Transcript Evolution of Populations and Speciation

• Pick up a copy of the notes.
Evolution of Populations and
Speciation
Variation of Traits
• Population genetics
– The study of evolution from a genetic
viewpoint.
– Evolution is a gradual change of genetic
information over time.
• In a population individual may vary in
observable traits.
– The distribution of these traits can be
demonstrated using a bell curve.
Variation of Traits
• What are different causes for genetic
variation?
• Mutation
– Flawed copies of individual genes.
• Recombination
– Independent assortment during crossover.
• Random fusion of gametes
– The random chance of which gamete succeeds.
Sexual vs. Asexual Reproduction
• Sexual Reproduction – Uses the process
of meiosis to create gametes. Fertilization
results when the embryo receives alleles
from both parents.
– Genetic variability is a result of independent
assortment, recombination of chromosomes,
or mutations. Gametes are produced with
alleles arranged in new ways.
• Genetic changes or variability result in the
transcription and translation of new
proteins that may improve an organism’s
opportunity for survival.
• These beneficial proteins (traits) will be
passed on to the next generation.
• Asexual reproduction – Involves only one parent
that produces the offspring that are for the most
part genetically identical to that parent.
– Variation only occurs through mutations passed to the
offspring.
– The asexual reproduction rate is much faster than the
sexual reproduction rate.
– Asexual reproduction may have a disadvantage in
changing conditions (environment) because all
individuals are identical.
Allele Frequencies and the Gene
Pool
• Gene pool
– The total genetic information available in a
population.
• Allele frequency
– The frequency that a certain allele appears in
a population.
– May be a percentage or decimal.
– Example: Two forms of the A allele (A and a)
• If there are 20 gametes and 5 are “a” then the
allele frequency for “a” is .25.
Allele Frequencies and the Gene
Pool
• Phenotype frequency
– The total number of individuals with a
particular phenotype divided by the
number of individuals in the population.
– Example: There are 20 plants in a
population. If four are pink then the
frequency of pink is .20.
Hardy-Weinberg Genetic
Equilibrium
• Wilhelm Weinberg and Godfrey Hardy
independently showed that allele
frequencies tend to remain the same
from generation to generation unless
acted on by outside influences.
Hardy-Weinberg Equilibrium
• Based on a hypothetical population that is
not evolving.
• 5 Assumptions for evolution not to occur.
– No net mutations. Allele frequencies never
change from mutation.
– Individuals never enter nor leave a population.
– The population is large.
– Individuals mate randomly.
– Selection does not occur.
• When the equilibrium is disrupted evolution
occurs.
Disruption of the Genetic
Equilibrium
• Mutation
– Mutations occur spontaneously and
constantly at low rates and under normal
conditions.
– Beneficial mutations are a vital part of
evolution.
Disruption of the Genetic
Equilibrium
• Migration
– Immigration – the movement of individuals
into the population.
– Emigration – the movement of individuals
out of the population.
• Migration results in gene flow, the
process in which genes move from one
population to another.
Genetic Drift
• Genetic Drift
– Allele frequencies in a population change
as a result of random events.
• Small populations are more damaged
by genetic drift than large populations.
Nonrandom Mating
• Many species do not mate randomly.
• Mate selection can be influenced by
geographic proximity.
– This can result in mating between related
individuals.
– Amplifies certain traits.
• Assortative
– The selection of a mate based on the
similarity of characteristics.
Natural Selection
• Stabilizing Selection
– Individuals with the average form of a trait have
the highest fitness.
• Directional selection
– Individuals that display a more extreme form of a
trait have greater fitness than individuals with an
average form.
• Disruptive selection
– Individuals with either extreme variation of a trait
have greater fitness than individuals with the
average.
Natural Selection
• Sexual selection
– Individuals tend to mate with individuals
displaying certain more attractive traits.
– Example: The peacock.
Directional Selection
Microevolution vs Macroevolution
• Microevolution is evolutionary change
within a species.
• Macroevolution is evolutionary change
that transcends species and causes new
species to emerge.
Formation of Species
• Speciation
– The process of species formation.
• Morphological concept of species
– Species classification was a result of the
appearance and structure of the internal and
external of the organism (or morphology).
– Convenient b/c morphological characteristics are
easy to observe.
– Limitations
– Species may display different phenotypes within a
population.
Formation of Species
• The Biological Species Concept
– A species that can successfully interbreed
but cannot breed with another group is a
species.
– Problems
• Doesn’t help to determine extinct species.
• Doesn’t help to determine species that do not
reproduce sexually.
Isolating Mechanisms
• Speciation begins with isolation.
– Two formerly interbreeding populations
stop interbreeding.
• Geographic isolation
– Populations may be physically separated
when their original habitat divides.
• Example: River changes course.
Formation of Species
• Reproductive Isolation
– Prezygotic isolation
• Happens before fertilization.
• Incompatible behavior reduces the chance for a hybrid.
• Example: Different mating calls.
– Postzygotic isolation
• Happens after fertilization.
• Offspring of the interbreeding are either underdeveloped
or sterile.
• Example: Mules
Rates of Speciation/Evolution
• Speciation can require millions of years or it
can happen rapidly (several thousand years).
Gradual verses punctuated rates.
• Gradualism
– Gradual changes over long periods of time.
• Punctuated equilibrium
– When long periods of stability are followed by
instant changes in the species (several thousand
years)
Patterns of Evolution
Coevolution – The change of 2 or more
species in close association with
eachother.
Examples: Parasites/hosts,
predator/prey, plants/pollinators
• Cuckoo Bird
Yucca Moth and the Yucca Plant
Patterns of Evolution
• Convergent Evolution – Organisms that
appear to be very similar, such as a shark
and a porpoise but are in fact very
different developmentally.
Patterns of Evolution
• Divergent Evolution – Two or more
related populations or species become
more and more dissimilar.
Divergent
Evolution
Patterns of Evolution
• Adaptive Radiation- Many related
species evolve from a single ancestral
species.
Patterns of Extinction
• Gradual Extinction – occurs at a slow
rate and may be a result of other
organisms, climate change, or natural
disasters.
• Mass Extinction – Usually occurs when a
catastrophic event suddenly changes the
environment.
Selection Scenarios
• In a population of lizards, the larger than
average individuals may be more easily
spotted, captured, and eaten by a
predator. On the other hand, lizards that
are smaller than average might not be
able to run fast enough to escape. What
might a population curve look like for a
population of lizards? What type of
selection is occuring?
• Anteaters feed by breaking open termite nests,
extending their sticky tongue into the nest and
lapping up termites. Suppose that an area was
invaded by a new specise of termite that built
very deep nests. Anteaters with long tongues
could more effectively prey on the termites.
What would the population curve look like for the
distribution of tongue length in anteaters? What
type of selection is occuring?
• The shell color of limpets (marine animals)
varies from pure white to dark tan. White
shelled limpets that are on rocks covered with
barnacles that are also white are at an
advantage because birds (predators) have
difficulty distinguishing lampets from barnacles.
On bare, dark colored rocks, dark shelled
limpets are at an advantage because birds
cannot distinguish them from the rocks either.
Those limpets in between in color are at a
disadvantage because they don’t blend in with
the rocks or the barnacles. What type of
selection is occuring? What would the
population curve look like?