Lecture 13: Variation and Evolutionary Forces
November 10, 2011
Next Time: Evolutionary Forces and Sex
Recall Last Lecture:
At a locus one allele may be dominant over another allele. The dominance status
of an allele describes its effect on the phenotype of the heterozyotes… NOT
whether the allele is helpful, harmful or neutral.
o Most genetic disorders are associated with recessive alleles. Why?
A. In general, recessive alleles tend to be rare
B. By definition, recessive alleles reduce an individuals alleles
C: Selection eventually weeds out all copies of a harmful
dominant allele from the population, but a harmful recessive
allele can “hide” from selection in heterozygous genotypes.
Dominance and recessive status of an allele aren’t defined based on the fitness
effects, but the dominance status of an allele is going to be important in
determining how thoroughly selection can change the frequencies of a particular
In other words, the dominant status of a particular allele turns out to be
important. It turns out to be a potential CONSTRAINT on the degree to which
selection can favor the spread of a dominant allele. How effectively selection can
weed out a deleterious allele. The dominant status of an allele can affect the
speed of evolution as well as the possible outcome of evolution.
For example, a new favorable allele, beneficial to the population. A new
favorable allele starts in a population starts off with low frequency. This allele is recessive. If it is recessive, that is the other deleterious allele is dominant, then
eventually selection is going remove every single of the dominant alleles,
harmful dominant alleles. The beneficial allele should reach fixation and replace
all of the deleterious dominant alleles.
HOWEVER, if a new beneficial mutation arises, that is DOMINANT to the other
alleles at that locus, some small proportion of those harmful recessive alleles can
be sheltered or hidden from selection, because there is a diploid population.
Those deleterious recessive alleles can be carried by heterozygous individuals.
These individuals are still going to be healthy because they will carry the
dominant allele, but selection is isn’t going to see and weed out every copy of
the recessive allele.
The dominant allele spreads faster in the population compared to recessive.
o There is a greater expression of dominant alleles. Recessive alleles can be
only found in heterozygous individuals. Therefore, takes longer time for
recessive allele to grow in the population. Selection is constraint by the
dominant status alleles and by diploids.
Although powerful enough to cause adaptive evolution, there are many
constraints on selection:
1. Selection can be constraint by genetic correlations among traits.
i.e. Deeper bird beaks selectively favoured during dry
Cannot select beak depth, without changing other components
(correlation) such as body size. Genetic correlation between
traits might affect not only beak depth, but body size, beak shape
and etc. This is due to genes being pleiotropic – one gene change
affects another gene that may or may not be related.
2. Selection can be constrained by time.
Environment may be changing more rapidly than evolution can
A population may continue to adapt to past conditions,
but if environment changes fast, those conditions will not
3. Selection can be constrained by available genetic variation.
Very little genetic difference between cheetahs
Can affect the population if a disease can affect each individual.
Natural selection requires genetic variation Why is Genetic Variation so important?
Genetic Variation affects the population’s evolutionary potential. The ability it
will have sufficient genetic variation for selection to have something to work on.
Shaping individual fitness and population fitness because it affects evolutionary
Quantifying Genetic Variation
Of an individual
o Proportion of HETROZYGOUS loci
The HIGHER the proportion your heterozygous at, the HIGHER
o Inbreeding coefficient
Of a population
o Proportion of POLYMORPHIC loci
What % of population exhibits MORE than ONE allele
o Alleles per locus
How many alleles are there at each loci
More alleles means more diverse population
Directional selection refers to selection AGAINST on one
extreme end of the graph
o Ex. Yellow side of the graph
Selection is AGAINST short tail lengths
Selection FAVOURS birds with longer tails
If directional selection sustained for a long time,
eventually, we will see a decrease in Genetic Variation
We see Genetic Variation because of the environments
o Every year, environment can change (i.e. water cycle)
o Selection causes animals to adapt that
o Therefore, Genetic Variation occurs
Spatial Variation in Selection
Selection does not always result in loss of variation
When environments vary, spatially, certain alleles may be
favoured in a particular environment Population maintains HIGH amount of genetic variation
In stabilizing selection, selection works AGAINST BOTH
extreme ends of the graph.
MIDDLE part of graph is FAVOURED
We see a reduction in Genetic Variation
o Ex. Bird Tail