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The Four Forces of Evolution and Variation

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Evolution refers to the process by which inheritable characteristics of living organisms change from one generation to another over a given period. Evolution usually results into biological diversity amongst populations. Evolution occurs at two levels namely microevolution and macroevolution. Microevolution refers to various changes that occur in gene alleles within the gene pools of a species whereas macroevolution concerns changes in the gene pools of populations. Microevolution often leads to macroevolution after a long period. Microevolution usually results from the four forces of evolution like mutation, natural selection, gene flow and genetic drift.

Species and Population

A species is a group of organisms that are able to interbreed and reproduce fertile offspring. Organisms belonging to the same species usually possess identical biological and physiological traits such as genes and physical appearances. According to Freidman (2007), a species refers to members of a population that are capable of reproducing naturally. In relation to evolution, a species is a group of organisms with similar genetic materials that are transferable to new generations through inheritance.

On the other hand, a population is a group of organisms living together or close to each other with definite number and pattern of genes, and in which members of opposite sex can mate and reproduce. A population is composed of members of the same species living together within a given geographical area.

The Four Forces of Evolution

The four forces of evolution refer to various processes that involve changes in the genes of an organism due to gene flow, genetic drift, natural selection and mutation. These forces of evolution explain why variations exist within and amongst populations.

a)Natural Selection

Natural selection refers to the process by which organisms that are best-adapted to the environment are able to survive, live longer and give birth to more offspring than the least-adapted ones, thus ensuring continuity of their generations. The offspring of these best-adapted organisms will also carry forward the genes for adaption to their offspring. According to Mayr (2010), natural selection enables organisms to evolve by passing on heritable traits to their offspring from one generation to another. These heritable traits that are passed on to offspring facilitate survival as well as reproduction within the population. Natural selection concerns adaptation of organisms to the environment, thus the environment plays a key role during the evolution process.

b)Gene Flow

Gene flow concerns changes in allele frequency of genes that develop as a result of movement or migration of members of a population. Mayr (2010) defines gene flow as the movement of genes between members of a population due to migrations amongst the organisms. Through gene flow, suitable allele frequencies are carried from one population to another. For example, in plants, gene flow may involve movement of pollen grains with new genetic material from one population of plants to another in different regions. Gene flow facilitates development of similar characteristics or traits amongst populations.


Mutation refers to changes in the sequences of DNA in genes. Mutations usually involve duplication or replication of genes thus leading to production of additional copies of the genes. The additional copies created are then recombined and used to produce or generate new genes that have new genetic materials, sequences and functions; hence leading to evolution. Certain pre-existing traits or characteristics are usually carried forward to the new generations because the new genes are produced from copies of already existing genes. This implies that mutations do not result into evolution of populations with purely new characteristics but rather populations with modified characteristics from previous generations. The genes produced are then carried forward to new generations through genetic inheritance. According to Kucherlapati and Smith (2008), the ability of mutations to spread from one generation to another usually depends on its effects on the organisms (advantageous or disadvantageous) and dominance on the genes. Mayr (2010) further asserts that gene sequences or alleles can only be produced through mutations. Mutations usually occur randomly leading to varied genetic variations amongst populations.

Although some mutations may be beneficial to the population, most mutations are usually harmful to the organisms because mutations lead to alterations or modifications of the gene sequences; hence the genes become less functional. For example, the sickle cell anemia is a suitable example of a disease that results from harmful mutations. In human beings, mutations may lead to changes in physical and biological features of a person, thus leading to changes in physical appearances, behavior and physiological functions of the body.

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d)Genetic drift

Genetic drift refers to the occurrence of random changes within the gene sequences that leads to variations in the gene alleles. The new gene alleles created often have better chances for survival. For genetic drift to occur effectively, the population under consideration has to be very small. Genetic drifts can be explained with the help of bottleneck effect, founder effect and inbred populations theories. Through the bottleneck effect, drastic changes occur within gene alleles as a result of negative impacts of natural disasters such as earthquakes and floods. The organisms that manage to survive during these natural disasters will be overrepresented within the population. Consequently, a new collection of gene alleles will dominate the populations, thus leading to genetic variations. According to the founder effect theory, a small portion of the population may move and settle in a new and isolated environment. Due to inhabitation of new habitats, this portion of the population develops new gene alleles from alleles inherited from the original population. This results into genetic variations between this portion of the population and the original population. With regard to inbred populations, closely related organisms in a population mate accidentally leading to production of homozygote offspring with varied genetic materials. The homozygous offspring usually have higher chances of developing negative traits.

Variations Within and Between Populations

Genetic variations refer to differences in alleles of genes that exist within and between members of a population. Genetic variations are caused by mutations that often lead to irreversible changes in the chemical composition or structure of genes of the organisms. Genetic variations within a population usually result from changes in the chemical composition and sequences of genes of the members of the population and can be identified by observing quantifiable characteristics amongst the organisms and determining the quantity of nucleotide in genes.

On the other hand, genetic variations between populations usually results from inhabitation of different geographical regions by members of the population. For my part, genetic variations between populations usually stem from genetic drift and founder effect. When members of a population occupy different geographical areas, the habitats exact selective pressure on them, forcing them to develop new genes in order to cope with these environmental pressures as well as to facilitate their adaption. Generally, variations within and between populations are caused by the four forces of evolution discussed above.

Isolating Mechanisms and Speciation

Isolation mechanisms refer to reproductive traits that thwart mixing or fusion of organisms. Isolation mechanism usually prevents mixing of genes within a population. Gerald (2010) defines isolation mechanisms as behavioral, morphological or genetic characteristics that bar species from interbreeding. There are two types of isolation mechanisms; namely pre-mating and post-mating isolation mechanisms.

Pre-mating isolation mechanisms are those factors that prevent mating of organisms within a species prior to reproduction. Examples of pre-mating isolation mechanisms include temporal isolation, ecological isolation and behavioral isolation and mechanical isolation. Temporal isolation entails inactiveness of mating partners during certain periods whereas ecological isolation concerns inhabitation of different geographical areas in order to avoid mates. Behavioral isolation concerns choosing mates selectively while mechanical isolation concerns inability to transfer sperms during copulation attempts.

Post-mating isolation mechanisms involve methods such as gametic incompatibility, zygotic mortality and hybrid sterility that aim at preventing production of fertile offspring with undesired mates. For example, a mating between a donkey and horse usually produces a mule which is infertile and cannot procreate.

On the other hand, speciation refers to the formation of new species of a population during evolution. It is the process that leads to coming up of new species. Speciation may occur naturally through allopatric, parapatric and peripatric mechanisms or artificially through animal husbandry or breeding techniques and laboratory experiments. Organisms usually use various isolation mechanisms during speciation to ensure that they reproduce offspring with desired characteristics such as high adaptation abilities. For example, women are highly selective when choosing their husbands in order to ensure that they give birth to children with most desirable characteristics such as high intelligence, handsomeness, good height and bravery. In addition, the four forces of evolution discussed above may also lead to speciation.

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