Evolution
The Theory of Evolution, originally proposed by Charles Darwin, describes the way that organisms change over time. It is, according to most biologists, the single most important concept in the study of living things.
Why Do We Study Evolution Last?
If evolution is so important, why is it the last thing we study? Well, in order to understand evolution, you need to have an understanding of genetics. You need to know about the structure and function of DNA. You need to know about cells so that you can see the similarities shared by all living things.
What Did Darwin Observe?
Charles Darwin was a naturalist, a person who studies nature. When he finished his schooling, he signed up to be the naturalist aboard the HMS Beagle. The Beagle sailed around the world for five years. Along the way, Darwin observed many different plants and animals. Darwin made his most important observations in the Galapagos Islands. The Galapagos Islands are located about 600 miles offshore of the South American country of Ecuador.
Darwin noticed that the finches that lived in the Galapagos were similar to the finches in Ecuador but that there were slight differences. He also noticed that there were slight differences in the finches from island to island. One of the differences between the populations of finches was the shape and size of their beaks. Darwin thought that the finches beaks were related to what kind of food they ate. Small, narrow beaks are good for eating insects, but wide, strong beaks are better for eating hard seeds. Darwin thought that the finches had evolved over time, and that the different beaks represented adaptations that helped the finches to survive in their environment.
Darwin's Theory of Evolution by Natural Selection
When his voyage on the Beagle came to an end, Darwin returned to England. He spent years recalling his experiences, going over his notes and drawings, and thinking about what it all meant. Finally, he published his book: On the Origin of Species by Means of Natural Selection.
In formulating his theory, Darwin relied on other scientific evidence. Most scientists in Darwin's time thought that the Earth was young. Some geologists, though, were producing evidence that the Earth was older, much older, than previously thought. Darwin knew that if the Earth was older, there would be time for evolution to occur. Darwin also used ideas about populations. A scientist named Thomas Malthus noticed that babies were being born faster than people were dying, and that the human population was growing. Malthus believed that the population could grow faster than the food supply, resulting in food shortages. Malthus believed that starvation, disease, and other problems would limit the size of the human population. Darwin reasoned that the same would be true of other organisms. Darwin also knew that farmers and breeders were able to produce plants and animals with desired traits through selective breeding. Selective breeding, Darwin knew, could cause significant changes in the traits expressed in a population. Darwin connected the dots and his theory was born.
How Does Natural Selection Work?
There are four steps to natural selection: overproduction, genetic variation, struggle to survive, and successful reproduction. Let's have a look at each of these.
Overproduction
Overproduction means that so many offspring are produced that it is not possible for all of them to survive. This is closely related to the belief of Malthus that the human population would eventually be limited by the amount of food available. In order for natural selection to operate, there must be winners and losers. In order for there to be losers, there needs to be overproduction of offspring.
Genetic Variation
Genetic variation means that there are differences in the DNA among the offspring in a population. If there were no differences in the DNA, then all of the organisms in a population would be the same, and no organism would hold an advantage in terms of survival. These differences might be obvious (eye color, or fur color, for example), or they might be hidden.
Struggle to Survive
In the natural world, organisms are constantly engaged in a struggle to survive. They are competing with members of their own species and with members of other species for space and for food. Sometimes, this struggle is obvious. You have all seen nature videos where the cute gazelle is ferociously attacked by a cheetah. Sometimes, the struggle is not so obvious. In a forest, seedlings that are surrounded by large trees will not survive, because they cannot get enough sunlight.
Successful Reproduction
If an organism "wins" the struggle to survive because of a genetic advantage, it must reproduce for that genetic advantage to be passed on to the next generation. No matter how advantageous a particular genetic variation might be, it will not act to cause a population to evolve unless it is passed on to the next generation.
Natural Selection in Action
One of the most common examples of natural selection is the story of the peppered moths in England. Peppered moths spend much of their time on the trunks of trees. The trunks of the trees in England before the Industrial Revolution were covered with lichens. Many of the peppered moths had light colored wings, and they were able to camouflage themselves because their wings looked a lot like the tree trunks covered with lichens. Here is a picture of a light colored peppered moth.
During the Industrial Revolution, huge amounts of soot were poured into the atmosphere from factories. The lichens, unable to cope with the pollution, died. The tree trunks became covered with this soot, and over a relatively short period of time the tree trunks became much darker. Suddenly, the light colored peppered moths were at a disadvantage. The light coloring that allowed them to blend in with the tree trunks now made them stand out like a sore thumb, making them easy targets for birds.
Not all peppered moths had light colored wings. Some had dark colored wings. Here is a picture of a dark colored peppered moth.
Because of the changes to the color of the tree trunks, the dark-winged moths now had the camouflage advantage. The number of dark moths increased and the number of light moths decreased. Before the Industrial Revolution, about 98% of the peppered moths had light wings. After the Industrial Revolution, about 98% of them had dark wings.
The story of the peppered moths does not end here, though. In modern times, we have learned to better control emissions of soot from factories. As the air has become cleaner, so have the tree trunks. With reduced pollution, the lichens are coming back, too. So, the light-winged peppered moths are better able to camouflage than the dark-winged peppered moths. With these environmental changes, the population of light-winged moths is increasing and the population of dark-winged moths is decreasing.
So what happened? There was a population of peppered moths. Within that population was genetic variation. One of the genetic differences between the moths was the color of their wings. When the environment included tree trunks that were covered with lichens, the light-colored moths had an advantage in the struggle to survive, and the dark-colored moths had a disadvantage. Birds were able to more easily see the dark-colored moths, and the dark-colored moths got eaten before they could reproduce. The light-colored moths, because they were harder to see, successfully reproduced in higher numbers, passing their DNA on to the next generation. But when the environment changed, the light-colored moths were at a disadvantage compared to the dark-colored moths. Now, it was the light-colored moths that were being eaten by birds, and the dark-colored moths that were successfully reproducing, passing the DNA that produces dark wings on to the next generation.
You can play several different computer simulation games that show how natural selection works. Each of the links below will take you to a different game. Some of these links use Shockwave Flash, so they might not all work if you are using an Apple product.
Peppered Moth Evolution Simulation
Natural Selection Simulation from Glencoe
Who Wants to Live One Million Years?
The Evidence for Evolution
Darwin published his theory in 1859. Back then, our knowledge of biology was very different. We knew that organisms inherited traits from their parents, but we didn't know how. We didn't know about chromosomes or DNA. We had microscopes that could help us to see cells, but we couldn't see the organelles inside of them, and we didn't know how those organelles keep cells alive. We hadn't yet invented Carbon 14 or other radioactive dating, so we couldn't tell how old bones or fossils were. But with each passing year, and new technologies are invented and new discoveries are made, the evidence supporting Darwin's theory grows. Let's have a look at some of this evidence.
Fossils
Fossils are typically found in sedimentary rock. Sedimentary rock is formed when particles of sand or soil are left behind. These particles form in horizontal layers. Newer layers of rock form on top of the older layers. It's a lot like the way garbage is piled in a dump. The oldest garbage is at the bottom of the dump and the newest garbage is on top. When we look at fossils, we see that newer fossils more closely resemble present day organisms. Older fossils, from deeper in the rock layers, are less similar to present day organisms. The fossil record provides clear evidence that organisms have changed over time.
Comparative Anatomy
When you compare the anatomy of different organisms, you find that there are many physical similarities among related organisms.
In this picture from your textbook, you can see that the bone structure of a human arm, cat leg, dolphin flipper, and bat wing are all similar to one another. This is evidence that humans, cats, dolphins, and bats share a common ancestor. Over time, as the functions of these structures changed (arms, legs, flippers and wings do not have the same functions as one another), changes also occurred in the bone structures.
Comparing DNA
All organisms have DNA, and the structure of DNA is the same in all organisms. There is no difference between the chemical makeup of DNA in a human and the chemical makeup of the DNA in an amoeba. What is different is the amount of DNA and the actual sequence of the nucleotide bases (A, C, G, and T). By comparing the sequence of nucleotide bases found in the DNA of an organism to the sequence in another organism, scientists can see how closely related they are. Even though we humans all have a slightly different sequence from each other, the sequence is closer between two brothers than it is between two cousins. It is closer between two cousins than it is between two unrelated people. It is closer between two unrelated people than it is between a human and an ape. The sequence in a human is closer to an ape than it is to a giraffe. It is closer to a giraffe than it is to a lizard. It is closer to a lizard than it is to a sunflower. In other words, the more closely the sequence of nucleotide bases matches between two organisms, the more closely they are related.
How a New Species is Formed
If you've paid careful attention in class, understood the videos we've seen, and have a basic understanding of heredity and genetics, you should be able to see how natural selection can change organisms over time. But how do new species arise? How can there be such a wide variety of organisms? The answer is that evolution by natural selection can sometimes change two populations of an organism so much that they can no longer mate and produce fertile offspring. In other words, they are not the same species anymore.
This process, called speciation, has three components to it. Let's take a look at each of these.
Separation
Speciation can occur if two populations of a species become separated from one another. The finches that garnered so much of Darwin's attention are an example. At one time, all of the finches lived together in Ecuador. Some finches, though, made it to the Galapagos Islands. They became separated from one another. Separation can occur in different ways. Separation can occur when a lake, canyon, or mountain range separates a population. Populations can also become separated due to the movement of tectonic plates.
Adaptation
When a population becomes separated, environmental conditions may not be the same. Let's use Darwin's finches again as an example. The environment in the Galapagos was different from the environment in Ecuador. The differences in DNA that provided an advantage in Ecuador were different from the differences in DNA that provided an advantage in the Galapagos. So, the two populations of finches evolved, or changed, in different ways.
Reproductive Isolation
Two separated groups can become very different from one another over time. Sometimes, even if the groups are reunited, they have changed so much from one another that they can no longer breed and produce fertile offspring. Once this happens, they are no longer considered to be the same species. The illustration below from your textbook shows how speciation may have occurred with Darwin's finches.
The Role of Mutations
Mutation. It's kind of a scary word. There aren't many people who would want to be known as a mutant. Really, though, all of us are mutants. Each one of us has DNA that is slightly different from everyone else. Those differences are the result of the DNA that we inherited from our parents or the result of a mutation. A mutation is just a change on our DNA. It might have happened when a mistake was made while DNA was being copied. It might have happened when we were exposed to a chemical, to radiation, or even to sunlight.
There are two kinds of mutations, and they work very differently from one another. You may remember that we discussed the way that a mutation can lead to cancer. In the animation that we viewed in class, a mutation occurred in a cell as it was copying its DNA just prior to mitosis. The "broken" DNA became part of the daughter cells, the daughter cells transferred the mutation when they underwent mitosis, and so on. In cancer, cells reproduce wildly. Instead of reproducing only to replace dead cells or for normal growth, they keep reproducing over and over and over again. Eventually, these abnormal cells develop their own blood supply, starving the surrounding tissues of nutrients. This mass of "crazy" cells is known as a tumor. If a cell from a tumor makes its way into the blood stream, it can travel to other parts of the body and start new tumors. No, not a good thing. This kind of mutation, though, cannot be transferred to offspring.
The type of mutation that is important to evolution is a mutation that takes place in the sex cells -- the sperm or eggs -- because those are the cells that contain the DNA that will be passed on to the next generation. Remember that a mutation is just a change in the sequence of the nucleotide bases that make up DNA (cytosine, adenine, guanine, and thymine). Sometimes, the mutation can be the result of a mistake in copying the chromosomes during meiosis. Or, the mutation might be the result of a physical change to a chromosome like a breakage.
Some mutations cause very serious problems. If the mutation is serious enough, a fertilized egg may not grow into an embryo. Or, it may grow into an embryo that dies before it is born. It might produce an offspring with a serious physical or mental deformity. On the other hand, a mutation might have no effect at all.
Links to Videos
Here are some links to videos that will help you to understand evolution and natural selection. We watched some of these in class, and others are closely related to topics we covered in class. There are also two videos from Khan Academy. The Khan Academy videos will help you to understand some of the information in the other videos, and the other videos will help you to understand the Khan Academy videos. Sorry, but that's just the way science works.
Khan Academy - Genetic Variation
PBS - Isn't Evolution Just a Theory?
What is a theory? Many people will say that a theory is an educated guess. A theory is much more than an educated guess! The scientific evidence for evolution is strong, and it grows stronger the longer we study it. This video explains how scientific theories are formulated and why the "just a theory" criticism doesn't hold water.
How did Darwin go from an ordinary naturalist to the creator of the most important idea in biology?
PBS - How Do We Know Evolution Happens?
Is there really proof of evolution? Is the proof easy to understand? The answer to both questions is yes.
PBS - How Does Evolution Really Work?
How do new species arise? The answer to this question is key to understanding evolution. This video will help you to understand.
Have we evolved the same way that other organisms have?
Learn about how our brains evolved.
Many people wonder how a structure as complex and "perfect" as the eye could be the result of the random mutations that, according to Darwin, cause organisms to evolve. Some answers can be found in this excellent video.
PBS - Why Does Evolution Matter Now?
Why does understanding evolution matter so much? Just like the poisonous newts caused evolutionary changes to take place in the garter snakes, things that humans do are causing evolutionary changes to take place in bacteria and viruses. Some of these changes could endanger our existence.
This is one of the videos that we watched in class. One proof of the genius of Charles Darwin is that he formulated his theory without any knowledge of chromosomes or DNA. Learn how modern scientific techniques are providing more proof of the mechanisms of evolution.
PBS - Intelligent Design on Trial
Is the idea of Intelligent Design consistent with science? Does Intelligent Design help to answer some of the questions that Darwin's Theory of Evolution hasn't answered? This excellent PBS documentary will help you to understand the ongoing debate between Creationism and Evolution.
Are all eyes the same? Why do flies have eyes that are different from ours if they are both used for the same thing? This is a great video that explores the ways that eyes have evolved.
This is the Bill Nye video that we watched in class.
A video from our friends at Crash Course Biology.