Transcript Evolution - NIU Department of Biological Sciences

Evolution
•
Basic tenet of science: everything in
the natural world has a natural cause
and explanation.
•
How did life originate?
•
There are many forms of life,
displaying many similarities and
differences. How did they get that
way?
•
Aristotle and his medieval European
followers believed that all living things
could be arranged on a scale, from
lowest to highest: the Great Chain of
Being. In this view, the work of
biologists was to find the place of each
organism in the Chain.
Geology
•
The theory of evolution grew out
of geology. Specifically, the
realization that the Earth is very
old.
•
Current estimate of the age of the
Earth is 4.6 billion years.
Geology
•
James Hutton (late 1700’s) and Charles Lyell in the 1820’s and 1830’s developed the
theory of Uniformitarianism: that the geological processes we see today are the
same ones that operated in that past. For example, we can see rivers slowly eroding
rocks. Carried on for a long enough time, a river can erode its way through a mile of
rock to form the Grand Canyon.
•
Another example: high mountains are raised gradually through a series of
earthquakes and volcanic eruptions. Oceans lay down sediments that are washed off
the land, and eventually they build up to miles deep deposits.
•
If the geology we see is due to the same slow forces in operation today, the Earth
must have been here for a very long time
•
The contrasting theory in Lyell’s time was Catastrophism: specifically, the great Flood
of Noah caused the visible geology of the Earth in a very short period of time.
Fossils
•
Fossils are the remains of
ancient life, turned to stone
by chemical processes.
•
They have been known since
ancient times, but their
significance wasn’t clear until
the 1800’s. Fossils can be
created by burying the
remains of living things in
sediments that slowly
compress into layers of rock.
↑
The first good geological
maps of the layers of rocks in
England showed that each
layer had its own distinct set
of fossils. And, the older the
rocks—the deeper the layer
was—the simpler the fossils
were.
Fossils
•
Long before rocks could be dated,
the geological ages were defined
based on their characteristic
fossils.
•
Most fossils come from the hard
parts: bones, teeth, shells. These
structures first appeared about
600 million years ago. Fossils
before this time are much more
difficult to detect: mostly
microscopic. And, very old rocks
are hard to find and often very
distorted. But, the earliest fossils
come from about 3.8 billion years
ago, in almost the oldest rocks
known, close to the beginning of
the Earth,
3.5 billion year old Apex Chert microfossils
Radioactive Dating
•
Ancient rocks are dated by looking
at radioactive isotopes. There are
numerous methods, and ages are
often estimated by several
different methods.
•
We will discuss the potassiumargon dating method. This
method is used to find the age of
volcanic deposits, and it has been
heavily used to date the remains
of ancient human ancestors in
east Africa.
Radioactive Dating
•
Potassium is a element that is
common in volcanic rock. The isotope
potassium-40 (atomic weight of 40
protons + neutrons) is radioactive: it
decays into argon-40. The rate of
decay is steady and not affected by
external conditions at all. It takes 1.25
billion years for ½ of the potassium-40
to convert into argon-40.
•
Argon is a “noble” gas: it has 8
electrons in its outer shell and doesn’t
combine with other atoms to form
compounds. Under all earthly
conditions, argon is a gas.
•
When a volcano erupts, the lava is
molten, and all of the argon gas is
released into the atmosphere. When
the rock freezes, it contains
potassium-40 but no argon-40. Over
time, argon-40 builds up from the
decay of the potssium-40. By
measuring the ratio of potassium-40 to
argon-40, the amount of time since the
rock was molten can be determined.
Radioactive Dating
•
Other dating methods use
uranium and lead, or carbon-14,
or a variety of other radioactive
isotopes.
Radioactive Dating
•
The time scale used in
evolutionary studies is calibrated
by counts of tree rings. In some
places this scale goes back 9000
years.
Lamarck
•
By the end of the 1700’s, it was clear
from the diversity of life and the fossil
record that organisms had changed
over time. The force driving the
change remained mysterious.
•
One common idea was the
“inheritance of acquired
characteristics”, a theory associated
with Jean Lamarck. This theory
states that during the course of an
individual’s life, various needs and
desires bring about changes in the
body’s internal state, and that these
changes are then passed along to the
offspring. For example, if you exercise
heavily, your children will be born with
heavier muscles than you were. Or,
giraffes stretch their necks to reach
leaves on tall tress, so their offspring
have longer necks.
•
This theory is known to be wrong.
Inherited changes do occur, but they
are random in nature, not influenced
by the needs of the individual. The
cells that generate the sperm and
eggs are separate from the rest of the
body’s cells.
Lamarck
•
Many demonstrations disproving
inheritance of acquired
characteristics: in the late 1800’s
one researcher cut the tails off a
groups of mice at birth for 200
generations. Even after that time
their tails were as long as in the
original generation.
•
Another: take blood from black
rabbits and transfuse it into white
rabbits. The offspring of the white
rabbits are still white, even though
the blood was supposed to carry
the germs of the acquired
characteristics.
•
The real problem for Lamarck is
that no one was able to do an
experiment that demonstrated
inheritance of acquired
characteristics under controlled
conditions. If a theory is
repeatedly tested and never
successful, people start to doubt
its truth.
Darwin and Wallace
•
Charles Darwin and Alfred Russel Wallace independently came up with
the key ideas of evolution through natural selection in the 1830’s -1850’s.
Both spent years traveling to exotic locations and examining the plants
and animals there. Darwin went first, but he spent years slowly thinking
and writing. He was only prodded to publish when Wallace showed him
his manuscript.
Charles Darwin
Alfred Wallace
Evolution through Natural Selection
• Three basic conditions for natural selection to occur:
– 1. There must be variation within the species.
– 2. The variations must be inherited.
– 3. Some variants must have a better ability to survive and
reproduce than others.
• Fitness = ability to survive and reproduce. More fit
individuals have a better chance of producing offspring
than less fit individuals.
• This means that the alleles present in the more fit
individuals will increase their share of the population with
each generation.
• In time, the alleles that increase fitness take over the
population, and the less fit alleles disappear.
Artificial Selection
• In artificial selection,
humans define fitness
(ability to survive and
reproduce) by only
allowing the desired
individuals to mate, then
selecting the best
offspring.
• Darwin noted many
examples of this: he was
devoted to pigeon
breeding.
Natural Selection
• It is easy to find examples in
nature where one original type
has been converted to a
number of similar types that
differ in small ways. Darwin
found a group of finches on the
isolated Galapagos Islands
that fit this description.
↑ Another example: moths in
England had 2 main varieties,
dark and light. In the 1800’s,
the dark form was most
common. At that time, trees
were covered with soot from
burning coal, and the dark
moths were hard for predators
to see. In more recent times,
coal soot has decreased, so
tree trunks are lighter. Now
the lighter moths are more
common, because the dark
ones stand out.
Summary of Natural Selection
• 1. Any population can reproduce beyond the capacity of the
environment to sustain it. Some resource will be in short supply if
the population gets too large. In this case, individual organisms will
compete with each other for that resource.
• 2. Within a species there are a number of different genetic variations
(alleles) for many genes. Some of these alleles help the organisms
outcompete other members of the population: increase their ability
to obtain critical resources to survive and reproduce.
• 3. The individuals possessing the better alleles will have a better
chance of reproducing, so their offspring will make up a larger
portion of the next generation.
• 4. This process (called microevolution) increases the frequency of
the more fit alleles in the population. Over time the population
becomes better and better suited for its environment.
Gene Pool and Allele Frequencies
•
Selection (artificial or natural) involves changes in the “gene pool”: the
genetic resources of the entire population that can breed with each other.
•
The gene pool is counted in terms of “allele frequencies”: how many copies
of each allele are present in the population.
•
Aa
aa
AA
aa
Aa
Aa
For example, consider a gene with 2 alleles: A and a. In one population of
100 organisms, there are 30 AA individuals, 60 Aa individuals, and 10 aa
individuals.
Aa
Aa
AA
Aa Aa
Aa
Aa Aa AA Aa Aa
Aa AA Aa
Aa
AA
AA
Aa
AA
aa
Aa AA
Aa
AA Aa
Aa
AA
Aa
AA
Aa
aa Aa
AA
aa
Aa
aa
AA
Aa aa
Aa Aa
aa
Aa
Aa AA
Aa
Aa
Aa
AA
Aa
aa
AA
AA Aa
Aa Aa
AA
Aa AA
Aa
Aa Aa
Aa
Aa
Aa
Aa
Aa
Aa
Aa
AA
Aa
AA
AA Aa
Aa
Aa Aa Aa
AA
AA
AA
Aa
AA
aa Aa
Aa
Aa
AA
Aa
AA
Aa AA
Aa
Aa
AA
Gene Pool and Allele Frequencies
•
•
•
•
Counting copies of A: 2 in each AA individual (30 x 2 = 60 A alleles) plus 1
in each Aa individual (= 60 more A alleles), for a total of 120 A alleles in this
population
Counting copies of a: 2 in each aa individual ( 10 x 2 = 20 a alleles) plus 1 in
each Aa individual (= 60 more), for a total of 80 a alleles.
There are 200 total alleles ( 2 in each of the 100 individual organisms), so
the frequency of A is 120 / 200 = 0.6, and the frequency of a is 80 / 200 =
0.4
Note that the total of the frequencies equals 1: 0.6 + 0.4 = 1. A frequency of
1 means that every allele is either A or a.
Aa AA
Aa
Aa
Aa
Aa
Aa
Aa
Aa
AA Aa
Aa
Aa AA Aa
Aa
AA
AA
A
aa aa
Aa AA
Aa
AA Aa
Aa
AA
Aa
a
AA
Aa
AA
Aa
AA
aa
Aa
aa
aa
AA
Aa
Aa Aa
aa
Aa
AA
Aa
Aa
Aa
aa Aa
Aa
AA
aa
AA
AA Aa
Aa Aa
AA
Aa
Aa
Aa
Aa
AA Aa
Aa Aa
Aa Aa
Aa
Aa AA Aa AA Aa
AA Aa
Aa Aa Aa
AA Aa AA
AA
Aa
aa Aa Aa Aa
AA
AA
AA
Aa
Aa AA Aa AA Aa
Aa
AA
aa
Selection Changes Allele
Frequencies
• Individuals can’t change which alleles they have, but the frequency
of different alleles in a population can change. “Individuals don’t
evolve—populations do.”
• As an example of fitness, assume that the aa homozygotes have a
genetic disease that makes them less fit than AA or Aa individuals.
In an extreme case, assume that the aa individuals die before they
can reproduce. Their fitness is 0, since they can’t pass their genes
on to future generations.
• So, only the AA and Aa individuals mate. They still generate some
aa offspring ( ¼ of the offspring when Aa x Aa), but the number of
aa’s is low. The frequency of A increases while the frequency of a
decreases.
• This change in allele frequencies within a species based on
differences in fitness is called “microevolution”. It is the source of
slow changes in the characteristics of a species.
Populations in Equilibrium
•
Many populations are more or
less in equilibrium: allele
frequencies stay constant
between generations, no
selection occurs because all
the genotypes are equally fit.
•
These equilibrium populations
are governed by the HardyWeinberg rule, which
determines how many
homozygotes and
heterozygotes there will be,
based on the allele
frequencies.
Populations in Equilibrium
•
Hardy-Weinberg rule:
–
–
–
–
–
if the frequency of A is called p and
the frequency of a is called q, then
the frequency of AA individuals is p2,
the frequency of Aa individuals is 2pq, and
the frequency of aa individuals is q2.
Hardy-Weinberg Equation:
p2 (AA) + 2pq (Aa) + q2 (aa) = 1
Populations in Equilibrium
phenotypes
genotypes
AA
Aa
aa
For example,
– if the frequency of A is 0.6 and
– the frequency of a is 0.4, then
– a population in equilibrium will
have (0.6)2 = 0.6 x 0.6 = 0.36
as the frequency of AA’s.
– The Aa’s will be at a frequency
of 2pq = 2 x 0.6 x 0.4 = 0.48.
– The aa’s will be at q2 = 0.4 x
0.4 = 0.16 frequency.
number of
plants
(total =500)
genotypes
frequencies
180
180
500
# of alleles in
gene pool
(total = 1000)
240
= 0.36 AA
x2
360 A
allele frequencies
p = frequency of A = 0.6
240
500
= 0.48 Aa
80
500
= 0.16 aa
x2
240 A
600
1,000
80
=
0.6 A
240 a
160 a
400
1,000
=
0.4 a
q = frequency of a = 0.4
Conditions Necessary for
Equilibrium
• Once a population is in equilibrium, the
frequencies of the different genotypes doesn’t
change between generations.
• To be in equilibrium, a population must fulfill
several conditions:
–
–
–
–
–
1. population must be very large
2. all mating must be random
3. no fitness differences between individuals
4. no migration in or out of the population
5. No new mutations
Effects of Hardy-Weinberg
Conditions
• Mutation. Mutations are the raw material of evolution. They occur
at a slow but steady rate, and they provide alleles of greater or
lesser fitness. However, mutations by themselves don’t influence
allele frequencies significantly.
• Migration. The movement of organisms in and out of populations
keeps a species from fragmenting into several different species.
When two populations mix, the combined population has allele
frequencies that are a mix of the two original sets of frequencies.
• Random mating. Most organisms don’t mate at random, especially
among the animals. Mate selection is a constant factor. The main
form of non-random mating is called “assortative mating”, which
means mating with someone similar to you. Tall people with tall
people, smart people with smart people, etc. Disassortative mating,
where the types differ (“opposites attract”) is also found.
Non-random Mating
•Inbreeding, mating with close blood relatives, is another form of nonrandom mating. In humans, almost all cultures forbid brother-sister
marriages, but different cultures have different views on first cousin
marriage. Some cultures encourage it and others forbid it.
•Overall, humans are inbred are approximately equivalent to
everyone marrying a third cousin (i.e. common great-great
grandparents). In the US population, the inbreeding equivalent is
approximately that of fifth cousins (common great-great-great-great
grandparents: mine were born about 1780).
•Why does inbreeding occur?
• Desire to keep wealth or power in the family
• Most people live near where they were born: ½ of all living
descendants of the people who came to the US on the
Mayflower in 1620 live within 50 miles of their landing spot at
Plymouth Rock, Massachusetts.
• Opposites don’t attract: even distant, unknown relatives share
cultural characteristics and appearance details that are attractive
to each other.
Directional Selection
•
The simplest form of selection is
directional selection: one extreme
phenotype is less fit than the rest
of the phenotypes.
•
Plot the distribution of the trait
being selected on a graph.
Usually get a bell-shaped curve
(normal distribution, Gaussian
distribution)—most individuals are
more or less average, with a few
extremes at each end.
•
Don’t let individuals at one
extreme breed.
•
In later generations, the
population average shifts away
from the less fit extreme.
Examples of Directional Selection
• Pesticide resistance. In the absence of pesticides, a few insects
are naturally resistant. They have a combination of genes that does
them no particular good in the absence of pesticides. However,
when pesticides are used, suddenly these insects are the only ones
who survive. Many of their descendants get the same resistance
genes. After several generations of spraying pesticides to kill the
insects, all the insects are resistant.
• Antibiotic resistance. Same phenomenon applied to diseasecausing bacteria. If the antibiotic leaves some of them alive, their
descendents are all resistant and the antibiotic no longer works. A
growing problem—antibiotic resistance is spreading faster than new
antibiotics are being developed.
• Cancer cell progression. If chemotherapy doesn’t kill all the tumor
cells, the ones left living are the most resistant ones. They multiply,
creating a new tumor that isn’t sensitive to chemotherapy.
• One solution: use multiple drugs—much more difficult to be resistant
to several different antibiotics.
Stabilizing Selection
•
Selection can act to favor the most
common type, the middle of the
distribution. This can happen when
both extreme types are attacked. The
status quo is maintained.
•
Human birth weight: a small baby will
survive the birth process better than a
big one (and so will the mother).
However, small baby will have a
harder time surviving after birth than a
big one. Leads to opposing forces that
result in an average size of baby.
•
Example: The fly Eurosta solidaginis
lays its eggs on goldenrod plants. The
larvae hatch, then bore into the stem,
creating a gall, a swollen area of the
stem. The larvae have 2 predators
that eat them. One is a wasp that can
only penetrate small galls. The other
is a woodpecker that can penetrate
any gall, but prefers the larger ones.
So, both small galls and large galls are
attacked. The intermediate ones have
the highest rate of survival. Selection
favors intermediate sizes.
Disruptive Selection
•
Disruptive selection is the opposite of
stabilizing selection. In disruptive selection,
the average type is the least fit. Only the
extremes survive, creating a population with
two different alternatives. This is one of the
forces that drives the splitting of one species
into 2.
•
This would seem to be an odd form of
selection. Often it occurs with sudden
changes in the environment.
•
Example: grasses that find themselves near
mines. Mine tailings containing heavy
metals are toxic to most plants. Some
grasses are resistant to heavy metal
poisoning, so they can grow on the mine
tailings. Less resistant members of the
species grow on uncontaminated ground.
Since heavy metal resistance is expensive,
the resistant plants are less successful on
uncontaminated ground. So, the species is
being cut into 2 groups: the resistant variety
growing on the mine tailings and the
sensitive variety growing on clean soil.
Sexual Selection
•
No trait is more important to
producing offspring than finding a
mate. And, selecting a high
quality mate increase the chance
that your offspring will survive to
adulthood. For these reasons,
much selection is aimed at
increasing attractiveness to the
opposite sex and signaling (or
faking) good genes and good
health. This is sexual selection.
•
In the situations common in birds
and mammals, the males must
drive away their rivals, and the
females must pick the “best” male
as a mate.
•
This leads to ritual fighting among
males in many species: male elk
fighting during mating season is
an example. The winner drives off
the loser, leaving the winner in
possession of a territory that he
can keep females in.
More Sexual Selection
• To show that they are in good health, one sex often puts on a showy
display. The bright coloration of male birds is a good example. It serves
no end other than to attract females, and it makes them easier prey for
predators. But, it can lead to strong selection for very extreme
appearances.
Heterozygote Advantage
•
Sometimes selection works on the
single gene level, by conferring an
advantage on the heterozygotes.
This allows 2 alleles to be
maintained in a population even
though both homozygotes are less
fit than the heterozygote.
•
Good example: sickle cell
hemoglobin. The heterozygote is
resistant to malaria and is
otherwise normal under most
conditions. Homozygotes for
sickle cell hemoglobin die early
with severe anemia. Homozygous
normal people die of malaria. The
relative death rates between these
3 genotypes causes an allele
frequency of about 0.2 for sickle
cell hemoglobin in areas affected
by malaria. (That is, 20% of the
alleles in the population are the
sickle cell type.)
Neutral Mutations
• In Darwin’s time, natural selection was thought to be the only force
driving evolution. This led to “social Darwinism”, theories that
glorified competition above all else. The catchphrase “nature red in
tooth and claw” comes from this era.
• However, it became clear that many traits seem to be selectively
neutral, or at least there was no obvious selection pressure on them.
For example, much of the DNA is not part of any gene. How could a
base change mutation in DNA outside of a gene affect fitness?
Genetic variants that don’t affect fitness are called “neutral
mutations” and they are said to be “selectively neutral”.
• An example: detached vs. attached earlobes. Hard to see how this
would affect fitness.
• Another example: blood type.
Japanese Blood Type Personality Chart
My Boyfriend is
Type B
Type A
Best
Traits
Conservative, reserved, patient, punctual,
perfectionist and good with plants.
Worst
Traits
Introverted, obsessive, stubborn, self
conscious, and uptight
Type B
Best
Traits
Creative and passionate. Animal loving.
Optimistic and flexible
Worst
Traits
Forgetful, irresponsible, individualist
Type AB
Best
Traits
Cool, controlled, rational. Sociable and
popular. Empathic
Worst
Traits
Aloof, critical, indecisive and unforgiving
Type O
in Korean, written and directed
by Choi Seok-Won
Best
Traits
Ambitious, athletic, robust and self-confident.
Natural leaders
Worst
Traits
Arrogant, vain and insensitive. Ruthless
Genetic Drift
•
•
•
•
Genetic drift is the random change in allele frequencies due to chance alone. It happens in all
populations, but it is most significant in small populations.
Genetic drift is largely the result of sampling error: the variation in results that occurs when too
small a sample is taken. For example, if a population contains only 3 individuals, 2 males and
1 female, the allele frequencies of the next generation will be heavily influenced by the
female’s choice of a mate. Any alleles found only in the unmated male will be lost.
If a population stays small, genetic drift leads to the fixation of one allele and the loss of other
alleles. Fixation = the allele has a frequency of 1, all alleles in the population are of this one
type.
The time until fixation is random, but it is proportional to the size of the population. In large
populations fixation by genetic drift is rare.
Bottlenecks
•
A bottleneck is a severe
reduction in the size of a
population. Whatever
the allele frequencies
were before, the allele
frequencies after the
bottleneck are based
solely on the alleles
found in the survivors.
Bottlenecks
•
Small islands are especially subject to
bottlenecks—it is very easy to lose a
significant portion of the population. A
typhoon hit Pingalap Atoll, a small Pacific
Ocean island, in 1780. All but 30 people
died. One of the 9 male survivors was a
high ranking man who had the recessive
genetic condition achromatopsia. This
causes complete colorblindness, cataracts,
and often complete blindness due to retinal
degeneration. Today between 4 and 10% of
the people on this island are homozygous
for the disease. The allele frequency before
the typhoon was low, but due to the vagaries
of survival, it became very high.
•
The entire human population is thought to
have gone through a bottleneck about
100,000 years ago. One consequence is
that there is less genetic variation among
the 6 billion humans than there is among, for
example, one 55-member social group of
chimpanzees living in West Africa.
•
Cheetahs in Africa went through a very bad
bottleneck within the last 10,000 years.
Their population was reduced to fewer than
100 individuals. The population has since
grown much larger, but the amount of
genetic diversity among them is very low
because there were so few founders.
Founder Effect
•
•
•
The founder effect is very similar:
when a small group leaves a large
population, the allele frequencies in
the newly established population
are based on which alleles were in
the small group of founders.
Example of founder effect: the
Amish are a group descended from
30 Swiss founders who renounced
technological progress. Most Amish
mate within the group. One of the
founders had Ellis-van Crevald
syndrome, which causes short
stature, extra fingers and toes, and
heart defects. Today about 1 in 200
Amish are homozygous for this
syndrome, which is very rare in the
larger US population.
Note the effect inbreeding has here:
the problem comes from this
recessive condition becoming
homozygous due to the mating of
closely related people.