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II. Theories of Evolution
A. Development of Theories
1. Jean-Baptiste Lamarck (1744-1829)
First person to suggest that life evolved
a) The law of Use
and Disuse
b) Inheritance of
acquired
characteristics
Lamarck believed that giraffes stretched their necks to reach
food. Their offspring and later generations inherited the
resulting long necks
Charles Darwin
2. Darwin’s Theory: Natural
Selection (Based on his observations)
a)
b)
c)
d)
Variation exists within all species
Populations always tend to increase
The amount of resources is limited
There will be a struggle for the
available resources
e) Organisms who are most fit will live
longer and have more offspring. (The
offspring will inherit advantageous
traits)
Most giraffes used to have short necks, but some had slightly
longer necks. When the food on the lower branches was eaten,
many of the giraffes with shorter necks died of starvation,
leaving the ones with slightly longer necks to survive and
reproduced. Through many generations, the giraffes with longer
necks became the most common.
3. Origin of Variation
• Genetic Recombination
(Sexual reproduction. Meiosis,
Crossing over and random
fertilization)
• Mutation (Produces New
Genes)
B. Mechanisms of Evolution
1. Species and Populations
• Evolution cannot be seen in
an individual because their
genes do not change.
a) Evolution is a change in the
genetic make up of a
population
b) Gene Pool- all the different
alleles (gene forms) in a
population
2. Hardy-Weinburg principle
Gene Pool
2.
Population Genetics Or
Fun with Hardy-Weinburg!
The Math!
Let P = the frequency of Dominant Genes in a population
Let q = the frequency of Recessive genes in a population
Therefore:
P+q=1
All the dominant genes + all the recessive genes = all the genes
Individuals can be:
Genotypes
Hardy-Weinburg Notation
Homozygous Dominant
Heterozygous
Homozygous Recessive
Therefore:
PP or P2
p
Pq + qP or 2Pq
q
qq or q2
p
q
P2
Pq
qP
q2
P2 + 2Pq + q2 =1
All the Homozygous Dominant + All the Heterozygotes + All the
Homozygous Recessives = All the Population
With these formulas, you can solve all Hardy-Weinburg problems
C. Factors which Change Genetic
Equilibrium (ie. Cause Evolution)
1. Natural Selection: Favoring one genotype
over another. Ex. H. B. Kettlewell
In England, before the industrial revolution, tree bark was a lighter
color and lighter moths out numbered darker moths 9:1
Coal caused the tree bark to darken during the industrial revolution. The
darker moths then numbered the lighter moths 9:1
Geospiza fortis
Study by Peter and Rosemary Grant
Antibiotic Resistant Strains of Bacteria are
a Prime Example of Evolution
Types of Selection
a) Directional
Selectionconstant genetic
change of one
type. Ie. Giraffe’s
necks getting
longer. Dinosaur
size increasing
Types of Selection
b) Stabilizing
SelectionSelection that
favors the average
and eliminates the
extremes. The
population
becomes more
alike
Types of Selection
c) Disruptive
selectionSelection that
favors the extremes
and eliminates the
average.
Population
becomes more
different. This can
lead to new species
Types of Selection
C. Factors which Change Genetic
Equilibrium (ie. Cause Evolution)
2) Gene flow- Movement of organisms in or
out of a population
3) Mutation- change in a gene form that is
unequal. Ie. Change form gene A to a more
than a to A.
4) Genetic Drift- change in gene frequency
due to random chance
5) Isolation- separation of a population so it
can’t interbreed. Either genetic or
geographic
Patterns of Evolution
1. Divergent evolution-Two separate
species becoming more different due to
different selective pressures in different
environments
a) Adaptive Radiation- process by which
species adapt to a variety of habitats
b) Speciation- formation of two separate
species from one
2. Convergent
Evolution- the
process by
which distantly
related
organisms
become more
similar due to
similar selective
pressures in
similar habitats