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Review Questions
Natural Selection
1. Who discovered the theory of natural selection?
The theory of natural selection was discovered by Charles Darwin and
Alfred Russell Wallace. Darwin first described the process of natural
selection in his diary back in 1837. By 1844, he had fleshed out the idea
into its finished form. Darwin sat on his discovery for more than a
decade. His big plan was to write a book called “Natural selection” that
would explain his idea and provide evidence of evolution. He knew his
ideas would create a firestorm so he planned for his book to be
published posthumously.
In 1858, Darwin received a manuscript from Alfred Russell Wallace
outlining natural selection. Wallace was a young naturalist working in
Malaysia at the time. Independent of Darwin, he came up with the same
idea. Darwin was floored. He wanted to give Wallace all the credit and
fade into the background. Friends of Darwin recommended he co-author
a paper with Wallace and present their discovery to the scientific
community and to the world. So Darwin and Wallace published a paper.
Soon after, Darwin wrote “On the Origin of Species”.
2. Explain artificial selection
The first chapter of “On the Origin of Species” is not about the evidences
of evolution (e.g., fossil record, comparative anatomy, etc.) as you might
imagine. Instead, Darwin begins his book explaining the origin of the
numerous varieties of domesticated animals and plants. He describes in
detail the process by which farmers selectively breed their crops and
livestock to improve their valuable traits.
The process of selective breeding goes like this. Let’s say we want to
grow bigger and bigger pumpkins. A pumpkin farmer would only breed
her biggest pumpkins. On average, each succeeding generation would
be bigger. Over many generations, pumpkin size would increase
enormously. Right now, the world record for a giant pumpkin is over
1600 lbs! Selective breeding, Darwin called it “artificial selection”, implies
that humans control the gene pool of a population. Humans select what
organisms breed based on the traits the humans value.
Darwin looked around and saw the transforming power of artificial
selection in the dozens of varieties of domesticated plants and animals
existing at the time. He realized that living organisms were remarkably
plastic, that artificial selection can make some astonishing changes in
body form, coloration, temperament, behavior, quality and yield.
Darwin saw the remarkable diversity of living forms in nature. Darwin
reasoned that if artificial selection, where humans controlled
reproduction, was so successful in creating so many new breeds, then
natural selection, where environmental conditions determined
reproductive success, could produce the millions of species we see on
earth.
3. Explain the process of natural selection. Describe the five premises
and result of Darwin’s theory of natural selection.
Let’s examine Darwin’s line of reasoning for natural selection. The theory
is based on five premises resulting in adaptation. The first premise is
that every species has an enormous biotic potential. All species have
such great potential fertility that their population size would increase
exponentially if all individuals that are born reproduced successfully. For
example, a single fern can produce fifty million spores in a single year. If
all of those spores germinated and produced mature ferns. After a few
generations, we would be up to our earlobes in ferns. There are lots of
examples of organisms that produce huge numbers of offspring. Darwin
wondered if a slow reproducer could also reproduce exponentially. In the
Origin, Darwin calculates the biotic potential of elephants. These
massive beasts are pregnant for two years before giving birth and
generally have a maximum of 6 offspring. Starting with two elephants,
Darwin calculated that with their biotic potential, the population would
reach 19 million in just 750 years. From this, Darwin reasoned that biotic
potential was universal.
The second premise of Darwin’s theory is that each ecosystem has a
finite amount of resources and can support only a certain
community size (carrying capacity). If we examine natural
populations, we find that their numbers don’t fluctuate much and
generally stay fairly constant generation after generation.
Darwin recognized that these two premises were at odds with one
another. Organisms produce more offspring than ecosystems can
support. This leads to intense competition among individuals. Just a tiny
fraction of them survive to adult and reproduce. Darwin called this
competition “the struggle for existence”. Competition is his third
premise.
In the struggle for existence, what determines which individuals survive
to reproduce? Darwin argued it was an individual’s inherited traits. If an
organism had a set of traits that gave a slight edge over a competitor,
that could make a big difference in survival. Back in the mid 19th Century
genetics was still a mystery. Darwin knew that in the natural world,
useful traits were retained and harmful traits were eliminated from a
population generation after generation. Where did these traits come from
and why weren’t they the same in all the members of a population?
Darwin observed variation in natural populations. In fact, to confirm this
observation, Darwin spent 8 years studying and classifying barnacles.
He saw variation first hand. So why was there variability in populations?
Darwin knew about mutations. He called them “unsolicited novelties”. He
recognized that mutations were the raw material for variation in a
population. He also saw that populations vary constantly due to the
reshuffling of traits in sexual reproduction. These two mechanisms
caused individuals to vary from one to another. Variation is his fourth
premise.
Darwin connected these two ideas: competition and variation, and
arrived at his fifth premise: differential success in reproduction.
Those individuals that survived the competition, meaning they had
inherited characteristics that better fit them to the environment, passed
their winning traits onto the next generation. Biologists measure this as
fitness.
The final result of this train of logic is adaptation. Darwin argued that the
results of this constant struggle, variation, inheritance, and reproductive
success generation after generation would gradually change a
population to be better and better adapted to their environment. This
constant tinkering, testing and accumulating favorable characteristics
was the powerful force that created the diversity of life as we know it.
4. Give two examples of natural selection.
Antibiotic resistance is a major problem in medicine and agriculture. We
now have “superbugs”, pathogenic bacteria that are resistant to multiple
antibiotics. We are in an arms race with infectious microbes. There is a
rule in medicine called “two and twenty”. It takes, on average, two years
for resistance to show up after the introduction of a new antibiotic. It
takes twenty years for a new antibiotic to be discovered, tested, and
placed on the market.
So how do bacteria become resistant? Let’s say your physician
prescribes you an antibiotic for an infection. If you read the instructions
on the bottle, it says you must take all the pills. What happens if you stop
prematurely? If you stop, you might have infectious bacteria still alive in
your system. These are the survivors. They have won the competition
because of their resistance traits. You have killed off the sensitive ones.
The resistant ones pass on their resistance traits to the next generation.
If we want to slow resistance, we need to use antibiotics judiciously. If
you have a cold or the flu, don’t ask for an antibiotic. Colds and flu are
caused by viruses. Antibiotics only work on bacteria. Using an antibiotic
when you don’t need one only selects for more resistant bacteria.
We see resistance pop up in other things too. The misuse of
antiretroviral medication for HIV infections has also caused resistant
strains of HIV to appear. Pesticides also become ineffective because of
the natural selection for resistance. Where do these resistance genes
come from? Well, they come from random mutations. Some think
wrongly that the antibiotic creates the mutation that bestows resistance.
In studies, where cultures are grown in the presence of an antibiotic or in
the absence of antibiotics, resistant mutations show up in both. The
resistant mutations occur independently of the antibiotic. The difference
is that in the antibiotic-free cultures, the resistant mutations do not
bestow any benefit to the bacterium. However, there is a huge benefit to
those grown with an antibiotic.
Another great example of natural selection comes from Australia. In the
mid 1800’s, a Sydney man imported twelve European hares from
England and released them onto his land. From these original twelve,
the population exploded. By the end of the century, these pest rabbits
had spread across the Australian continent. The rabbits denuded vast
areas of land with their warrens. Cattle and sheep farmers couldn’t
compete and turned to the government for help. Australian biologists
learned of a virus found in the lungs of South American hares, where it
was benign, that when introduced to a European hare by a mosquito
vector would cause a lethal lung disease called myxomatosis. The
myxomatosis virus seemed to be a perfect biological control for
European rabbits. The virus didn’t harm humans, livestock, or wildlife
and killed the rabbits in just a few days. So in the 1950’s, scientists
released millions of myomatosis virus-laden mosquitoes in Australia.
They had great success. Thousands of rabbits died.
However, the mortality rate was not 100% (in fact it was 99.9%). This
was bad news for the biologists. By not killing all of the rabbits, the
biologists realized a couple of things were happening. First, there were
some resistant rabbits surviving. They would be passing their resistance
genes to the next generation to breed even more survivors. And second,
some of the viruses were not killing their hosts. If you are a parasite, let’s
say, the last thing you want to do is kill your host before you reproduce.
The virulent viruses were doing just that. They killed their host and in
turn eliminated themselves. Less virulent strains (created by random
mutations) allowed their rabbit hosts to live and therefore passed their
more benign traits down to the next generation. Today, the mortality rate
of myxomatosis is around 40%.
5. What are microevolution, speciation, and macroevolution?
Up until now, I have given you examples of changes in the frequency of
alleles made by natural selection. This is microevolution. If selection
pressures are directional or divergent and are sustained over time,
natural selection causes enough of a change in a population to
distinguish it as a new species. The pace of speciation is slow (much
longer than a human life span). But we can easily see examples of
populations in the process of diverging. Over longer spans of time,
natural selection can produce evolutionary change above the species
level, including the appearance of evolutionary developments, such as
flight, that we use to define higher taxa. We call this macroevolution.
6. Clarify the following misconception concerning evolution and natural
selection: how can random changes lead to order?
A common argument often heard against evolution is: how could an
organism have evolved through random chance. Fred Hoyle, a British
cosmologist, posed the same question this way. He used an analogy.
Imagine a large junkyard in Southern Florida. Now, picture a hurricane
sweeping through the junkyard and out pops a 747 jet, fully assembled
and working. Like most of the general public, Hoyle has a
misunderstanding of natural selection. It is true that there is randomness
in the process. Mutations do occur at random. But the process of
selection is particularly non-random. Only beneficial mutations are kept
while useless mutations are eliminated. Each beneficial mutation is
saved and retained by the process. The accumulation of beneficial
mutations results in complicated organisms. We can often predict what
mutations will be retained and which will be eliminated. That makes the
selection part of the equation non-random. Most people forget the
selection part.
Let me give you another example. Opponents of evolution have
sometimes used the following analogy. Imagine you wanted to see how
long it would take to randomly replicate the famous phrase from
Shakespeare “To be or not to be.” Let’s say you had one million
monkeys with typewriters, how long would it take for them hitting their
keyboards at random to come up with that phrase? Mathematically, we
would calculate that it would take ~78,000 years if each monkey typed
one phrase per second. That sounds really improbable. But let’s take the
same analogy and throw in the non-random selection. How long would it
take if each time a correct letter appeared, it was saved? Then, our
same task would only take 90 seconds. This same process could
produce Hamlet in 4.5 days. Remember, natural selection is a two part
process: random changes and non-random selection.
7. Clarify the following misconception concerning evolution and natural
selection: how can complex structures evolve through a gradual
accumulation of small changes?
Living organisms are incredibly complicated. Even the smallest bacteria
are super complex. How could all this complexity arise through a series
of tiny changes? For the general public this can be hard to fathom. How,
for example, could wings have evolved in a series of small steps? What
good is half a wing for an organism? Finding these functional
intermediates seems like an impossible task.
You may have heard of the term “intelligent design”. Proponents argue
that some structures are so complicated (irreducibly complex), there is
no way natural selection could have built it. So they argue that there
must have been a “designer”. They don’t really tell you who the designer
is but it is implied that it is God.
There are some problems with this proposal. First off, they are proposing
a supernatural explanation. This violates the natural causality
assumption of science. So the hypothesis is not scientific. Second, they
are ignoring Occam’s Razor. They are positing a more complicated
being (explanation) than their structure. They are opting for an
explanation with more assumptions and thus more unlikely. Third, a
designer hypothesis is intellectually lazy. In fact, this logical fallacy is
called “the argument from ignorance” or “the god of the gaps”. “If I can’t
figure out how a structure evolved, then a designer must have done it.”
Does this mean that someone else might not find the answer? Lastly, a
supernatural hypothesis doesn’t get you anywhere. It doesn’t tell you
how or why or when a structure was made? You are stuck back at
square one. It is intellectually lazy.
Evolutionary biologists have had several success stories in figuring out
the path natural selection followed to create a complicated structure. We
have a pretty good idea how bacterial flagella evolved. We have an
explanation on how the blood clotting mechanism came to be. One way
biologists figure out these evolutionary pathways is to look around at
other organisms that have intermediate forms. A classic example is the
vertebrate eye. We can see examples of invertebrates with all the
intermediate stages leading to the vertebrate eye: eyespots, cupped
eyes, pinhole eyes, etc. One really interesting tool in understanding the
power of natural selection has been computer algorithms. Computer
scientists and engineers have modeled natural selection in cyberspace
and have created wonderfully complicated virtual organisms that solve
problems in new and novel ways without any human input. I think it is
easy to underestimate the power of natural selection.