Jean-Baptiste de Lamarck (1744–1829), a French biologist, claimed that animals changed according to natural laws. He said that animals could pass on traits they had acquired during their lifetime to their offspring, using inheritance. Today, his theory is known as Lamarckism. Its main purpose is to explain adaptations by natural means. He proposed a tendency for organisms to become more complex, moving up a ladder of progress, plus "use and disuse".
Lamarck's idea was that a giraffe's neck grew longer because it tried to reach higher up. This idea failed because it conflicts with heredity (Mendel's work). Mendel made his discoveries about half a century after Lamarck's work.
Charles Darwin (1809–1882) wrote his "On the Origin of Species" in 1859. In this book, he put forward much evidence that evolution had occurred. He also proposed natural selection as the way evolution had taken place. But Darwin did not understand about genetics and how traits were actually passed on. He could not accurately explain what made children look like their parents.
Nevertheless, Darwin's explanation of evolution was fundamentally correct. In contrast to Lamarck, Darwin's idea was that the giraffe's neck became longer because "those with longer necks survived better". These survivors passed their genes on, and in time the whole species got longer necks.
Alfred Russel Wallace OM FRS (1823–1913) was a British naturalist, explorer, biologist and social activist. He proposed a theory of natural selection at about the same time as Darwin. His idea was published in 1858 together with Charles Darwin's idea.
An Austrian monk called Gregor Mendel (1822–1884) bred plants. In the mid-19th century, he discovered how traits were passed on from one generation to the next.
He used peas for his experiments: some peas have white flowers and others have red ones. Some peas have green seeds and others have yellow seeds. Mendel used artificial pollination to breed the peas. His results are discussed further in Mendelian inheritance. Darwin thought that the inheritance from both parents blended together. Mendel proved that the genes from the two parents stay separate, and may be passed on unchanged to later generations.
Mendel published his results in a journal that was not well-known, and his discoveries were overlooked. Around 1900, his work was rediscovered. Genes are bits of information made of DNA which work like a set of instructions. A set of genes are in every living cell. Together, genes organise the way an egg develops into an adult. With mammals, and many other living things, a copy of each gene comes from the father and another copy from the mother. Some living organisms, including some plants, only have one parent, so get all their genes from them. These genes produce the genetic differences which evolution acts on.
Darwin's "On the Origin of Species" has two themes: the evidence for evolution, and his ideas on how evolution took place. This section deals with the second issue.
The first two chapters of the "Origin" deal with variation in domesticated plants and animals, and variation in nature.
All living things show variation. Every population which has been studied shows that animal and plants vary as much as humans do. This is a great fact of nature, and without it evolution would not occur. Darwin said that, just as man selects what he wants in his farm animals, so in nature the variations allow natural selection to work.
The features of an individual are influenced by two things, heredity and environment. First, development is controlled by genes inherited from the parents. Second, living brings its own influences. Some things are entirely inherited, others partly, and some not inherited at all.
The colour of eyes is entirely inherited; they are a genetic trait. Height or weight is only partly inherited, and the language is not at all inherited. Just to be clear: the fact that humans can speak is inherited, but what language is spoken depends on where a person lives and what they are taught. Another example: a person inherits a brain of somewhat variable capacity. What happens after birth depends on many things such as home environment, education and other experiences. When a person is adult, their brain is what their inheritance and life experience have made it.
"Evolution only concerns the traits which can be inherited", wholly or partly. The hereditary traits are passed on from one generation to the next through the genes. A person's genes contain all the traits which they inherit from their parents. The accidents of life are not passed on. Also, of course, each person lives a somewhat different life: that increases the differences.
Organisms in any population vary in reproductive success. From the point of view of evolution, 'reproductive success' means the total number of offspring which live to breed and leave offspring themselves.
Variation can only affect future generations if it is inherited. Because of the work of Gregor Mendel, we know that much variation is inherited. Mendel's 'factors' are now called genes. Research has shown that "almost every individual in a sexually reproducing species is genetically unique".
Genetic variation is increased by gene mutations. DNA does not always reproduce exactly. Rare changes occur, and these changes can be inherited. Many changes in DNA cause faults; some are neutral or even advantageous. This gives rise to genetic variation, which is the seed-corn of evolution. Sexual reproduction, by the crossing over of chromosomes during meiosis, spreads variation through the population. Other events, like natural selection and drift, reduce variation. So a population in the wild always has variation, but the details are always changing.
Evolution mainly works by natural selection. What does this mean? Animals and plants which are best suited to their environment will, on average, survive better. There is a struggle for existence. Those who survive will produce the next generation. Their genes will be passed on, and the genes of those who did not reproduce will not. This is the basic mechanism which changes a population and causes evolution.
Natural selection explains why living organisms change over time to have the anatomy, the functions and behaviour that they have. It works like this:
There are now many cases where natural selection has been proved to occur in wild populations. Almost every case investigated of camouflage, mimicry and polymorphism has shown strong effects of selection.
The force of selection can be much stronger than was thought by the early population geneticists. The resistance to pesticides has grown quickly. Resistance to "warfarin" in Norway rats ("Rattus norvegicus") grew rapidly because those that survived made up more and more of the population. Research showed that, in the absence of "warfarin", the resistant homozygote was at a 54% disadvantage to the normal wild type homozygote. This great disadvantage was quickly overcome by the selection for "warfarin" resistance.
Mammals normally cannot drink milk as adults, but humans are an exception. Milk is digested by the enzyme lactase, which switches off as mammals stop taking milk from their mothers. The human ability to drink milk during adult life is supported by a lactase mutation which prevents this switch-off. Human populations have a high proportion of this mutation wherever milk is important in the diet. The spread of this 'milk tolerance' is promoted by natural selection, because it helps people survive where milk is available. Genetic studies suggest that the oldest mutations causing lactase persistence only reached high levels in human populations in the last ten thousand years. Therefore, lactase persistence is often cited as an example of recent human evolution. As lactase persistence is genetic, but animal husbandry a cultural trait, this is gene–culture coevolution.