You can't talk about evolution without talking about Charles Darwin (1809-1882), a would-be physician and theologian whose fascination with natural history and geography led him to accept a position as gentleman's companion to the captain of the HMS Beagle, a ship bound for South America with the purpose of mapping the area and sending plant, animal, and fossil specimens back to England. The voyage lasted five years, from 1831 to 1836.
Several things led Darwin to speculate about the changes that might occur in species over time: the diversity of life he observed on his voyage, the geographical patterns whereby different yet obviously related species were found in close proximity to one another, and the fossils he collected that made it clear present-day species weren't the ones that had been present in the past.
Darwin returned to England in 1836, already well known in the scientific community for the specimens and detailed notes that he had sent back. By 1838, Darwin had developed in more detail his theory of how gradual changes resulting from natural selection could result in changes in existing species as well as the formation of new ones. Over roughly the next 20 years, Darwin continued to develop and refine his ideas. In 1859, he published his seminal work, On the Origin of Species, which laid out the foundations of evolutionary theory. The following sections hit the highlights of Darwin's ideas. His other works include The Descent of Man, and Selection in Relation to Sex (1871) and The Expression of the Emotions in Man and Animals (1872).
Find the title On the Origin of Species a bit cryptic? Roll the full title around your mouth for a while: On the Origin of Species by Means of Natural Selection, or the Preservation of Favoured Races in the Struggle for Life. The shorter version may not be as descriptive, but it certainly is easier to remember — and say! You can read it (and all of Darwin's other works) at http:// darwin-online.org.uk/.
Darwin didn't use the word gene; instead, in his work, he referred to characters. Yet because his ideas focused on heritable characters (that is, those that can be passed from parent to offspring), his "characters" are directly linked to genes.
One of Darwin's big ideas was what he called natural selection, the mechanism that he proposed to explain what he called "descent with modification" — that is, changes in an organism through subsequent generations. (Today, we'd say that natural selection explains how gene frequencies could change over generations.) This big idea, explained in depth in Chapter 5, is both remarkably insightful and remarkably simple, which explains why it's stood the test of time.
Basically, Darwin recognized that some characters get passed from one generation to the next and others don't. What he wanted to understand was how descent with modification could have occurred. What was the underlying driving force? He concluded that the driving force was the process of natural selection: Not all individuals in a given generation have an equal chance of contributing to the next generation. Some are selectively favored; some are selected against.
Darwin surmised that natural selection worked the same way as the process of artificial selection used in animal husbandry and agriculture:
1 Artificial selection: Since before Darwin's time, people have been selectively breeding animals and plants: chickens that lay more eggs, cows that make more milk, pansies that are brighter and last longer . . . and the list goes on. Essentially, humans have been pretty apt hands at spurring evolution in agriculturally important plants and animals. We decide which genes are more likely to make into the next generation. The cows that produce more milk are the ones that we selectively breed to produce better dairy cows; the ones that make less milk, we eat. As a result of the choices we humans make as selecting agents, we can dramatically alter in a relatively short period the characteristics of the organisms we breed.
1 Natural selection: Darwin realized that if humans, by the process of artificial selection, could create such major differences over the extremely short period of time, then the natural environment, acting over a much longer time scale, could have produced much larger changes. Darwin called his process natural selection because the natural environment, not humans, was the selecting agent.
In artificial selection, farmers and breeders determine which characters they like and work to propagate in their produce and livestock. In natural selection, the same type of selection occurs, but the selecting factor isn't man, but nature, or the environment in which the organism exists. To help you understand the difference between artificial and natural selection, consider the cow. In the barnyard, farmers selectively favor the cow that makes the most milk; in the wild, natural selection favors the cow that can make enough milk to feed its calf and still do all the other things the cow needs to do to survive on its own.
Whether natural selection favors an individual is a function of the individual's particular heritable characters. Some heritable characters increase the probability that the individuals containing them will contribute to the next generation; some characters decrease the probability that individuals will contribute to the next generation. What all this means is that organisms in the first category reproduce more than do the organisms in the second category. That's what makes one generation different from the next, the next different from the one that follows it, and the one that follows it different from the one that comes later . . . and so on and so forth, ad infinitum.
In case you're curious: Survival of the fittest
Although he didn't coin the phrase survival of the fittest, Darwin did make it a household term. Many people assume (erroneously) that it means the natural order mandates that the strong survive and the weak die away. But to Darwin and other evolutionists, survival of the fittest is simply synonymous with natural selection. In other words, those organisms that possess selectively favored heritable characters are the ones that pass their genes into the future with the most success.
This sidebar marks the first and the last time you'll see this phrase in this book. Why?
I It's problematic. The phrase doesn't clarify the concept Darwin was trying to explain (although he no doubt thought it did; otherwise, he wouldn't have used it). To express the concept more clearly, Darwin could have used the term survival and differential reproduction of the fittest, but that's just not as sexy.
I It doesn't make much sense semantical^. In beginning a study of evolution, students often say, "Well, if evolution is survival of the fittest, and the fittest are the ones that survive, that seems pretty circular." Indeed, it is.
I Even evolutionary biologists never use it.
Don't let any of this get in the way of your developing an understanding of the term fitness, however; that word is crucially important to understanding evolution. Head to the section "How 'fitness' fits in with natural selection" for details.
Here's an example: Imagine a population of lions. Half the lions have the work-hard-run-fast-and-catch-lots-of-gazelles character. The other half have the sit-around-and-be-lazy character. It's tough in the Serengeti, and only the lions with the work-hard-run-fast character manage to store up enough energy to reproduce and raise offspring successfully. If you reanalyze this population after a few generations have passed and find fewer lions with the lazy character, that's evolution driven by natural selection!
Darwin realized that because individuals differ in the characters they have, and because these differences affect their chances of survival and reproduction, some characters are more likely to get into the next generation than others. He also realized that as a result of this process, the frequency of characters changes over generations. Pass through enough generations, and the sum of all the little evolutionary changes may result in an organism that's evolved into an entirely different species.
Here's a quick example: Imagine you have two populations of the same animal. Each population lives in a different place, and the populations rarely interbreed. The selective forces in those two places — the combination of things we call the environment — is different. In one environment, it's good to have a long beak; in the other environment, a short beak is better. Other significant environmental differences exist as well. It's very wet on one side of the mountain range and very dry on the other, for example. In two such different environments, gene frequencies change in one way in the first location and another way in the second. Over a long period, the two populations become so different that they can no longer interbreed. They have become different species.
Today, scientists can identify all the stages of speciation in the natural world. They can find pairs of species that seem to have diverged from a single species very recently, and they can find pairs of populations that appear to be on the verge of becoming separate species. In some cases, the two populations are so close to becoming different species that all it would take is some minor habitat change to push them that last little bit and turn one species into two. For more detailed information about speciation, head to Chapter 8.
The idea of speciation got Darwin into a lot of hot water, and it's a hot-button issue today because it links organisms to common ancestors, which is all well and good for things like fish, oak trees, and invertebrates. But when you throw humans into the mix — whoa, Nelly. To read more about the conflict between evolutionary science and those who deny it, head to Chapter 22.
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