Describe evolution by natural selection and how this may lead to speciation?

A given population of biological organisms will show considerable genetic variation. This is initially (relatively speaking) a result of random mutation. However, as factors affecting survival and reproduction, such as disease or the competition for scarce resources, exert their effect on this population, exert their effect on such a population, differential survival and reproduction occur. Those individuals with phenotypes (and therefore underlying genotypes) that give them characteristics which are advantageous for survival, and more importantly reproduction (i.e. the passing on of genetic material) in their environment, are selected for, whereas those organisms with "disadvantageous" phenotypes are selected against. In this way, advantageous characteristics accumulate in a population (e.g. sharp teeth in predators). Of course, such characteristics are encoded by the genotype and can be represented by allele frequency within the population. Sometimes, populations can be separated into two groups that have no 'reproductive contact' with each other - e.g. a lake being split into two separate lakes, and fish in one no longer being able to mate with fish in the other. When this occurs, each sub-population begins to chart its own unique genetic evolution, and eventually, the accumulation of genetic differences between the two sub-populations will prevent the successful mating of these sub-populations, even if the fish from the two populations in the lake example were again put in the same body of water. Successful here means the production of fertile offspring. At this point, when cross-breeding no longer produces fertile offspring, it is said that speciation has occurred - i.e., the two populations have become genetically different enough to be classified as two separate species.