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1.- Natural Selection

Natural Selection leads to an evolutionary change when some individuals with certain traits in a population have a higher survival and reproductive rate than others and pass on these inheritable genetic features to their offspring. Evolution acts through natural selection whereby reproductive and genetic qualities that prove advantageous to survival prevail into future generations. The cumulative effects of natural selection process have giving rise to populations that have evolved to succeed in specific environments. Natural selection operates by differential reproductive success (fitness) of individuals.

The Darwin’s Finches diagramillustrates the way the finch has adapted to take advantage of feeding in different ecological niches:

2.- 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.

In population genetics, 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 fertilization.

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.