12 Health Allied – 14
Jericho Rodel A. Valiente
Evolution
Evolution was first coined by Charles Bonnet in the year 1769 that he described as changes
taking place as time goes by (Dela Pena, Gracilla, & Pangilinan, 2016). Other scientists had challenged
and used Bonnet’s ideas over the course of many years such as Lamarck, Weismann, Wallace, andmost notable- Darwin. They, among many other scientists, had discovered the truths behind evolution
and had presented their evidences and mechanisms of evolution that are still being studied today
Figure 1. Darwin's Evolution of Man
Evidences of Evolution
1. Fossil Evidence
Different kinds of fossils include mineralized bones,
teeth, shells, seeds, spores, and other durable body
parts are used to identify organisms that had existed
in the past and how long they had existed. They were
identified through the process of radiometric dating.
Radiometric dating calculates the age of a fossil
Figure 2. Rodhocetus kasrani
through its radioisotope content and its daughter
elements (Evers, Starr, & Starr, 2015). For humanity
today to discover and know the creatures of the past,
we must find a fossil of it. Afterwards, the discovered
fossils are to be carefully examined to look for any
similarities and/or differences to organisms of the
Figure 3. Dorudon atrox
present. A sample of this is a discovered fossilized
skeleton, Rodhocetus kasrani and Dorudon atrox (see
Figures 2 and 3), that are thought to be ancestors of whales because of similarities in the skull
bones.
2. Drifting Continents
Long ago, all the continents of today were compacted together as a supercontinent called
Pangaea. Over time, it split into fragments of land masses and drifted apart. This theory of
drifting continents was initially met with skepticism until discoveries of ridges and trenches
were discovered in the late 1950s that later birthed to the plate tectonics theory (Evers, Starr,
& Starr, 2015). These geologic changes had a profound impact on the living continents that
were once together in Pangaea. As the continents drift apart, same species of an organism may
be separated and are forced to adapt to the physical changes in its environment and new
species it must coexist with. This, in turn, result to fossils of the same species being found in
different locations that are now very distant with each other. The case of the Mesosaurus is a
great example of this. Fossils of this organism were found in both South Africa and Eastern
South America (refer to Figure 4).
3.
Evidence in Form (Physiological Structure)
This evidence refers to the similarities of body parts of
between species of separate lineages or ancestors.
Morphological divergence refers to homologous
structures where body parts appear similar because
they evolved from the same ancestor but have changed
in its structure. An example of this suggests that many
vertebrates have descended from ancient stem reptiles
that had diversified over millions of years (Evers, Starr,
& Starr, 2015). Morphological convergence, however,
Figure 4. Fossils of Extinct Organisms
refers to independent evolution of similar body parts
found in different continents
but of different lineages that are often called as
analogous structures. An example of analogous structures between animals is the presence of
wings. Many animals have wings and may have the same function, but their physiological
appearances greatly differ from each other. A bird’s wings is different to a bat’s wings as it
different to a dragonfly’s wings.
4. Evidence in Function
In general, the more closely related animals are, the more signs of similarities would show in
their development. An example of this is that all vertebrates undergo a stage in their
development during which a growing embryo has four limb buds, a tail, and a visible
endoskeleton that show the divisions of the body. They have similar patterns in embryonic
development but the reason behind the difference in their adult stages is the different homeotic
genes, called Hox, that mold the growing body in the early stages of development. For example,
insects have a Hox gene called antennapedia that leads to a different growth in its embryo
compared to other vertebrates.
Figure 5. Embryonic Development of Vertebrates (Fish,
Salamander, Tortoise)
Figure 6. Mechanisms of Evolution
Mechanisms/Processes of Evolution
1. Natural Selection
a. Directional Selection – forms at one end of a range of phenotypic variation becomes more
common over time
b. Stabilizing Selection – an intermediate form of a trait is favored, and extreme forms are
selected against
c. Disruptive Selection – forms of a trait at both ends of a range of variation are favored,
and intermediate forms are selected against
d. Sexual Selection – focus on the matter of outreproducing because of better security in
finding mates
2. Genetic Drift
It is the change in allele frequency brought about only by chance. It makes nondistinctive
populations particularly susceptible and vulnerable to the loss of genetic diversity. The concept
of the founder effect and inbreeding are also related to genetic drift. If a small population
establish a new one and does not have the original allele frequencies, then it will not be a
representative or a direct descendant of the of the original. This outcome is called as founder
effect and often, the species involved are closely related and underwent the process of
inbreeding. Inbreeding refers to mating between two closely related organisms that are more
harmful than not.
3. Gene Flow
It refers to the movement of alleles between populations that may change or stabilize allele
frequencies. This process of evolution is closely related to the genetic drift. However, gene flow
focuses on how the transport of alleles (via food sources or chance) influence great changes in
an organism.
4. Speciation
This is the process of evolution where an entirely new species arise from countless mutations,
genetic drift, gene flows, and natural selection. This is somewhat a mix of all other mechanisms
and as such, it is a rare occurrence in the natural world.
References
Dela Pena, R. A., Gracilla, D. E., & Pangilinan, C. R. (2016). General Biology. Pasay City, Manila,
Philippines: JFS Publishing Services. Retrieved August 17, 2019
Evers, C. A., Starr, C., & Starr, L. (2015). Biology Today & Tomorrow (Fifth ed.). Boston,
Massachusetts, USA: Cengage Learning. Retrieved August 17, 2019