Transcript Mechanisms of Population Evolution

Mechanisms of Population
Evolution
The History of Evolutionary Biology
• When Darwin developed his theory of
evolution, he did not understand how
heredity worked!
– He didn’t know how traits were passed onto the next generation.
– He didn’t know how variation in populations appeared.
• Since the 1930s, we know now that traits
are passed via DNA from parents to
offspring and these traits that are passed
are dependent on natural selection.
Populations Evolve, not Individuals
• An individual organism
cannot evolve its
phenotype in response
to its environment.
• Each individual has
genes that characterize
the traits of their species,
and they exist as pairs of
alleles on a
chromosome.
• Evolution occurs as a
population’s genes and
their frequencies
change over time.
How do a population’s genes change?
• Picture all the alleles of a population
together in a pool, called a gene pool.
• The percentage of any specific alleles
in the gene pool is the allelic
frequency.
– Relative frequency is the number of times
that an allele occurs in a gene pool.
• A population in which the frequency of
alleles remains the same over
generations is in genetic equilibrium.
• Therefore, Evolution is defined as the
change in the relative frequency of
alleles in a population over time.
– Think of a change in the environment in
which the recessive allele is favored, what
would eventually happen to the entire
population?
2 Main Sources of Genetic Variation
• Mutations. There is any change in the
sequence of DNA.
– This can occur naturally though mistakes in DNA
replication or as a result of unnatural radiation
or chemicals in the environment.
• Can be positive – Increase fitness and propagate into
the population.
• Can be negative - Those individuals cannot survive or
reproduce.
• Can be neutral - Sometimes mutations have no affect
on the phenotype.
Changes in Genetic Equilibrium:
A.k.a. Evolution
• A population in genetic
equilibrium is NOT evolving.
• One mechanism for evolution
is mutation.
• Genetic Drift can occur
where rare events can alter
allelic frequencies.
• Can be prominent in very
small populations.
• Gene Flow is the input or
output of genes due to
individuals moving around.
Genetic Drift in Action
2 Main Sources of Genetic Variation
• Gene Shuffling – variation in offspring due to
gene shuffling in the production of gametes.
– Remember during Meiosis a process called crossing over
(Chiasmata) occurs during prophase I causing genes to get
mixed up.
– As a result, the 23 pairs of chromosomes found in humans
can produce 8.4 million different combinations of genes!
– Sexual reproduction can produce many different
phenotypes but does not change the relative frequency of
alleles (or genotypes) in a population.
• Think a card game, you can get different hands, but it’s always the
same deck of cards. The likelihood of pulling an ace is still 4/52 or
1/13 no many times you shuffle the deck.
Calculating Phenotypic/Allelic
Frequencies
• w3.dwm.ks.edu.tw/bio/
activelearner/18/ch18c5.html
5 Conditions to Maintain
Genetic Equilibrium
• In order for a population to not evolve, the
following must happen.
1. Mating must be random. (No sexual selection.)
2. There is a very large population size. (No bottlenecks.)
3. There is no movement into or out of the population. (No
immigration or emigration.)
4. No mutations. (No changes in the DNA.)
5. No natural selection. (No changes in the environment or
presence of predators.)
• But as you know of geologic time and the reality
of the world these do not always occur.
Natural Selection, the major
driving force of Evolution!
• Recall variations can
increase OR decrease
an organism’s chance of
survival in its
environment. (Fitness)
• The factor that lends to
particular success or
death is called the
selection factor.
– What was the selecting factor
for Darwin’s Finches?
• There are 3 outcomes to
these selection factors.
Stabilizing Selection
• Favors average
individuals in the
population.
• Those average in the
population survive and
reproduce, flooding the
gene pool.
• Reduces variation of
individuals in a
population.
• Keeps all individuals
towards the middle and
stabilizes the species.
Directional Selection
• Favors 1 extreme of
individuals in the
population.
• Those extreme in the
population survive and
reproduce, flooding the
gene pool.
• Can lead to rapid
evolution of a population.
• What can you think of
that would cause this?
Disruptive Selection
• Favors either extreme of
individuals in the
population.
• Those 2 extremes of the
population survive and
reproduce.
• Leads to no intermediate
forms of the trait and the
evolution of 2 new species.
• How could the breakup of
Pangaea cause disruptive
selection?
Outcomes to Selection
• Speciation occurs when
members of similar
populations no longer
interbreed and produce
fertile offspring within
their natural environment.
– Are mules a species?
• Three ways to cause
speciation:
– Geographic Isolation
– Reproductive Isolation
– Temporal Isolation
Models of Evolution
• Gradualism – species originate
through a gradual change of
adaptations.
– Fossil record
– Slowly and steadily
throughout time
• Punctuated Equilibrium –
speciation occurs quickly, in
rapid bursts, with long periods of
genetic equilibrium in between
– About 10,000 years or less
– Fossil Record
• Both end up with Speciation
depending on the circumstances
Patterns of Evolution
• Adaptive Radiation – an
ancient species radiated out
into many species adapted to
their environment.
• Divergent Evolution – One
species diverged to become
distinct species.
• Convergent Evolution –
unrelated species occupy the
similar environments in a
different part of the world.
– Because of similar environmental
pressures, they hare similar
pressures of natural selection
despite their common ancestry.