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Transcript
Genes and Variation
March 9th/10th, 2009
Remember…
• In order for natural selection to occur,
there MUST be variation.
• Variation in the genes then creates
variations in the phenotype.
• Natural selection then “chooses” those
phenotypes that are most favorable.
Genetic variation is studied in
Populations
• A population is a group of individuals of
the same species that interbreed.
• Because members of the population
interbreed, they share a common group of
genes called a gene pool.
• A gene pool consists of all the genes,
including all the different alleles, that are
present in a population.
• The relative
frequency of an
allele is the
number of times
that the allele
occurs in a gene
pool, compared
to the number of
times other
alleles for the
same gene
occur.
What are the main sources of
genetic variation in a population?
•
There are two main sources of genetic
variation in a population;
1. mutations
2. the shuffling of genes that results from
sexual reproduction.
Mutations
• A mutation is any change in a sequence
of DNA (insertion, deletion, point shift, etc.)
– Mutations can be caused by replication errors,
radiation, and/or chemicals in the
environment.
Gene Shuffling
• Gene shuffling is the
random mixing-up of the
genetic information.
– Occurs during gamete
formation (meiosis) when
chromosomes cross over,
as well as when they are
randomly pulled apart
during anaphase I and II.
• Each of these babies has inherited a
collection of traits. Some, such as hair
color, are visible, while others, such as the
ability to resist certain diseases, are not.
Single-Gene Traits and Polygenic
Traits.
• A single-gene trait is a trait that is
controlled by a single gene that has two
alleles.
• For example, widow’s peak in humans.
– The allele for widow’s peak (W) is dominant
over the allele for no widow’s peak (w).
• Many traits in humans are controlled by
two or more genes, and are therefore
called polygenic (many-genes).
• A polygenic trait can
have many possible
genotypes and
phenotypes.
– For example, height.
People are not either
tall or short. They can
be tall, short, and
everything in between.
How Does Natural Selection Work
on Polygenic Traits?
• Class activity: Student Height
1. Directions: Using meter sticks, record the height
of each student at your table to the nearest
inch. Record the measurements in your data
table.
2. When you are done, have a representative from
your group come up to the front and record your
numbers in the data table on the board.
3. Make sure to record each groups’
measurements in your data table, as they are
written on the board.
Types of Selection
• Natural selection can affect the
distributions of phenotypes in any of three
ways: directional selection, stabilizing
selection, or disruptive selection.
Directional Selection
• Directional Selection – when individuals
at one end of the curve have a higher
fitness than individuals in the middle or
other end.
For Example: Really tall
people are more fit than
average or really short
people. Those that are very
tall are then selected for, all
others are selected
against.
Stabilizing Selection
• Stabilizing Selection – when individuals
near the center of the curve have a higher
fitness than at either end of the curve.
For Example: Average
height people are more fit
than really short or really
tall people. Those that are
average are then selected
for, all others are selected
against.
Disruptive Selection
• Disruptive Selection – when individuals
at the upper and lower ends of the curve
have a higher fitness than individuals near
the middle.
For Example: Very tall and
very short people are more
fit than average height
people. Those that are very
tall or very short are then
selected for, all others are
selected against.