UNIVERSITY OF DUBLIN
TRINITY COLLEGE
TEU00442 - DECODING GENETICS:
THE BUILDING BLOCKS OF LIFE
DECLARATION FOR ASSIGNEMENT
I have read and I understand the plagiarism provisions in
the General Regulations of the University Calendar for the
current year, found at http://www.tcd.ie/calendar.
I have also completed the Online Tutorial on
avoiding plagiarism ‘Ready Steady Write’, located
at http://tcd- ie.libguides.com/plagiarism/readysteady-write.
I declare that this Assignment does
not contain material which has been
PLAGIARISED and have NOT USED
ARTIFICIAL INTELLIGENCE (AI)
technology to generate this essay
Signed: Kaushalraj Ranga Janardhanan Thayumanaswamy
Dated: 30/04/2023
The word count of this document (excluding the
References section and figure legends) is:
1183 words
Signed: Kaushalraj Ranga Janardhanan Thayumanaswamy
Dated: 30/04/2023
The shaping of human genetic variation by selection
The genetic makeup of living beings has been affected by various forces of evolution over the
course of millions of years, and natural selection is one of the main forces. Natural selection is
a major driving factor in how genetic variation is caused in populations which leads to the
evolution of various organisms. It can be defined as the change in allele frequency due to
differences in reproductive fitness among individuals in a population. When an allele is seen
to be more beneficial to the reproduction of the population, the members of the population with
the particular variation would pass it on to future generations, and the frequency of the variation
would increase. The basis of natural selection is genetic mutations as random occurrences of
changes in alleles result in them being naturally selected into a population due to their
advantageous nature. Natural selection mainly is influenced by the organisms’ phenotype,
which leads to the specific genotype becoming more prevalent in its future progeny.
Human genetic variation is the differences in the genomes of every human being arising from
various genetic traits, such as hair colour, and skin pigmentation. A huge aspect of these
variations has been caused due to natural selection, and it has had a huge impact on human
evolution. This essay will discuss the basis and different aspects of natural selection and
explore its effect on various human traits.
The concept of natural selection was first introduced in Charles Darwin’s book, On The Origin
of Species (1859). His inference on natural selection was that variations in the survival and
reproduction of individuals in a population were not random and resulted in the increase in the
proportion and rate of a specific trait being transferred into the next generations of the
population. This inference was based on Darwin’s observations of how individuals were not
similar and had varying traits, and that these traits were hereditary and passed on from one
generation to the next. This inference created the basis for research on natural selection and led
to the concept of biological fitness.
Fitness describes the reproductive success of an individual organism. It can be defined as a
holistic quantity that shows how majorly phenotypes, and therefore its genotypes, with regard
to its organism’s environment affect its offspring and possibly reflects the census size of a
population (Sober, 2000). Fitness is relative to each organism and heavily depends on the trait
and the atmosphere of the organism’s population is in. The fitness of the trait determines how
natural selection would act upon it and how it propagates into future populations. Hence it can
be seen how heavily natural selection is affected by it.
Natural selection can be classified into 3 types based on the expression of traits – Directional
selection, disruptive selection and stabilizing selection
Directional selection is when an extreme phenotype is preferred
above other variations in a population. The likelihood of
individuals with the advantageous trait surviving and reproducing
increases in this process, increasing the frequency of that trait in
the population. This kind of selection can happen in reaction to
environmental changes, such as the appearance of a new predator.
For instance, if a predator moves more quickly, individuals with
greater speed will be more likely to live, procreate, and pass on
their genes for increased speed to subsequent generations. The
population will gradually begin to favour the quicker phenotype. A population's features can
change quickly as a result of directional selection, making it an impactful factor in evolution.
Disruptive selection is a process in which extreme variations of a trait are preferred over
intermediate forms (Rueffler et al., 2006). Natural selection of this type can take place when
various selective pressures act on the two opposite ends of the spectrum of a property. Due to
the rejection of individuals with intermediate traits, disruptive selection can result in the
emergence of two distinct subpopulations with divergent traits. The two subpopulations might
eventually stop reproducing with one another, creating a new species. Disruptive selection is a
major force behind evolution because it fosters population divergence and chances for unique
adaptations in a variety of contexts. This type of selection can have significant impacts on the
ecology and traits of a species.
Stabilizing selection is a type of natural selection in which intermediate variants of a trait are
favoured over extreme variations (Charlesworth, 2013). When individuals with average traits
have more biological fitness than those with extreme ones, this selection takes place. As a
result, the population has a higher frequency of the intermediate trait. Stabilizing selection is a
key force in evolution since it contributes to the continuity of a population's traits over time. It
can take place in a range of situations. As it works to preserve the balance of the population
rather than encouraging change, this kind of selection can be thought of as an equalising force.
Stabilizing selection is often observed in situations where extreme traits are associated with
high costs or risks.
An example of stabilizing selection in humans is the birth weight of babies. New-born babies
with low body weights lose heat more quickly and are more likely to contract infections,
whereas babies with higher body weights are more challenging to deliver. An infant with a
more average or mean weight has a substantially higher survival rate, due to the aforementioned
problems (Karn and Penrose, 1951).
Skin pigmentation is another trait, found in humans,
which can be explained by natural selection.
Researchers suggest that, as early Asians and
Europeans started migrating from Africa, colder
temperatures and low sunlight environments began to
influence their genetic traits that control the
phenotype of skin pigmentation or the amount of
melanin in the populations’ skin, and this resulted in
the natural selection of this trait. Positive selection has been seen in the genes of Europeans
and Asians alone, and there has been retention in the trait for dark skin tone in African
populations when sunlight exposure is very high.
Another example of natural selection is the trait of lactose tolerance in specific human
populations. The enzyme to digest lactose, which is majorly present in milk, in many mammals,
including humans, reduces significantly have they mature and reach adulthood. But it has been
seen in populations with European ancestry, that there has been a genetic modification in their
LCT gene which allowed them to produce the enzyme to digest lactase into adulthood and this
is now prevalent in a majority of the population. This mutation is speculated to have affected
the population 5000 to 10000 years ago. This could be due to the increase in the availability of
cow’s milk and the domestication of cattle by early Europeans. It also provides them with an
additional source of vitamin D, as the sun exposure is lower.
In conclusion, it can be seen that natural selection has had a major impact on shaping human
genetic variations. There are many speculations that natural selection has been affected due to
humans surpassing many barriers of natural evolution, but the examples mentioned do show
the effect of natural selection on humans which still do affect us.
References
Darwin, Charles (1859), On the Origin of Species by Means of Natural Selection, or the
Preservation of Favoured Races in the Struggle for Life (1st ed.), London: John Murray
Sober, E. (2000) ‘The Two Faces of Fitness’, in Thinking about Evolution - Historical,
Philosophical, and Political Perspectives Cambridge University Press, Cambridge. 2000
Rueffler, C., Van Dooren, T.J.M., Leimar, O. and Abrams, P.A. (2006). Disruptive selection
and then what? Trends in Ecology & Evolution, [online] 21(5), pp.238–245.
doi:https://doi.org/10.1016/j.tree.2006.03.003.
Charlesworth, B. (2013). Stabilizing Selection, Purifying Selection, and Mutational Bias in
Finite Populations. Genetics, 194(4), pp.955–971.
doi:https://doi.org/10.1534/genetics.113.151555.
KARN, M.N. and PENROSE, L.S. (1951). BIRTH WEIGHT AND GESTATION TIME IN
RELATION TO MATERNAL AGE, PARITY AND INFANT SURVIVAL. Annals of
Eugenics, 16(1), pp.147–164. doi:https://doi.org/10.1111/j.1469-1809.1951.tb02469.x.
Schaffner, S. & Sabeti, P. (2008) Evolutionary adaptation in the human lineage. Nature
Education 1(1):14