Biology 2483A Study Guide - Final Guide: Trophy Hunting, Bighorn Sheep, Allele Frequency

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Published on 15 Dec 2017
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Unit 2: Evolutionary Ecology
Chapter 6: Evolution and Ecology
Trophy Hunting and Inadvertent Evolution: A Case Study
Bighorn sheep populations have been reduced by 90% through hunting, habitat loss, and introduction of cattle
Hunting is now restricted; permits for a large “trophy ram” cost over $100,000
Trophy hunting removes the largest and strongest males –the ones that would sire many healthy offspring
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
oLasting effect because larger males have disproportionate number of offspring
oWe are trying to be modest in hunting but when it does occur  big horned males hunted
Decline in size (inadvertently caused directional selection –more later on)
This is also being observed in other species E.g. rock shrimp are all born male and become females when they
are large enough to carry eggs.
oCommercial harvesting takes the largest individuals, which are all females
oGenes for switching sex at a smaller size became more common
oResults in more females
oSmaller females lay fewer eggs
What is Evolution?
Evolution can be viewed as genetic change over time or as a process of descent with modification (features are
maintained and modified)
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As a population accumulates differences over time and a new species forms, it is different from its ancestors
oNew species has many of the same characteristics as its ancestors and resembles them
Example of evolution of fish seen through fossil record
oGeneral shape of skeleton is the same but the pelvic bone develops and changes showing descent with
modification (natural selection is responsible for modification)
Many other examples can be seen that lead to interesting traits and visually different species
Mutations
Different alleles arise by mutation: change in DNA  it is the raw material for evolution (source of new alleles)
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
oIf mutation did not produce new alleles, all members of a population would have identical genotypes
and evolution could not occur
Mutations are actually very rare
oIn a generation, one mutation would occur in every 10,000 to 1,000,000 copies of a gene
oIn one generation, mutation acting alone causes virtually no change in allele frequencies of a population
Mechanisms of Evolution
Natural selection, genetic drift & gene flow can cause allele frequencies in a population to change over time
Natural selection: individuals with certain heritable traits survive and reproduce more successfully that other individuals
Phenotype: observable characteristics that are determined by the genotype
oIndividuals differ from one another in part because they have different alleles for genes
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Three types of natural selection
1. Directional selection: individuals at one phenotypic extreme are favored
E.g. drought favored large beak size in medium ground finches  most food sources were larger,
smaller seed sizes died out from drought, bigger beak = better tool for bigger seeds
2. Stabilizing Selection: individuals with an intermediate phenotype are favored
E.g. Parasitic wasps select for small gall size of Eurosta flies; while birds select for large gall size 
since both extremes are favored by different predation types intermediate size is selected for
3. Disruptive Selection: individuals at both phenotypic extremes are favored
E.g. African seed-crackers (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 –this leads to a
scenario where the intermediate size is at no advantage
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