Transcript Evolution of Populations

EVOLUTION OF
POPULATIONS
Objectives:
12.1 Identifying ways in which the theory of
evolution explains the nature and diversity of
organisms
12.2 Describing natural selection, survival of the
fittest, geographic isolation, and fossil record
AOD B.12.4 Describe evidence of species variation
due to climate, changing
SECTION 1: GENES AND VARIATION
 Review:

Mendel:
Pea plants
 Punnett squares
 Crosses between 2 heterozygous parents predict:

Phenotype ration of 3:1
 Genotype ratio of 1:2:1

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
Lamark
Darwin:
Made observations of species (living and fossils)
while on the HMS Beagle.
 Could NOT:

explain how traits were passed from one
generation to another.
 How new species appeared and others became
extinct.

SECTION 1: GENES AND VARIATION
 Sources
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
of Variation:
Mutations --- changes in nucleotide bases
Gene shuffling --Due to independent assortment of chromosomes
during meiosis
 23 pairs of chromosomes (humans) can yield 8.4
million different combinations of genes! Then
add in crossing over!!!!
 Kind of like a deck of 52 playing cards --- your
chance of drawing an ace at any time is 4/52, or
1/13, no matter how many times you shuffle the
deck.

SECTION 1: GENES AND VARIATION
 Gene
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Pools
Single-gene activity
Single-gene trait vs. polygenic trait

Polygenic will yield many more possible combinations
 Variation


and Gene Pools
Gene pool: all the genes, and their different
alleles, present in a population
Relative frequency: the number of times an
allele occurs in a gene pool, compared to the
total numbers of alleles present; expressed as a
percentage.
Use our class gene pool data as an example.
 Again, how would an introduction of non-tonguerollers affect our population?

SECTION 2:
EVOLUTION AS GENETIC CHANGE
 Review:
What is evolutionary
“fitness”?
Answer: the ability of an organism to
pass on its traits/genes to its offspring
 So an evolutionary adaptation is any
genetic change that increases an
individual’s chances of passing on its
genes.

 Evolution
is any change over time in
the relative frequencies of alleles in a
population. (Refer to our gene pool
activity?)
SECTION 2:
EVOLUTION AS GENETIC CHANGE
 Natural
Selection on Single-Gene Traits:
Can lead to changes in allele frequencies in a
population, which is what we call
_______________ .
 In your camouflage experiment, did you
have any alleles (colored beads) disappear
from your population?

Why?
 What did this do to the gene pool for “bead color”?
 What was the cause of this change?
 Lizard color mutation example on pp.397-398 of
textbook (next slide)

WHAT ARE SOME POSSIBLE REASONS THE
RED MUTATION DID NOT SURVIVE, BUT THE
BLACK MUTATION SURVIVED AND THRIVED?
SECTION 2:
EVOLUTION AS GENETIC CHANGE
 Natural

Selection on Polygenic Traits:
Normal distribution of polygenic traits
resembles a bell curve:
Source: http://sixminutes.dlugan.com/good-public-speaker-average/
SECTION 2:
EVOLUTION AS GENETIC CHANGE
 Natural
selection can affect the
distribution of polygenic phenotypes in
three ways:
Directional selection – shifts toward one
end or the other
2. Stabilizing selection – favors individuals in
the middle of the graph, putting more
individuals there, and making the
distribution curve more narrow
3. Disruptive selection –
1.
a.
b.
conditions favor extremes at end of bell
curve, so curve will dip in the center
Can result in two separate curves
Source:
http://cosbiology.pbworks.com/w/p
age/24299272/1302%20%E2%80%93%20Evolution
%20as%20Genetic%20Change
SECTION 2:
EVOLUTION AS GENETIC CHANGE
 Genetic
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Drift:
In small populations, results may not be those
predicted by a Punnett square (think of
purposeful breeding of species, or populations
without an influx of “outsiders”).
An allele can become more or less common simply
by chance, if an allele is passed on more
frequently than another.
The random change in allele frequency is called
genetic drift.
Occurs commonly when a small population
“settles” a new habitat --- called founder effect.
Which type(s) of selection would a graph of the
phenotype(s) resemble?
SECTION 2:
EVOLUTION AS GENETIC CHANGE
 Evolution
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
Hardy-Weinberg principle: allele
frequency will remain constant unless
one or more factors cause it to change.
Five conditions required to maintain
genetic equilibrium:
1.
2.
3.
4.
5.

vs. Genetic Equilibrium:
Random mating
Very large population
No movement into or out of the population
No mutations
No natural selection
What are the chances of these conditions
ALL being met??
SECTION 2:
EVOLUTION AS GENETIC CHANGE
Mutiny on the Bounty and Pitcairn Island in
the South Pacific
 How would each of these conditions affect
allele frequency, if not met?
 Give an example of how each of these
conditions could be met.

SECTION 16-3
THE PROCESS OF SPECIATION
 Speciation:
the formation of a new
species
 What is the definition of a species?
 For a new species to form, enough
genetic change must occur so that the
“new” species can no longer
effectively reproduce with the “old”
species.
 This is termed reproductive isolation,
and the two species now have
separate gene pools.
SECTION 16-3
THE PROCESS OF SPECIATION
 Reproductive
forms:
isolation can take several
Behavioral isolation --- the species are
capable of interbreeding, but do not
respond to each other’s overtures
(different mating rituals or reproductive
strategies).
2. Geographic isolation --- may or may not
lead to separate species; natural
selection does work separately on each
group.
3. Temporal isolation --- different mating
times (EX: pollen production)
1.
DARWIN’S FINCHES & THE GRANT’S
Source: http://myweb.rollins.edu/jsiry/Grants'finch-study.html
SECTION 16-3
THE PROCESS OF SPECIATION
 How
speciation worked in the
finches:
Founder effect: A few finches flew
from the South American mainland to
one of the Galapagos Islands --probably blown there in a storm.
2. Geographic isolation: At some point,
some of these birds flew to another of
the islands. The two populations were
separated by open water, creating two
separate gene pools.
1.
SECTION 16-3
THE PROCESS OF SPECIATION
 How
speciation worked in the finches:
Changes in gene pool: As birds better
suited to the environment on each island
survived better, natural selection resulted
in directional selection --- especially for
beaks.
4. Reproductive isolation: Finches are choosy
about partners and inspect beaks closely.
Once the beaks were different, they would
not mate with each other ---- EVEN
THOUGH THEY PROBABLY COULD!
(Separate gene pools = separate species)
3.
SECTION 16-3
THE PROCESS OF SPECIATION
 How
speciation worked in the finches:
Ecological competition: When the two
species live on the same island, they
compete for food. (survival of the fittest)
6. Continued evolution: As the pattern
repeated itself many times, 13 different
species (or more) have evolved, each
specialized to survive in different
niches.
5.
SECTION 16-3
THE PROCESS OF SPECIATION
 Why
do we care?
 Discussion of antibiotic-resistant
bacterial strains and pesticideresistant pests. (Optional reading:
Issues in Biology, p.403)