Transcript 15.3: Patterns of Evolution

15.3: Patterns of Evolution:
• Microevolution
• Macroevolution
15.3: Microevolution
• When the relative frequencies of alleles
in a population change over a number
of generations, evolution is occurring
on its smallest scale (microevolution)
There are several potential
causes of microevolution
• Genetic drift is a change in a gene pool due to chance
(smaller populations)
– Genetic drift can lead to the founder Effect and cause the
bottleneck effect
• Gene flow can change a gene pool due to the movement
of genes into or out of a population
• Mutation changes alleles
• Nonrandom mating
• Natural selection leads to differential reproductive
success
Genetic Drift
• Natural selection is not the only source of
evolutionary change.
• The smaller a population is, the farther the
results may be from what the laws of
probability predict. This kind of random change
in allele frequency is called genetic drift.
• How does genetic drift take place?
– 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 in a
population.
Genetic Drift
Sample of
Original Population
Descendants
Founding Population A
Founding Population B
Chapter
15
Evolution
15.3 Shaping Evolutionary Theory
Founder Effect
 Occurs when a small sample of a
population settles in a location separated
from the rest of the population
 Alleles that were uncommon in the original
population might be common in the new
population.
Chapter
15
Evolution
15.3 Shaping Evolutionary Theory
Bottleneck
 Occurs when a population declines to a
very low number and then rebounds
Hardy-Weinberg Equilibrium:
• If a population’s gene pool
remains constant, then the
population will not evolve.
(Hardy-Weinberg Equilibrium)
Hardy-Weinberg principle
• The Hardy-Weinberg principle states that
allele frequencies in a population will remain
constant unless one or more factors cause those
frequencies to change.
• The situation in which allele frequencies remain
constant is called genetic equilibrium (juh-netik ee-kwih-lib-ree-um).
• If the allele frequencies do not change, the
population will not evolve.
Hardy-Weinberg Equation:
• Used to calculate the frequency of alleles
p2 + 2pq + q2 = 1
• Frequency of WW + Frequency of Ww + Frequency of ww = 1
• The combined frequencies of all alleles
must be 100%
Five conditions are required for HardyWeinberg equilibrium
Evolution v/s Equilibrium
• Five conditions
are required to
maintain genetic
equilibrium from
generation to
generation
• The population is very
large
• The population is
isolated
• Mutations do not alter
the gene pool
• Mating is random
• All individuals are equal
in reproductive success
The Hardy-Weinberg equation is useful in
public health science
• Public health scientists use the HardyWeinberg equation to estimate frequencies
of disease-causing alleles in the human
population
– Example: phenylketonuria (PKU)
Adaptive change results when natural
selection upsets genetic equilibrium
• Natural selection results in the accumulation
of traits that adapt a population to its
environment
– If the environment should change, natural
selection would favor traits adapted to the new
conditions
VARIATION AND NATURAL SELECTION
Variation is extensive in most populations
• Phenotypic variation may be environmental
or genetic in origin
– But only genetic changes result in evolutionary
adaptation
How natural selection affects
variation
• Natural selection tends to reduce variability
in populations
– The diploid condition preserves variation by
“hiding” recessive alleles
– Balanced polymorphism may result from the
heterozygote advantage
The evolution of antibiotic resistance in
bacteria is a serious public health
concern
• The excessive use of antibiotics is leading to
the evolution of antibiotic-resistant bacteria
– Example:
Mycobacterium
tuberculosis
– MRSA
Figure 13.22
Checkpoint Questions:
1. Describe how natural selection can affect traits
controlled by single genes.
2. Describe three patterns of natural selection on
polygenic traits. Which one leads to two distinct
phenotypes?
3. How does genetic drift lead to a change in a
population’s gene pool?
4. What is the Hardy-Weinberg principle?
5. How are directional selection and disruptive
selection similar? How are they different?
15.3: Patterns of Evolution
• Macroevolution
refers to the
large-scale
evolutionary
changes that take
place over long
periods of time.
• Six important patterns
of macroevolution
–
–
–
–
–
–
mass extinctions
adaptive radiation
convergent evolution
Coevolution
punctuated equilibrium
changes in
developmental genes.
Mass Extinctions:
• New fossil studies show
that those mass
extinctions not only
extinguished species but
also wiped out whole
ecological systems,
disrupting energy flow
throughout the biosphere
and causing food webs to
collapse.
• Many paleontologists
think that most mass
extinctions were
caused by multiple
factors.
• For the survivors,
there was a new
world of ecological
opportunity.
• Often, the result was
a burst of evolution
that produced an
abundance of new
species.
Adaptive Radiation
• Studies of fossils or of living organisms
can show that a single species or a
small group of species has evolved into
several different forms that live in
different ways.
• This process is known as adaptive
radiation.
– Implies common descent
Convergent Evolution
• Unrelated organisms that come to
resemble one another, is called
convergent evolution.
• Natural selection may mold different body
structures, such as arms and legs, into
modified forms, such as wings or flippers.
– EX: Streamlined body of penguin, shark,
dolphin
Coevolution
• The process by which two species evolve in
response to changes in each other over time
is called coevolution.
• An evolutionary change in one organism may
also be followed by a corresponding change in
another organism.
• EX: Many flowering plants, for example, can
reproduce only if the shape, color, and odor of
their flowers attract a specific type of pollinator.
Punctuated Equilibrium
• Evolution has often
proceeded at different
rates for different
organisms at different
times during the long
history of life on
Earth.
(Rate of Evolution)
• Gradualism - slow,
steady change in a
particular line of
descent.
• Punctuated
equilibrium - long,
stable periods
interrupted by brief
periods of more
rapid change
Developmental Genes and Body Plans
• First, molecular studies show that homologous hox
genes establish body plans in animals as different as
insects and humans
• Second, major evolutionary changes—such as the
different numbers of wings, legs, and body segments in
insects—may be based on hox genes.
• Finally, geneticists are learning that even small changes
in the timing of genetic control during embryonic
development can make the difference between long legs
and short ones
Changes in
developmental genes
are one major pattern of
macroevolution.
• Fossil evidence shows
that some ancient insects
(top left) had no wings, but
others (top right) had
winglike structures on
many body segments.
• In modern insects
(bottom), genes may turn
off wing development in all
except one or two body
segments.