Introduction
There are two main reasons why geneticists scientists want to identify selection. Firstly, they are interested in our evolutionary past, and secondly they want to understand the fundamental processes that control molecular evolution. They believe molecular evolution is where it all began
In this article, we will journey back in time to 2001, when the first human genome was published in a public database and learn about the process of natural selection together.
What is Natural Selection?
Ever since Charles Darwin published his classic work "On the Origin of Species" one hundred and fifty years ago, the evolutionary paradigm of adaptation by natural selection among the available gene diversity of individuals has been celebrated.
Natural selection is the process by which certain traits become more or less common in a population over time. This happens because some individuals are better adapted to their environment than others, and they are more likely to survive and reproduce. As a result, their advantageous traits are passed on to their offspring, while less advantageous traits are not.
Genome Sequencing
Over the past forty years, scientists’ research in this area has concentrated on calculating the relative weights of Darwinian selection and random genetic drift in determining the degree of variation within and across species.
However, as evidence for selection’s significant role grows, efforts are focusing more on pinpointing and describing specific examples of selection and adaptation at the molecular level.
Identifying genes in humans that have recently undergone selection, specifically in relation to important human attributes like cognitive ability, has attracted a lot of interest.
Scientists are gradually getting better at the process of annotating a genome sequence—which involves locating genes, repetitions, single nucleotide polymorphisms (SNPs), micro-RNAs, and other genomic features. Although there is still much to learn, cytogeneticists are beginning to shed light on the function, history, and origins of genomic patterning.
These discoveries improved awareness of the evolutionary mechanisms that shape the human genome, expanded our understanding of gene function, and hold promise for improving our knowledge of the genetic roots of diseases and evolution.
Examples of Natural Selection
One example of human adaptation as a result of exposure to an environmental selective pressure is found in populations living on high-altitude plateaus.
Several studies have analysed genomic signals of high-altitude-related natural selection such as the Andean Altiplano in South America, as a means of investigating altitude adaptation.
Low-altitude humans experience a complex, plastic physiological response to high altitude, which is characterized by an immediate decrease in plasma volume, an increase in ventilation that occurs over days, and an increase in the production of red blood cells that occurs over weeks to months.
Taken together, this adaptation has been passed down through generations, making it easier for people living in these areas to survive. These adaptations serve to raise hemoglobin (Hb) concentration and help offset the reduced arterial O2 content because of the lower inspired partial pressure of oxygen (pO2).
Another example of genetic diversity and natural selection is the great genetic heterogeneity of pharmacological responses in humans, which are caused by genetic variants in numerous genes involved in drug absorption, distribution, metabolism, and excretion (ADME genes).
The way a person responds to drug therapy is important in determining how well they will respond to treatment since atypical responses to pharmaceuticals may result in a lack of therapeutic effect or an unfavorable drug reaction, which can have medical and financial consequences if not properly understood.
To what extent genetic diversity in the ADME genes may account for variations in drug reactions among human cultures still remains unknown as of today.
One last example is the classical research that looked at the distributions of physiological characteristics in humans around the world, such as body size and shape, basal metabolic rate, and pigmentation. It revealed that recent human evolution may have been influenced by climate natural selection.
Strong relationships between allele frequency, the word that we use to describe the alternative form or versions of a gene, and climate factors were discovered to be linked with common metabolic diseases.
The Role of Negative and Positive Selection
Assuming that all mutations are either harmful or neutral, scientists Eyre-Walker and Keightley, estimated that at least 38% of all new amino-acid changing mutations in the human genome are being removed by negative selection (that is, having no effect on organism fitness).
This was an underestimate, as stated by the authors of this work, and later studies have revealed that as many as 70–75% of mutations that modify amino acids are subject to moderate or strong negative selection. A significant portion of this selection may act at the level of gametogenesis, on mature gametes, or in the early stages of development meaning that it happens and passes unnoticed as those with unfavorable mutations aren't born.
This suggests that only mutations with a minor negative effect are likely to persist in a population. Natural selection will rapidly eliminate mutations that have a severe impact on fitness, leaving only mutations that, at most, have a modest negative impact on fitness.
Positive selection, on the other hand, happens when a new or previously uncommon mutation gives its carriers a fitness benefit. This kind of selection has received a lot of interest because it leaves molecular traces of evolutionary adaptation.
Positive selection is generally useful when trying to understand processes or species-specific adaptations that have to do with dynamic interactions between an organism and its environment over time.
By examining how positive selection has changed the human genome over the past few hundred thousand years, we can learn a lot about the biological drivers of the evolution of modern humans.
Conclusion
Natural selection is a powerful force in the process of evolution and understanding how it works, geneticists can better understand how species have adapted to their environment over time.
This knowledge is important for understanding the genetic basis of not only genetic change but also phenotypic change. As we continue to face the challenges of climate change, it is more important than ever to understand the role of natural selection in shaping the world around us.
References
Hancock A., et al(9) “Adaptations to Climate-Mediated Selective Pressures in Humans” - PLoS Genet. 2011 April; doi: 10.1371/journal.pgen.1001375. PMCID: PMC3080864. PMID: 21533023. https://doi.org/10.1371/journal.pgen.1001375
Oleksyk T., Smith M. & O'Brien S. “Genome-wide scans for footprints of natural selection” - The Royal Society Publishing. 2010 January 12; doi: 10.1098/rstb.2009.0219. PMCID: PMC2842710. PMID: 20008396. https://royalsocietypublishing.org/doi/full/10.1098/rstb.2009.0219
Nielsen R., et al(4) “Recent and ongoing selection in the human genome” - Nature Reviews Genetics. 2007 November; doi: 10.1038/nrg2187. PMCID: PMC2933187. PMID: 17943193 https://www.nature.com/articles/nrg2187#Bib1
Bustamante C., et al(13) “Natural selection on protein-coding genes in the human genome” - Nature Reviews Genetics. 2007 October; doi: 10.1038/nature04240. PMID: 16237444. https://www.nature.com/articles/nature04240
Figure (1) Retrieved April 12, 2023, from: https://www.science.org/doi/10.1126/science.abl3533
Figure (2) Retrieved April 16, 2023, from: https://www.frontiersin.org/articles/10.3389/fgene.2020.00743/full
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