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Mechanisms of Microevolution

Have you ever wondered why humans share certain similarities with monkeys, even though we belong to different species? This intriguing question introduces us to the concept of evolution, a process that shapes the distinctions among living organisms from their common ancestors, resulting in the diversity of species we observe today. Evolution encompasses a broad spectrum of changes, ranging from the vibrant colors of fish to the complex evolutionary journey of humans. These changes are classified into microevolution and macroevolution. Microevolution involves shifts in allele frequencies, driven by mechanisms such as mutation, natural selection, non-random mating, gene flow, and genetic drift.

Mutation

In a gene pool, the mutation is the only source of new alleles. It is a random change that occurs in the inheritable DNA of the individual. The occurrence of a mutation is not influenced by the consequences it might have. Mutation can be neutral, beneficial, or harmful. Neutral mutation does not benefit or harm the organism, which usually occurs in the non-coding region. Even if a beneficial mutation occurs, the subsequent mutations still have a chance of yielding outcomes unrelated to the first mutation or, in some cases, resulting in harmful mutations. Therefore, beneficial mutation is very rare. However, mutations are mostly harmful mutation.

Natural selection

In a certain environment, natural selection is a process that selects advantageous heritable traits, passing the features to the following generations. It acts on the result of mutation which can provide new sources of genes because new alleles do not arise from natural selection itself.  It chooses the traits that can greatly help increase fitness which can enlarge the population. Here, "fitness" refers to the traits that can provide reproductive success for the next generation compared with others.


Directional selection results in a shift of one extreme aspect of a trait. For instance, the long-necked giraffes are selected for and the short-necked giraffes are selected against. In another scenario, the ground finches mostly have large and small beaks, this is because the finches with medium beaks couldn’t get food and therefore died. It shows the disruptive selection in which the extreme traits are selected and acts to remove the most common characteristics.


Another mode of natural selection is stabilizing selection, it favors the intermediate trait and selects against the traits that differ from the most extremes. A good example of stabilizing selection is that human infants are always born at a medium weight, since either too small or too large is hard to survive.

Non-random mating

Non-random mating states that any male and any female have equal chances of mating, regardless of phenotype. Individuals tend to mate with those who live nearer to them, when closely related individuals breed together, it is inbreeding. They share similar genotypes so the frequency of the homozygous genotype increases. However, harmful recessive alleles are more frequent and more likely to be expressed. The preferred phenotype is selecting a mate based on physical and behavioral traits. Both inbreeding and preferred phenotype do not change the allele frequency since they do not introduce new alleles or eliminate existing ones. However, they will increase the number of expected homozygous. Therefore, they do not cause microevolution by themselves but contribute to the mechanism.


When certain physical and behavioral characteristics are actively sought out by one sex, sexual selection occurs. It results in sexual dimorphism a marked difference between males and females. The difference between sexual selection and natural selection is that it does not necessarily focus on an organism surviving longer, just its ability to pass on its traits. All individuals are still mating in preferred phenotypes so allele frequency does not change, whereas only specific individuals are mating in sexual selection so allele frequency does change.

Gene flow

Gene flow is the movement of genes from one population to another by the movement of individuals. It needs to have a migration of a fertile individual to a new population, or the transfer of gametes, between populations. It alters the allele frequency, increasing the genetic diversity of the population. For example, male lions leave to form new colonies with genetically unrelated females, thus creating more genetic variation.


Genetic drift

Genetic drift is a change in gene or allele frequencies in small populations. Alleles can be lost randomly from the population, causing dramatic changes in the genetic makeup of the population. It can result in alleles disappearing completely or becoming very common. The smaller the sample, the greater the uncertainty of the results. The bottleneck effect is when a large and temporary reduction in the population may cause significant genetic drift. Also, the founder effect is that a small number of dispersed individuals establish a new population at a distance from the original population by chance, acting on genetic drift.



Reference

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