The birth-and-death evolution of multigene families revisited

For quite some time, scientists have wondered how multigene families come into existence. Over the last several decades, a number of genomic and evolutionary mechanisms have been discovered that shape the evolution, structure and organization of multigene families. While gene duplication represents the core process, other phenomena such as pseudogene formation, gene loss, recombination and natural selection have been found to act in varying degrees to shape the evolution of gene families. How these forces influence the fate of gene duplicates has ultimately led molecular evolutionary biologists to ask the question: How and why do some duplicates gain new functions, whereas others deteriorate into pseudogenes or even get deleted from the genome? What ultimately lies at the heart of this question is the desire to understand how multigene families originate and diversify. The birth-and-death model of multigene family evolution provides a framework to answer this question. However, the growing availability of molecular data has revealed a much more complex scenario in which the birth-and-death process interacts with different mechanisms, leading to evolutionary novelty that can be exploited by a species as means for adaptation to various selective challenges. Here we provide an up-to-date review into the role of the birth-and-death model and the relevance of its interaction with forces such as genomic drift, selection and concerted evolution in generating and driving the evolution of different archetypal multigene families. We discuss the scientific evidence supporting the notion of birth-and-death as the major mechanism guiding the long-term evolution of multigene families.