The Evolution of Populations

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Transcript The Evolution of Populations

The Evolution of Populations
Natural selection acts to
promote traits and
behaviors that increase
an organism’s chances of
survival and
reproduction, while
eliminating those traits
and behaviors that are to
the organism’s detriment.
Modern Synthesis
- Relationship between
natural selection and
- How evolutionary
processes can affect a
population’s genetic
- Useful in many branches
of biology and medicine
Allele (Gene) Frequency
Quick Review of Mendelian Genetics
BB = Brown mouse
Bb = Brown mouse (heterozygous)
bb = Beige mouse (recessive)
What happens when a
BB mouse is crossed
with a bb mouse?
What happens when a
Bb mouse is crossed
with a bb mouse?
What happens when a
Bb mouse is crossed
with a Bb mouse?
Gene Pool = the sum of all the alleles in a population
Genetic Diversity is good for populations
Genetic Diversity
Serves as a way for populations to adapt to changing environments.
Plant Disease Resistance is a genetic trait that allows plants to survive against
infections. High genetic diversity allows for plant population to respond to
environment stimuli, unlike low diversity in which the few organisms of the species
may perish if they can’t adapt to new environments.
Changes to allele frequency within a gene pool (evolution) can result
from five key processes we call “Evolution Mechanisms”:
1. Mutation: A random change in the genetic composition of an
organism due to changes in the DNA base sequence
2. Gene flow: The movement of alleles into, or out of, a population
3. Sexual reproduction: New gene combinations and alter allele
frequencies if mating is assortative
4. Genetic drift: The change in the composition of a gene pool as a
result of a chance or random event
5. Natural selection: The change in the composition of a gene pool as
a result of differentially selective environmental pressures
Gene Flow
- movement of
alleles, migration
- increases variation
- can prevent
Sexual Reproduction - increases variation
Nonrandom mating (individuals choose) decreases diversity → speciation?
Refers to changes in allele frequencies, usually in small populations
Individuals die due to random chance.
Bottleneck Effect
A form of genetic drift
that reduces the
The population that
regrows has less
Founder Effect a few individuals
start a new
The founder effect is an
example of genetic drift
where rare alleles or
combinations occur in higher
frequency in a population
isolated from the general
Dwarfism in Amish
Due to few German founders
Types of Selection
Natural selection only acts
on the population’s
heritable traits: selecting for
beneficial alleles and thus
increasing their frequency
in the population, while
selecting against
deleterious alleles and
thereby decreasing their
frequency—a process
known as adaptive
1. Directional Selection
One phenotype is favored
over another
Causes a shift in the
overall appearance of the
Ex: horses get larger
occurs when extreme
phenotypes are
eliminated and the
phenotype is favored.
Human babies have an average size
Too big and they can't get through birth
Too small and they have low survivability
DISRUPTIVE SELECTION -- occurs when extreme phenotypes are
favored and can lead to more than one distinct form.
En parejas responder lo siguiente:
• ¿Cómo se relaciona el pool genético con la cantidad de
organismos de una especie?
• ¿Qué sucede cuando una especie llega a un punto donde la
diversidad genética es mínima?
• Según el siguiente pasaje, ¿cuál de los dos felinos presenta
mayor diversidad genética?
“When the captive [cheetah] felines at an Oregon breeding colony for
large cats were exposed to a potentially deadly virus, it swept through the
cheetah population, killing about 50% as a direct or indirect result of the
virus — but none of the lions even developed symptoms.”
“Low Genetic Variation." Understanding Evolution. University of
California Museum of Paleontology. 22 February 2017 < >.
• En la historia de “El Arca de Noé”.
¿Qué mecanismo evolutivo se llevó a cabo?
• ¿Por qué no es buena idea salvar solamente dos individuos de
cada especie?
• Si una especie evoluciona para reproducirse asexualmente ¿qué
consecuencias podría traer?