The Evolution of Populations and Speciation
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Transcript The Evolution of Populations and Speciation
Reference: Campbell 7th Ed. Chapters 23 & 24
THE EVOLUTION OF
POPULATIONS AND SPECIATION
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VARIATION OF TRAITS IN A POPULATION
Evolution by natural selection gains wide
acceptance
Early 1900’s birth of genetics field
Questions resurface about evolution and
natural selection
“Population Genetics”: study of
evolution from genetic point of view
Involves gradual changes in genetic
material over generations, in groups of
organisms
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VARIATION OF TRAITS IN A POPULATION
A Population is the smallest unit in which evolution occurs
(“microevolution”)
Individuals may vary in observable traits
Studying variation in a single trait – use a large sample
Quantitative traits in a population (height, weight) show
variation in a bell-shaped “normal” curve
Ex. Body length in a population of fish
X axis: fish length (cm)
Y axis: # of fish
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VARIATION OF TRAITS IN A POPULATION
What causes variation in traits?
Environmental
factors & Hereditary can account
for different phenotypes within a single family
Genotypes (alleles) come from same parents
but in different combinations can account for
variations in successive offspring due to
formation of gametes & how they fuse
(Segregation of Alleles)
Ex: Rr x Rr = ?
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CAUSES OF VARIATION
Mutation: flawed copies of individual genes
Recombination: reassociation of genes in diploid
individual (occurs during meiosis)
Segregation of alleles
Independent assortment (nonhomologous)
Crossing over (homologous)
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CAUSES OF VARIATION
Random fusion of gametes: chance game
played by gametes
Millions of sperm in mating
“Chosen One” fertilizes egg
Ensures variation in offspring
No exact copies of parents, or other offspring
likely
Try this game: The Great
Sperm Race
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ALLELE FREQUENCIES AND GENE POOL
“Gene pool”: total genetic information available in a
population
“Allele frequency”: percentage of allele in gene pool (expressed
as a decimal)
Ex: If there are ten individuals in a population and at a given
locus there are two possible alleles, A and a, then if the
genotypes of the individuals are:
Population 1: AA, Aa, AA, aa, Aa, AA, AA, Aa, Aa, and AA
Then the allele frequencies of allele A and allele a are:
pA = (2+1+2+0+1+2+2+1+1+2)/20 = 0.7
pa = (0+1+0+2+1+0+0+1+1+0)/20 = 0.3
*remember, gametes are haploid, and carry only one form of allele
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PREDICTING PHENOTYPE
Phenotypes are controlled by which alleles are
inherited (genotypes)
Phenotype frequency: ratio stating number of
times a specific phenotype occurs in a
population in a single generation
example, F2:
red 1/6 = 0.17
pink = 3/6 = 0.50
white = 2/6 = 0.33
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HARDY-WEINBERG
British mathematician Godfrey Hardy
German physician Wilhelm Weinberg
Independently showed that allele frequencies in a
population “tend to remain the same from
generation to generation unless acted on by
outside influences” when populations are in
“genetic equilibrium”.
Hardy-Weinberg Equilibrium
Based
on set of assumptions about ideal hypothetical
population that is not evolving
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HARDY-WEINBERG CONDITIONS:
1) No mutations occur
Allele
frequencies do not change overall
2) Individuals don’t migrate
3) Population is large
4) Individuals mate randomly
5) Natural selection does not occur
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HARDY-WEINBERG EQUATION:
Equation:
p2+2pq+q2=1.0
p2
= homozygous dominant
condition; AA
q2 = homozygous recessive; aa
2pq = heterozygous ; Aa
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HARDY-WEINBERG
Theoretical state in which allele frequencies
remain the same over generations (P = F1 = F2 =
F3, etc)
Showed what forces disrupt genetic
equilibrium and led to evolutionary change
Real populations usually violate HW
conditions, causing gene frequencies to
fluctuate
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MUTATION
Evolution results from the change of
population’s allele frequencies (genetics) over
generations
Any violation of 5 conditions of Hardy-Weinberg
Equilibrium results in evolution
Mutagens can cause increase/decrease in allele
frequency
Spontaneous mutations occur constantly
Mutations can produce new alleles for trait
Affect genetic equilibrium
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MUTATION
Spontaneously introduces new allele variants into a
population
Natural selection is often slow to eliminate harmful
recessive mutations
Natural selection operates only when genes are expressed
(phenotypes); often not when “carried”
Beneficial mutations are vital to evolution in long run
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MUTATIONS: BENEFICIAL OR NOT?
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MIGRATION
Gene frequency changes
Gene flow: process
of genes moving
from one
population to
another
ex: Baboons
Immigration: movement of
individuals into a population
Emigration: movement of
individuals out of a population
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GENETIC DRIFT
Genetic Drift: allele frequencies in a population change
as result of random events or chance.
Example:
Small population can be affected by single organism’s
ability to reproduce low or high
Small populations are much more susceptible. Why?
Abrupt changes in alleles shows high genetic drift
Large population
Retain stable allele frequencies; low genetic drift
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GENETIC DRIFT
Small population loses
genetic variability and
becomes vulnerable to
extinction
“Bottlenecking” a
population
Northern Elephant
Seals
Cheetahs = very little
genetic variability left
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GENETIC DRIFT POPULATION BOTTLENECK
FOUNDER EFFECT
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NONRANDOM MATING
Most species do not mate randomly
Geographic proximity is a factor
Matings of related individuals can amplify traits
& result offspring with disorders
Similar
recessive genes (carried, masked) often
present in genomes of related individuals
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NONRANDOM MATING
Physical Characteristics (similar genes)
Assortative Mating: selection of mate based on
similarity of characteristics
Nonrandom mating can affect genotypes (combination
of alleles) of population
May not affect on overall allele frequencies
Blue and white snow geese
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NATURAL SELECTION
Ongoing process in
populations
Single most significant factor
that disrupts genetic
equilibrium
Individuals reproduce more
successfully as result of
natural selection
Contribution of genes to next
generation
Stabilizing, Directional,
Disruptive and Sexual all
cause evolution in a
population (microevolution)
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STABILIZING SELECTION
Stabilizing Selection: average form of trait
causes organism to have an advantage in
reproduction; high fitness
Lizard size
Small
lizard runs too slow
Large lizard easily spotted and captured
Selection reduces size range
Most common type of selection
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DIRECTIONAL SELECTION
Directional Selection:
individuals that display
more extreme form of trait
have higher fitness than
individuals with average
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DISRUPTIVE SELECTION
Disruptive Selection : individuals with either
extreme variation of trait have higher fitness
than average form of trait
Limpets
Shell
color
Pure white to dark tan
White
on rocks with goose barnacles
Dark tan on bare rocks blend in
Intermediate color at disadvantage
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SELECTION CHARTS
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SEXUAL SELECTION
Sexual selection: preferential choice of a mate
based on specific phenotypic trait
Females tend to choose males they mate with
due to certain traits male expresses
Genes of successful reproducers rather than of
merely successful survivors are amplified
through natural selection
The Tale of the Peacock
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CONCEPT OF SPECIES
Total # of species today is inaccurate due to numerous
undiscovered species
Currently, scientists have named and successfully classified over
1.5 million species. It is estimated that there are as little as 2
million to as many as 50 million more species that have not yet
been found and/or have been incorrectly classified.
Remote locations: Rainforests and Oceans
New species discovered while others become extinct at fast rate
One species can become two through process of speciation
Speciation results in many related populations
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CONCEPT OF SPECIES
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MORPHOLOGICAL SPECIES CONCEPT:
Morphology: study of
internal and external
structure and form of
an organism
Using the MSC, species
are defined by structure
and appearance
Aka “Phenetic” species concept: a species is a set of organisms that are
phenotypically similar and that look different from other sets of organisms.
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LIMITATIONS TO MSC
Adult & juvenile herring gulls
Mallards (Anas platyrhynchos)
Phenotypic
differences
may exist among
individuals in one
population.
American Black duck (Anas rubripes)
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BIOLOGICAL SPECIES CONCEPT
Organisms may appear different enough to belong to different species.
How different do they have to be to be considered a unique species?
Biological Species Concept: A species is often defined as a group of
individuals that actually or potentially interbreed in nature. In this sense, a
species is the biggest gene pool possible under natural conditions.
Defines a species as those organisms that can produce viable offspring
together. Same chromosome #
Issues:
What about hybrids?
What about plants, etc that reproduce asexually?
What about extinct species?
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GEOGRAPHIC ISOLATION
Geographic Isolation: physical separation of members of
population
Allopatric Speciation
Populations physically isolated by an extrinsic barrier
Gene flow between them stops
Natural selection and genetic drift cause divergence
Individuals of two populations can no longer interbreed
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REPRODUCTIVE ISOLATION
Reproductive Isolation: results from barriers to
successful breeding between population groups in
same area
Parapatric Speciation
Two or more separate gene pools form, and eventually
these diverge into different species
Two broad types
Prezygotic: before fertilization
Postzygotic: after fertilization
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REPRODUCTIVE ISOLATION
Types of postzygotic isolation
Offspring of interbreeding species are underdeveloped,
die early, or are not fertile
If death or infertility occurs parents have wasted
gametes from evolution standpoint
Prezygotic
Incompatible behavior
Reduce chance of hybrid formation
Mating times, calls
Frogs,
birds
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RATES OF SPECIATION
Gradualism –vs- Punctuated Speciation
Speciation usually takes millions of years,
but some species form more rapidly
“Gradualism” - Fossil record indicates
many species existed without change for
long periods
Fossil evidence seems to indicate that
“instant” changes can occurred within
few thousand years (Hox genes)
Punctuated Equilibrium: theory that
speciation occurs during brief periods of
rapid genetic change, interspersed with
long equilibrium periods
In 1972 paleontologists Niles Eldredge and
Stephen Jay Gould published a landmark
paper developing this idea.
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