The amateur scientists, the real experts, both scientific and medical who don’t always agree, the TV pundits and the social media no-bell winners are having a field day. The world’s knowledge on viral outbreaks is at an all-time high with at least 3,258 conflicting theories about the origin, the transmission and the treatment of the novel coronavirus Covid-19. But very few people really understand where life comes from and how it has evolved. To understand that we have to go back to Charles Darwin and Alfred Russell Wallace and the very start of the exploration into The Origins of the Species 250 years ago.
The Evolution of Darwin’s Theory of Evolution
Darwin’s theory is based on the notion of variation. It argues that the numerous traits and adaptations that differentiate species from each other also explain how species evolved over time and gradually diverged. Variations in organisms are apparent both within domesticated species and within species throughout the natural world. Variations in colours, structures, organs and physical traits differentiate a multitude of species from one another. Heredity is the mechanism that perpetuates variations, Darwin argues, as traits are passed from parents to offspring. What is important about these variations to Darwin, though, is the way they allow species to adapt and survive in the natural world. He gives numerous examples of variations that illustrate the wondrous adaptations that allow species to survive in their natural environments: the beak that allows the woodpecker to gather insects, the wings that allow the bat to fly, the paddles that allow the porpoise to swim and so on. Darwin hypothesizes that the minor variations we see within a single species—such as variations in size, shape and colour of organisms—are related to the more distinct variations seen across different species. His theory of evolution explains how variations cause the origin of species.
But does this apply to viruses? Are they really a lifeform that evolves?
For about 100 years, the scientific community has repeatedly changed its collective mind over what viruses are. First seen as poisons, then as life-forms, then biological chemicals, viruses today are thought of as being in a grey area between living and non-living: they cannot replicate on their own but can do so in truly living cells and can also affect the behaviour of their hosts profoundly. The categorization of viruses as non-living during much of the modern era of biological science has had an unintended consequence: it has led most researchers to ignore viruses in the study of evolution. Finally, however, scientists are beginning to appreciate viruses as fundamental players in the history of life.
What would Darwin have made of this if he had known about viruses?
Natural selection is the key component of Darwin’s theory, as it explains the relationship between variation and the eventual evolution of a species. Borrowing from Thomas Malthus’s principle of exponential population growth, Darwin argues that the possibility of infinite growth of population sizes is checked by the limits of geography and natural resources, which will not allow an infinite number of beings to survive. As a result of limited food, water, shelter and so on, species must engage in a “struggle for existence,” creating competition for survival. What decides, then, which species will survive and which will become extinct? Here is where “natural selection” comes in. Darwin argues that organisms exhibiting “advantageous variations”—variations that will allow them to adapt to their environment better than other organisms do—will be more likely to survive. Through heredity, these advantageous variations will be passed on to the organisms’ offspring. Eventually, natural selection will allow those species best adapted to their environments to survive and prosper, while species without these advantageous adaptations will lose the struggle for existence and become extinct.
But if viruses are not “alive” do these rules apply? They seem to, although viruses do not strictly produce “offspring”.
It is easy to see why viruses have been difficult to pigeonhole. They seem to vary with each lens applied to examine them. The initial interest in viruses stemmed from their association with diseases—the word “virus” has its roots in the Latin term for “poison.” In the late 19th century researchers realized that certain diseases, including rabies and foot-and-mouth, were caused by particles that seemed to behave like bacteria but were much smaller. Because they were clearly biological themselves and could be spread from one victim to another with obvious biological effects, viruses were then thought to be the simplest of all living, gene-bearing life-forms.
So Darwin’s idea that species have a process of “natural selection” could also apply to viruses?
Natural selection is the mechanism that leads to “descent with modification,” (Darwin’s term for the process of evolution). Organisms will continually give birth to offspring that carry variations, some of which are advantageous and some of which are not. As advantageous variations are naturally selected and become perpetuated through successive generations, organisms carrying these advantageous variations will diverge from the original species, eventually becoming a species of their own. Continual modification and divergence, then, create a branching scheme of evolution, in which new species continually branch off from old ones. The “branches” help biologists link later species back to an original parent species, identifying the point at which different species are related to one another. Darwin notes that existing classification systems developed by naturalists already show these relationships between species. Darwin’s theory of descent with modification, then, simply provides an explanation for why many species seem so similar: Either they evolved from one another or they both evolved from a common parent species.
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Darwin also attempts to explain how variations occur in species, driving natural selection and the creation of new species. Geographical isolation is a key component of Darwin’s theory. Darwin hypothesizes that because all species originated from one or a few original beings, species needed modes of transportation to migrate between geographical areas throughout the world. Barriers such as oceans and mountain ranges restrict the ability of organisms to migrate, and the few that manage to do so play a large role in shaping the evolution of species on islands and in geographically isolated areas. Geographical isolation accounts for the plethora of unique species on islands, as well as the wider distribution of species across continents.
The spread of viruses in human and animal hosts is, therefore, the perfect way to overcome this challenge, and isolation of hosts is the answer to limiting the spread since the virus cannot “live” outside the host.
The seemingly simple question of whether or not viruses are alive, which many people are now asking, has probably defied a simple answer all these years because it raises a fundamental issue: What exactly defines “life?” A precise scientific definition of life is an elusive thing, but most observers would agree that life includes certain qualities in addition to an ability to replicate. For example, a living entity is in a state bounded by birth and death. Living organisms also are thought to require a degree of biochemical autonomy, carrying on the metabolic activities that produce the molecules and energy needed to sustain the organism. This level of autonomy is essential to most definitions.
Of course this argument started much earlier and brought the scientific and the religious leaders to blows.
Darwin’s theory challenged not only the prevailing view of the independent creation of species but also larger claims of religion and science. Darwin explicitly denied the validity of natural theology, which posited that species’ adaptations to their environments, was proof of their “intelligent design” by a creator. It was natural selection, not independent creation, that resulted in these adaptations, Darwin argued. Moreover, Darwin’s use of scientific methodology to prove his theory amounted to an explicit critique of naturalists who would attempt to ignore the scientific validity of his theory because of its controversial nature.
While the text of The Origin of Species did leave room for religious theology, Darwin’s overall commitment to scientific rationale rather than theological reasoning pitted him against religious doctrine. Darwin’s text was controversial when it was published, and it remains controversial today. However, his theory of natural selection has stood the test of time in scientific circles, and it remains the leading scientific explanation for the origin of species.
So by all standards viruses are a “species” and follow most of Darwin’s rules, but with a difference…
Viruses parasitize essentially all bimolecular aspects of life. That is, they depend on the host cell for the raw materials and energy necessary for nucleic acid synthesis, protein synthesis, processing and transport and all other biochemical activities that allow the virus to multiply and spread. One might then conclude that even though these processes come under viral direction, viruses are simply non-living parasites of living metabolic systems. But a spectrum may exist between what is certainly alive and what is not.
A rock is not alive. A metabolically active sack, devoid of genetic material and the potential for propagation, is also not alive. A bacterium, though, is alive. Although it is a single cell, it can generate energy and the molecules needed to sustain itself, and it can reproduce. But what about a seed? A seed might not be considered alive. Yet it has a potential for life, and it may be destroyed. In this regard, viruses resemble seeds more than they do live cells. They have a certain potential, which can be snuffed out, but they do not attain the more autonomous state of life.
Fascinating stuff and provides a good explanation as to where we are today, but also where we are heading, the evolution of SARS to MERS to Covid-19 is certainly only the beginning, somewhere out there is already Covid-Plus virus ready to take the battle to an even higher plain. Darwin told us 250 years ago. Why didn’t we listen?
Written by Alistair Speirs-20, a slightly improved version of Alistair Speirs-19, with huge assistance from Wikipedia and Scientific American.