In 1953 Watson and Crick, of Cambridge University, published the first big breakthrough in our understanding of how genetic variation works on evolution. Before this, we knew that there is genetic variability, and that it is the driving force of evolution, but we didn't know the actual mechanism. We shall come back to Watson's and Crick's work on DNA in a moment.
The most common mistake made in the understanding of evolution is to confuse intra-species adaptations with evolution. Ever since Lamarck published his work in the early 1800s, this confusion has hampered our understanding. It is important to know exactly what we are talking about.
Every individual in a species is unique. As we know from animal and plant breeding techniques, adaptations can be selected for within a species. This does not constitute evolution. A dog is a dog, whatever shape it has been bred to be.
Evolution entails the development over time of new species, not just varieties. The main difference between a species and a variety is that varieties can interbreed, whereas hybrids (the products of inter-species reproduction) are always sterile.
The two things are not completely unconnected. Adaptations to the environment can be selected for and contribute to the:
'aggregates of tiny adaptive changes (which)can generate complex organ systems'
and therefore to evolution. But it is not the case that adaptive changes in the parents will translate itself into their genes and be passed on to the offspring.
To understand the way in which variation acts on evolution, we have to go down to the level of the genes, beginning with chromosomes.
Each member of a species has exactly the same set of chromosomes, varying only in the sex chromosomes, females having 2 'X' chromosomes, and males an 'X' and a 'Y'. The chromosomes dictate the basic template of a species. It is not until there is enough change at the chromosomal level to create new species, that we can call it 'evolution'.
Contained within the chromosomes are the genes. Genes are made up of DNA (deoxyribonucleic acid). An individuals DNA is a protein made up of amino acids which tend to vary slightly in their constitution, causing polymorphism, or variations of form within a species. Most of these variations in amino acids are infinitesimal, and can be passed on to offspring, without changing anything at the chromosomal level. When big enough sets of these variations have been selected for and enhanced over many generations, as is the case with dog breeds, the differences in forms can be thought of as varieties. The varieties of a species can interbreed successfully. The bred-for characteristics can be added to, or diluted, by further interbreeding.
Eventually, over many, many generations of reliable selection for particular attributes, changes in the DNA accumulate to become changes in the genes and ultimately changes in the chromosomes so that we have a new species.
But again, these changes at a genetic level are not caused directly by the parents adaptations to their environment being passed on to their offspring. If we go further down, and back to Watson and Crick, we find that in fact, changes at this level are caused by genetic mutation.
Genetic mutation is happening all the time. It can be caused by radiation, mutagen chemicals and, most commonly, by spontaneous 'events' in the copying of DNA during reproduction. As Watson and Crick demonstrated, DNA is arranged in strands. These strands are arranged in 'base pairs' in a double helix. There are only 4 bases (adenosine, cytosine, thymine and guanine) but they can be arranged in any sequence and the length of the strands varies within species.
When cells divide, eg after fertilization, the strands split down the middle and each half copies itself to form a new whole. It is here that mutations can occur. There is a one in a billion chance under normal circumstances of the copies being slightly inaccurate, leading to a slight change in form. If the change is a favorable one, offering the individual a greater chance of survival or of being selected as a mate, it will have more chance of being passed on to offspring and the process of evolution is begun. The vast majority of these mutations don't make it, if they don't enhance the individuals survival, they won't be selected for.
In summary, life tends to variation. Some of these variations are caused purely by an individuals ability to adapt to its particular environment. At a deeper level, members of a species can vary because of superficial variation of the amino acids that make up their DNA. These changes cannot be said to constitute evolution on their own.
Variation also occurs via genetic mutation, changes in the base pairs making up an individuals DNA. These changes cause changes of form which, if useful, will lead to evolution.
Steve Jones and Borin Van Loon, 'Introducing Genetics', Icon Press 2005
Philip Whitfield, 'Evolution: The Greatest Story Ever Told, Marshall Press 1993
Richard Dawkins, 'The Ancestors Tale', Weidenfield and Nicholson 2003
Mark Ridley, 'Evolution', www.blackwellspublishing.com