**5. Evolutionary perspective on antimicrobial peptides**

In this chapter, we proposed a structural classification of antimicrobial peptide families considering the diversity of their structures and then we reviewed the traditional function and biological activities. Finally, we propose new insights into the functions of antimicrobial peptides that could provide a large body of research to create new classes of antimicrobial therapeutics. AMPs are widespread molecules throughout the animal and plant taxa, this fact suggest its relevance in the evolution of immune response. Traditionally, their basic molecular and biochemical nature is related to the disruption of membrane potential and/or structure with the ensuing cell death. However, the diversity in the structure and biological properties (above mentioned) of AMPs within and between species suggest that these molecules have different functions in immune response.

The immune system of living organisms is formed by a set of cells, molecules and reactions. All of these features are continuously evolving to resist (attack and eliminate) pathogen invasion and to limit the negative (in terms of host survival and reproduction, i.e. fitness) consequences of the infection (Hoffmann and Reichhart, 2002). On the other hand, pathogens success depends upon overcoming the selective immune pressures brought about by the host. As a consequence, both, host and pathogens, evolve traits and strategies to increase the fitness of each one. Van Valen (Van Valen, 1973) proposed this co-evolutionary arm races as an evolutionary theory called "The Red Queen Hypothesis". The theory was proposed citing Lewis Carrol's Red Queen, where it takes all the running somebody can do to keep in the same place. Given this situation, immunologically, there is never a "best" solution to pathogens infection. To understand this scenario, we must consider (1) pathogen's short generational cycles, that may provide enough time to adapt to the host's immune response. As an outcome there will be grounds for high variability in immune response. (2) Differences in the kind and burden of pathogens, where divergent hostpathogen interactions for each species are possible and can be reflected in the course of action of taxa immune response (Read and Taylor, 2001). (3) Virulence differences, where there can or cannot be a harm imposed on a host (for example *Bacillus anthracis* vs. commensal microbiota). As long Hypothesis, an immune effecter that has the ability to be produced under different infection circumstances could have a selective advantage, antimicrobial peptides could have this property, as the host has to deal with the problem of maximizing fitness under the Red Queen pressure.
