The theory of evolution essentially explains life as we know it. But how well do you know the theory? In this article, I explain the fundamental principles behind evolution as well as its history in scientific thought, and hopefully provide some insight into man's origins and oversights.
Introduction
Over the last 150 years, the concept of evolution has changed from a novel and speculative idea into one of science's most important and comprehensive theories. Like evolution itself, its integration into the mainstream of knowledge has been both punctuated by new discoveries and a process of gradual acceptance. Modern evolutionary theory combines the insights of Darwin and Mendel with molecular genetics to show us why evolution happens, and uses natural selection and speciation to demonstrate how it works. But, even with this wealth of information, the theory is far from uncontested. Evolution continues to play a role in the age-old tension between science and religion, and many of its intricacies remain a mystery.
What is Evolution?
The term “evolution” generally refers to a gradual change towards increasing complexity, but has become specifically associated with the process of biological evolution. In this context, evolution is a change in the heritable traits of a population over successive generations. While the idea is often interchanged with that of natural selection, the terms are not synonymous, Natural selection serves as one mechanism of evolution, but must act on the necessary forces of genetic variation in order to bring about its change. Evolution encompasses the entire process, from universal common descent, through mutation, genetic drift, adaptation, natural selection and speciation.
Types of Evolution
It can also be broken down into subcategories. Convergent evolution, for instance, refers to the same traits that have developed independently in different species due to similar environmental pressures. Another type, co-evolution, exists when the evolutionary history of two species is closely entwined, such as barnacles on whales. A third, adaptive radiation, occurs when one ancestral species quickly branches into many new species to fill an environmental niche. The marsupials of Australia or birds of New Zealand effectively demonstrate such a scenario.
History of Evolutionary Thought
When evolution exploded into the scientific community in 1859, however, it was more simple. Charles Darwin's On the Origin of Species Through the Means of Natural Selection focused solely on natural selection. Based on twenty years of observations, notably in the Galapagos Islands, Darwin stated that all individuals compete for limited resources within their environment. Some possess slightly different traits that give them an advantage in obtaining these resources, and these “evolutionarily fit” individuals will have a higher success rate in living to reproduce. When they do, those traits will then be passed to some of the offspring, which will continue the cycle by dominating individuals lacking the beneficial trait. Eventually, the frequency of the trait will increase throughout the entire population, leaving the entire group better adapted to its environment and potentially on its way to being a new species.
Darwin's theory of natural selection aptly described the mechanism for evolution, but could not explain how the determining differences in traits came to be. For this, the scientific community waited until the 1930s, and breakthroughs in genetics based on the studies of Gregor Mendel. In the 1860s, Mendel studied the properties of inheritable traits in pea plants. He discovered that traits were discrete (that is, either wrinkled or round, not a combination) and passed from one generation to the next in a distinct pattern. From these beginnings, 20th century biologists eventually gained a greater understanding of genes and their role in evolution. The combination of this ever-increasing knowledge of genetics and Darwin's ideas constitutes the current theory of evolutionary biology, the “modern synthesis,” or neo-Darwinism.
Neo-Darwinism
Neo-Darwinism begins with genes, which are integral to the evolutionary process. Changes in alleles (the various types of genes, i.e. the “blonde” or “brown” types of the hair color gene) provide the variation upon which natural selection acts. These changes occur in three primary forms. The most common is mutation, in which a part of the cell's genetic material (the DNA or RNA) is altered. This may take place as a “copying error” during cell division, or due to external sources such as carcinogens, radiation, UV light, chemicals or viruses. While most mutations are neutral and have no effect on the individual, some cause alterations to its functioning or appearance that may be either harmful or advantageous to its survival. Recombination, another mode of genetic change, effects similar results. In sexual organisms, alleles from both parents are shuffled in the offspring, producing new combinations and therefore new traits in the new generation. Finally, genes are occasionally altered by horizontal gene transfer. In this process, organisms transfer genes to individuals that are not their offspring, generally through viruses or bacteria. Recently perceived as a major force in cell evolution, the prominence of horizontal gene transfer has reshaped the concept of the “tree of life,” and instead promoted the idea of an interconnected web.
