Evolution

This article provides an overview of the Evolution of populations and the mechanisms that derive it.

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.

Evolution has two main perspectives: microevolution and macroevolution. The evolution of populations is best understood in terms of phenotype, genotype and allele frequencies.

Understanding The Evolution

The understanding of evolutionary biology began with the 1859 publication of Charles Darwin's On the Origin of Species. In addition, Gregor Mendel's 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.^[1]

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,
Mutation,
Migration,
Competition,
Speciation.

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.

Natural Selection

Charles Robert Darwin in 1880, while he was still working on his contributions to evolutionary thought that had had an enormous effect on many fields of science. Source: Wikipedia

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 other, while allow better adapted individuals to survive and reproduce.^[1]

Genetic Drift

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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 case of a population getting through a bottleneck, genetic drift can become a major evolutionary force. After the 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. ^[2]

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. ^[3]

Gene flow

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 variability 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.

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Mutation

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

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.

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. ^[4]

Competition

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. ^[5]

Speciation

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 specie 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 gradual change within the years so that a specie is changed so much that is considered to be a different at a later time. 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.

There are four modes of natural speciation, based on the extent to which speciating populations are geographically isolated from one another: allopatric, peripatric, parapatric, and sympatric ^[6]

References

↑ ^1.0 ^1.1 Solomon, E. P., Berg, L. R., & Martin, D. W. (2002). Biology, sixth edition. (N. Rose, Ed.). Stamford, CT: Thomson Learning.
↑ http://anthro.palomar.edu/synthetic/synth_8.htm
↑ Servedio, M.R. 2000. Reinforcement and the genetics of nonrandom mating. Evolution. 54:21-29.
↑ Hartl D. L. et al., Genetics: analysis of genes and genomes (Jones and Bartlett, 2001) – 5^thed.
↑ Raghukumar S. and A. C. Anil, 2003. Marine biodiversity and ecosystem functioning: A perspective. Current Science,vol.84, NO. 7, 10.
↑ Purves, W. K. et al. Life, the science of biology (Sinauer Associates and WH Freeman, 2001)

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