UNDER THE MICROSCOPE:TODAY MARKS the 200th anniversary of the birth of Charles Darwin. Darwin's discoveries had a profound influence on the modern world, more so than the ideas of any other scientist. Darwin's big discovery was natural selection, which explains design in the living world, the origin of species and is the driving force behind evolution. Darwin also explained that man is descended from an ape-like animal – a most uncomfortable idea at the time.
In today’s article, I will describe the theory that Darwin proposed and next week I will describe the influence this theory has on the modern world.
DARWIN WAS THE son of a wealthy physician. He studied medicine in Edinburgh for two years but wasn't interested and moved to Cambridge University where he graduated with a BA in 1831 in preparation for life as a clergyman. On impulse, he enrolled as naturalist on the voyage of HMS Beagle. On the five-year voyage (1831-1836) around the world, Darwin collected evidence he later used in formulating his theory of evolution through natural selection.
Darwin didn’t invent the concept of evolution. The principle of evolution – that living species change their characteristics considerably over long time spans – was widely accepted by naturalists at the time. Jean-Baptiste de Monet, Chevalier de Lamarck (1744-1829) proposed a broad theory of evolution in which simpler forms developed into more advanced forms by inheritance of acquired characteristics.
Most scientists and philosophers in Darwin’s day were Christians. A powerful argument for the existence of God, the argument from design, was developed by the Protestant clergyman William Paley (1743-1805). Paley argued that a designed object implies a designer and since nature is full of wonderful biological designs, eg the eye, there must be a super-intelligent designer, viz, God.
Darwin studied Paley’s argument as an undergraduate and was very impressed. He later destroyed Paley’s argument by showing how “design” in nature arises naturally and unconsciously through natural selection.
Natural selection was Darwin’s big discovery, and in the phrase “The Theory of Evolution through Natural Selection”, the word theory refers mainly to natural selection and not to evolution. Natural selection works as follows. Variation naturally exists in any biological population. Some variations are better equipped to thrive and breed than others and will therefore leave more offspring than their fellows, thereby ensuring that their desirable characteristics are passed on more effectively to the next generation. This “natural selection” automatically proceeds generation after generation automatically selecting those varieties best suited to the environment – “designed” to fit the environment. There is no need to invoke a supernatural designer.
THE FAITHFUL inheritance of characteristics is central to the working of natural selection. Darwin’s understanding of the mechanism of inheritance was wrong. The correct mechanism was worked out by the Augustinian monk Gregor Mendel (1822-1884), who showed that the expression of biological characteristics is controlled by “factors” that we inherit from our parents and these factors independently sort and segregate as they pass from generation to generation.
Today we call these factors genes. Chemically genes are made of DNA, long information-rich molecules composed of four different kinds of nucleotides, termed A, T, G and C. The information content of DNA is encoded in this four-letter alphabet organised into three-letter words.
DNA controls the day-to-day chemistry of our cells and is also the hereditary material passed from one generation to the next. Errors (mutations) naturally creep randomly into the information encoded in DNA and this new information is passed on to the next generation. Mutations are usually harmful to the organism and are weeded out by natural selection.
However, sometimes a mutation confers a new characteristic on an organism that is beneficial and such mutations are preserved and spread by natural selection. The combination of this gradual trickle of novelty through mutation in the genetic material, combined with natural selection, is sufficient to explain how organisms can change slowly but significantly over many generations and how new species arise. Evolution is often said to proceed through chance (mutations arising at random) and necessity (natural selection, a lawful process).
Life began on Earth about 3.8 billion years ago, probably as a simple single form, and it has been evolving ever since, eventually producing the myriad forms of life, including man, that today have colonised every environmental niche on the planet. Humans are descended from an ape-like ancestor – the human line of descent branched off from the ape line several million years ago.
THE THEORY OF evolution through natural selection is supported by a massive amount of evidence, including the fossil record, comparative anatomy, patterns of embryonic development, vestigial structures, and molecular biology. It is the central unifying theory of biology. In next week’s article I will discuss the influence of Darwin’s ideas on the modern world.
William Reville is associate professor of biochemistry and public awareness of science officer at UCC; http://understandingscience.ucc.ie