Definition of Species and Mysteries of Evolution

We have already discussed that the nucleotide composition at each letter position of the codon is highly skewed from a completely random composition, which allows organisms to exist in a very stable manner. Furthermore, each species has a target nucleotide composition, and the nucleotide composition in individual organisms must be in the vicinity of that target. In other words, if you plot the fitness of an organism against the nucleotide composition from the entire genome sequence, it will be maximal at the composition of the target. Then, when mutations are introduced into the intracellular factors that determine nucleotide composition, the target’s nucleotide composition jumps, and a new species is expected to be born. When we calculated the distance of nucleotide composition in the evolutionarily closest species pairs, we actually observed a jump in the phase diagram of life, the nucleotide composition space. When we calculated the distances for many genome sequences, the interspecies distances within a genus were clearly different from the intergenus distances. This leads to the hypothesis about evolution that the birth of a species is a random walk in the phase diagram of life. Furthermore, we discussed some of the mysteries of evolution on the basis of the phase diagram of life.