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Lecture 5

Biology 2483A Lecture Notes - Lecture 5: Greater Prairie Chicken, Trophy Hunting, Allele Frequency

Course Code
BIOL 2483A
Hugh Henry

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Bighorn sheep populations have been reduced by 90% by hunting, habitat loss, and
introduction of cattle. Hunting is now restricted; permits for a large "trophy ram" costs over
Trophy hunting removes the largest and strongest males - the ones that would sire many
healthy offspring.
You are going to have smaller males reproducing because of artificial selection
against the bigger males.
Trends of smaller and smaller males every year.
In one population, 10% of males were removed by hunting each year, the average size of
males and their horns decreased over 30 years of study.
Rock shrimp are all born male, and become females when they are large enough to
carry eggs. Commercial harvesting takes the largest individuals, which are all females.
Genes for switching sex at a smaller size become more common, resulting in more
females, but smaller females lay fewer eggs.
Trophy Hunting an Inadvertent Evolution: Case Study
Evolution can be viewed as genetic change over time or as a process of descent with
As a population accumulates differences over time and a new species forms, it is different
from its ancestors.
But the new species has many of the same characteristics as its ancestors, and resembles
Different alleles arise by mutation: a change in DNA
Mutations can result from copying errors during cell division, mechanical damage, exposure to
chemicals (mutagens) or high-energy radiation.
Formation of new alleles is critical to evolution. If mutation did not produce new alleles, all
members of a population would have identical genotypes and evolution could not occur.
In a generation, one mutation would occur in every 10,000 to 1,000,000 copies of a
Mutations are actually very rare.
Lecture 5: Evolution & Ecology
Thursday, October 1, 2015
8:05 PM
Lecture Slides Page 1

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In a generation, one mutation would occur in every 10,000 to 1,000,000 copies of a
In one generation, mutation acting alone causes virtually no change in allele frequencies
of a population.
Natural selection, genetic drift, and gene flow can cause allele frequencies in a population to
change over time.
Individuals with certain heritable traits survive and reproduce more successfully than
other individuals.
Populations change over time though natural selection:
Phenotypes: Observable characteristics that are determined by genotype.
Individuals differ from one another in part because they have different alleles for genes.
Mechanisms of Evolution
Drought favoured large beak size in medium ground finches.
Directional Selection: individuals at one phenotypic extreme (e.g. large size) are
Example: Parasite wasps select for small gall size of Eurosta flies; while birds
select for large gall size.
Stabilizing Selection: Individuals with an intermediate phenotype are favoured.
Three types of natural selection:
Lecture Slides Page 2

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Example: African seedcrackers (birds) have two food sources - hard seeds that
large beaks are needed to crack, and smaller, softer seeds that smaller beaks are
more suited to.
Disruptive Selection: Individuals at both phenotypic extremes are favoured.
Genetic Drift: occurs when chance events determine which alleles are passed to the next
A branch falling down on a flower gets crushed. These flowers could have had the most
favourable phenotype but by chance their alleles don't get passed on.
It is significant only for small populations.
It acts by chance alone, thus causing allele frequencies to fluctuate at random. Some
may disappear, others may reach 100% frequency (fixation).
Genetic drift has four effects on small populations:
Various animals have figured out that
there are flies in here. The small galls
are eaten by wasps and the bigger ones
are eaten by birds. Therefore, the
intermediate size is favoured.
Lecture Slides Page 3
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