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Evolution: A Brief Introduction and History Lesson

Ever wondered why dog breeds look so different from each other, how plants can photosynthesize, how bacteria become resistant to antibiotics, or how viral strains such as SARS-COV-2 emerge?

The simplest answer to all these questions and many, many more is “because they evolved”.

What is evolution?

Evolution is the natural process in which heritable characters of a population change across successive generations. Heritable characters encompass any trait (i.e. phenotype) which can be passed down to the next generation, such as colour, height, shape, heat tolerance, chemical resistance, and the presence or absence of a body structure. Meanwhile, a population refers to any biological grouping, although it is commonly used for a species or a group of individuals. For example, humans can evolve, but you yourself cannot.

One important thing for you to remember is that - without evolution, you would not be here. In fact, evolution has made life diverse, whereby animals, plants, protists, fungi, bacteria and even viruses have evolved.

Evolution by natural selection

Figure 1: Statue of Charles Darwin at the Natural History Museum, London.

The current concept of evolution was most famously introduced by Charles Darwin and Alfred Russell Wallace (Figure 1). Both had extensively studied the world’s biodiversity. Darwin worked in the Galapagos islands of South America and the natural laboratory (which was his backyard). Meanwhile, Wallace laboured in the hot and humid Malay Archipelago. Both had also read Thomas Malthus’ An Essay on the Principle of Population, a socio-economic work arguing that the exponential growth of the human population and linear growth of its food supply would eventually lead to famine and death. Synthesizing their observations with Malthus’ work, they proposed that evolution could indeed happen, specifically by a mechanism called natural selection.

Assuming that nature with its limited resources can only support so many organisms, natural selection will bring about the “survival of the fittest”. Specifically, organisms best adapted to their environment will have a higher likelihood of survival and reproduce, meaning that they will be able to overcome selection pressures imposed on them. On the other hand, those that are not selected will die and their traits will be eliminated along with them. If all members of a population die, it is said to be extinct. Of note, evolution does not actively generate new traits in a population, instead it changes the relative numbers of pre-existing traits. With natural selection, Darwin and Wallace had theorized a means for life to naturally change and diversify.

Mendelian inheritance and the genetic basis of evolution

At this time, scientists still did not know how traits were inherited, how this standing variation of traits came about, and the fundamental biomolecules behind this. Darwin proposed a particle which could be inherited in a Lamarckian fashion. In simpler words, he supported the idea that traits acquired during a parents’ lifetime will be passed down to offspring as “pangenes”.

Contrary to this, years before, an abbot called Gregor Mendel had proposed a Mendellian inheritance. From his studies on pea plant coloration, Mendel theorized that parent plants passed down “discrete hereditary factors” which remained unchanged during their lifetime.

It took mathematicians such as the Ronald Fisher and J. B. S. Haldane in the early 20th century to prove that Mendellian inheritance was the form of inheritance compatible with natural selection. It was also during this time that modern genetic terminologies were coined. Hereditary factors were replaced with “genes”, while variants of the same gene were called “alleles”. The genetic changes which fuel evolution were called “mutations” and the total number of genes in a population were referred to as “gene pools”. As a result, the outcome of evolution could now be expressed as an altered gene pool (Figure 2). This joining of natural selection and genetics would be termed the Modern Synthesis, an unified theory of evolution. The final piece of the puzzle was what genes and mutations were made of.

Figure 2: How natural selection drives changes in gene pools. Imagine a population of diploid moths with 2 colours (orange and green). They are coded by a single gene with 2 coloration alleles, A and a, which are initially equal in frequency in the gene pool. Orange is coded by the presence of the A allele, otherwise the moth will be green. In this example, orange moths are more likely to be targeted by predators than green moths. With this, most of the detrimental A allele is removed from gene pool of the original population. Following mating and reproduction of the survivors, a new generation of moths is created. However, its gene pool is likely to have more a alleles and fewer A alleles, significantly differing from the original population’s 50:50 distribution.

Several decades later, the biochemical composition of genes would be discovered, that being DNA (see A Brief Introduction to DNA and its History for more details), and RNA in some viruses. Genes would come to represent the sequences coding for proteins, which are themselves building blocks for naturally selectable heritable characters. Mutations would now be defined as altered coding sequences due to replication mistakes, such as point mutations, insertions, deletions, or rearrangements, resulting in altered protein sequences.

Coupling this increase in biochemical knowledge and advancements in technological complexity, several revolutions in the field of evolution would further emerge. These include the recombinant DNA revolution, where the structure and functions of genes could be elucidated through molecular cloning and genetic modification, and the advent of sequencing technologies, where entire genomes can be analyzed on a computer for signs of evolution.

Mechanisms of evolution

Natural selection is not the same as evolution. Rather, it is the most famous mechanism of evolution as there are many simple yet concrete examples. Selection pressures include biotic factors such as competition and predation, as well as abiotic factors such as climate and resource availability. However, it is not the only mechanism of evolution. In fact, there are three other recognized ways that evolution can occur.

First is through the survival of selectively neutral mutations, which are neither beneficial nor detrimental to survival. Secondly, populations can evolve due to genetic drift. This concept explains that certain genes can be gained or lost by an entire population due to chance survival of individuals, regardless of whether the genes are beneficial, neutral, or detrimental. Catastrophic events or random environmental factors can cause this. Thirdly, gene flow (i.e. migration) between different mating populations can also cause an evolutionary change by altering their respective gene pools. These different mechanisms of evolution can, and often do, happen in tandem with each other, creating evolutionary puzzles for life scientists, such as evolutionary biologists, conservation biologists and population geneticists, to tackle.

Why should you care?

“Nothing in biology makes sense except in the light of evolution.”

Theodosius Dobzhansky

Evolution is a process which has enabled every conceivable form of life to exist, and it is from this diversity of life which every biochemical on the planet has been derived from. The many mechanisms and directions in which different populations evolve(d) provide many variations on themes, outside-the-box solutions to problems in life, and hypotheses ready to be explored in new contexts.

Here is just a selection of cases where evolution may be relevant to your daily life.

1) Implicated in major health and agricultural problems

The evolution of pathogens is of grave concern for agriculture and human health. There are many cases where human treatments have acted as selective pressures driving the evolution of resistant strains. Some important examples include the evolution of antibiotic resistance by pathogenic bacteria and pesticide resistance by crop-damaging insects. Additionally, previously harmless organisms can evolve into pathogens. Most recently, an evolved coronavirus originally infecting bats has evolved mechanisms to invade human cells, causing the COVID-19 pandemic. Following the emergence and spread of additional mutations, more infectious variants have evolved during the course of the pandemic.

Fortunately, evolutionary thinking has given rise to newer, longer-term solutions. Examples of solutions to the aforementioned problems include the development of new drugs, limiting antibiotic usage, releasing genetically modified pests with reduced reproductive capacity, and creating vaccines based on slower-evolving regions of the viral genome. Predictions based on evolutionary theory will inevitably spur pre-emptive solutions for future problems in disease and pest control.

2) Applied to improve important proteins

There exists a Nobel-prize winning biochemical application of evolution in active use today. Inspired by natural selection, a procedure called directed evolution is used to improve proteins such as antibodies, vaccines, AAV (adeno-associated virus) vectors for gene therapy and industrially-important enzymes. This begins with a protein with an already validated function. The gene encoding this protein is then subjected to random mutations to create a library of variants. These variants are subsequently expressed and screened for their desired function, and the most effective variants are preserved for even more refinement.

This method is used as an alternative to rational design of proteins and de novo generation of proteins from protein structural predictions. Critically, these other methods may yield non-functional proteins when translated, and are difficult to perform if the proteins lack pre-existing structural or functional information.

3) Associated with climate change

Lastly, you should care about evolution because it plays a big role in the current climate crisis. The Earth’s temperature is rising more quickly than before, creating a growing selective pressure on many organisms in the planet. Some organisms can survive this, whether it be through an innate tolerance of heat, quicker evolution of thermal tolerance, or the ability to migrate. But others cannot and are at risk of extinction.

A particularly concerning case is the bleaching of coral reefs. Coral reefs are important to humans as they are integral shelter for millions of fish and crustaceans, provide coastal protection from harsh storms and fuel tourism industries. Corals rely on photosynthetic algae to provide food in exchange for shelter but will reject these algae if stressed by excessive heat, dying as a result. Dead corals appear white; hence they are “bleached”.

While heat tolerance has evolved in some populations of corals, they are slow to evolve, and most coral reefs risk may go extinct if the current rate of climate change continues. Without coral reefs, many people will lose their food source, homes, and jobs. This is but one example why climate change should not be ignored, and why you should do every part to slow it down.

Author: James Tan Sheng Yi, BSc Biological Sciences

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