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Darwin & Microevolution
Chapter 19-20
Charles Darwin (1809-1882)
Former divinity and medical student Secured an unpaid position as ship's
naturalist on the H.M.S. Beagle Voyage provided Darwin a unique
opportunity to study plants, animals, and their environment
Gathered a great deal of evidence he would later incorporate into a theory of evolution
EQUATOR
GalapagosIslands
Voyage of the Beagle
5 yr. mission to chart South America
Darwin’s Theory of Natural Selection Individuals in a population have variable levels
of success in reproducing Left unchecked, populations tend to expand
exponentially, leading to a scarcity of resources In the struggle for existence, some individuals
are more successful (fit) than others, allowing them to survive and reproduce
Those organisms best able to survive and reproduce will leave more offspring
Darwin’s Theory of Natural Selection Over time there will be heritable changes in
phenotype (genotype) of a species These changes may result in a transformation of
the original species into a new species similar to, but distinct from, its parent species
Common Descent, due to these changes similar species have common ancestors. This means that nearly all of life is linked
What is Evolution?
Evolution is a process that results in heritable changes in a population spread over many generations
Evolution can be precisely defined as “any change in the frequency of alleles within a gene pool from one generation to the next”
Populations, not individuals evolve Traits within a population vary among individuals
Variation Within a population most phenotypic traits are
polymorphic, they have two or more forms Those traits that have many forms show
continuous variation Individual inherit different combinations of alleles
leading to different phenotypes All these genes & their alleles within a
population is known as the gene pool. This variation is the raw material for
evolution This variation is also what allows for natural
selection
What Determines Alleles in an Individual? Mutations, many are lethal, but some can be
neutral & some may confer an advantage Crossing over at meiosis I, shuffles alleles Independent assortment, genes that may work
together, but are on different chromosomes will not be inherited together
Fertilization, sexual reproduction Change in chromosome number or structure,
often deleterious, but can be advantageous
Genetic Equilibrium
Point when a population is not evolving The opposite of evolution Allelic frequencies are not shifting Calculated using the Hardy-Weinberg equations
p + q = 1p2 + 2pq + q2 = 1
Where p = frequency of Dominant allele (A) and q = frequency of recessive allele (a)
Genetic Equilibrium
Five conditions need to be met: No mutations Random mating Gene does not affect survival or
reproduction Large population No migration
Hardy-Weinberg Equilibrium
AA(p2) Aa(pq)
Aa(pq) aa(q2)
Ap aq
Ap
aq
p2 = frequency of AA (Homozygous dominant) 2pq = frequency of Aa (heterozygous) q2 = Frequency of aa (homozygous recessive)
490 AA butterfliesDark-blue wings
420 Aa butterfliesMedium-blue wings
90 aa butterfliesWhite wings
Hardy Weinberg Equilibrium: Example
Starting population:
Frequencies in Gametes:
A A A a a a
0.49 AA 0.42 Aa 0.09 aa
0.49 + 0.21 0.21 + 0.09
0.7A 0.3a
F1 genotypes:
Gametes:
Hardy Weinberg Equilibrium: Example
490 AA butterflies
THE NEXT GENERATION
420 Aa butterflies
90 aa butterflies
THE NEXT GENERATION
490 AA butterflies
420 Aa butterflies
90 aa butterflies
NO CHANGE
NO CHANGE
STARTING POPULATION
490 AA butterfliesDark-blue wings
420 Aa butterfliesMedium-blue wings
90 aa butterfliesWhite wings
Hardy Weinberg Equilibrium: Example
Mechanisms of Evolution
Evolution of a population over time may occur as a result ofNew mutationsNatural selectionNonrandom mating (Sexual selection)Genetic drift because of small
populationGene flow – immigration and emigration
(Opposite of Genetic Equilibrium)
Mutations Mutations that alter protein structure enough
to impact its functionmore likely to be harmful but may be beneficialour genome is product of thousands of
generations of selection Fuel for evolution Mutant allele may enable an organism to fit
its environment better & increase reproductive success
especially likely if environment is changing
Natural Selection Difference in the survival & reproductive success
of different genotypes and/or phenotypes Over time, the alleles that produce the most
successful phenotypes will increase in the population
Less successful alleles will become less common
Change leads to increased fitness Selection is not a “force” it is merely the favoring
of some genetic changes over others
Types of Natural Selection
Directional Selection Stabilizing Selection Disruptive Selection
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Range of values for the trait at time 1
Range of values for the trait at time 2
Range of values for the trait at time 3
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Directional Selection Shift in the variation in a
consistent direction within the phenotypic range
Examples: Pesticide resistance in
insects Antibiotic resistance in
bacteria
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Range of values for the trait at time 1
Range of values for the trait at time 2
Range of values for the trait at time 3
Stabilizing selection Loss of extreme
forms with stabilization of an intermediate form
Example: Infants greater or less than 7.5 lbs have increased mortality
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Range of values for the trait at time 1
Range of values for the trait at time 2
Range of values for the trait at time 3
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Disruptive Selection favors individuals at
the extremes with a reduction of intermediate forms
Example: African Finches selection favors small or large beak size
Sexual Selection
A type of natural selection Selection that is driven by the competition
for mates
Gene Flow
Movement of alleles into a population Tends to keep the gene pools of
populations similar Counters changes due to mutation, natural
selection, and genetic drift
Genetic Drift
Random change in allele frequencies brought about by chance
Effect is most pronounced in small populations
This can cause similar, but isolated populations to become dissimilar due to the loss or fixation of alleles
Bottleneck
Genetic drift is most pronounced when small populations grow into larger ones, usually after a catastrophe
A bottleneck as only a few alleles survive and are now disproportionally expressed in the population
A founder effect results in rare or even disadvantageous alleles being found in a population at a level higher than normally expected
Macroevolution:Evidence for evolution Biogeography Fossil Record Comparative anatomy Comparative embryology Molecular Biology
Evolution evidence: Biogeography
Geographical distribution of species Indicates that populations that are isolated from
one another geographically evolve separately
Evolution evidence: The Fossil Record
Fossils are created when organisms become buried in sediment, bone and other hard tissue is converted to rock
The fossils contained in sedimentary rock layers reveal a history of life on earth
Evolution evidence: Comparative Anatomy Comparing body forms and structures of
major lineages By studying homology or how similarly
derived body parts have evolved, we can put together an evolutionary tree and find common ancestors even if the body parts no longer serve the same function
Evolution evidence: Comparative anatomy Homologous
structures Similar anatomy,
different functions Indicate divergent
evolution
Evolution evidence: Comparative anatomy Analogous structures
Similar function, different anatomy
Indicated Convergent Evolution
Example: bird’s wing vs. fly’s wing
Evolution evidence: Comparative anatomy Vestigial Organs
Remnants of evolution Organs that were
required in ancestor that are not needed in present-day organism
Examples: appendix, tailbone
Evolution evidence: Comparative Embryology Development of early
embryos of resemble each other due to a common developmental plan
During later embryonic development this plan becomes modified to create the different body types we see
Evolution evidence: Molecular Biology Tracking mutations in
sequences of DNA and/or proteins can trace the evolutionary history of organisms
The more nucleotides/ amino acids in common, the more closely related the species