Download CHAPTER 16 EVOLUTION OF POPULATIONS

A. Darwin’s Ideas revisited
- it was more than 50 years after Darwin started to develop
his theory of evolution before biologists could determine how
evolution takes place
- about 1910, biologists realized that genes carry the
information that determine traits
- with this knowledge, they combined Mendel’s work on
inheritance with Darwin’s work on evolutionary theory
- today, we combine genetics, molecular biology, and
evolutionary theory to explain how inheritable variation occurs
and how natural selection operated on that variation
B. Gene Pools
- to study evolution, biologists focus on one particular
population
- populations share a common group of genes called a
gene pool
- gene pools contain the combined genetic information
of all the members of a particular population
- they contain two or more alleles for each inheritable
trait
- the relative frequency of an allele is the number of
times that allele occurs in a gene pool compared with the
number of times other alleles occur
C. Sources of Genetic Variation
Two main sources include:
1. mutations
- any change in DNA sequence
- occur due to mistakes in replication of DNA or from
radiation or chemicals in the environment
- may be limited to one or a few DNA bases or may
affect long segments of a chromosome
- do not always affect an organism’s phenotype, but
many do
- those that do may or may not affect an organism’s
fitness, or its ability to survive and reproduce
2. gene shuffling
- most inheritable differences result from gene
shuffling that occurs during the production of gametes
- because each chromosome (all 23 pairs) move
independently during meiosis, 8.4 million different
combinations of genes can occur
- crossing over further increases the number of
different genotypes that can occur in offspring, making
sexual reproduction a major source of variation within
populations
- sexual reproduction produces different phenotypes,
but does not change the relative frequency of alleles in a
population
Gene Shuffling
D. Single-gene and polygenic traits
- the number of phenotypes produced for a given trait
depends on how many genes control the trait
Single-gene trait (Ex. widow’s peak)
- controlled by a single gene with two alleles
and only two outcomes; the widow’s peak or no widow’s
peak
a.
b. polygenic traits
- most traits are controlled by two or more genes
- each gene has two or more alleles
- therefore, one polygenic trait can have many possible
genotypes and thus, even more phenotypes
Ex. height
- a bell curve is typical of polygenic traits (shows
normal distribution)
16-2
Evolution as Genetic
Change
A. Natural Selection on Single-gene traits
- leads to changes in allele frequencies and thus, to
evolution
See Fig. 16-5 pg. 397
Ex. organisms of one color may produce fewer
offspring of other colors
B. Natural selection on polygenic traits
- more complex
- fitness varies from one end of the bell curve to the
other; where fitness varies, natural selection can act
- distribution of phenotypes are affected in 3 ways:
1. directional selection
- occurs when individuals at one end of the curve have
higher fitness than individuals in the middle or at the other end
- causes the curve to move as the character trait changes
- evolution causes an increase in the number of organisms
with the trait at one end of the curve (See Fig 16-6)
2. stabilizing selection
occurs when organisms at
the center of the curve have higher
fitness than those at either end
keeps the curve at its
current position, but narrows the
overall graph (See Fig 16-7)
3. disruptive selection
occurs when organisms at the upper and lower ends of the
curve have higher fitness than those in the middle
selection acts more strongly against organisms of an
intermediate type
if natural selection pressure is strong enough and lasts long
enough, it can split the graph in two creating two distinct
phenotypes (See Fig 16-8)
C. Genetic Drift
- a random change in allele frequency that occurs in
small populations
- it occurs when:
individuals that carry a particular allele may have
more
descendants than other individuals; may cause an
allele to become more common in a population
- may occur when groups of individuals colonize a new
habitat; they may carry alleles in different relative
frequencies than the larger populations they came from
- the new population may become genetically different
from the parent population (See Fig 16-9)
the founder effect is a change in allele frequency that
results from the migration of a small subgroup of a population
Ex. fruit flies on the Hawaiian Islands
D. Evolution versus Genetic Frequency
Are there any conditions under which evolution will not
occur?
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 the
same is called genetic equilibrium
Five conditions are necessary to maintain genetic
equilibrium from generation to generation:
1. random mating
- all members of a population have an equal
opportunity to produce offspring and an equal chance of
passing on its alleles
- in natural populations, it is rarely random
Ex. lions and wolves select mates on certain traits
such as size or strength making genes for these traits not in
equilibrium and under strong selection pressure
2. large populations
- important in maintaining genetic equilibrium because
genetic drift has less effect on large populations
3. No movement into or out of the population
- movement would bring new alleles into a population
- the population gene pool must be kept together and
kept separate from that of other populations
4. No mutations
- mutations can result in new alleles being introduced
into the population and allele frequencies will change
5. No natural selection
- all genotypes must have equal probabilities of
survival and reproduction with no phenotype having a
selective advantage over another
16-3
The Process of
Speciation
- the formation of a new species
A. Isolating Mechanisms
- as new species evolve, populations become
reproductively isolated from one another
- reproductive isolation occurs when two populations
cannot interbreed and produce fertile offspring
- the populations develop separate gene pools and
respond to natural selection or genetic drift as separate units
Ways in which reproductive isolation occurs:
1. behavioral isolation
- occurs when two populations are capable of
interbreeding but have differences in courtship rituals or
other types of behavior
Ex. Eastern and Western meadowlarks Fig 16-11
Western
eastern
- they have different mating songs even though their ranges overlap
2. geographic isolation
- two populations are separated by geographic barriers
such as rivers, mountains, or bodies of water (See Fig 16-12
the Abert squirrel)
- does not guarantee the formation of a
new species
- separated populations may mix due to
lakes that become linked or land bridges that
form temporarily between islands
3. temporal isolation
- two or more species reproduce at different times
Ex. Closely related species of fireflies mate at
different times of night or different species of plants have
different flowering seasons
B. Testing Natural Selection in Nature
- these processes can be observed in nature as was
evidenced by Darwin and his observation of finches in the
Galapogos Islands (See Fig 16-13)
- tested finally by Peter and Rosemary Grant
studying non hibernating finches
1. Variation
- the Grants identified and measured as many birds as
possible on a single island and found great variation among
inheritable traits
2. Natural Selection
- individual birds with different beak sizes had different
chances of survival during a drought
- when food was scarce, finches with the largest beaks
survived
- beak size also played a role in mating behavior with bigbeaked birds mating with other big-beaked birds
- this resulted in an increase in beak size in that population
(directional selection)
3. Rapid Evolution
- by documenting natural selection in the wild, the
Grants provided evidence of the process of evolution; the
beak size got bigger in the next generation of finches
- they found natural selection takes place frequently
and sometimes rapidly
C. Speciation in Darwin’s Finches
- speciation occurred in this way:
1. Founders arrived
- a few finches traveled from S. America to one of the
islands, survived, and reproduced
2. Separation of populations
- some birds crossed to a second island and the two
populations no longer shared a gene pool
3. Changes in the gene pool
- seed sizes on the second island favored birds with
bigger beaks, therefore, the population on the second island
evolved into one with bigger beaks
4. Reproductive isolation
- a few big-beaked birds crossed back to the first island and
would not mate with the birds there; now there were separate
gene pools and two separate species
5. Ecological Competition
- as the two species on the first island complete for seeds,
they continue to evolve. Some of the original birds from the
second island may migrate to a third island
6. Continued Evolution
- the process continued leading to the formation of 13
species of finches on the Galapogos Islands (See Fig 16-17)