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
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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
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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
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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
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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
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“Gene pool”: total genetic information available in a
population
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“Allele frequency”: percentage of allele in gene pool (expressed
as a decimal)
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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
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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
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example, F2:
red 1/6 = 0.17
pink = 3/6 = 0.50
white = 2/6 = 0.33
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HARDY-WEINBERG
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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
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 Based
on set of assumptions about ideal hypothetical
population that is not evolving
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HARDY-WEINBERG CONDITIONS:
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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
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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
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Affect genetic equilibrium
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MUTATION
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Spontaneously introduces new allele variants into a
population
Natural selection is often slow to eliminate harmful
recessive mutations
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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
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Gene frequency changes
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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
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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
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 Similar
recessive genes (carried, masked) often
present in genomes of related individuals
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NONRANDOM MATING
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Physical Characteristics (similar genes)
Assortative Mating: selection of mate based on
similarity of characteristics
Nonrandom mating can affect genotypes (combination
of alleles) of population
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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
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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
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 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
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 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
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The Tale of the Peacock
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CONCEPT OF SPECIES
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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
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Organisms may appear different enough to belong to different species.
How different do they have to be to be considered a unique species?
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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.
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Defines a species as those organisms that can produce viable offspring
together. Same chromosome #
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Issues:
 What about hybrids?
 What about plants, etc that reproduce asexually?
 What about extinct species?
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GEOGRAPHIC ISOLATION
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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
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Reproductive Isolation: results from barriers to
successful breeding between population groups in
same area
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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
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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
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Prezygotic
Incompatible behavior
 Reduce chance of hybrid formation
 Mating times, calls
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 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)
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Punctuated Equilibrium: theory that
speciation occurs during brief periods of
rapid genetic change, interspersed with
long equilibrium periods
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In 1972 paleontologists Niles Eldredge and
Stephen Jay Gould published a landmark
paper developing this idea.
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