Evolution is an important concept within Biology.
To modern Biologists it is not seen as just a theory, i.e. an unproved idea, but an explanation based on sound scientific principles, and it supports most of the topics within Biology.
All forms of life can be traced back to other living organisms. Each species is seen to have originated from another pre-existing species, and consequently they share a wide range of features - not just their external appearance but also at the cellular and biochemical level.
This diversification ought to be reflected in their classification and naming which is a human construct and so it sometimes appears questionable.
Most living organisms do not exist independently, but as members of a population.
Within a population, most individuals share certain features. For example, they look similar, but are rarely identical.
This is partly because they share mostly the same genes but in different combinations. Genes have alternative forms called alleles.
Genes/alleles are fairly reliably passed on from one generation to the next with little modification, but individuals inherit different combinations of them from their parents.
The observable characteristics of an individual - its phenotype - are produced as a result of interactions between its genotype (genes/alleles mentioned above) and the environment.
The main source of genetic variation is mutation. This is a random small change in the organism's genome, generally made of DNA. It is likely to be caused by chemicals or radiation.
The changes may involve the loss of a nucleotide base or cause it to be mis-read as a completely different one. A missing base, or an extra base, results in a different base sequence which causes the remaining sections of DNA to be read in a different way, resulting in a different protein product, which perhaps may not work properly, although it may occasionally be an improvement. A gene is a portion of DNA which results in a particular protein product, and so the change in the DNA can be seen as a new version of the gene - a different allele.
If a DNA molecule in the reproductive organs becomes altered in this way, it may result in sex cells (gametes) that are different, and if one of these takes part in fertilisation, the resulting zygote will grow into an organism with a different genetic makeup than the parents.
Very infrequently, problems with cell division in meiosis may result in chromosomes breaking and re-forming so that sections may be doubled. Duplicated genes sometimes develop different functions so that they add to the number of different alleles in the population
The halving of chromosome numbers as a result of meiosis during gamete formation and the random fertilisation of gametes during sexual reproduction both produce further genetic variation.
Within a population there will be a number of individual organisms with differences in their genetic makeup. These differences will cause a 'spectrum of variation' in their phenotypes which can remain in the background for some time, and form the basis for evolutionary change when selective pressure occurs. Many features which show continuous variation, such as overall height, or limb length, are caused by polygenic inheritance, in which many genes (scattered within the genome of individual organisms) combine to have an effect.
If selective pressure exerts a negative effect on either end of the spectrum of variation it is called stabilising selection; this tends to reduce variation and give advantage to organisms which are close to the average for that population.
If it acts more at one end of the spectrum it is said to be directional selection, causing a gradual change in the average for the population.
If it acts against the average or mid point of the spectrum it is called disruptive or diversifying selection, and this tends to make one population turn into two populations.
Evolution - the gradual change by which types (varieties, subspecies, species, genera) of organisms develop from earlier ones - is caused by a change in allele frequency within a population.
Natural selection involves the enhancement of survival rate of certain individuals who have phenotypic features which are advantageous (in some way more suited to their environment): more able to escape from predators, to withstand disease and to compete with others for food and other resources.
This survival of the fittest is a major factor in altering the frequency of alleles in a population.
The most successful individuals in a population are more likely to reproduce and have more offspring than others which are less successful. This is called differential reproductive success.
It ensures that organisms are able to respond to changes in the environment - either as a result of climatic change, or in the colonisation of a new section of the planet.
In some circumstances (small populations) allele frequency can change as a result of genetic drift.
In such small populations, some alleles may not be passed on to the next generation as a result of chance, rather than a selection process. This is likely to reduce genetic diversity, which is seen as a way for populations to adapt to changing environments.
Populations within a species may become isolated from one another in a number of ways.
Changes in the environment such as flooding of an area or the development of a desert, or mountains or islands forming by geological action - together with migration of (parts of) populations into these areas - can lead to their geographic isolation.
Changes in reproductive behaviour - different mating or flowering seasons, different patterns of pre-mating display, and biochemical incompatibility mechanisms may affect reproductive success so that two populations exist alongside one another but do not interact. This is reproductive isolation.
In both these cases there will be no gene flow between the two populations. As a result each population will have a separate gene pool.
Over time genetic differences will build up between these populations. Small differences can result in different varieties, or races.
When these accumulate and result in the inability to interbreed between members of different populations (and produce fertile offspring), new species are said to have formed.
Subspecies are at an intermediate stage. Further changes result in different genera, and so on.
The process of forming a new species is called speciation. It is often said that evolution may lead to speciation, but in reality speciation is only part of the process.
Allopatric speciation occurs as a result of geographical isolation, and sympatric speciation is a result of (purely) reproductive isolation within the same area, caused by disruptive selection.
Further explanation and examples of these two processes are given in separate topic files on this website. See links below.
Evolution is usually thought to take place over long periods of time, but the effects of natural selection can sometimes be seen over quite short time intervals. It does not stop at producing new organisms at the species level, and is responsible for the diversity of life on the planet.
There are several examples of adaptive radiation, a process in which organisms diversify rapidly from an ancestral species into a number of new forms. This is in response to a change in the environment which makes new resources available, effectively creating new environmental niches.
The diverse species of cichlid fishes of the East African lakes (Lakes Victoria and Malawi and especially Tanganyika) are prime examples.