Transcript Evolution Lecture #2

Mechanisms of Evolution
15.3
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 Changes
in allele frequencies
causes evolution
 3 Types of natural selection
 How new species form
 Speed of Evolution
 Patterns of evolution
I. Allele Frequencies
A. Review: What is an allele?
One form of a gene that causes
different traits to appear
(Ex: pink, red, or white petals)
B. Allelic Frequency- The % of a
specific allele in the gene pool.
C. Gene Pool- ALL possible alleles in
a population
I. Allele Frequencies
D. Genetic Equilibrium- A
population in which the allelic
frequency stays the same
1. Evolution is NOT occurring!!!
II. Genetic Drift
A. Genetic Drift- An evolution
mechanism in which the allelic
frequencies are changed due to
chance.
1. Mutations occur by chance,
not caused by UV rays,
chemicals, radiation, etc.
II. Genetic Drift
II. Genetic Drift
2. Greatly affects SMALL
populations.
a. Ex: Amish populations
1) 30% of settlers carried a
recessive allele for extra fingers & toes.
2) Today 1/14 Amish carry the
allele compared to 1/1000 non-Amish
United States Citizens
III. Natural Selection
A. Allele frequencies change
because organisms with traits
that fit their environment
survive and reproduce (others
die).
B. Affects small AND large
populations
III. Natural Selection
C. Types of Natural Selection (WHO
DOES IT FAVOR???)
1. Stabilizing Selection- Favors
average individuals
a. Big spiders are seen by birds
b. Small spiders can’t find food
c. Medium spiders are favored
(see graph on pg. 408)
III. Natural Selection
2. Directional selection- Favors
ONE extreme for a trait.
a. Insects are found deep in
a tree trunk.
b. Woodpeckers with short &
medium length beaks
have trouble getting to the
insects. c. Long beaks are
favored.
III. Natural Selection
3. Disruptive Selection- Favors BOTH
extremes, but not intermediate
phenotypes.
a. A beach has white and dark
brown rocks
Remember
the green
b. White and dark brown
car?
snails cannot be seen by birds
c. Tan snails can be seen on
either color of rock and are eaten
IV. Speciation
A. Species- Group of organisms
that look alike, interbreed and
produce fertile offspring.
B. Speciation- The evolution of
new species.
IV. Speciation
C. Causes of speciation
1. Geographic Isolation- When
a physical barrier divides a
population and the two pops
evolve differently.
a. Different alleles appear in
the two populations
IV. Speciation
IV. Speciation
2. Reproductive IsolationWhen closely related organisms
can no longer produce fertile
offspring.
a. Genetic material becomes
too different
b. Reproductive behaviors
become too different
IV. Speciation
Rana boylii
Breeds late
March to May
Rana aurora
Breeds January
to March
IV. Speciation
3. Change in Chromosome #
a. Polyploid- Individual or
species with more than the
normal set of chromosome
(3n, 4n, 5n etc)
b. Caused by mistakes in
meiosis
IV. Speciation
c. Organisms with different #
of chromosomes cannot
produce fertile offspring
d. Polyploids CAN interbreed
among themselves if they
have the same # of
chromosomes
IV. Speciation
D. Speciation Rates
1. Gradualism- Evolution occurs at
slow, steady rate
2. Punctuated EquilibriumEvolution occurs in short bursts
with long periods of equilibrium in
between.
(See Elephants on pg. 411)
IV. Speciation
3. Both gradualism and
punctuated equilibrium are
accepted today.
4. Side Note: Phylogenic Tree
V. Patterns of Evolution
A. Adaptive Radiation- One
species evolves into an array
species to fit different niches
(mini habitats)
1. Diversity allows for more
flexibility and larger numbers
2. Supports the idea of natural
selection
V. Patterns of Evolution
V. Patterns of Evolution
B. Divergent Evolution- Similar
species become increasingly
different.
C. Convergent Evolution- Very
distantly related organisms
become more similar
1. They share similar
environments and selection
pressures
Recap
A. Changes in allele frequencies
causes evolution
B. 3 Types of natural selection
1. Stabilizing
2. Directional
3. Disruptive
Recap
C. Speed of evolution
1. Gradualism
2. Punctuate equilibrium
Recap
D. How new species form
1. Geographic Isolation
2. Reproductive Isolation
3. Changes is chromosome #
E. Patterns of evolution
1. Adaptive Radiation
2. Divergent Evolution
3. Convergent Evolution