This entry is based on a short public talk I saw by a former lecturer of mine last year (Professor Tom Tregenza). He put across a simple way to argue that evolution is a process that cannot be denied. I chose to write about it because of it’s simplicity and elegance. I start by defining evolution and then expanding on the points made by Tom.
The theory of evolution was defined by Darwin as “Descent with modification.” That is, characteristics descend along a lineage (from parent to offspring) and, between generations, can become different from the ancestral characteristic. It is, in my opinion, only a theory by scientific definition, just like the theory of gravity. I know that if I stand on the surface of the earth and drop an apple, without applying any other forces, it will fall to the ground because of gravity. The existence of evolution is equally as certain, yet many do not accept it.
The aim of Tom’s talk was to provide a logical basis, supported by fact, upon which the only outcome that makes sense is to accept evolution. All of the following questions can only be answered with “yes” and thus irrefutably support evolution. I do not provide hard evidence for many of these points because the answers are so obviously true.
1. Do living organisms reproduce?
The very definition of life states that all living organisms reproduce. Every single species on the tree of life reproduces in some capacity. Right from simple clonal reproduction of single celled organisms through to the sexual reproduction found in many eukaryotes, and all sorts of weird and wonderful variations in-between and beyond. The answer to this question is undoubtedly yes.
2. Do some individual organisms have more offspring than others?
There is high variation in reproductive success even within species. Looking to humans we know that some people die before reaching a reproductive age, some are infertile, some chose not to have children, and some chose to have many. Looking across species we see even greater variance from very few offspring per parent to many thousands. Again, the answer to this question is yes.
3. Do individuals vary in their characteristics?
Look around you, there is variance in height, hair color, eye color, between species there is variance in the number of legs used to walk, the number and type of limbs species have, body size, shape, senses, color, behavior, and much more. So, is there variation between individuals? Yes.
4. Are characteristics heritable?
Again look at humans. We more often see that the characters of offspring resemble their parent’s characters more than the homologous character of a random organism. Tall people tend to have tall children. Short people tend to have short children. Humans tend to produce offspring that look more like humans than an elephant, and elephants tend to produce offspring that look more like elephants than humans. Obviously there is heritability in traits (and much of this is due to the passing of genes along a lineage) so the answer is thus yes.
5. Are traits causing variance in reproductive success heritable?
An example of this would be the genetic inheritance of disease. Some diseases kill people before they have reached the end of their natural reproductive window, the point where they can no longer viably reproduce. Cancer has been linked to genetic mutations which can be passed from parent to offspring (for example the BRCA1 gene is linked to high incidences of breast cancer). If a cancer, passed along the lineage, stops an individual from producing as many offspring as possible then a heritable characteristic has affected the variation in reproductive success. Traits affecting reproductive success need not be detrimental, a mutation might arise that increases may fertility for example. In such a case the male would be expected to sire more offspring in the next generation, thus there will be an increased proportion of the population with that mutation. Therefore the answer to this fifth and final question is also yes.
These five points are nothing pioneering or controversial, they just simply support the definition of evolution by showing that heritable changes can occur in a population and as a result the population may become different over time. *Technically, excluding point five, we can still say that evolution can only be true. Combing the evidence from points one to four, shows that traits are transmitted from one generation to the next in such a way that could cause a change over time – this is via genetic drift. Point five simply invokes selection for adaptation as a mechanism of evolution.*
The discussion over the truth of evolution is largely fuelled by religious groups who see it as opposing their beliefs, but eventually I am sure that religions will come to accept the theory of evolution. In the face of growing evidence, just as when we realized that the earth was round and not at the centre of the universe, those religions will be forced to adapt, to evolve.
* This section has been added after initial publication