What is Natural Selection

What is natural selection? Darwin’s main contribution was not to establish that evolution occurs, although he helped in that regard. His great achievement was his theory of natural selection. In a nutshell, Darwin’s theory runs as follows:

• There is heritable variation in a population.

• More individuals are born in a population than can survive.

• Individuals that do survive and reproduce are not a random subset of the population. These individuals possess traits that somehow make them better at surviving and reproducing in a particular environment.

• The heritable adaptive traits would become increasingly represented in a population over time, and thus shape the phenotypic characteristics of that population.

There are a couple of additional points. Natural selection can only act on existing heritable variation. If there is no variation for a particular trait, then selection simply cannot do anything with it. This is why the generation of variation in the context of mutation and genetic recombination is so central to our understanding of evolution.

Another important point is that although natural selection acts on individuals, its evolutionary consequences occur in populations. Individuals do not evolve. Evolution is measured as change in the average characteristics of individuals within a population. I think this, in a nutshell, answers the question.

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Darwin and Evolution - Part 2

Let's continue here what we left in our left article about Darwin and evolution. It was his opportunity to serve as the naturalist on the Beagle that provided Darwin with his first insights into how evolution might work. In fact, it was this voyage that changed his worldview and convinced him that evolution occurred. Darwin began to observe and document the kinds of evidence for evolutionary change that I mentioned in my previous post: geological change, homologies among species, relationships between fossil forms and living forms.

Particularly illuminating to Darwin were groups of animals and plants occurring on islands. He would find that if you went to a group of islands, you will often find, on different islands, species that were similar enough to obviously be related to each other, and yet different enough to be considered other species. Furthermore, these species occurring on islands would often resemble a species occurring on the mainland.

Darwin’s Finches

The most celebrated example of this phenomenon is a closely related group birds species, now collectively known as Darwin’s finches, that are found on the Galapagos Islands, about 700 kilometers off the coast of Peru. Each island has a different species that appears to be uniquely suited for that habitat. These are really representative of Darwin and evolution itself.

In particular, these species differed largely in the size and shape of their beaks. They used their beaks in different ways to feed on different kinds of material. For example, there are some species with very large beaks that appear to be suitable for crushing large and hard seeds. Other species have smaller beaks that are more suitable for handling smaller seeds. The size and shape of the different beaks did correspond to what Darwin observed about their feeding ecology.

Darwin wondered how is it possible that there are different species on islands that were similar and yet clearly different, and that the differences related so clearly to the environment in which those species lived. This pattern made perfect sense to Darwin if the various species were all descended from the same common ancestor. This was presumably and ancestor that had come from the mainland. Eventually, all of the island species had gradually evolved and diversified in a way that matched the habitats in which they live. Darwin called this pattern “descent with modification”.

Working on the Species Problem

Darwin spent five years traveling around the world, collecting animals and plants and making observations. He returned from his trip in 1836, loaded with specimens and notebooks of his observations. He spent more than 20 years working on what he called the “species problem”. During the time that he was working on this problem, he spent most of his time in England. He nonetheless continued to amass evidence by talking to naturalists who were collecting plants and animals from other parts of the world, and specially by looking at the effects of domestication on species.

These observations convinced him not only that evolution occurred, but also suggested a particular mechanism by which evolution could occur. Darwin proposed this mechanism, the theory of natural selection, as early as 1844. He wrote a paper, but he never published it. He was urged by friends and colleagues to publish it, even by his wife, but Darwin preferred to perfect his ideas. He didn’t want to propose this idea until he had amassed so much evidence that the idea could simply not be refuted. So, he spent another ten or more years revising his ideas, collecting more specimens, evaluating more data.

The Publication of the Origin

Darwin hand was forced, however, in 1858, when he received a manuscript from Alfred Russell Wallace, a well-known naturalist. Wallace’s manuscript essentially presented the same idea of natural selection that Darwin had been working on for 20 years. Wallace asked Darwin to submit this manuscript he sent for publication.

I could not imagine what Darwin felt at that moment. He wrote to his friend Charles Lyell asking him for advice about what to do. Lyell arranged for Wallace’s paper, and an excerpted synopsis of Darwin’s 1844 manuscript to be published simultaneously, giving credit for the discovery of the theory of natural selection to both men.

Darwin then quickly finished the book-length version of his ideas, which was published the following year, 1859, with the title: “On the Origin of Species by Means of Natural Selection”.

What’s in On the Origin of Species

The essential observations behind the theory of natural selection are very straightforward. Darwin was struck by three things. First, he was struck by how much variation he observed among individuals of the same species. All individuals of the same species look alike, but no two were exactly alike. Darwin also recognized that some of this individual variation is passed on from parents to offspring. He observed that traits are heritable. Darwin didn’t know the mechanism responsible for this, but he argued that a mechanism must exist. We today know that DNA is the molecule that holds information in living things, how it is replicated, and all the stuff that makes it much clearer to us.

Darwin’s next important observation was that most species produced more offspring than ever survive. In this, Darwin was influenced by the writings of Thomas Malthus, who was an economist. Malthus wrote an essay arguing that much of human suffering was inevitable as the result of the fact that human population would always grow faster than the available resources needed to support it. In other words, the size of the human population is limited by competition. Darwin saw that this was true in animals and plants as well.

These observations led Darwin to the following conclusions. First, given that more individuals are born in a population that can ever live, there must be competition for limited resources, so that only some of those individuals are going to survive and eventually reproduce. Second, because individuals in a population differ in their characteristics, not all individuals are expected to be successful in this competition. Individuals with the more favorable variations would be more likely to survive and reproduce. Therefore, these naturally selected individuals, as Darwin called them, would contribute more offspring to later generations.

Finally, if the traits that contributed to the success of the parents are heritable, then over time, there would be more individuals possessing those traits. We call traits that evolved this way “adaptations”. Over time, natural selection is thought to cause populations to change their characteristics in such a way that would increase adaptation to the prevailing environment.

The first edition of Darwin’s book was published in 1859. All 1000 copies were sold on the first day. People knew this idea was coming and were eager to digest it. Despite the original controversy that it caused, Darwin’s work established the fact that evolution occurred. Darwin came to be considered one of the most famous and celebrated scientists of his day.

By the end of the century, pretty much every biologist accepted the idea that evolution occurs.

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Darwin and Evolution

The words Darwin and evolution are nowadays associated in our minds. Nowadays,also, biologists take for granted that evolution occurred, but that wasn’t the case 150 years ago, when Charles Darwin introduced his ideas. As a boy, young Charles developed a keen passion for nature. He was the kind of kid who loved to walk in the woods, collect bugs, go hunting and fishing. He generally spent time learning about different kinds of plants and animals.

When he was only 16, he was sent to medical school. He found it “distasteful” and soon left the university without a degree. It wouldn’t do for a young man of Darwin’s social status to not study for some career. So, Darwin enrolled in Cambridge University to study theology, with the goal of entering the clergy. The fact that Darwin went to complete a bachelor’s degree in theology may sound a little surprising to us, but it isn’t really. The study of natural history in the early 1800’s was a monopoly of the clergy. Biology was done in what was called natural theology, describing nature in order to more fully appreciate the glory of God. Some of the most important natural historians of the time were clergymen. Mendel himself was a monk.

This seemed to Darwin to be a good way to pursue his true passions. Darwin excelled at his studies. After graduating he was offered a unique opportunity, he was asked to serve as the official naturalist on a five-year long voyage around the world on the Beagle. Darwin’s job was twofold; first, he was to provide interesting conversation to the ship’s captain for five years. Second, and more importantly, he was to acquire and catalog plant and animal specimens from every place the ship visited. This voyage changed forever the history of Darwin and evolution.

When Darwin started his voyage around the world, a long held view in science, and one that Darwin himself almost certainly subscribed to, was that all organisms were formed by a special creation, much as it is described in the book of Genesis. More importantly, it was generally held that species remained immutable throughout all time. This view isn’t just a Christian idea, some Greek philosophers talked about evolution, but Plato and Aristotle argued that species must be immutable. The idea that species living today were completely unchanged throughout time had been a dominant view in western culture for several thousand years.

Early Evidence for Evolution

Early in the 1800’s, there were a few scientific findings that began to suggest this view of immutability of species was not entirely correct. There were three general kinds of evidence that were challenging to the idea of immutable creation. The first sort of evidence came from the field of geology. Creationist views of the world argued that the Earth was relatively young. Archbishop Ussher calculated that the Earth was created in 4004. Geologists who studied landforms, however, saw evidence that convinced them that the Earth had to be a lot older than this. Many people today still challenge these well established views, as well as Darwin and evolution, arguing that Ussher's date was right.

Geological analysis showed that the Earth might be millions of years older. Second, geologists saw that landscape features had obviously undergone many radical transformations. You only have to drive along an interstate highway to see the folding in the rocks. Third, geologists began to realize that physical forces at work in nature today could explain the transformations in landforms that must have occurred in the distant past. For example, they saw how erosion, if given enough time, could lead to landscape transformation, such as the creation of a canyon.

These ideas were developed completely by the leading geologist of Darwin’s day, Charles Lyell, who in the 1820’s formally developed the idea of geological gradualism. The theory of gradualism argued that large-scale changes in the geological features of the Earth could be explained by the gradual accumulation of many small changes over a very long period of time. Charles Lyell theory and his book had a profound influence on Darwin and evolution.

The word evolution simply means change. What geology did was to show that the physical world, at least, could have changed over a long period of time.

Homologues and Vestigial Structures

Another kind of evidence that began challenging the immutability of biological species came from the work of comparative anatomists. At the time, biology was largely a descriptive enterprise, involving the collection, dissection and description of different kinds of plants and animals. In the process of doing this kind of description, anatomists noticed that very different-looking animals shared some of the same basic parts. For example, if you look at the forelimbs of different kinds of mammals, it is easy to see that each animal has one with a quite different shape, but they nonetheless share components. Despite their differences in shape and function, if you dissect these forelimbs, you can see that they include similar arrangements of bones, tendons, muscles and so forth.

These structures are different in each species, but they share sufficient similarities to appear as though they had been modified from some original common form. We refer to anatomical features having these kinds of similarities as homologues structures. These kinds of homologues structures could be best explained if living forms were not static, but instead had been transformed from some common form that was shared. That is, they are best explained by darwin and evolution.

An even more problematic case for immutability was the appearance of structures that had no apparent function at all, the so called vestigial organs or structures. An example of this might be the tiny pelvic bones that are still found in whales. If species were the result of a single perfect creation event, then why should such structures exist at all? A simpler explanation would be that species had changed over time in such a way that previously useful structures had lost their usefulness.

What About the Fossil Record?

Another sort of evidence that species were not immutable came from the branch of science we call paleontology today. Paleontologists are people who study fossils. In the early 1800’s, paleontologists began to describe fossils of species that were no longer living on the present day Earth. The existence of these fossils made it obvious that species weren’t constant.

There was another observation from the field of paleontology that was hard to explain away. It was obvious that some fossil species were similar enough in their anatomy to be related to living species, while at the same time differed enough to clearly have to be classified as a different species. It was possible to find series of fossil species occurring at different times in the fossil record which seemed to be connected, from one ancient form that looked one way, to one that existed today, with a number of intermediates in between. This was called the Law of Succession, and was consistent with the ideas of darwin and evolution.

These kinds of observations, coming from geology, comparative anatomy and paleontology were much discussed in the early part of the 19th century. Some scientists were beginning to suspect and even suggest in writing that species were not immutable. The idea that evolution might occur was beginning to be accepted. What was not at all clear was how evolution could occur. What would be a mechanism that could account for the evolutionary transformation of species?

Darwin’s main contribution, as it turns out, was not to suggest that evolution occurs, this was already out there. His real contribution was to understand the mechanism by which evolution could occur, which was embodied in his theory of natural selection. We'll see his theory in more detail in the next part of this article: Darwin and Evolution, Part 2.

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Darwin’s Theory of Evolution

Nowadays, biologists take for granted that evolution occurred, but that wasn’t the case 150 years ago, when Darwin’s theory of evolution was introduced. Evolution is a simple and obvious idea if you think about it, but we needed Darwin to discover it. This series of articles covers much about Darwin’s life and work.

1. What Was There Before Darwin: We are particularly interested in our own origin. All cultures have their creation myths. Since the Greeks started doing science, however, some men tried to give a rational explanation to the origin of life’s diversity. Let’s review what they had to say.
   
   
2. De Maillet's Theory of Evolution: Among the earliest people to suggest that life had developed from simple to complex forms was Benoît de Maillet, who lived from 1656 to 1738. He realized his ideas were over the top for his day, so, he didn’t just come right out and declared the evolution of life to be his view.
   
   
3. Darwin and Evolution: As a boy, young Charles developed a keen passion for nature. He was the kind of kid who loved to walk in the woods, collect bugs, go hunting and fishing. He generally spent time learning about different kinds of plants and animals. When Darwin started his voyage around the world, a long held view in science, and one that Darwin himself almost certainly subscribed to, was that all organisms were formed by a special creation, much as it is described in the book of Genesis.
   
   
4. What is Natural Selection: Darwin’s main contribution was not to establish that evolution occurs, although he helped in that regard. His great achievement was his theory of natural selection. We call natural selection a theory, but it is a testable theory and has been tested many times.
   


5. Examples of Natural Selection: Some examples of natural selection to make it clearer how it works in nature.

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Theories of Origins Before Darwin

What were the theories of origins before Darwin? What was there before Darwin? What did people think about the origin of life and the different species on Earty? One thing characterizes people, as Descartes would say, they think. As thinking creatures, we have always wondered about how the universe and things in it originated. We are particularly interested in our own origin.

The first chapter of Genesis contains the Christian creation account. It tells of God creating the heaven and the Earth, plants and animals, and then man in God’s image. All in six days. The Bible doesn’t state when this creation occurred, but most early Christians probably assumed that this did not occur too long ago. In the 1600’s, the Anglican bishop James Ussher fixed the date of creation at 4004 B.C.E. This is the established biblical view that continues to the present.

The Early Scientific Accounts of Origins

Over the past 2000 years, this creationist account did not exist alone within the western tradition. Religious accounts of origins, at least for the past 2000 years, have competed with scientific accounts of origins. Science began with the Greeks, about 600 B.C.E. At this time we find the firsts scientific explanations for natural phenomena.

Although many Greeks retained religious theories about nature founded on revelation or mythical stories, some philosophers proposed materialistic explanations founded on reason. What do I mean by materialistic? This means that they explained natural phenomena without recourse to God or the supernatural. These philosophers said that natural phenomena can be explained as the result of physical matter moving in accord with natural law, with God, at most, as the remote creator of the primordial matter and the laws of motion. We find this sort of account in Plato, for whom God created the primordial matter and its laws, and then left it operate.

Biological origins posed a particular puzzle for Greeks who tried to devise purely materialistic explanations for natural phenomena. Biological organisms, people specially, seemed much more intricate and intelligently designed than just rocks or mountains. They seemed created, and creation implies a creator.

So, to explain the origin of biological organisms, early natural philosophers, like Anaximander and the so-called atomists, proposed crude theories of evolution. They are not very detailed, but they had the idea that there was some sort of spontaneous generation of life and somehow species could evolve over time. They weren’t worked out very well.

Aristotle critiqued these ideas. Aristotle himself was an atheist, and first and foremost, a biologist. He was a very avid observer of life, particularly of fishes. Based on his close study of animals, Aristotle defined a species as a breeding group. A group of particular animals or plants that can breed, and produce offspring that eventually could reproduce. He concluded that species were fixed.

Rejecting both creation and evolution, Aristotle simply saw the species as eternal. They always existed. Later Christian philosophers tried to integrate Genesis with Aristotle. They typically viewed each species as created by God in the beginning, but then, using Aristotelian authority, asserted that these species remained fixed for all time in a perfect (albeit fallen) creation.

This was the dominant view for a millennium in the West. It began to break down, though, when religious authority began to break down.

Deist and Atheist Accounts

The breakdown of religious authority finally occurred during the Enlightenment, in the 1700’s. Notions of evolution began creeping back in. This happened particularly in France, where natural philosophers again struggled to devise purely materialistic explanations for life. Seeking to push God back to the beginning, deists proposed a variety of ideas. They proposed that the solar system was created not by God, but rather a comet once hit the sun and knocked off a bunch of matter, which separated, each piece becoming a planet.

They also proposed ideas for the origin of species. They said that the tremendous array of species evolved from a few common ancestral types. Some of the French natural philosophers were even more atheistical. Denis Diderot, for example, a committed materialist, proposed that all living forms developed by random chance mutations from spontaneously generated organisms.

Probably the most influential natural philosopher from this period was the astronomer Pierre Laplace, who proposed a purely materialistic explanation for the origin of the solar system. He said that the solar system was once a big rotating gas nebula, and as it rotated, centrifugal and centripetal forces would pull in matter to the center, which became the sun, but as it pulled in, it left little blobs of material that collapsed into the different planets. This was called the nebular hypothesis.

When Laplace described his theory to Napoleon, he was asked “how does God fit into it?”, Laplace famously responded “I have no need for God in my hypothesis”.

All the ideas that we’ve gone over were highly speculative and were driven more from philosophy than empirical scientific research. There were a few discoveries at the time, though, that reinforced these ideas.

For example, Abraham Trembley detected that polyps, which are very simple sea creatures, could regenerate. By cutting them into pieces they regenerated the whole. They could be flipped inside out and still operate. People saw this as “almost spontaneous generation”. Philosophers took this as scientific evidence for their speculations.

Overall, however, the empirical research during this period cut the other way. Even if these ideas were speculated about, when people actually did the experimental and observational work in nature, most of them opposed the evolutionary ideas. The generation after the Enlightenment reacted against the speculative nature of evolutionary ideas. They returned to creationism, although not the creationism of the Bible, but a creationism based on scientific evidence. I will discuss this in my next post, when we see how Georges Cuvier founded modern biology.

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