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This article provides an overview of the Evolution of populations and the mechanisms that derive it.
What is Evolution?
The theory of evolution explains how populations of organisms have changed over time. Evolution does not refer to changes that occur in an individual within its lifetime but it refers to changes in the characteristics of populations over the generations. These changes, which include modifications in structure, physiology, ecology and behaviour, may be so small that it is difficult to detect them or such great that the population differs from its ancestral population noticeably. Eventually, two populations may diverge to such a degree that we refer to them as different species.
Understanding The Evolution
The understanding of evolutionary biology began with the 1859 publication of Charles Darwin's (12 February 1809 – 19 April 1882) On the Origin of Species. In addition, Gregor Mendel's (July 20, 1822 – January 6, 1884) work with plants helped to explain the hereditary patterns of genetics.
Although Charles Darwin is universally associated with evolution, ideas of evolution predate Darwin by centuries. Aristotle (384-322 B.C.) saw much evidence of natural affinities among organisms. This led him to arrange all of the organisms he knew in a “Scale of Nature” that extended from the exceedingly simple to the most complex. His was vague to the nature of this “movement toward perfection” and certainly did not propose that the process of evolution was driven by natural processes.
Evolution - Deriving Mechanisms
Two major mechanisms drive evolution; the first is natural selection and the second is genetic drift. Other mechanisms evolving in the evolutionary process are:
- Non random mating,
- Allele (gene) flow,
Each population possesses a gene pool including all the alleles for all the genes present in it. Allele and/or genotype frequencies may be changed by the evolution deriving mechanisms.
Charles Darwin, in his 1859 seminal book, “On the Origin of Species”, named the differential survival and reproductive success of individuals as natural selection. By natural selection, individuals of a population that enjoy more successful adaptations to the environment have more chances to survive and reproduce. Natural selection fights back seriously abnormal phenotypes as well as eliminates harmful mutations or reduces them. Over successive generations, the proportion of favourable alleles cause the population to increase. In contrast with other microevolutionary processes, natural selection leads adaptive evolutionary change.
Natural selection results in the preservation of individuals with favourable phenotypes and elimination of those with unfavourable phenotypes. Individuals that are able to survive and produce fertile offspring have a selective advantage.
Natural selection explains why organisms are well adapted to the environments in which they live but also helps to account for the diversity of life. Natural selection enables populations to change, thereby adapting to different environments and different ways of life. The mechanism of natural selection does not cause the development of “perfect” organisms; it wipes out those individuals whose phenotypes are not as well adapted to the environment as others, while allowing better adapted individuals to survive and reproduce.
The production of random evolutionary changes in small breeding populations is known as Genetic drift. It results in changes in allele frequencies in a population from one generation to another. One allele may be eliminated from the population purely by chance, regardless of whether that allele is beneficial, harmful or of no particular advantage or disadvantage. Genetic drift, although it tends to increase the genetic differences among different populations, it can also decrease genetic variation within a population.
In the case where a population is forced through a bottleneck, then genetic drift can become a major evolutionary force. After this bottleneck, as the population increases again in size, many allele frequencies may be quite different from those in the population preceding the decline.
Non random mating
In random mating, a population’s reproductively active individuals mate with one another without regard to their respective genotypes. This is not the case though for non-random mating where the chance of two genotypes or phenotypes breeding is determined by their frequencies in the population.
In agriculture and aquaculture, animal breeders do essentially non-random mating, when they intentionally try to improve varieties or create new ones by carefully making sure that mating is not random. When they select mates for their animals or fish based on desired traits, farmers hope to increase the frequency of those traits in future generations. As the discriminated traits are genetically inherited, evolution is usually a consequence.
Non-random mating can act as an ancillary process for natural selection to cause evolution to occur. Any departure from random mating upsets the equilibrium distribution of genotypes in a population. A single generation of random mating will restore genetic equilibrium if no other evolutionary mechanism is operating on the population. However, this does not result in a return to the distribution of population genotypes that existed prior to the period of non-random mating. 
When in the natural environment, during non-random mating, females could prefer to mate with males carrying a population-specific trait, a preference model, or to mate with males that share their own trait phenotype, an assortative mating model. 
Gene flow is closely related to migration. It occurs when migrating individuals breed in their new location. Immigrants may add new alleles to the gene pool of a population or may change the frequencies of alleles already present if they come from a population with different allele frequencies.
Gene flow can have major evolutionary costs. As alleles “flow” from one population to another, the amount of genetic diversity inside the recipient population is usually increased. If there is sufficient gene flow between two populations, this can work against the mechanisms of natural selection and genetic drift, sometimes causing populations to become discrete.
Variations in populations are introduced through mutations; they are the source of all new alleles within the population.
All mutations happen spontaneously and unpredictably. The rate of mutation is rather constant for a gene but may be different among genes within a single species and among different species. Not all mutations are inherited from one generation to the next. In the case when a polypeptide is enough altered to change its function, the mutation is usually harmful.
Mutations are considered as the raw material for evolution; without them evolution would not exist. Nevertheless, evolution is not directly driven by mutation; genetical mutations provide the genetic diversity on which natural selection and other mechanisms act.
Mutations do not determine the direction of evolutionary change. They cause changes in allele frequencies, smaller than those which the Hardy-Weinberg principle has predicted. As an evolutionary force, mutation is usually insignificant, but it is important as the ultimate source of variation for evolution.
Migration of individuals or (local) populations is similar to mutation in the sense that new alleles can be introduced into a population, although the alleles derive from another subpopulation and are not new as in mutations.
Regarding fish, many types of fish migrate on a regular basis, on time scales ranging from daily to annual, and over distances ranging from a few meters to thousands of kilometers. Fish usually migrate because of diet or reproductive needs, although in some cases the reason for migration remains unknown 
Migration is the main motor for gene flow. In the absence of migration, the allele frequencies in each population can change independently. In this way, also populations can change independently and undergo considerable genetic differentiation.
Genetic differentiation among subpopulations means that there are differing frequencies of common alleles among the populations or that some populations possess certain rare alleles which are not found in others. The accumulation of genetic differences among subpopulations can be minimised if the subpopulations exchange individuals by migration.
Just a relatively small amount of migration among subpopulations is usually sufficient to prevent the accumulation of high levels of genetic differentiation. On the other hand, genetic differentiation can occur in spite of migration if other evolutionary forces, such as natural selection for adaptation to the local environment, are sufficiently strong. 
The type of interaction between two species that use the same resources and one can harm the fitness of the other is competition. In the case of plants, the limited environmental resources are often light, water and nutrients whilst for animals it tends to be food, shelter, nesting sites, and mates.
Competition among individuals of the same species (intraspecific competition) can cause reduced growth and reproductive rates for some of them or may exclude some from better habitats or cause death of others. Competition among individuals of different species, (interspecific competition) can affect entire species, for example keeping them out of habitats where they cannot compete successfully.
According to the competitive exclusion principle, species less suited to compete for resources should either adapt or die out. According to evolutionary theory, this competition within and between species for resources plays a critical role in natural selection. Species that are endowed with better competitive abilities than others in utilizing a given limited resource, e.g. space or nutrients will ultimately outgrow the less efficient ones, exclude them from the habitat and reduce the diversity. Absence of competitive exclusion will lead to a higher diversity. 
Speciation is the process by which one species splits into two species, which thereafter evolve as distinct lineages. In other words, speciation is the evolution of a new species or the process by which populations diverge and become reproductively isolated so that they develop into different species. Not all evolutionary changes end up in creating new species.
Species arise in two ways. The first is a gradual change over many years so that a species has changed so much that it is considered to be a different species. The second is diversification, when a prior species gives rise to two or more descendant species. This occurs when populations genetically differentiate, become reproductively isolated and are said to have speciated. Speciation frequently involves, at least partial, physical isolation.
Evolution and Biodiversity
Biodiversity found on Earth today is the result of 4 billion years of evolution. The origin of life has not been definitely established by science, however some evidence suggests that life may already have been well-established a few hundred million years after the formation of the Earth. Until approximately 600 million years ago, all life consisted of archaea, bacteria, protozoans and similar single-celled organisms.
The history of biodiversity during the Phanerozoic (the last 540 million years, see the second time scale), starts with rapid growth during the Cambrian explosion—a period during which nearly every phylum of multicellular organisms first appeared. Over the next 400 million years or so, global diversity showed little overall trend, but was marked by periodic, massive losses of diversity classified as mass extinction events.
The apparent biodiversity shown in the fossil record suggests that the last few million years include the period of greatest biodiversity in the Earth's history. However, not all scientists support this view, since there is considerable uncertainty as to how strongly the fossil record is biased by the greater availability and preservation of recent geologic sections. Some (e.g. Alroy et al., 2001) argue that, corrected for sampling artifacts, modern biodiversity is not much different from biodiversity 300 million years ago. Estimates of the present global macroscopic species diversity vary from 2 million to 100 million species, with a best estimate of somewhere near 13–14 million, the vast majority of them arthropods.
Most biologists agree however that the period since the emergence of humans is part of a new mass extinction, the Holocene extinction event, caused primarily by the impact humans are having on the environment. It has been argued that the present rate of extinction is sufficient to eliminate most species on the planet Earth within 100 years.
New species are regularly discovered (on average between 5–10,000 new species each year, most of them insects) and many, though discovered, are not yet classified (estimates are that nearly 90% of all arthropods are not yet classified).
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