The macroevolutionary patterns noted above with respect to novelty in defense strategies among animal lineages could not occur if there were not microevolutionary processes ultimately involved in www.frontiersin.org FIGURE 2 | One scenario for early in the divergence of animals is that different lineages (A-D) have fundamentally similar immune systems such that they all are colonized by the same parasites. In the case shown at the top, an immunological innovation occurred in lineage A, that allowed it to resist these parasites. This may have permitted a subsequent radiation in "parasite-free space" in this host lineage. At the bottom, lineage D has acquired a lineage-specific parasite different from those previously experienced. This requires an immunological accommodation that causes the immune system of lineage D1 to diverge. Both the host lineage and the lineage-specific parasites along with them may subsequently diverge. generating them. These microevolutionary processes occur below the species level, within and among populations of either host or parasite species, and might culminate in speciation of either participant. Speciation may be accompanied by colonization of new habitats, and further divergence to create major new lineages. Starting with the process of parasite diversification, the sections below discuss how the involvement of immunology in microevolutionary processes could lead to events that can help explain the macroevolutionary patterns discussed above.