• Genetic variation= wide range of phenotypes
• Why is variation in a population beneficial?
– Genetic variation in a population increases the chance that
some individuals will survive
• Gene pool- all genes, including different alleles, that are
present in a population
• Allele frequency- # of times that the allele occurs in a gene
pool, compared to other alleles for same gene
– Expressed as %
– Nothing to do with allele being recessive or dominant
• What are the main sources of genetic variation in a
population?
– Mutations and gene shuffling from sexual reproduction
• Main sources of genetic variation= mutations and
genetic shuffling from sexual reproduction
• Mutations- any change in sequence of DNA
– May affect organism’s fitness or ability to survive and
reproduce
– May have no effect on fitness
• Gene shuffling- occurs during production of
gametes
– Crossing over
– Sexual reproduction- major source of variation within
many populations
• Natural selection acts on
distributions of traits
– Traits not undergoing natural
selection have normal
distribution
• Natural selection can
change the distribution of a
trait in one of three ways
• Microevolution- evolution
within a population
– Observable change in allele
frequencies
– Can result from natural
evolution
• Natural selection can take one of three paths
• Directional selection- individuals at one end of curve have
higher fitness than individuals in the middle or at end
• Stabilizing selection- takes place when individuals near the
center of curve have higher fitness than individuals at either
end
• Disruptive selection- individuals at the upper and lower ends
of the curve have higher fitness than individuals near middle
• Natural selection- not only source of
evolutionary change
• Gene flow- movement of alleles between
populations
– Individuals join new populations and reproduce
– Keeps neighboring populations similar
– Low gene flow increases chance that two
populations evolve into different species
• Genetic drift is a change in allele frequencies due
to chance
– Causes a loss of genetic diversity
– Most common in small populations
• How do populations become small enough for
genetic drift to occur?
– Bottleneck effect- an event greatly reduces the size of
a population- overhunting
– Founder effect- small number of individuals colonize a
new area- Old Order Amish community
• Sexual selection occurs when
certain traits increase mating
success
• Why does it happen?
– Due to higher cost of
reproduction for females
– Females are more limited in
potential offspring each cyclethey want a good return
• Two types of sexual selection
– Intrasexual selection: competition
among males
– Intersexual selection: males
display certain traits to female
• Hardy-Weinberg principle- allele frequencies in a pop
will remain constant unless one or more factors cause
those frequencies to change
• Genetic equilibrium- allele frequencies remain
constant
• Conditions required to maintain genetic equilibrium
–
–
–
–
–
Random mating: no sexual selection
Population must be large: no genetic drift
No movement into or out of population: no gene flow
No mutations: no new allele added to gene pool
No natural selection: all traits aid equally in survival
• Real populations rarely meet all five conditions
• Speciation- formation of new species
• What must happen for a species to evolve into
two new species?
– Gene pools of two populations must become
separated
• Reproductive isolation- members of two pop
cannot interbreed and produce fertile offspring
– Behavioral isolation
– Geographic isolation
– Temporal isolation
• Behavioral isolation- two populations have
differences in courtship rituals or other
reproductive strategies that involve behavior
– Eastern and western meadowlarks
• Geographic isolation- two pops are separated by
geographic barriers, like rivers, mountains, bodies
of water
– Abert and Kaibab squirrel
– Do not guarantee formation of new species
– May separate some organisms but not others
• Temporal isolation- two or more species
reproduce at different times
– Orchids in rain forest
• Evolution through natural selection is not random
– It can have direction, results cumulative
• Different species often must adapt to similar environments
– Convergent evolution- evolution toward similar characteristics in
unrelated species
– Divergent evolution- closely related species evolve in different
directions, become increasingly different
• Species can shape each
other over time
– Coevolution- two or
more species can evolve
together
– Species evolve in
response to changes in
each other
• Species can become
extinct
– Extinction- the elimination
of a species from Earth
– Background Extinction
• Caused by local changes in
environment
– Mass extinction
• More rare but more
intense
• At least five mass
extinctions in last 600
million years
• Speciation often occurs in
patterns
• Punctuated equilibriumepisodes of speciation occur
suddenly in geologic time
– Followed by long periods of
little evolutionary change
• Adaptive radiation- many
species evolve from one
species
– Ancestral species diversify
many descendent species
– Usually adapted to wide
range of environment
• Why is understanding evolution important?
– Drug resistance in bacteria and viruses
– Pesticide resistance in insects
• Evolutionary theory helps us understand and
respond to these changes in ways that
improve human life