Evolution is a difficult concept to grasp. The earliest explanation offered for our existence, usually by an elder in the family, is that ‘we were created by a higher power’. This explanation is often accompanied by descriptions of fantastical beasts and awe-inspiring natural phenomena that seem easier to explain as the whimsy of an all-powerful being.

Often, students of science get their first taste of the theory of evolution by natural selection in high school. Unfortunately, the nuances of this theory are often left unexplored at this stage. Most students are left equating the theory with four words ‘survival of the fittest’. But these words do no service to the elegant mechanism by which this theory explains all the diversity of life on earth.
Foundations of the theory
Independently proposed by Charles Darwin and Alfred Russell Wallace, the theory of evolution by natural selection is based on the premise that organisms have traits that are variable and inheritable. Simply expressed, trait refers to an attribute of an organism.
Traits can vary at every level of organization of the living world (see Box 1). For example, the human trait of height is expressed as a range — people can be tall, short, or of medium height. This range may be different for different ethnic groups. For example, the tallest people in Southeast Asia may be shorter than the tallest people from northern Europe. This means that the trait of height is highly variable in humans (see Box 2).
Traits are also heritable. Children of tall parents are more likely to be tall themselves. Like height, there can be a number of other inheritable traits — like eye colour, skin colour, even the presence of dimples in your cheeks! Traits are transmitted from one generation to the next through genes (see Box 3). Our genes contain the underlying code for each and every trait that is present in an organism. These codes are inherited by offspring from parents.
Individuals with traits most suited to a certain environment are better equipped than others in the same population to survive and reproduce in it. Thus, an organism’s environment acts as a filter, increasing the likelihood of individuals with certain traits to be present in each successive generation (see Box 4).
What does natural selection mean?
When variable and inheritable traits pass through the sieve of the environment, those most favourable for survival and reproduction are found at a higher frequency in the next generation. This process is called selection.
An example of how a change in environment over space creates selection pressures can be seen in the connection between malaria and sickle-cell anaemia. Malaria, caused by a singlecelled parasite that is transmitted by mosquito bites, is more prevalent in some geographies (hot, humid, tropical) than others (cold, dry, temperate). Sickle-cell anaemia, on the other hand, is caused by a mutation in genes coding for hemoglobin (a protein that binds and transports oxygen to every cell of the body). This mutation contorts otherwise doughnut — or uddinavada-shaped (with a depression in the middle instead of a hole) red blood cells (RBCs) into sickle — or crescent-shaped cells with reduced capacity to bind oxygen (see Fig. 2). It so happens that a sickle-shaped cell is bad not just for humans, but for the malarial parasite too. The parasite requires healthy, round RBCs to complete its life cycle. Consequently, in areas endemic to the malarial parasite, humans with sickleshaped RBCs are more likely to survive and have children than humans with only normal RBCs. In other words, the trait of sickle-shaped RBCs is selected for in an environment where malaria is prevalent.
Similarly, the effectiveness of the pesticide dichlorodiphenyltrichloroethane (DDT) offers an example of how changes in environment over time can create unique selection pressures. This extremely toxic chemical was initially very effective in controlling mosquitoes in urban areas. But, soon, it was observed that increasingly higher concentrations of the pesticide were needed to achieve the same level of control. Why? Let us suppose that coded into the genetic material of mosquitoes, was an inheritable, variable trait for pesticide resistance that confers no–, low–, or high–tolerance for DDT-like pesticides (see Fig. 3). In an environment with low concentrations of DDT, mosquitoes with no form of resistance to the pesticide die. Those that can tolerate the pesticide survive and reproduce. Since the trait of resistance to pesticides is heritable, most mosquitoes in the next generation will carry some form of the resistance gene. Consequently, higher concentrations of the pesticide will be needed to kill them. However, any increase in the dose of pesticide will only kill mosquitoes with the lowest tolerance for it. Mosquitoes with a higher tolerance for the pesticide will continue to survive and reproduce. Therefore, each increase in pesticide concentration will cause a corresponding increase in the frequency with which the trait for high tolerance to pesticide is seen in the next generation of mosquitoes. This, in turn, will mean that increasingly higher concentrations of the pesticide will be needed to control each new generation of mosquitoes. Once the concentration of pesticide required for effective control of mosquitoes reaches a level where it has toxic effects on humans, DDT becomes unsafe to use.
Fig. 2. A pictorial representation of sickle-shaped and normal RBC's.
Credits: BruceBlaus, Wikimedia Commons. URL: https://en. wikipedia.org/wiki/File:Sickle_ Cell_Anemia.png. License: CC-BY-SA.
The key to the evolution of a trait is the transmission of its genes from one generation to the next. Thus, how fast traits evolve in different organisms depends on how fast they can reproduce (see Box 5). The reproductive fitness of any individual in a population is determined by the number of its offspring that survive into adulthood. In this sense, ‘survival of the fittest’ does not refer to, say, an individual that emerged victorious from a fight because of its brute strength. Instead, it refers to individuals with traits that improve their chances of survival into adulthood, and the effectiveness with which they transmit their traits to the next generation, under prevailing environmental conditions.
Fig. 3. The effect of varying concentrations of DDT on mosquito populations showing variability in the trait for DDT resistance. The black mosquitoes in the image represent individuals with no resistance to DDT. The blue ones represent individuals with low resistance, and the red ones represent individuals with high resistance to DDT. When low concentrations of DDT are sprayed, the black mosquitoes die out. But the blue and red mosquitoes are able to survive and reproduce. This increases the frequency of blue and red mosquitoes in the population. If higher concentrations of DDT are sprayed on this population, the blue mosquitoes die out. The surviving population is dominated by red mosquitoes or individuals with high-resistance traits. Increasing the concentration of DDT beyond this point might have negative impacts on other organisms, including humans.
Credits: Geetha Ramaswami. License: CC-BY-NC.
How do different species arise?
Based on their ecological requirements and evolutionary origins, the term ‘species’ can be defined in many ways. In the ‘biological species’ concept, two sets of organisms are thought of as different species if they cannot interbreed to produce fertile offspring (see Box 6). This is why leopards and cheetahs, which may seem very similar in their carnivorous habits and spotted fur, are thought of as two different species. In contrast, a Doberman pinscher and a Labrador retriever may seem very different in their behavior and appearance, but belong to the same species (see Fig. 4).
An estimated 8.7 million species (some described, many yet unknown to man) inhabit the earth. These species are further classified under five large groups of living organisms (called kingdoms) — Archaea, Bacteria, Protists, Plants, and Animals. In spite of this remarkable diversity, we know that all life emerged over 3.5 billion years ago, from the same primeval ancestral form, through a process called speciation. The splitting of ancestral forms into daughter species is known as diversification (see Box 7). Diversification is caused, primarily, by prolonged reproductive isolation.
Let us imagine a population that has evolved a set of traits suitable for mostly arboreal habits on a densely forested landmass. Say, some individuals of this population happen to venture over a temporary land bridge (formed, for example, by the sea freezing up during an extremely cold climatic event) to another landmass with much fewer trees. Once the land bridge melted, these individuals would be cut-off from the rest of the population. This would mean that the individuals on either side would no longer have any way of inter-breeding — a large, uncrossable water body would lie between them. Natural selection would take its course — selecting for more and more arboreal traits on one landmass, and for less and less arboreal traits on the other (see Box 8). 
Fig. 4. Understanding the ‘biological species’ concept: A leopard (a) and cheetah (b) look similar, but cannot interbreed to produce viable offspring. They are, therefore, considered to be different species. A Labrador retriever (c) and Doberman pinscher (d) look very different but can interbreed to produce viable offspring. They are, therefore, thought of as belonging to the same species.
Credits: (a) Stephen Temple from Cape Town, South Africa, Wikimedia Commons. URL: https:// commons.wikimedia.org/wiki/File:1M2A5681_ (45925689112).jpg. License: CC-BY-SA. (b) Wegmann, Wikimedia Commons. URL: https:// commons.wikimedia.org/wiki/File:Cheetah_ Umfolozi_SouthAfrica_MWegmann.jpg. License: CC-BY-SA. (c) SixtyWeb, Wikimedia Commons. URL: https://commons.wikimedia.org/wiki/ File:Yellow_Labrador_Retriever_2.jpg. License: CC-BY-SA. (d) YamaBSM, Wikimedia Commons. URL: https://commons.wikimedia.org/wiki/ File:0Doberman-40172501920.jpg. License: CC-BY.
Credits: (a) Peter Griffin. URL: https://www. publicdomainpictures.net/en/view-image. php?image=54539&picture=human-evolution. License: Public Domain. (b) Charles Darwin. URL: https://commons.wikimedia.org/wiki/File:Darwins_ first_tree.jpg. License: Public Domain.
A more accurate representation of speciation is in the form of a tree that shows how a common ancestral species with certain traits split into daughter species with their own set of traits (see Fig. 5b). This process has repeated itself over time, with each daughter species splitting into another set of daughter species, and so on. Seen from the perspective of an evolutionary tree, it becomes clear that humans came, not from apes, but from an ancestral species that we share with apes.
If these two populations remained separated from each other for long enough (~ thousands or millions of years), they would lose the ability to interbreed altogether and form two different species (see Box 9). This process, where a natural barrier causes the genetic isolation that gives rise to a new species, is known as allopatric speciation.
Physical barriers are not the only reason two populations become genetically isolated over time. In some cases, environmental conditions may be localized in such a way that two different types of selection pressures act on different members of a population at the same time (see Box 10). If these different sets of individuals do not cross-breed for long enough, this process of sympatric speciation could give rise to a new species.
Parting thoughts
The ever-changing environment is constantly selecting for traits that make organisms more likely to survive and reproduce than others. When selection acts on reproductively isolated groups of individuals, it can lead to speciation. This process can be used to explain the vast diversity of life that we see on earth today. New species and their evolutionary histories continue to be discovered every day, leading us one step closer to unravelling the mystery of how life originated on earth.
Key takeaways
• The theory of evolution by natural selection can be used to explain the vast diversity of living organisms on earth.
• This theory is based on the premise that all organisms have variable and inheritable traits.
• Changes in the environment of an individual (over space and/or time) act as filters or sieves that select for certain traits.
• Traits that are favourable for survival and reproduction in a certain environment are likely to be inherited by a higher percentage of individuals in the next generation.
• Evolution is not a linear process where one species changes into another. Instead existing life forms split from their ancestral life forms at specific times in the earth’s evolutionary history.
• The splitting or diversification of species is caused by the reproductive isolation of ancestral populations.
• Reproductive isolation between individuals of the same species can occur because of physical barriers or highly localized environmental conditions.
Note: Source of the image used in the background of the article title: https://en.wikipedia.org/wiki/File:Haeckel_Muscinae.jpg. Credits: Haeckel Muscinae (Mosses) by Ernst Haeckel, Kunstformen der Natur or Art forms in Nature (1904), plate 72: Muscinae, Wikimedia Commons. License: CC-BY-SA.

Geetha Ramaswami heads SeasonWatch (www.seasonwatch.in), a citizen science project aimed at understanding seasonality through tree phenology, based at the Nature Conservation Foundation (NCF), Bengaluru. She can be contacted at geetha@ncf-india.org.

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