The Reasons Why We Know That Evolution Is True

Introduction
The development of the concept of evolution is, in essence, the story of the development of the scientific method. It begins with observation, continues through the development and testing of hypotheses, adds replication, and finally, affirmation of the strength of the theory. The fact that evolution is one of the most-challenged theories in all of science has ended up being one of its strengths. Because so many have sought to reject it, to undermine it, and denigrate it, the theory of evolution has been forced to stand up to all of these challenges. And it has. The reality is that the theory of evolution is one of the most robustly supported theories in the whole of scientific study, and it had to be, in order to withstand these challenges. The constant testing, adaptation and yes, evolution of the theory of evolution over the years has made it the perfect story of the validity of all science.
The Basic Theory of Evolution
Charles Darwin set out his theory of evolution in On the Origin of Species as a means of explaining the differences between species and subspecies of plants and animals. At the outset of the book, he observes that cultivated plants differ significantly from their wild forebears, but also from each other. In other words, apples produced in one geographic region will take on a character of their own, different from apples produced in another region. So different not only from wild applies, but from other cultivated varieties, having adapted over successful generations to the particular conditions in which they live. This basic observation is then extended by Darwin to the natural world, based on his observations there.
He was writing at a time characterized by intense exploration – both of the physical world but also of the natural world, and his work incorporated both of these. He traveled widely collecting specimens and making observations. From this, and his initial observation about cultivated plants, that plants and animals in nature may also derive from an original root species, and if they are segregated by geography or other factors, may end up over time sufficiently different from their forebears. In some cases, the forebear might still exist, while in other cases it may not. But in either case, the animal or plant in question would have developed into a unique species over time, and with sufficient segregation from the forebear species. Thus he observed many relatively common animals in different variants around the world.
He then outlines the theory of natural selection, in particular as distinguished from selection by man. Selection by man is a clear point of differentiation that has helped Darwin formulate his theory. He identifies an observation in a population, and is curious to see if that same phenomena can be observed in another population, in this case one unaffected by man's intervention. Without man to influence the development of the unique species, there are nevertheless unique species that have developed. At this point, Darwin is starting to flesh out the hypothesis on which the theory of natural selection rests. He further observes that for all the mutations that have resulted in different species, there is typically some underlying reason for the change in phenotype – and by extension genotype, of course. The differences between species are result of specific traits being selected over time. If an animal migrates to a colder climate, it grows a thicker coat. If that same animal lives in the desert, it might need a thinner coat, but also better hearing because it may prefer to hunt in the cool of night. These types of observations, taken individually, demonstrate little other than that differences between species exist, and that there is a reason for the differences. What is missing at this point – the point where dozens or hundreds of observations have been made- is any theory to bind the observations together.
That is where natural selection and evolution come into play, because they argue that the observable trait differences in these different species have come about specifically over the course of time, because the defining differences in traits are valuable to the survival of the species within the context of its environment. In other words, nature selects over time for those traits because animals or plants that lack the desired traits survive to reproducing age at a lower rate than those animals or plants that possess the desired traits. The ones that survive at a higher rate to reproducing age will, over time, pass along the genes associated with the very traits that have allowed for that survival. Each animal or plant, therefore, is optimized to survival within the context of that environment.
The final component is that the process is a work in progress. This is what is meant by evolution – the process of evolution in most species is ongoing, or at the very least subject to change if there is a change in the environment. If no change in the environment, a species can exist for a very long time without evolving – most sharks are a good example – but in a changing environment a species either evolves, leaves that environment, or gradually dies out. While we see species in the context of what they are today – a brief moment in time – evolution typically takes place over a much longer period, and is a random, ongoing process, controlled by nothing more than the passing on of successful genes.
Testing the Hypothesis
Darwin laid out the theory of evolution based on natural selection, and immediately the world took notice. He did not perform empirical tests on the theory – it was based on his observations, and logical reasoning that tied that observations together. A theory is nice to have, but proves nothing in and of itself. A theory is really only the second step in this scientific process. Evolution would quickly come under challenge, by those whose power was undermined by the theory's very existence, as they lacked the intelligence to adapt the stories from which they derived their power to the emerging science. These challenges, arguably, created an imperative to test the theory of evolution. Not only was there tremendous curiosity with respect to proving the theory true, just from a scientific point of view, but there was also an imperative to do so, as evolution would prove to be a flashpoint, a cultural battleground, between those who wish to see the world for what it is, and those who fear loss of power should they be demonstrated charlatans.
So it was that the testing of the theory of evolution began. The best test for a theory that covers all life on earth is to find out if it can predict what will happen. Evolution cannot be proven in any single one-off experiment, after all. Natural selection must be the basis for a wide variety of experiments. If its predictions hold true time and again, then this strengthens the evidence for the theory. Ayala (2009) notes that natural selection is the most important component of Darwin's work because it provides the basis for tests today – the underlying principle of natural selection is still part of experiments. This is not to say that longevity is what gives natural selection its power, but the fact that it has been used to successfully predict the outcomes of thousands of different experiments. There are few scientific theories that can claim as many successful predictions as natural selection.
It was more than imperative of course, that has allowed for this. Most theories also do not cover all living beings. It is easy to design thousands or tens of thousands of experiments to test the theory, if given enough scientists and enough time. Natural selection has had both – and it has successfully predicted a statistically valid number of test. Indeed, it has never really failed to predict.
Preponderance of Evidence
A theory requires testing, but then there is the question of how many tests should a theory be subject to before it is accepted as truth. This is where mathematics become involved. There is a point, when measuring different observations, that the weight of those observations takes on verifiable meaning. When enough studies confirm that a theory holds true, versus whatever studies might fail to confirm this, there comes a point when there is statistical validity, that point at which the chance that the theory does not hold true is infinitesimal. The theory of natural selection is well past that point – the preponderance of evidence supports natural selection, and this in turn is the core of the theory of evolution.
Philosophical Thoughts
The philosophy community does not have any particular argument against the core matter of evolution. As Millstein (2017) notes, philosophers tend to accept the scientific method as a robust mechanism for determining truths. This may not have been the case in Darwin's time, but it certainly is today. Further, core concepts such as heritability are accepted. Millstein notes that philosophical questioning of evolution is more around the fringes, such as whether the definition of heritability is too narrowly defined in evolution as simply relating to genes, as opposed to things like culture and learned behavior. Extensions of the discussion of theology versus evolution are also present in philosophical discourse though at this point they are tangential.
Philosophical discourse, therefore, does not seek to counteract the evidence produced by the scientific community. Where there are questions, they relate to the implementation of our understanding of evolution. Where discussions about existentialism and such subjects occur, they do not appear to intersect with the questions of evolutionary biology. Yet there might be a valid intersection, in that, while the theory of evolution does not make a hypothesis about the nature of life, it seems to imply that survival is the nature of life . If life exists simply to perpetuate itself – does that change at the individual or collective levels, and what is the merit of existing simply to continue to exist?
Theology
Not all religion has opposed the theory of evolution. The theory was more controversial when it first appeared, for a couple of reasons. One is simply that it had less supporting evidence than it did today; it was easier to call into question. Over time, with the emergence of not just more evidence for evolution but for the scientific method itself, many faiths have had to accept that their holy texts may not have been the divine truths that their adherents had always pretended to be. Some faiths, however, persist in challenging evolution, testing the gullibility of their adherents, apparently.
This resistance is interesting, because it has been the pattern basically since Darwin's time that science has proved to be a robust explanatory system for natural phenomenon, evolution and natural selection included. The relative transparency of the scientific method has shown to be a breath of fresh air for people who otherwise would have had no source of knowledge other than their faith leaders. Education for the masses, broader-based understanding of scientific principles, and a growing interest in critical thinking have all led people to understand how science works, or at least to accept that scientific consensus should be trusted. That is not to say that all educated people understand – they certainly do not in all parts of the world – but that there has been progressively greater understanding over time.
Many faiths have also adopted, or shall we say evolved, in their response to science in general. Evolution makes for a great flashpoint for the conflict between faith (religious and otherwise) and science because all cultures have an origin story of some sort, and all of those origin stories can be challenged. Some religions have proven more adaptable than others to scientific inquiry. The Dalai Lama has written several books about science and its intersection with his religion. Yet there are other faiths that cling to the notion that science is a threat to their power, and have openly targeted the theory of evolution as emblematic of the threat that science poses. When people seek out knowledge, to learn that which they could not previously explain, perhaps that is a challenge to faith. Perhaps the faiths that saw the scientific method as a threat were right – this progression of acceptance for scientific principles has coincided with the downfall of organized religion.
This of course calls into question which will prevail – science or faith – assuming that the two are, ultimately, mutually exclusive. Will natural selection pick the winner as science without dispute, will it allow for religious faith that accepts science, or will natural selection demonstrate a sense of irony by favoring precisely those faiths that reject it? All options are possible, but the trend is showing that humankind, increasingly, has lost the need for organized religion, if not spirituality altogether. That the faiths that are most challenged by evolution are also challenged by other science, like climate science, will only further lead to their downfall as people see them for what they are – people whose lack of willingness to accept demonstrable fact has contributed to humanity's downfall.
Sociology
One of the challenges at the sociological level, and perhaps why there are still these obtuse pockets of resistance, the lingering cognitive dissonance that afflicts some people, is the sociological problem. Social conditioning allows for influence of ideas, regardless of whether they are proven. Many such individuals – not all by any means but most – are taught not to question their faith, yet somehow they manage to question scientific inquiry.
Perhaps more important than these people are the larger body of people that tacitly accepts the theory of evolution, without having a meaningful understanding of it, and certainly not having an understanding of how it can has been proven that evolution is true. It can be quite worrisome the lack of scientific literacy, because critical to understanding both the claims and limitations thereof of science, is knowing how scientific consensus on a matter is built. It is not achieved because groups of scientists nod heads in agreement, or because they collude, but by virtue of many independent and verifiable, replicable studies over time. Failure to understand this, or the math that demonstrates that all of these studies say the same thing, and that there are enough of them to validate a theory, is a social weak point. While the majority will attest to believing the theory of evolution is true, their actual understanding of that fact is limited, and their support for it thus not founded on any particularly strong ground.
Personal Journey
Clearly, I have not journeyed as far as other in the sense of having no particular theological background. This does not mean that I possess some inherent trust in the scientific method; it's actually quite the opposite. A natural sceptic, I still had to learn about how the scientific method works. I still had to learn the process by which science arrives at its conclusions. Like most people, I wasn't really raised with any particular understanding of scientific inquiry, in particular the mechanisms by which the theory of evolution can be validated. I certainly believed that evolution was true, but I didn't arrive at that belief through scientific means; I arrived at it the same way a person of faith arrives at the conclusion that Darwin was a heretic and evolution is false. I listened to those around me, and parroted their views. My beliefs were the result of social conditioning and nothing more. In other words, not true belief based on genuine understanding.
One of the points learned from walking through the process of how evolution has been proven is actually taking the time to understand how the scientific process works. We do not say that evolution is true or that natural selection is true because that is what we were taught to believe. We can, and as human beings with our own agency should, verify such claims for ourselves. It is not at all unreasonable to simply accept scientific consensus, at least once you understand what that phrase actually means. Such an understanding is much stronger than accepting faith blindly for the same reasons. But the right way to approach examining if you should accept the theory of evolution as true is to approach the issue as a sceptic.
Start with understanding how the theory was constructed, how it came about. Then move into how it was tested, and what tests have been done over the years. An examination of what that evidence says, especially on the aggregate level, is important to demonstrate that a theory has in fact been replicated time and time again, past the point where doubts can linger. This is the meaning of scientific truth. The theory of evolution is not incontrovertible. It is true, in the sense that the best evidence we have today shows that it is true; if better evidence comes along tomorrow to suggest otherwise, that will also be accepted into our understanding of evolution. The process of understanding evolution should continue to evolve in this way – we continually build on the past with new evidence.
What Evolution is Not
This paper has spent quite some time discussing what evolution is, but it is important to realize what it is not. When one merely accepts evolution without understanding the scientific process, it is difficult to understand what it is not. The scientific process makes clear what evolution isn't. It's not some great limitless truth, for one. It is a theory, one for which there is overwhelming supportive evidence. There is little if any evidence to support any alternative theories. So we can, with a very high degree of confidence, say that evolution is true.
We cannot extrapolate from that a host of other conclusions, for which evolution has been accused. Evolution does not disprove the existence of God – nor does the Big Bang theory for that matter. Evolution may refute the timeline of certain holy texts, but it does not invalidate the entirety of their findings – the theory of evolution and indeed most of science simply does not address them. The theory of evolution is not an assault on anyone's faith – it is the result of a rigorous method of inquiry; the findings may be expressed with a very high degree of confidence but that does not mean one's faith is invalidated as the result. While those religions that steadfastly see their power as deriving specifically from opposing science continue to do so, the reality is that the threat to their power stems from their lack of willingness to adapt to a changing environment. Should they prefer to survive in the long-run, this adaptation should be embraced because humans are the agents of their own change. We don't need to pass our genes through successive generations in order to adapt to new circumstances. Power structures and belief systems capable of accepting scientific truth are going to be better positioned in the long run than those that fail to adapt. So as a threat to some, or at least their power, evolution is that. But we'll pass no judgement, because evolution isn't about value judgements. It's just a theory for which there is overwhelming support.
References

Ayala, F. (2009). Darwin and the scientific method. PNAS. Vol. 106 (S 1) 10033-10039.

Darwin, C. (1859) On the origin of species. Retrieved March 20, 2018 from http://darwin-online.org.uk/converted/pdf/1861_OriginNY_F382.pdf

Millstein, R. (2017). Evolution. Stanford Encyclopedia of Philosophy. Retrieved March 20, 2018 from https://plato.stanford.edu/entries/evolution/