Download The evolution of Populations

THE EVOLUTION OF
POPULATIONS
WHAT IS A POPULATION?
• A group of organisms
of the same species
that live in a specific
geographical area and
interbreed
POPULATION SIZE
• The number of individuals in a
population
• Can affect the population’s ability
to survive
VERY SMALL POPULATIONS
• Most likely to become extinct
– Random events or natural disturbances,
such as a fire or flood, endanger small
populations
• Tend to experience interbreeding
– Only relatives may be available as mates
– Reduces the population’s fitness
GENETIC VARIATION
• Gene pool: collection of alleles found in
all of the individuals of a population
• Allele frequency: proportion of one
allele, compared with all the alleles for
that trait, in the gene pool
GENETIC VARIATION COMES
FROM SEVERAL SOURCES
• Mutation: a random change in the DNA of a gene
– Mutation rates in nature are very slow
– Genes mutate only about 1-10 times per 100,000 cell
divisions
– Mutation is the source of variation and makes evolution
possible
• Recombination: new alleles that form in offspring
during meiosis
NATURAL SELECTION ACTS ON
DISTRIBUTIONS OF TRAITS
• Normal distribution:
distribution in a
population in which
allele frequency is
highest near the mean
range value and
decreases
progressively toward
each extreme end
NATURAL SELECTION CAN
CHANGE THE
DISTRIBUTION OF A TRAIT
IN ONE OF THREE WAYS
DIRECTIONAL SELECTION
• Pathway of natural selection in which one
uncommon phenotype is selected over a
more common phenotype
STABILIZING SELECTION
• Pathway of natural selection in which
intermediate phenotypes are selected
over phenotypes at both extremes
DISRUPTIVE SELECTION
• Pathway of natural
selection in which two
opposite, but equally
uncommon,
phenotypes are
selected over the
most common
phenotype
GENE FLOW IS THE MOVEMENT OF
ALLELES BETWEEN POPULATIONS
• Increases the genetic variation
of the receiving population
• Caused by the migration of
individuals to or from a
population
• New individuals (immigrants) add
alleles
• Departing individuals (emigrants)
take away alleles
GENETIC DRIFT IS CHANGE IN ALLELE
FREQUENCIES DUE TO CHANCE
• Small populations, like small sample
sizes, are more likely to be affected be
chance
• Some alleles will likely decrease in
frequency and become eliminated
• Genetic drift causes a loss of genetic
diversity in a population
GENETIC DRIFT
• Example: the cheetah
• Undergone drastic
population declines in the
last 5000 years
• Those alive today are
the descendants of only
a few individuals, and
each cheetah is almost
genetically uniform
BOTTLENECK EFFECT
• Genetic drift that
results from an
event that
drastically
reduces the size
of a population
• Example: northern
elephant seals
FOUNDER EFFECT
• Genetic drift that occurs after a small
number of individuals colonize a new area
• Example: Older Order Amish
EFFECTS OF GENETIC DRIFT
• Can cause several problems for populations
• Population loses genetic variation
• Individuals less likely to be able to adapt
to a changing environment
• Lethal genes could become more common
due to chance
SEXUAL SELECTION OCCURS WHEN
CERTAIN TRAITS INCREASE MATING
SUCCESS
• Males
– Make sperm continuously
– Value of each sperm is relatively small
• Females
– Limited to the number of offspring they can
reproduce in each reproductive cycle
– Each investment is more valuable
– “The choosy sex”
SEXUAL SELECTION
• Selection in which certain traits enhance
mating success
• Traits are passed on to offspring
• Intrasexual selection: competition among
males for the female (i.e. male deer)
• Intersexual selection: when males display
certain traits to attract the female (i.e.
male bird of paradise)
• Some traits that were originally selected
for quality and good health are now
exaggerated through sexual selection
(i.e. the male peacock feathers)
HARDY-WEINBERG EQUILIBRIUM DESCRIBES
POPULATIONS THAT ARE NOT EVOLVING
• Genotype frequencies stay the same in a population if
certain conditions are met:
– Very large population
– No emigration or immigration
– No mutations
– Random mating
– No natural selection
• Real populations rarely meet all 5 conditions
THE HARDY-WEINBERG EQUATION IS
USED TO PREDICT GENOTYPE
FREQUENCIES IN A POPULATION
2
p
+ 2pq +
2
q
•
p2 = Frequency of individuals that are homozygous for allele A
•
2pq = Frequency of heterozygous individuals with alleles A and a
•
q2 = Frequency of individuals that are homozygous for allele a
Know that: p + q = 1
HARDY-WEINBERG EXAMPLE
HARDY-WEINBERG PRINCIPLE
• The principle that states that the
frequency of alleles in a population does
not change unless evolutionary forces
act on the population
• IMPOSSIBLE IN NATURE!
THERE ARE FIVE FACTORS THAT
CAN LEAD TO EVOLUTION
• Genetic drift
• Gene flow
• Mutation
• Sexual selection
• Natural selection
SPECIATION THROUGH
ISOLATION
• The isolation of
populations can lead to
speciation
• Reproductive Isolation:
Final stage in speciation, in
which members of isolated
populations are either no
longer able to mate or no
longer able to produce
viable offspring
POPULATIONS CAN BECOME
ISOLATED IN SEVERAL WAYS
• Behavioral isolation: due to differences in
courtship or mating behavior
• Geographic isolation: due to physical
barriers
• Temporal isolation: due to barriers related
to time, such as differences in mating
periods or differences in the time of day
that individuals are most active
EXAMPLE: REPRODUCTIVE ISOLATION
• Kaibab squirrel
– North Rim of the Grand Canyon, Arizona
– Black belly
• Abert squirrel
– South Rim of the Grand Canyon, Arizona
– White belly
• Have been isolated for about 10,000 years
and cannot interbreed
• Some biologist consider them different
species
A NEW SPECIES
• Divergence: the accumulation
of differences between
groups
• Leads to the formation of
new species
• Speciation: The formation of
new species as a result of
evolution by natural selection
SUBSPECIES
• Populations of the same species
that differ genetically because
of adaptations to different living
conditions
• The members of newly formed
subspecies have take the first
step toward speciation
• Eventually, the subspecies may
become so different that they
can no longer interbreed
successfully…they would then be
separate species
SUBSPECIES
EVOLUTION THROUGH NATURAL
SELECTION IS NOT RANDOM
CONVERGENT EVOLUTION
• Evolution toward similar characteristics
in unrelated species, resulting from
adaptations to similar environmental
conditions
DIVERGENT EVOLUTION
• Evolution of one or more closely related
species into different species; resulting
from adaptations to different
environmental conditions
SPECIES CAN SHAPE
EACH OTHER OVER TIME
• Beneficial relationships through
coevolution
• Evolutionary arms race
SPECIES CAN BECOME EXTINCT
• Background extinctions
• Mass extinctions
SPECIATION OFTEN
OCCURS IN PATTERNS
• Punctuated equilibrium: theory that
states that speciation occurs suddenly
and rapidly followed by long periods of
little evolutionary change
• Adaptive radiation: process by which
one species evolves and gives rise to
many descendent species that occupy
different ecological niches