Chapter 16
What is evolution?
•Evolution is a process that results in heritable changes in
a population spread over many generations.
•“Evolution can be precisely defined as any change in the
frequency of alleles within a gene pool from one
generation to the next."
- Helena Curtis and N. Sue Barnes, Biology, 5th ed. 1989 Worth Publishers, p.974
How does Evolution Occur?
• Jean Baptiste Lamarck was one of the
first to propose that organisms change
through time
– He proposed that a change in the
environment produced a need to
change in the animals
– Thus if an animal uses one part of
its body frequently, that part will
become stronger
– Conversely, if they don’t use a part,
it will become smaller and
disappear.
– He assumed these acquired traits
could be passed on to offspring.
animation
Charles Darwin
• Influences:
– geologist Charles Lyell
• Wrote Principles of Geology and
claimed that geologic forces produced
changes on Earth in the past and
those same forces will continue to
produce changes in the future.
– Economist Thomas Malthus
• Wrote in Essay on the Principle of
Population that “population growth
would always overpower food supply
growth, creating perpetual states of
hunger, disease, and struggle”
If evolution was a car, the theory of natural
selection would be the engine
• The basic ideas of evolution were
discussed long before there was any
scientific research done to support them.
• The evolutionary concept was never able
to gain any real steam because it lacked a
mechanism.
– Scientists wanted to believe that
species evolved from one form to
another, but had no plausible process
to make it happen.
– The theory of natural selection
provides that reasonable method of
evolution.
animation
Darwin’s Theory of Natural Selection
1. Overpopulation-
All species have the potential to
overpopulate the earth, but
usually remain stable over time.
2. Competitiona large # of the population must die at an
early age.
3. VariationDue to variation, different individuals
among populations have different traits
Darwin’s Idea Cont.
4. Survival of the Fittestmembers of a population whose trait makes
them better adapted to their environment
are more likely to survive to reproductive age
and produce more offspring.
5. Reproductionmembers of a population whose trait makes
them better adapted are more likely to
reproduce.
6. Speciationgiven time, the process of evolution by natural selection can account
for the formation of new species and thus for the diversity of life on
earth.
Vocabulary to understand:
• Population
– a group of individuals living in the same geographical area
and sharing a common gene pool
• Gene Pool
– the sum of all genetic information carried by the members of
a population
• Mutation
– any heritable change in DNA (gene-level and the
chromosome-level)
• Gene Flow
– The transfer of alleles between populations through
interbreeding
• Natural Selection
– Differential survival and reproduction of organisms with a
result of increase in frequency of best adapted traits.
• Nonrandom Mating
This stud
bull isselection
the product of generations of selective breeding
– artificial
that has resulted in a double muscled freak that can hardly walk
Populations and Gene Pools
What’s the difference
between microevolution
and macroevolution?
• Microevolution is change within
species which can occur over
dozens or hundreds of
generations.
• Macroevolution usually involves
much longer periods of time and
includes the origin of new
species.
What is a gene pool?
The collection
of alleles
available
among
reproductive
members of a
population
Evolution occurs on the population level
• Remember, a population is a group
of individuals of the same species
in a given area whose members
can interbreed.
• Because the individuals of a
population can interbreed, they
share a common group of genes
(gene pool).
• Each gene pool contains all the
alleles for all the traits of all the
population.
• For evolution to occur in real
populations, some of the gene
frequencies must change with time.
Gene Frequencies and H-W
• Recall, the gene frequency of an allele is the
number of times an allele for a particular trait
occurs compared to the total number of
alleles for that trait.
Gene frequency = the number of a specific type
of allele / the total number of alleles in the
gene pool
• An important way of discovering why real
populations change with time is to construct a
model of a population that does not change.
– This is just what Hardy and Weinberg did.
– Their principle describes a hypothetical
situation in which there is no change in the
gene pool (frequencies of alleles), hence
no evolution.
Recall: The Hardy-Weinberg Law
This law states an equilibrium of allele frequencies in a gene pool (using a
formula p2 + 2pq + q2) remains in effect in each succeeding generation of a
sexually reproducing population if five conditions are met:
1.
2.
3.
4.
5.
Large populationThe population must be large to minimize random
sampling errors.
Random matingThere is no mating preference. For example an AA male
does not prefer an aa female.
No mutation
The alleles must not change.
No migration –
Exchange of genes between the population and another
population must not occur.
No natural selection- Natural selection must not favor
any particular individual.
• In nature, the conditions of the Hardy-Weinberg law are
rarely met
• Allele frequencies in the gene pool of a population do
change from one generation to the next
– thus evolution happens.
• The H-W equation provides a baseline by which to judge
whether evolution has occurred.
Remember that the effect of natural selection on gene frequencies can be
quantified.
36% homozygous dominants (AA)
48% heterozygotes (Aa) and
16% homozygous recessives (aa)
If a change in allele frequencies occurs over time, you can assume evolution is
happening.
A hypothetical “gene pool”
long as the
the conditions
of Hardy-Weinberg
are
met,athe
population
can increase
•As Find
Frequencies
of
A
and
and
the
in size and the gene frequencies of A and a will remain the same.
genotypic frequencies of AA, Aa and aa.
Solution:
Thus, the gene pool does not change.
f(A) = 12/30 = 0.4 = 40%
f(a) = 18/30 = 0.6 = 60%
Then, p + q = 0.4 + 0.6 = 1
and p2 + 2pq + q2 = AA + Aa + aa
= .16 + .48 + .36 = 1
• Now, suppose more 'swimmers' dive in as shown in
What will the gene and genotypic frequencies be?
Solution:
f(A) = 12/34 = .35 = 35 %
f(a) = 22/34 = .65 = 65%
f(AA) = .12, f(Aa) = .23 and f (aa) = .42
• The results show that H-W Equilibrium was not maintained.
– The migration of swimmers (genes) into the pool
(population) resulted in a change in the population's gene
frequencies.
– If the migration were to stop and the other agents of
evolution (i.e., mutation, natural selection and nonrandom mating) did not occur, then the population would
maintain the new gene frequencies generation after
generation.
– It is important to note that a fifth factor affecting gene
frequencies is population size.
• The larger a population is, the number of changes that occur by
chance alone becomes insignificant.
• In the previous analogy, a small population was used to simplify
the explanation.
How much variation does a large
population of sexually reproducing
organisms have in their gene pool?
•Typically about 0.5% of the DNA bases
are different
• In fruit flies there are 165 million base
pairs, so ~1 million nucleotides sites
differ.
•In humans with 3 billion base pairs, this
means there are about 15 million
different/variable nucleotides).
Mutations and Peppered Moths
In 1850, f(L)= .95 and f(l) = .05
•light colored moths on light colored
trunks were camouflaged
In 1900, f(L) = .05 and f(l) = .95
50 generations later:
•light lichen was killed by pollution
•So the dark tree trunk was exposed
• Therefore, light colored moths became easy prey for
birds
Positive, Negative, or Neutral:
• Mutations are random and unpredictable
• Some are lethal and kill individuals before
they are born.
• Some are harmful, but masked by a
dominant allele
– each of us carry 7-8 lethal recessive genes
• Some are neutral and have little to no affect
on an organisms long term survival.
• Some are beneficial and help the organism
survive to reproductive age and reproduce
Gene Flow
• the exchange of alleles
– Alleles move through populations when organisms
interbreed or migrate
– Gene flow can increase variation within a population by
introducing new alleles
Genetic Drift
• random changes in the gene pool
– Genetic drift causes gene pools of two isolated
populations to become dissimilar
– some alleles get lost and other get fixed
– This causes a reduction in the frequency of the
heterozygotes over time.
– The smaller the population size the faster the effect is
seen
Why does it affect small
populations more?
• In a small population, allele
frequencies are likely to be atypical
just by chance.
• If you were to toss a coin 1000 times
and get heads 750 times you would
be very surprised.
• If you tossed a coin 4 times and got 3
heads, you would not be so surprised!
• Remember, chance has a greater
influence upon gene frequencies in a
small population
When is genetic drift likely to occur in
nature?
• Most populations are large enough so genetic
drift does not occur
• Some populations crash in numbers due to
natural disasters or over-harvesting by
humans(“bottleneck effect”)
• Another example is when a new habitat (e.g.
an island) is colonized by just a few
dispersing individuals (“founder effect”)
• The suddenly small population may, purely by
chance, contain a different frequency of
different genotypes that the original large
population.
Founder Effect
• Genetic drift can also occur when a small
number of individuals from a large population
emigrate to a new area.
•The small number
of emigrants are
likely to have an
genetic structure
that differs, purely
by chance, from the
main population
An Example of Founder’s Effect:
•In the 1700’s 200 German Amish immigrated to Pennsylvania to start a
community.
•These people carried a high concentration of a mutation which causes a
form of polydactylism.
•Individuals in this group tended to marry within so there's a greater
likelihood that the recessive genes of the founders will come together in the
cells that produce offspring.
Inbreeding
Breeding between close relatives.
This causes the gradual increase in homozygosity (or
loss of heterozygotes.
Example:
In most species, related individuals share about 80% of the same genes.
With cheetahs, this figure rises to approximately 99%.
The genetic inbreeding in cheetahs has led to low survivorship (a large
number of animals dying), poor sperm quality, and greater susceptibility to
disease.
The Bottleneck Effect
• From an original large population, only few survive
to repopulate the habitat.
• The few survivors are likely to have a genetic
structure (a set of genotype frequencies) that is
unrepresentative of the ancestral large population
Example of a genetic
bottleneck
Northern elephant seals have reduced genetic variation probably
because of a population bottleneck humans inflicted on them in the
1890s.
Hunting reduced their population 20 individuals at the end of the 19th
century.
Their population has since rebounded to over 30,000—but their genes
still carry the marks of this bottleneck:
they have much less genetic variation than a population of southern
elephant seals that was not so intensely hunted
Comparing the 2
Effects:
Bottleneck Effect is sampling error
as a result of only a few individuals
surviving a population crash.
Founder Effect is sampling error
as a result of movement of a few
individuals away from the main
population
What’s inbreeding
depression?
The fertility and survival are reduced
compared with populations that are not inbred.
Caused by an increase in homogous
individuals.
Ex.
Outbreeding enhancement/hybrid
vigor is when they manage a species
and bring back heterogeneity.
In the adder case, introducing
snakes from other populations
a population of 40 adders
experienced inbreeding depression
when they were isolated them from
other adder populations.
As a result, a higher proportions of
stillborn and deformed offspring
were born in the isolated
population than in the larger
populations
Artificial Selection
• Much of what Darwin
learned about Natural
Selection, came from
his observations from
selectively bred crops
and domestic animals.
• This showed that
continued selection
was powerful enough
to bring about largescale changes within a
species.
A change in plant or animal
population by selective
breeding
Fitness
• The suitability of an organism
to a given environment
• This is often measured by the
number of offspring that an
individual has
• However, the offspring must
survive to contribute to the
following generation
•Consider two zebras:
• Both live in the savannah and must escape predation by lions while also
finding food & water. As babies, they were protected by the herd
•Zebra 1- had a mutation with deformed its rear leg
•Zebra 2- is fast and strong
• Soon after they both
became juveniles, a lion
pride attacked the heard.
– Zebra one, not being able to
run fast, was caught by the
lion pride and eaten.
– Zebra 2 escaped this, and
many more attacks. Lived to
have many offspring who
were also fast and strong
runners.
– So Zebra 2 was more fit than
Zebra 1, by surviving.
Adaptations that affect Fitness
• Camouflage– color is not necessarily relevant
– Pattern and shades matter
– Pattern breaks up an
organism’s outline
• Protective Coloration– Color of animal blends in with
environment
– However, this can allow the
organism’s shadow to reveal
the outline