Download Chapter 16 - variation Notes

Chapter 16
Evolution of Populations
16.1 – Genes and Variation
• Biologists studying evolution often focus on a
particular population.
Population - a group of individuals of the
same species in a given area.
• Because all members of a population can interbreed,
they share a common group of genes, called a gene
pool.
Gene Pool – the combined genetic information of
all the members of a particular
population.
Sources of Genetic Variation
There are two main sources of genetic variation:
• Mutations – any change in the sequence of DNA.
• Gene Shuffling – results from sexual reproduction.
Examples: production of gametes
crossing over
segregation
fertilization
O. I will understand the steps of speciation.
J. In a population the dominant
phenotype of a certain trait occurs 91%
of the time. What is the frequency of
the dominant allele? The recessive
allele?
Single-Gene and Polygenic Traits
• The number of phenotypes produced for a given trait
depends on how many genes control the trait.
Single-Gene Trait – a trait that is controlled by a gene
that has two alleles. Results in
discreet variation ;only two phenotypes
Ex. – widows peak
Fig. 16-3
Pg. 395
Polygenic Trait – traits that are controlled by two
or more genes. Results in
quantitative variation; characteristics vary along a
continuum
Ex. – Height in humans
Fig. 16-4
Pg. 396
How variation is preserved:
1. Diploidy – 2 copies of a gene
2. Balanced polymorphism – the
ability of natural selection to
maintain diversity in a population,
heterozygous protection or advantage
Ex. Sickle cell anemia –
ss – suffers from sickle cell
NS – immune to malaria
NN – normal blood, susceptible to malaria
Hybrid vigor – hybrid plants much hardier
16.2 – Evolution as Genetic Change
• Natural Selection on Single Gene Traits can lead to
changes in the frequencies of alleles which then lead to evolution.
Ex. – lizard survival
•
Natural Selection on Polygenic Traits
- When traits are controlled by more than one gene,
the effects of natural selection are more complex.
- The effect of selection on varying characteristics
in any of three ways:
• Directional Selection
• Stabilizing Selection
• Disruptive Selection
Directional Selection
- occurs when individuals at one end of the curve
have a higher fitness than individuals in the middle or
at the other end.
Fig. 16-6
Pg. 398
Stabilizing Selection
- occurs when individuals near the center of the
curve have higher fitness than individuals at either end
of the curve.
Fig. 16-4
Pg. 396
Disruptive Selection
- occurs when individuals at the upper and lower end of
the curve have higher fitness than individuals near the
middle.
Fig. 16-8
Pg. 399
16.3 – Speciation
• Isolating Mechanisms
As new species evolve, populations become
reproductively isolated from each other.
There are two modes of speciation base on how gene flow
among populations is interrupted.
They are:
• Allopatric Speciation
• Sympatric Speciation
Allopatric Isolation
- occurs when a geographical barrier that
physically
isolates populations initially blocks gene flow.
- Also called: Geographic Isolation
Sympatric Isolation - occurs when chromosomal changes (in
plants) and nonrandom mating (in animals) alter gene flow.
Includes:
Behavioral Isolation – occurs when two populations are
capable of interbreeding but have different courtship rituals or other
types of behavior.
Reproductive Isolation The eastern meadowlark (left)
and western meadowlark (right) have overlapping
ranges. They do not interbreed, however, because
they have different mating songs.
Temporal Isolation – when two or more species
reproduce at different times.
• Sympatric populations become genetically
isolated even though their ranges overlap.
Steps of Speciation
• Itext Activity (Page 409)
O. Understanding evolution of singlegene traits and polygenic traits.
J. What are the steps of speciation?
1. Population becomes isolated from
original population
2. Population changes
3. Population becomes reproductively
isolated – the 2 populations no
longer can mate together
• Question:
Are Darwin’s finches an example of
Allopatric or Sympatric Isolation?
Speed of Speciation
• Gradualism
- Species descended from a common ancestor
gradually diverge more and more through physical
changes as they acquire unique adaptations
• Punctuated Equilibrium
- a new species changes most as it buds from a
parent species, and then changes very little for the
rest of its existence.