The Origin of Species and Common DescentQUESTION: The Origin of Species and Common DescentANSWER:
Charles Darwin proposed that all forms of life as observed today can be traced back to a single universal common ancestor. What arguments are put forward in favour of this contention? Are these arguments valid? One line of often-cited evidence used to support the notion of common descent is the concept of DNA sequences which apparently have no function, yet appear in a variety of species as if they had been inherited in the course of evolution.
Caution is warranted when it comes to concluding that a particular feature is absolutely redundant, or without use. In principle, it is not possible to prove the functionless of a particular feature. For instance, because much of DNA consists of multiple repetitions of short sequences, for a long time it was thought that it was of no use. This led to it being labelled as ‘junk DNA’. More recently, however, it was noticed that even these repeated sequences may have an important role in gene regulation.
On the opposite side of the coin there are the non-homologies, especially those relating to embryological development. Such evidence seems to be strongly at odds with the notion of common descent. What do we mean by this? Let’s take an example. The vertebrae -- the major components of the vertebrate skeleton -- form embryologically in significantly different ways for different classes of vertebrate (such as mammals, birds, amphibians and fish), and even from different groups of early embryonic cells. In the course of embryological development, the members of different classes of vertebrate pass through a similar-looking stage (phylotypic stage) which often used as an argument for common descent. However, from the evolutionary point of view, it is somewhat puzzling that even though the phylotypic stage looks similar, and we would have expected it to be formed from a fertilised egg in substantially the same way, there is remarkable diversity, including some fundamental anomalies. The fact that the phylotypic stages are formed through fundamentally different pathways at least undermines -- if not completely negates -- the notion that they are derived from a common ancestor. There are similar confounding anomalies which need to be explained by the Darwinist in other phyla as well, not only the vertebrates.
The Origin of Species and common descent - The Conclusion Which of these two pieces of data carries more weight? If one were to take falsifiability seriously on this issue, then one ought to give more weight to the latter, namely the non-homologies, than to the circumstantial evidence based upon DNA sequences that may or may not be functionless.