Mechanisms of Evolutionary Change This unit we are learning that there are sever
ID: 53947 • Letter: M
Question
Mechanisms of Evolutionary Change
This unit we are learning that there are several mechanisms that lead to evolutionary change within a population or species: mutation, migration, genetic drift, non-random breeding and natural selection.
What is meant by “evolutionary change”; meaning what is it and how would you know when it has occurred?
Select two of the mechanisms listed above and explain how each can lead to evolutionary change as defined in Q#1. Be specific and use an example of each.
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Explanation / Answer
Evolutionary change:
Microevolution refers to the changes in allele frequencies within a single population. Allele frequencies in a population may change due to four fundamental forces of evolution: Natural Selection, Genetic Drift, Mutations and Gene Flow. Mutations are the ultimate source of new alleles in a gene pool.
Two of the most relevant mechanisms of evolutionary change are: Natural Selection and Genetic Drift. One of the main controversial issues in population genetics is concerned with the relative importance of both mechanisms in determining evolutionary changes.
Mechanisms of Evolutionary Change:
Genetic Drift:
Random Drift consists of random fluctuations in the frequency of appearance of a gene, usually, in a small population. The process may cause gene variants to disappear completely, thereby reducing genetic variability. In contrast to natural selection, environmental or adaptive pressures do not drive changes due to genetic drift. The effect of genetic drift is larger in small populations and smaller in large populations.
Genetic drift is a stochastic process, a random event that happens by chance in nature that influences or changes allele frequency within a population as a result of sampling error from generation to generation. It may happen that some alleles are completely lost within a generation due to genetic drift, even if they are beneficial traits that conduct to evolutionary and reproductive success. Allele is defined as any one of two or more genes that may occur alternatively at a given site (locus) on a chromosome. Alleles are responsible for variations in a trait.The population bottleneck and a founder effect are two examples of random drift that can have significant effects in small populations. Genetic drift works on all mutations and can eventually contribute to the creation of a new species by means of the accumulation of non-adaptive mutations that can facilitate population subdivision.
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for understanding:
Bottleneck effect occurs when there is a sudden sharp decline in a population’s size typically due to environmental factors (natural disasters such as: earthquakes or tsunamis, epidemics that can decimate the number of individuals in the population, predation or habitat destruction, etc.). It is a random event, in which some genes (there is not any distinction) are extinguished from the population. This results in a drastic reduction of the total genetic diversity of the original gene pool. The small surviving population is considerably be farther from the original one in its genetic makeup.
Founder effect is the loss of genetic variation that occurs when a new population is established by a small number of individuals that are cleaved from a larger population. This new population does not have the genetic diversity of the previous one. Because the community is very small and also geographical or socially isolated, some genetic traits are becoming more prevalent in the population. This leads to the presence of certain genetic diseases in the next generations. In some cases, founder effect plays a fundamental role in the emergence of new species.
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Mutation:
Mutation can be defined as a change in the DNA sequence within a gene or chromosome of a living organism. Many mutations are neutral, i.e. they can neither harm nor benefit, but can also be deleterious or beneficial. Deleterious mutations can affect the phenotype and in turn, reduce the fitness of an organism and increase the susceptibility to several illnesses and disorders. On the other hand, beneficial mutations can lead to the reproductive success and adaptability of an organism to its environment. These beneficial mutations can be spread and fixed in the population due to natural selection processes if they help individuals in the population to reach sexual maturity and to successfully reproduce. Mutations are, undoubtedly, a source of genetic variation and serve as a raw material for evolution to act. Germ line mutations occur in gametes (eggs or sperm cells) and can be pass on to offspring, whereas somatic mutations occur in non-reproductive cells and are not pass on to the following generation. Those mutations that occur in germ line are the most important to large-scale evolution because they can be transmitted to offspring.
Mutations can be spontaneous (errors during a normal process of DNA replication, spontaneous lesions and transposable genetic elements), but they can also be induced by numerous external or exogenous factors like environmental chemical agents or ionizing radiation, for example. According to their magnitude (mutations can occur at different levels), they can be divided into three different groups: Gene mutations, chromosome mutations and genome mutations. The DNA is constantly subject to mutations, thus its sequence can be altered in several different ways. A gene mutation can be defined as any change in the sequence of nucleotides of the genetic material of an organism. A chromosome mutation is a change in the structure or arrangement of the chromosomes. These mutations can involve duplications or deletions of chromosome segments, inversions of sections of DNA (reversed positions) and translocation. Genome mutations are alterations in the number of chromosomes in the genome. They can be classified into two groups: Aneuploidy and Euploidy. Aneuploidy is the losses and/or gains of individual chromosomes from the normal chromosome set arising from errors in chromosome segregation, and Euploidy refers to variations in complete sets of chromosomes.
Gene Flow:
Gene Flow (also known as gene migration) refers to the transfer of genes from the gene pool of one population to another. Gene flow may change the frequency and/or the range of alleles in the populations due to the migration of individuals or gametes that can reproduce in a different population. The introduction of new alleles increases variability within a population and allows for new combinations of traits. Horizontal gene transfer (HGT) also known as lateral gene transfer (LGT), is a process in which an organism (recipient) acquires genetic material from another one (donor) by asexual means. It is already known that HGT has played a major role in the evolution of many organisms like bacteria. In plant populations, the great majority of cases linked to this mechanism have to do with the movement of DNA between mitochondrial genomes. Horizontal gene transfer is a widespread phenomenon in prokaryotes, but the prevalence and implications of this mechanism in the evolution of multicellular eukaryotes is still unclear. Nevertheless, many investigations on HGT in plants have been carried out during the last years trying to reveal the underlying patterns, magnitude and importance of this mechanism in plant populations as well as its influence on agriculture and the ecosystem.Plant populations can experience gene flow by spreading their pollen long distances away to other populations by means of wind or through birds or insects (bees, for example) and once there, this pollen is able to fertilize the plants where it ended up. Pollen is a fine to coarse powder containing the microgametophytes of seed plants, which produce the male gametes (comparable to sperm cells). Of course, pollination does not always lead to Maintained gene flow also acts against speciation by recombining the gene pools of different populations and in such a way, repairing the developing differences in genetic variation. Thus, gene flow has the effect of minimizing the genetic differences between populations.Human migrations have occurred throughout the history of mankind and are defined as the movement of people from one place to another. However, in a genetic context, this movement needs to be associated with the introduction of new alleles into a population through successful mating of individuals from different populations.