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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.