Transcript Evolution How Natural Selection Shapes Populations

Evolution
How Natural Selection
Shapes Populations
Chapter 17 Miller Levine
Honors Biology NNHS 2015
Evolution & Natural Selection Work
via Genetics & Heredity
 Genotype
 Phenotype
 Alleles
 Describe how natural selection affects genotypes
by acting on phenotypes. (Hint- THINK BUNNIES!)
Definitions of Evolution
 Gradual process
(over millions of
years!) by which
modern organisms
have descended
from ancient
ancestors
 Evolution, in genetic
terms, involves a
change in the
frequency of alleles
in a population over
time.
Sources of genetic variation
 Mutation – random changes in the DNA sequence
 Gen Shuffling- during sexual reproduction
 Creating haploid cells
 Crossing over
Natural selection acting on a
single gene trait
 Can lead to changes in allele frequencies and, thus, to
changes in phenotype frequencies
 Think of those Naked Bunnies!
Polygenic traits
 If a trait is influenced by
more than one gene, it
is referred to as a
polygenic trait.
 Polygenic traits often
produce a normal
distribution curve of
variation.
 In these cases, we refer
to the phenotype as a
.
Polygenetic Trait- distribution of
phenotypes
Natural selection acting on
a polygenic trait
Natural selection can act on traits
produced by multiple genes in
 Directional selection
 Stabilizing selection
 Disruptive selection
Directional Selection
 Changes the trait in
one direction or the
other – bigger or
smaller, darker
colored or lighter
colored, faster or
smaller, etc.
3 Ways Natural Selection
Affects Genotypes
1. Directional Selection favors individuals at one end of the phenotypic range.
 Most common during times of environmental change or
when moving to new habitats.
 Example: Greyhounds
3 Ways Natural Selection
Affects Genotypes
Disruptive Selection
 Favors extreme over
intermediate phenotypes.


Occurs when environmental
change favors an extreme
phenotype.
If pressures are too strong and
last long enough they can cause
the curve to split.
3 Ways Natural Selection
Affects Genotypes
3. Stabilizing Selection

favors intermediate over extreme phenotypes.


Reduces variation and maintains the current average.
Example : Siberian Husky
Genetic Drift
 Natural selection is NOT the only source of
evolutionary change.
 Smaller populations do not always follow
laws of probability.
 Individuals may leave more descendants than
other individuals by chance
 A number of chance occurrences can cause an
allele to become common in a population.
Genetic drift
 Bottlenecks
 Founder effect
The Hardy-Weinberg Principle
 Genetic equilibrium – if a population is NOT experiencing
natural selection then the allele frequencies in a population
will remain the same
 IF nothing is disturbing equilibrium then: p2 + 2pq +q2 = 1.0
 5 conditions that can disturb genetic equilibrium:
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
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Nonrandom mating (sexual selection)
Small population size (genetic drift)
Immigration or emigration
Mutations
Natural selection
Applying Hardy-Weinberg
to Alleles
Example:
In a certain breed of fish, blue is dominant to yellow
Blue fish can have the following genotype(s):
BB
Bb
Yellow fish can have the following genotype(s):
bb
Genetic Variables:
p= the frequency of dominant allele B (blue)
(frequency = total # of B alleles/total number of alleles)
q= the frequency of recessive allele b (yellow)
(frequency = total # of b alleles/total number of alleles)
Since there are only two alleles in the population and everyone in
the population has one of the two alleles, the frequencies of the
two must add up to one.
p+ q = 1 (remember 1 --> 100%)
Hardy-Weinberg
Equation
Heterozygous fish Ex:
The equation:
1.0 = p2 + 2pq + q2
Where:
p2 = frequency of BB genotype
2pq = frequency of Bb genotype
q2 = frequency of bb genotype
B
b
BB
Bb
b Bb
bb
B
p
q
p2
pq
q pq
2
q
p
Let’s Watch a Video: http://ed.ted.com/lessons/five-fingers-ofevolution
 The Five Causes of Evolution (take notes!)
1. Natural Selection
1. Small Populations or GENETIC DRIFT
1. Non-Random Mating
2. Mutation
3. Gene Flow (Immigration and Emmigration)
Break save for next time…
What is a species?
 A species is a population
or group of populations
that can interbreed and
produce fertile offspring
 Speciation is the
formation of a new
species.
 Occurs thanks to
 Natural Selection
 Chance Events
 How does this happen?
Isolating Mechanisms
 The gene pool of two populations become
separated.
 Reproductive Isolation
 When members of two species cannot
 Interbreed
 Produce fertile offspring
 These two populations will respond to natural selection
and genetic drift as separate units
Types of Reproductive Isolation
 Behavioral isolation
 Geographic isolation
 Temporal isolation