What's the Big Idea? (eBook)

Chapter 2

Understanding Evolution

Evolution explains the harmony in nature and the diversity of life on Earth; and the secrets of evolution are death and time. Enormous numbers of life-forms that were imperfectly suited to the changing environment did not survive. It took millions of years for a long succession of small mutations, that were by chance adaptive, to produce the plants and animals we see on the Earth today.

Descent with Modification

Evolution is descent with modification. Today’s species are modified descendants of ancestors that inhabited the world in days past. The pattern of evolution is seen in data from a myriad of disciplines, from geology and physics, to biology and chemistry. These disciplines provide us with the facts of evolution, which include observations of how the natural world has changed. The process of evolution refers to the mechanisms that cause these patterns. It is only the mechanisms by which evolution occurs that are theoretical.

Until the mid-1800s, most people believed that every animal on Earth was created as we see it today, and that species do not change. Like every scientist, Darwin’s theory built upon old ideas and missteps of others. He looked at converging evidence from different scientific domains, such as geology and biology, and he saw that there were many holes in the record. Darwin’s five-year journey aboard the HMS Beagle and his trip to the Galapagos Islands, off the coast of South America in the Pacific, provided him with unique observations of the diversity of life. Darwin realized that the finches in the Galapagos descended from one ancestral species, and their diversity resulted from adaptations to specific islands. Darwin considered that adaptation to a specific environment might explain the evolutionary process.

Divergent and Convergent Evolution

The Galapagos finches demonstrated the diversity of variations between closely related species. But Darwin observed another branch of evidence supporting the theory of evolution that came from similarities, not differences. Evolution is the descent with modification from an ancestral organism as its descendants faced slightly different environmental conditions. Depending on the environmental conditions, anatomical structures presented in a common ancestor can be harnessed for different purposes by different species. These body parts are called homologous structures. One example of homology is found in development, when every vertebrate embryo starts with a tail and a throat pouch. They turn into gills in fishes, and turn into ears and throats in humans.

But not all similar structures across species indicate descent from the same ancestor. Some body parts result from convergent evolution. Whereas divergent evolution describes how the same ancestor can produce different species with different traits, convergent evolution describes the independent evolution of analogous structures in different species. This is a function of adaptations to similar environments. In convergent evolution, similar traits come from different ancestors.

Evidence for Evolution

Homologous and analogous structures are one type of evidence for the process of evolution. A second domain in which evidence is found is the fossil record. Scientists look to the fossil record to find out what types of species roamed the Earth at different points in time. As more and more fossils were uncovered, the data provided support for evolution. There is vast amount of evidence showing that past species not only differed from current species, but also were progressively less complex the farther back one goes in time. Critics of the theory of evolution sometimes point to holes in the fossil record as evidence that the theory has yet to be proved. But in fact, the fossil record is a remarkably rich source of evidence supporting the theory.

Another type of evidence for evolution comes from the geographic distribution of species. By understanding continental drift and the movement of landmasses over time, scientists can predict where fossils of particular organisms can be found and how one species can give rise to others. This is why some species that live on different continents can share a common ancestor.

Theory of Natural Selection

Darwin observed that there are individual variations in a species in terms of hereditary traits, and that species produce more individuals than their environments can support. As a result, many of these offspring fail to survive and reproduce, creating competition for survival among individuals in a population. From these observations, Darwin inferred that those individuals whose inherited traits give them a better chance of survival in their environment tend to leave more offspring than others. Over time, this ability of certain individuals to survive and reproduce shifts the proportion of the desirable traits in a population, leading to the gradual accumulation of changes adapted to the environment.

Natural selection is the process by which individuals with certain inherited traits produce more viable offspring, which leads to a better match between the species and its environment over time. Darwin coined the term “natural selection” to distinguish it from “artificial selection,” which farmers and breeders had already been practicing for hundreds of years by the time he proposed his theory. Whereas breeders artificially chose animals with desired traits, and bred them together to change the frequency of those traits in the population, natural selection does not work with a purpose in mind.

Natural selection works on a population, not on an individual. That is, it is the population that evolves, not a single organism. An individual might have a hereditary trait that makes him or her more likely to survive, but it is only across generations that one can see the effects of natural selection.

The Problem of Improbability

Natural selection succeeds as a solution to the problem of improbability. Complex life cannot be explained by chance alone. But complex life cannot be explain by postulating intelligent design either, because it raises an even bigger question: Who designed the designer? Natural selection is the only solution.

It succeeds because it is a cumulative process. It breaks the problem of improbability up into small pieces. Each of the small changes involved in evolution is slightly improbable, but not prohibitively so. When large numbers of these slightly improbable events are stacked up in a series, the end product of the accumulation is improbable far beyond the reach of chance. The mistake of creationist is that they insist on treating the genesis of statistical improbability as a single, one-off event.

The point can be expressed in a parable. One side of a mountain is a sheer cliff, impossible to climb. But on the other side is a gentle slope to the summit. On the summit sits a complex device, such as an eye. The absurd notion that such complexity could spontaneously self-assemble is symbolized by leaping from the foot of the cliff to the top in one bound. Evolution, by contrast, goes around the back of the mountain and creeps up the gentle slope to the summit.

Genetic Variation

Evolution would not happen if every individual in a species were exactly the same. Individual variability is a prerequisite for natural selection to take effect. This variability needs to be tied to something that gets passed down from one generation to the next. This is called genetic variation. It refers to the differences between individuals that are tied to their DNA, or the genetic code. Out genes are encoded on a set of 23 unique chromosomes, which are strands of DNA that contain many genes. We have two copies of each chromosome, one from our mother and one from our father, for a total of 46.

Therefore, we inherit two copies of each gene, and these genes are the blueprints for all of the proteins that we need for our cells to function. These two copies are called alleles. They can either be the same (homozygous) or different (heterozygous). Scientists look for changes in the frequency of one allele or another over generations. They want to know if one specific gene becomes more frequent in the population over time. If it does, genetic drift takes place. It is caused by random sampling and can lead to changes in allele frequency. Genetic drift is another mechanism by which the individual can adapt to the changing environment.

Survival of the Fittest

Natural selection provides an explanation for the diversity of life and for the observation that species are adapted to their environments. It is important to understand that evolution by natural selection is not random. Mutations are random, but sorting by natural selection favors some alleles over others because the environment favors the attributes of some organisms over others.

Evolution by natural selection is sometimes described as the survival of the fittest. But it is important to remember that scientists are talking about fitness as it relates to the ability to produce more viable offspring. Greater fitness might be demonstrated by the ability of a moth to blend into a tree, thereby avoiding being eaten long enough to have babies. It might be demonstrated in the skill of a barnacle in obtaining a greater proportion of available food, thereby depriving its neighbors of adequate nutrition and preventing them from reproducing.

Sexual Selection

Another contribution of Darwin was his description of sexual selection and its effects on a species....