Key term - Fitness: In evolution fitness does not mean "faster" or "stronger", but "better suited to the current environment". In this sense the fitter individual may actually be the smaller, weaker and slower organism. Fitness is entirely dependent on context.
Natural selection acts when there is variation in a population, especially in populations where organisms produce more offspring than are able to reach maturity. Individuals in a population compete over resources, whether it is food, water, space or mates. In some species, competition, especially in males, is often direct, resulting in combat. Competition can also come from other species which require the same resources.
The fitter members of the population are most likely to breed and pass on their genes to the next generation, thus passing on their own favourable variation. Natural selection concerns the organisms which are more able to pass on their genes. Success in breeding is often more important than success in survival, though the two are connected (a short life with lots of breeding can be a more successful strategy than a long life with little breeding).
A point to remember is that natural selection has no foresight and cannot select variation which are favourable in future environments; it can only select for the current environment. A change in the environment changes the context for fitness. Sometimes a trait which is fortuitous in the current environment is useful after the environment changes, sometimes for a new purpose, this is known as abaptation or preadaptation..
When there is continuous phenotypic variation (the phenotype is the outward appearance of an organism) there are three ways in which natural selection can act on a population: stabilising, directional and disruptive.
In stabilising selection the existing mean of the distribution of inherited variability is favoured. In other words, variance is reduced as extreme variants are selected out. For example, in a population both the largest and smallest individuals may be selected against, maintaining a mean between the two. This is often seen with birth rates in populations.
During directional selection the mean shifts as selection favours one extreme of the inherited variability distribution. This sort of selection is most common when a change in environment occurs. The famous peppered moth experiment demonstrated this as environmental change caused a shift in the mean colour of the moths from pale to dark.
Disruptive selection splits a population in two by favouring both the extremes of the variability distribution. From one phenotype emerges two distinct phenotypes. This process may be instrumental in the evolution of many new species as a population splits and adapts to new environments.
Some key points to remember about natural selection is that it is a non-random process and that it acts on very slight changes in phenotype. The chance aspect of evolution is genetic variation as mutation is a random process; natural selection does not act randomly but is instead differential reproduction, something which can be easily predicted. Changes which are too large in an organism most often result in being selected against as they are more likely to be poorly suited to the environment. Small changes do not stray far from the already favoured phenotype and so natural selection can favour those which provide even a tiny advantage, refining the adaptation.
When discussing natural selection it sometimes helps to make a key distinction. Natural selection acts on any phenotypic trait, whether it is heritable or not. Evolution requires heritable variation to be selected. The selection of a trait can also result in the selection of other connected traits, such as when a gene has multiple functions (pleiotropy).
Natural selection acts at all stages of the life cycle, the image below is useful in distinguishing the different types of selection at work:
As natural selection works on small changes it can take a long time. Each change it favours is a successful increase in fitness which accumulate bit by bit. Natural selection is a cumulative process and is therefore very important in the construction of complex adaptations in evolution.
Part 2 will go on to discuss speciation. Natural selection is a heavily studied phenomenon for which a lot can be found. I recommend The Blind Watchmaker by Richard Dawkins for any who want to know more, particularly with regards to natural selection being cumulative.