Book Club - Chapter 1

In the preface and the first chapter, Carroll spells out his intentions for the book. He makes the point that DNA evidence is used in courts to convict criminals. This technology works because of subtle differences in the sequences of DNA amongst different individuals. These same differences when read as sequences can reveal relatedness amongst individuals too. The further back in time we go, the less we share with parents, grand parents and great grandparents. This time line goes all the way back to the hypothetical Last Universal Common Ancestor (LUCA) of all species. The result is that the more related that we are to a species, the more similarity we have in our DNA sequences.

Carroll points out the main points of Darwinian evolution; that are descent with modification and the selection of the best adapted for the environment. This occurs through random mutations and selection of these mutations over large periods of time. Organisms are shaped by their environments – both physical and biological. They can therefore be seen as solutions to problems. To illustrate this principle of physiological ecology, the author uses the examples of ice fish. Their ancestors had to deal with the problem of a cooling ocean. To adapt to this, they evolved some antifreeze proteins (discussed in more detail in this post). These came about through the random mutation of part of the trypsinogen gene. This provides evidence for the capacity of organisms to create new genes from old ones (discussed here and here). That is descent with modification and is an essential feature of evolution that religious fundies often misrepresent. They claim that complex structures like eyes are supposed to have evolved in a generation or two. This however is not how evolution works. It works through many small cumulative (undirected) steps that use genes that are already there. A proper model for eye evolution can be found here. Around the same time, their genes for tubulin became modified in such a way that allowed them to tolerate lower temperatures. This allows their cells to retain their normal architecture and function at lower temperatures than would otherwise be tolerable.
One problem, which is not fully discussed is that the presence of antifreeze proteins increases the viscosity of the blood. This would strain the cardiovascular system. However, cold water carries more dissolved oxygen than warm water. This meant that the fish could get large quantities of oxygen from solution. This allowed the fish to decrease the number of circulating red blood cells (which carry oxygen) allowing them to decrease the viscosity of their blood. A reduced demand for red blood cells meant that there was no need for haemoglobin – the oxygen carrying component of red cells. This lead to the loss of these genes (see here). Other adaptations include a loss of scales and an increase in capillary number in the skin – this allowed the skin to take up oxygen as well as the gills. Finally, some species also lost their genes for the oxygen storing myoglobin. This is an other principle of evolution – you lose what you don’t need through random mutation (see here). The non-functional remnants of these genes can still be found in these fish. This tells a nice story of how genomes can change in response to environmental conditions

Carroll then goes on to outline his plans for the book, and finishes with a ca bit on why evolution matters. He concentrates on food production, but I felt he could have spent a bit of time on the evolution of antibiotic resistance, the role of animal models in medicine and the fact that it is important in its own right regarding the origins of species.

Happy Darwin day.