Cooperation and Competition in Mammalian Evolution

Mammalian genomes have had to endure the integration of exogenous DNA sequences over the course of time. In most cases, such events have proven harmful to individuals thus afflicted, but in the long-term gene domestication of exogenous DNA sequences, such as LTR retrotransposons, has also served as a driving mechanism in biological evolution. This is especially the case in eutherian mammals, in which two lines of domesticated genes increased in number in a common eutherian ancestor, eleven sushi-ichi-related retrotransposon homologs (SIRH)/retrotransposon Gag-like (RTL) genes and more than fifteen paraneoplastic Ma antigen (PNMA) genes. It is clear that these SIRH/RTL and PNMA genes were positively selected due to the advantage conferred on eutherian reproductive success. Thus, the principle of “competition among individuals within the same species” in the Darwinian theory of evolution is effectively at work in the domestication process. However, when the number of domestication events is taken into account, how could the common eutherian ancestor have acquired that many domesticated genes? We suggest that sexual mating across multiple generations of individuals with one or a small number of such domesticated genes may have been critically important for accumulating all of them into a single line, thus leading to the common eutherian ancestor. Then, we would like to propose that “cooperation among individuals within the same species” in the form of interactive behaviors of the individuals within a tightly delimited species is also at work in this process.