Definition:

Evolution is the spread of genes through the gene pool. This happens by a process of mutation, sexual reproduction and natural selection, these increase variation and pass changes through to the next generation.

Genes are the carrier of genetic information. They can be found in the DNA (deoxyribonucleic acid) which is situated in the chromosomes located in the nucleus of each cell. Humans have 46 chromosomes, we get 23 from each parent. DNA has the shape of a double helix, with two bands of 'sugar-phosphate' molecules as a backbone, joined together by four bases, adenine, thymine, cytosine and guanine (A,T,C,G) these always pair up A-T and C-G as shown:

The first person to examine heredity in a scientific manner was a monk known as Gregor Mendel. He did his work in the 1850's-60's on pea plants. He was able to find attributes in these plants that were simple enough to study. One such attribute is the height of the plant. This is controlled in pea plants by a single gene, for which there are just two alleles, one for tall plants and one for dwarf plants. It so happens that the gene for tallness is dominant, that is if either or both of the two genes received from the plants parents is for tallness, the plant will be tall. The only way for a plant to be small is if both the genes are for smallness. This is known as the recessive gene. If we represent the gene for tallness with a 'T' and for smallness with 't' (the recessive gene, by convention is the small letter of the dominant trait), then there are 4 possible gene combinations TT (genes from both parents are dominant) Tt (one dominant and one recessive) and tt (both parents passed on the gene for smallness). The fourth possible one is Tt also, but with the dominant coming from the opposite partner. The actual genes present is known as the genotype, but the appearance of the plant based on the genotype is known as the phenotype. With the two genotypes TT and Tt, the offsprings phenotype (how it will it look) will be tall, this is because of the 'T' gene being dominant, and with the tt genotype the phenotype will be small.

By very careful cross-pollination of dwarf varieties and tall varieties Mendel was able to show that traits survived unchanged through generations. He took the dwarf plants and crossed them with the tall ones. To examine his work, a few words of explanation are required. He crossed plants with geneotype 'TT' with plants with genotype 'tt'. The first generation of these plants were all tall, this can be shown diagrammatically like this:

The parents pass on one of their genes to each of their offspring, so there are four possible combinations. It can be seen that the genotype of all the offspring is Tt, meaning the phenotype of all of this generation were tall but the genotype of each contained a recessive gene.

He then took these first generation (f1) plants and crossed them with themselves. This can be shown like this:

It can be seen that 1/4 of the f2 plants were small, the other 3/4 were tall. The only small plant is the one with both recessive genes. The other 3/4 look tall, but 1/2 of them do carry the recessive gene.

This is the level of alleles in a population. If there are many animals in a population then the number of different alleles that are available are greater than if there is only a few. This assumes that all the animals can breed freely. For example, if a small population of a bird gets trapped on an island, then the gene pool available to that population is drastically reduced. If there are enough of the birds, with little competition this could lead to very rapid changes, but if there are too few, then the population cannot sustain itself and too much inbreeding will result in extinction of that population. This is one of the reasons people are concerned about endangered species, if the numbers get too few then the population will be in danger from the inbreeding in itself.

Mutation is the changing of the genetic information. The mutation for sickle cell amaemia is a single base change in the DNA that codes for haemoglobin. It is a recessive gene, and only survives in the gene pool in populations where its protection against malaria helps keep people with this mutation alive. Mutations are caused by many factors like radiation or chemicals, and because of the way DNA replicates any error gets copied to any daughter cell.

Most mutations are neutral, many have a negative effect on an organism, but positive mutations do happen. Experiments have shown that cultures of bacteria derived from a single bacterium become resistant to antibiotics.

What mutations do is increase the variation in the gene pool, meaning more alleles are available to be passed on to offspring.

Sexual reproduction

The process of sexual reproduction makes a great deal of changes in the chromosomes. The method of making the sperm or eggs is complicated, but in essence a complete cell copies half its chromosomes into a daughter cell, so, in humans there is a daughter cell with only 23 chromosomes. These copies are made from taking parts from each of the copies that the parent cell has. So, the parent cell has two no. 23 chromosomes (one from each of the organisms parents). The daughter cell gets only one copy of this no. 23 chromosome but made up of bits of both the parents no 23's. This is where most genetic variation comes from, the genes are shuffled around a great deal in the creation of a new organism.

Natural selection
It is a fact that all creatures produce more children than can survive into adulthood. What is it about the ones that survive that enable them to survive? They are more suited to their environment. In a simple example, consider a litter of lions, some will be better hunters than others. In a bad year, many young lions will die, and the best hunters will be more likely to survive to adulthood. If there is a genetic element to there skills, their children will have copies of these genes, and so will be more likely to be better hunters. It is the level of variation between good hunters and poor hunters that evolution works on to spread the genes through the population.

The above shows that the definition of evolution does not talk about speciation (the creation of new species). That is a consequence of evolution, not the cause.