Download Biology

Biology
Evolution II Notes
Evolution and Populations
Darwin’s Ideas Revisited
• Developed his theory of natural selection
which explains how species evolve
• Published theory in 1859
• Gregor Mendel’s findings were published in
1860 and the significance of Mendel’s work
was not realized until after 1900
• Without knowledge of genetics Darwin was
unable to explain two factors that were key to
understanding evolution
– What is the source of variation?
– How are inheritable traits passed on from one
generation to the next?
• Scientists were able to combine Darwin’s
theory with basic principles of genetics in
1910.
Gene Pools
• Gene Pool = combined genetic
information of all members of a population
– Typically contains two or more alleles for a
trait
• Relative Frequency = how often an allele
occurs within the gene pool compare to
other alleles
– Usually expressed as a decimal or a
percentage %
– All alleles for a trait must add up to 1 or
100%
Practice Problem: In a population of people
there are a total of 35 blue alleles, 40 brown
alleles, and 25 green alleles. What is the relative
frequency of each allele within the gene pool?
Lethal Recessive Alleles in Gene
Pools
• How are lethal recessive alleles
maintained within a gene pool?
Sources of Genetic Variation
Mutations
•
•
Gene Shuffling
Change made in DNA Sequence
•
– Change does not always affect
organisms phenotype
•
Changes that do affect an
organisms phenotype may also
affect an organisms fitness
– Can make an organism more or
less likely to survive and
reproduce successfully
– Give examples of mutations
affecting an organisms phenotype
that would make them more and
less successful!
Most inheritable differences are
due to gene shuffling
Chromosomes assort randomly
during meiosis resulting in new
combinations of genes
– Sexual Production therefore can
produce many genetic differences
between you, your parents, and
your siblings
Is Genetic Variation In a Species
Good or Bad?
• Good! Why?
• When natural disasters or changes in the
environment occur, it is more likely for
some members of the species to survive
because they may have a variation that
allows them to survive the change
(adaptation).
Single Gene vs Polygenic Traits
The # of phenotypes produced for a given trait depends on the #
of gene sites controlling the trait
Single Gene
Frequency of Phenotype
(%)
•
•
1 gene site controlling the trait
– Gene site has two or more
alleles
Few Possible Phenotypes
Example = Hairline
•
•
•
More than one gene site
controlling the trait
– Each gene site has two or
more alleles
Many possible phenotypes
Example = height
100
50
0
Widow’s
peak
No widow’s
peak
Frequency of Phenotype
•
Polygenic
Phenotype
(height)
Natural Selection On Single-Gene Traits
•
•
•
Leads to changes in allele frequencies and thus evolution (changing) of the
species
Less complex than polygenic traits
Give Example!
Natural Selection On Polygenic Traits
•Wide range of phenotypes for a trait
•Natural selection can act in 3 ways
•Directional Selection
•Stabilizing Selection
•Disruptive Selection
Directional Selection
•
•
•
One extreme for a trait is favorable, over time individuals in the population
will look more like that extreme
Can work in both directions depending on what is being favored by nature!
Example = speed with gazelles
– More fit to be fast than slow
– Population will get faster from generation to generation
Directional Selection
Food becomes scarce.
Key
Low mortality, high
fitness
High mortality, low
fitness
Stabilizing Selection
•
•
•
Average individuals are more fit, over time population will look more like the
Stabilizing Selection
average for a trait
Disadvantage to be at either extreme
Example = human birth size
– to small = health risks
– to large = difficulty in birthing process
Key
Low mortality, high fitness
High mortality, low fitness
Birth Weight
Selection against both
extremes keep curve
narrow and in same place.
Disruptive Selection
Disruptive Selection
Largest and smallest seeds become more common.
Key
Low mortality,
high fitness
High
mortality,
low fitness
Population splits
into two subgroups
specializing in
different seeds.
Beak Size
Number of Birds
in Population
•
Either is extreme is more favorable than being average, over time
population will look like both extremes with less individuals exhibiting the
average trait
Individuals being average for a particular trait are at a disadvantage
Number of Birds
in Population
•
Beak Size
Genetic Drift
• Random change due to chance (not
natural selection)
– Most likely to occur in a small populations
Founder Effect- small group separate from main
group
-gene frequencies differ from main group
Bottleneck Effect- large number of indivuals die
off.
-gene frequencies by chance (not selection)
are different from original group