Natural selection

Natural selection is a principle that was first proposed by Charles Darwin and Alfred Russel Wallace, who noted that organisms which were better adapted to their environment tended to survive longer and reproduce more than less well-adapted organisms. They predicted that over time favorable traits which are inheritable would become more common.

According to the theory of evolution, it is through a combination of natural selection and mutation that biological complexity and adaptation arise from earlier generations of life. This view is disputed by advocates of Intelligent Design. See also Creationism.

Variation

The prevailing theory in Biology is that natural selection is the mechanism by which allele frequency within a population changes over time due to genetic variation and selection pressures.

Natural selection is the mechanism in which members of a population exhibiting a specific phenotype have a reproductive advantage over other members of the population not exhibiting that phenotype. This is based on the current selection pressures in the environment. Due to this reproductive advantage, the group with the phenotype should produce more viable offspring that over time will increase their percentage in the overall population.

Genetic variation thus occurs due to several factors.

Spontaneous genetic mutations

These mutations change the genes within the chromosomes of the organism and can result in new polypeptides being produced. These abnormal polypeptides can have significant affects on the biochemistry of the organism, for example sickle cell anemia occurs because one base within the gene for beta-hemoglobin is altered. Mutations may occur for several reasons such as chemical mutagens or exposure to ionizing radiation, or because of errors made when DNA is copied inside a cell at the first stage of mitosis. Although most mutations are harmful due to their random nature (see the example regarding sickle cell anemia), beneficial mutations may also occur. Mutations will not be inherited unless the affected cells are gamete producing cells or gametes themselves.

For example, there may be a spontaneous mutation in a germline (gamete producing) cell of an agouti (brown) rabbit that changes the allele so that it produces a polypeptide that dyes fur white and produces an albino rabbit. There are also alleles that produce chinchilla and Himalayan coats^[1]

Meiosis and random fertilization of gametes

Meiosis is the division of diploid germline cells in the gonads of an organism to produce haploid gametes for sexual reproduction. During this process sections of homologous chromosomes (bivalents can get entangled and 'swap', in a process called crossing over, to form chiasmata. This process is random and produces unique gametes. Secondly, during the formation of gametes the distribution of the maternal and paternal chromosomes is random, causing further variation.

During fertilization, the genetic material (DNA) of a male gamete combines with the genetic material from a female gamete. The exact gametes which combine is also a random process. By these two processes gametes with unique genomes combine randomly to produce a unique offspring, and hence variation.

Continuing the rabbit example, whether the gamete coding for the albino polypeptide will fertilize another gamete containing the albino allele (this happens if both the sperm of the male and the egg of the female carry the albino allele) or not relies on coincidence. As the albino allele is recessive, there must be two copies present in the cells for it to be expressed.

Variation produces individuals within a population that have unique characteristics, such as different coat colors, blood groups or genetic disorders.

Selection pressures

Habitats place pressures upon their inhabitants. These can include, but are not limited to:

Predation;
Competition for food and water;
Disease;
Competition for space.

These environmental pressures can account for apparent over production by many species, especially those commonly regarded as prey species, and have large parts to play in population growth.

But what will determine which individual organisms will survive? The key factor is the genome of the individual. Some individuals may have variations of the species' genome that enables them to cope will with these selection pressures. For example, if large butterflies were more visible to predating birds the allele for large butterflies will be slowly removed from the population as large insects are predated on and killed before they can reproduce. Conversely, if small butterflies were seen as unattractive to the opposite sex they would be unable to mate and their allele for smallness would be removed from the gene pool. Such pressures provide a mechanism that changes the frequency of a particular allele within a population.

Another example carries on the theme of rabbit coat color: If the rabbits are living in a temperate region where there is plenty of foliage and the predominant color is green/brown the it is clear that the agouti rabbits will have better protection from predatory foxes than albino rabbits and would be selected against, reducing the albino allele frequency. However, if the climate were to change and become arctic the albino rabbits would have more of an advantage in camouflage and be selected for, increasing their allele frequency. This is, of course, assuming that the rabbits would be able to survive in arctic conditions.

Examples of natural selection

Following are good examples of natural selection in action.

Antibiotic resistance

As bacteria are exposed to increasing amounts of antibiotics only those that have or mutate genes for resistance survive as they are selected for. This is a problem in medicine as many antibiotic drugs are becoming less and less effective. The problem can be solved by:

Ensuring that patients complete the course prescribed by them by their doctor.
Only prescribing antibiotics when necessary - many people expect to be given a medicine when they visit their doctor even if one is not necessary.
Using two or more antibiotics at once can help ensure that those bacteria that are resistant to one of the drugs is killed by the other. This system has the additional advantage of ensuring that, in order to survive the antibiotic attack, non-resistant bacteria would have to simultaneously mutate in such way as to evade both antibiotics.

Industrial melanism^[2]

In England during the industrial revolution pollution killed mosses and lichens on tree trunks, turning them from light colors to dark brown. Before this occurred most specimens of the peppered moth Biston betularia had white wings with black spots - giving a peppered appearance. During the period with dark tree trunks (1849 to circa 1970) the white version of betularia was easily predated on by birds, decreasing the frequency of the white-wing allele. Individuals with dark wings caused by high concentrations of melanin became much more frequent and the allele frequency for melanic moths increased. When pollution levels were reduced in the late 1960s the trees gained more moss and lichens, making the trunks lighter and predation selected against melanic forms of the peppered moth. It is now thought that the proportions of white to melanic moths in England have now returned to pre-1849 levels.

Sickle cell anemia

This is a condition where red blood blood cells have a sickle shape caused by faulty hemoglobin genes. Suffers with full sickle cell anemia have two alleles for the condition and none of their red blood cells are formed correctly. These cells get stuck in blood capillaries and cannot carry oxygen properly. This is clearly not advantageous and many sufferers die early in life. Even though death occurs in some parts of East Africa up to 50% of babies carry the genes for sickle cell anemia and 14% have anemia^[3]. This can be explained as an individual with one normal allele and one sickle cell allele will have mostly normal red blood cells and a few sickle red blood cells. This condition is not harmful and the carriers can live a normal life. However, the protoctist plasmodium that causes malaria cannot survive in sickle red blood cells, thus giving an advantage to carriers in a high-risk malaria area.

Common misunderstandings about natural selection

The "goal" of natural selection

Natural selection does not have an ultimate goal or form and does not require guidance, but changes according to the environment. Since the environment is always changing, natural selection is an ever ongoing process.

"Perfection"

Natural selection will over the long term select organisms that are better adapted to the current environment. These organisms have to be able to survive long enough to reproduce, and produce viable offspring capable of the same, in order to ensure the continuing survival of their genetic traits. They do not have to be perfectly adapted to their environment; they only have to be able to survive in it.

Furthermore, an organism may be well adapted to one environment but not be able to survive in another, therefore there is no such thing as a perfect organism.