Sciences, 267 (1456), 1953-1957.
3) Charnov, E. L. 1982. The Theory of Sex Allocation. Princeton University Press,
NJ.
4) Cook, L. M., Dennis, R. L. H., Mani, G. S. (1999). Melanic Morph
Frequency in the Peppered Moth in the Manchester Area.
Proceedings: Biological Sciences, 266 (1416), 293-297.
6) Holdrege, Craig (1999). The Case of the Peppered Moth Illusion. Whole
Earth, 1-6.
7) Lees, D. R., Creed, E. R. (1975). Industrial Melanism in Biston
betularia: The Role of Selective Predation. The Journal of
Animal Ecology, 44 (1), 67-83.
8) Wells, Jonathan (1999). Second Thoughts about Peppered Moths: This
classical story of evolution by natural selection needs revising.
The Scientist, 13, 13-16.