Transcript Evolution of Populations

Evolution of Populations
CHAPTER 16
Chapter 16
Intro
•
Darwin didn't know how
heredity worked!
How do you think heredity
relates to evolution?
Gene Pools
• Gene pool-Combined
genetic information of all
member s of a particular
population
• Relative frequency- number
of times that allele occur in a
gene pool
Gene Pool
Relative frequencies of alleles Number of
times alleles show up in gene pool
Sources of Genetic Variation
• Mutations-any change in DNA sequence
• Gene shuffling- Occurs during
production of Gametes
• Gene pools are important to evolutionary
theory, because evolution involves changes in
populations over time.
In genetic terms,
evolution is any
change in the
relative
frequency of
alleles in a
population
Single gene trait
• Single gene trait- controlled
by a single gene that has two
alleles
• widow's peak—is a single-gene
trait.
Single gene trait
• The allele for a widow's peak is
dominant over the allele for a hairline
with no peak.
• As a result, variation in this gene
leads to only two distinct phenotypes,
Polygenic trait
• Polygenic trait has two or more genes
– Each gene of a polygenic trait often has two or
more alleles.
• One polygenic trait can have many possible
genotypes and phenotypes.
• Ex.) Human Height
– The symmetrical bell shape of this curve is
typical of polygenic traits.
• A bell-shaped curve is also called a normal
distribution.
Natural Selection on
Single Gene Traits
• Natural selection on single gene
traits can lead to a changes in allele
frequencies and thus to evolution
Natural Selection on Polygenic
Traits
• Directional
Selection
• Stabilizing
selection
• Disruptive
Selection
Natural Selection on Polygenic
Traits
• Directional
Selection-one
end has
higher fitness
Natural Selection on Polygenic
Traits
Stabilizing
selectionmiddle has
higher fitness
Natural Selection on Polygenic
Traits
• Disruptive
Selection-both
ends have
higher fitness
Natural Selection on Polygenic
traits
• Directional Selection- Individuals at one end
of the graph have higher fitness than
individuals in the middle or at other end
• Stabilizing Selection-Individuals near the
center of the curve have higher fitness than
individuals at either end of the curves
• Disruptive Selection-Individuals at upper
and lower ends have higher fitness than
individuals near the middle
Which type of selection?
Which Type of Selection?
Which Type of Selection?
Genetic Drift
• Genetic Drift- random change in allele
frequency
• In small populations, individuals that
carry a particular allele may leave more
descendants than other individuals do,
just by chance. Over time, a series of
chance occurrences of this type can
cause an allele to become common
Genetic Drift
• Founder effectallele frequencies
change as a
result of migration
of small
subgroups of
population
Evolution vs. Genetic
Equilibrium
• To clarify how evolutionary change operates,
scientists often find it helpful to determine
what happens when no change takes place.
• Biologists ask:
– Are there any conditions under which evolution will
not occur?
– Is there any way to recognize when that is the
case?
Evolution vs. Genetic
Equilibrium
• The answers to those questions are
provided by the Hardy-Weinberg
principle, named after two researchers
who independently proposed it in 1908.
Hardy-Weinberg Equilibrium
• Random Mating
• Large Population
• No Migration in or out of
Population
• No Mutations
• No Natural Selection
THIS NEVER HAPPENS!!!
The End!
Speciation
• Formation of new species is
called speciation
Isolating Mechanisms
• As new species evolve, populations
become reproductively isolated from
each other.
• When the members of two populations
cannot interbreed and produce fertile
offspring, reproductive isolation has
occurred.
Reproduction Isolation
• Reproductive isolation can develop in a
variety of ways, including
– behavioral isolation
– geographic isolation
– temporal isolation.
Behavioral Isolation
– Behavorial isolation-two populations are
capable of interbreeding but have differences in
courtship rituals or other reproductive strategies
that involve behavior.
– For example, the eastern and western
meadowlarks are very similar birds whose
habitats overlap in the center of the United States.
Behavioral Isolation
– Members of the two species will not mate with
each other, however, partly because they use
different songs to attract mates.
– Eastern meadowlarks will not respond to
western meadowlark songs, and vice versa.
Behavioral Isolation
Geographic Isolation
• Geographic isolation - two populations
are separated by geographic barriers
such as rivers, mountains, or bodies of
water.
• The Abert squirrel lives in the
Southwest. About 10,000 years ago, the
Colorado River split the species into two
separate populations.
Geographic Isolation
• Two separate gene pools formed. Genetic
changes that appeared in one group were not
passed to the other.
• Led to the formation of a distinct subspecies,
the Kaibab squirrel.
Albert Squirrel
Kaibab Squirrel
Geographic Isolation
Temporal isolation
• A third isolating mechanism is temporal
isolation, in which two or more species
reproduce at different times.
• For example, three similar species of orchid
all live in the same rain forest. Each species
releases pollen only on a single day. Because
the three species release pollen on different
days, they cannot pollinate one another.
Temporal isolation