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Chapter 16
Evolution of Population and Speciation
16-1 Genetic Equilibrium
Population Genetics
-The study of evolution from a genetic point of view. A population is the smallest unit in
which evolution can occur.
Mircroevolution- change in the collective genetic material of a population.
Variations
-Individuals in a population have variations.
-ex. Size in field mouse—small to large
Most Variations follow a Bell Curve
-most organisms fit in the average
form.
What causes genes to vary?
1) Mutations
2) Recombination of genes
3) Random Pairing of Gametes
Gene Pool
-The total genetic information available in a population. (All the genes that are available).
Allele Frequency
-Divide the total number of a certain allele by the total number of alleles in the
population.
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Example: Flowers (w/incomplete dominance)
RR = Red, rr = white, Rr = pink
In a population, there are 2 red (RR), 1 white (rr), and 2 pink (Rr)
Frequency of R = 6/10 = .60
Frequency of r = 4/10 = .40
Phenotype Frequency
-The number of individuals with a certain phenotype, out of the total number of
individuals.
Ex. 2 red, 1 white, 2 pink
Red = .4 (2/5)
White = .2 (1/5)
Pink = .4 (2/5)
To predict the phenotypes of the next generation, multiply the allele frequency.
Ex.
RR = .6 x .6 = .36
rr = .4 x .4 = .16
Rr = 1 -.36 - .16 = .48
For you people that enjoy equations,
The equation to determine allele frequencies:
Remember that p + q = 1
p2 + 2pq + q2 = 1
(RR + 2Rr + rr = 1)
Example: Tongue Rolling
Can roll= dominant (T),
Cannot roll = recessive (t)
In a class of 32 students, 28 students can roll their tongue, and 4 students cannot roll
their tongue.
Using the equation
p2 + 2pq + q2 = 1, T =p, t =q.
q2 (tt) = 4/32, = .125
q = √.125 = .35
because, p + q = 1, p + .35 =1,
p = 1-.35, p = .65
Fill in the equation
p2(.65 x.65)+2pq(2[.65x.35])+q2(.35x.35)=1
p2(.42) +2pq (.46) + q2(.12) = 1
Hardy-Weinberg Genetic Equilibrium
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For genetic equilibrium
1) no mutations occur
2) individuals do not enter or leave a population
3) population is large
4) random mating
5) selection does not occur
16-2 Disrupting Equilibrium
Mutations
-spontaneous mutations occur at relatively low rates
-exposure to mutagens can increase rates of mutations
Migration
-Immigration- individuals move in.
-Emigration- individuals move out
Ex. Male lions that take over the pride.
Ensures Gene Flow.
Gene Flow
-Genes moving from one population to another
-gene flow increases variation
Genetic Drift
-allele frequencies in a population change
-small populations are affected more by gene drift than large populations. Page 305
Ex. Elephant seals and Cheetahs- both species are homozygous for almost all traits
Nonrandom Mating
-Mate selection is influenced by:
-geographic proximity- can result in kinship
-similar physical traits (assortative mating) the mates would probably have similar genes
because they have similar physical traits.
Sexual Selection
-Females choose mates based on certain traits that indicate reproductive fitness
Ex. Birds- colors, dances, nest building ability
Elk/goats- strength of males
Stabilizing Selection
The average form of the trait has the highest fitness. Ex. Geckos in a forest of medium
green leaves- the medium green gecko has the advantage
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Directional Selection
-A more extreme form of the trait is favored. Ex. Flamingoes with longer legs and necks.
Disruptive Selection
-The extreme forms are favored
Ex. Limpets in light and dark colors have an advantage over medium colored limpets.
16-3 Formation of Species
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Speciation
-species formation
-forms many related populations of organisms
Morphological Species Concept
-using internal and external appearance and structures
-easy to observe
- phenotype variations can make identification difficult
Biological Species Concept
Ernst Mayr (1904-2005)
-Species- a group of organisms that can
successfully interbreed, but not breed with
other groups.
-does not work for extinct species
-does not work for asexual reproduction
-The modern definition of a species
includes both the morphological and
biological species concept.
-A species is a single type of organism.
Members of a species are morphologically
similar and can interbreed to produce
fertile offspring.
Speciation began with isolation
Geographic isolation
- a physical separation of a population
- canyons, rivers, islands, separated ponds...
Ex. Squirrels in the Grand Canyon
Allopatric speciation
Species arise as a result of geographic isolation
Reproductive Isolation
-Barriers to successful breeding
A) Prezygotic
-before fertilization
-different breeding times, different courtship behaviors
-will not waste gametes
B) Postzygotic Behavior
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-after fertilization
-offspring die or are sterile
-donkeys and horses mules
-wastes gametes
Sympatric speciation
Two subpopulations become reproductively isolated within the same geographic area.
They use slightly different niches.
Rates of Speciation
a) Gradual- slowly evolving
b) Punctuated Equilibrium- sudden changes, then long periods without change
-may be a combination of both.
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