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I. Population Genetics
Figure 1: Gene Pool
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Gene Pool:
a) In terms of the gene pool, evolution can be defined as a generation to generation change in the allele frequencies
within a population.
Figure 2: Macroevolution vs Microevolution
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Microevolution:
a) The accumulation of changes in a population’s gene pool over time leads to observable changes in the population
itself, a phenomenon known a Macroevolution.
II. Hardy-Weinberg
● Is a mathematical expression that predicts the current genotype frequencies within a population’s gene pool, given
the existing allele frequencies.
p2 + 2pq + q2 = 1
p2 = predicted frequency of homozygous dominant genotype
q2 = predicted frequency of homozygous recessive genotype
pq = predicted frequency of heterozygous genotype
1 = total number of allele for a given locus
Sample Problem: Albinism is a rare genetically inherited trait that is only expressed in the phenotype of homozygous
recessive individuals (aa). The average human frequency of albinism in North America is only about 1/20,000. Determine the
percentage of individuals who are: homozygous dominant, heterozygous carriers, & albinos.
Step 1: Determine the frequency of the recessive allele q.
Step 2: Determine the frequency of the dominant allele p (hint: for all traits controlled by two alleles, the frequency
of the dominant allele (p) plus the frequency of the recessive allele (q) must equal 100%. Therefore, p+q=1).
Step 3: Using the Hardy-Weinberg equation, determine the expected frequency of individuals who are homozygous
dominant, heterozygous (carriers), & recessive (albinos).
P2 =
2pq =
q2 =
*Conventionally, the homozygous recessive genotype is used to derive the frequency of the dominant allele, for its allelic makeup is known. The
dominant phenotype cannot be used to generate allele frequencies unless the frequency of the homozygous dominant & heterozygous genotype are
known (p = AA + ½ Aa).
Hardy-Weinberg Equilibrium
●The HW equation predicts that the aforementioned allele & genotype frequencies will remain CONSTANT from one
generation to the next (no evolution will occur) under the following set of environmental conditions:
a) Infinitely Large Population Size: when the size of a population is dramatically reduced, the frequencies of alleles
among the small number of individuals that remain will be much different compared to the original population =
Sampling Error.
b) Random Mating: assumes that all individuals mate with equal frequency. Thus all alleles within a gene pool have an
equal chance of being passed on & will thus be equally represented in the next generation.
c) No Mutations: prevents introduction of new alleles into a gene pool due to changes in the genetic code.
d) No Migration: prevents the introduction of new alleles into or the loss of alleles from the population.
e) Natural Selection: does not occur –prevents certain alleles from being eliminated from a population’s gene pool.
As long as these conditions are met, allele frequencies will NOT change & the population is said to be in a state of
Hardy-Weinberg Equilibrium for a given trait.
●
If ANY of these conditions are violated (not maintained), allele frequencies will change leading to microevolution. The
forces that can lead to changes in a population’s allele frequencies include the following:
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Hardy-Weinberg Condition
Direct Microevolutionary Violation
1) Infinitely Large Population
1a) Genetic Drift
2) Random Mating
2a) Nonrandom Mating
3) No Mutations
3a) Mutation
4) No Migration
4a) Gene Flow
5) No Natural Selection
5a) Natural Selection
*The net result of the direct violation of Hardy-Weinberg conditions is a change in the alleles frequencies of a gene pool, resulting in
microevolution (will eventually lead to observable macroevolutionary changes)
Agents of Microevolution: Violating H-W Equilibrium
Figure 3: Mutations
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Mutation:
Figure 3.1: Genetic Drift
Allele frequencies can change between generations due to the fact that fertilization is a random event. As a result,
some alleles may get passed on in slightly greater amounts than others to cause allele frequencies to change or Drift
from generation to generation.
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Figure 3.12: Genetic Drift Rates vs Population Size
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Genetic Drift:
a) Events that can influence the rate of genetic drift by altering population size include Bottlenecking & the Founder
Effect.
Figure 3.2: Influences on Genetic Drift: Bottlenecking
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Bottlenecking:
Figure 3.3: Influences on Genetic Drift: Founder Effect
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Founder Effect:
Figure 3.4: Gene Flow
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Gene Flow:
Figure 3.5: Nonrandom Mating
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Nonrandom Mating:
Figure 3.6: Natural Selection: Stabilizing Selection
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Stabilizing Selection:
Figure 3.61: Natural Selection: Directional Selection
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Directional Selection:
Figure 3.62: Natural Selection: Diversifying Selection
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Disruptive/Diversifying Selection:
Origin of Species
I. Macroevolution
Figure 1: Macro vs Microevolution
Generations of accumulated microevolutionary changes within the gene pool will ultimately result in macroevolution
to the point of Speciation. Speciation is said to have occurred when individuals of different populations can neither:
a) No longer physically mate OR
b) Physically mate but not produce fertile offspring
●
Figure 2: Interspecies Hybrids (Mules)
Horse
2n = 64, n = 32
Donkey
2n = 62, n = 31
Mule
2n = 63
Sterile due to inability to
make functional gametes.
II. Pathways to Speciation
● Natural selection can establish new species under two distinct circumstances:
a) Allopatry: promotes reproductive isolation (one of the conditions for speciation) via geographic barriers that
prevent gene flow (mating) between populations.
b) Sympatry: promotes reproductive isolation via behavioral differences between individuals within the same
population resulting to result in a decreased likelihood of mating.
Figure 3: Allopatric Speciation
●
Allopatric Speciation:
Figure 4: Sympatric Speciation
If some members of a population adopt different behaviors (through natural genetic variation), they may become
reproductively isolated from the rest of the group (even if living among the group itself!). Such was the case involving
the Hawthorn fly & Apple fly.
●
With the introduction of apple trees to North America, some individuals of the Hawthorn fly population began to live
in the apple trees instead. As a result of this preference, these individuals became reproductively isolated despite
living in close proximity to the rest of the population, ultimately developing into the Apple fly species.
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Sympatric Speciation:
II. Rates of Evolution
Figure 5: Gradualism vs Punctuated Equilibrium
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Gradualism:
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Punctuated Equilibrium:
a) Like gradualists, evolutionists subscribing to punctuated equilibrium believe that change occurs in a gradual
manner, only more rapidly in response to environmental changes.
III. Systematics & Taxonomy
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Systematics:
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Taxonomy:
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Binomial:
Figure 6: Binomial Nomenclature (Common Names vs Scientific Names)
Figure 7: Linnean Classification System
*This system is based on structural similarities between organisms (if they look alike, they are placed in the same
group). The main problem with this is such groupings can be misleading due to the existence of ANALOGOUS
STRUTURES. For example, the analogous body forms of dolphins & fish may result in them grouped together in this
system despite the fact (based on molecular evidence) that they are NOT closely related.
Figure 8: Phylogenetic Tree
Rules for reading a phylogenetic tree:
1) As you progress toward the “top” of the tree, you are progressing forward in time (older to more recent).
2) Each branch point on the tree represents a common ancestor to groups that arise from it. For example, “x” is the
common ancestor to species A & B, just as “y” is the common ancestor to species C, D, & E.
3) The species that are closest together on the tree share the greatest number of similarities (genetic) & are thus
more closely related compared to species more distantly situated about the tree. For example, species A is more
closely related to species B (& vice-versa) compared to any other species.