indicators and generalized first lines of defense that buy time while we populate our immune repertoire with more specialized antibodies tuned to the specific threats.

We can predict, however, that, during our lives, we will be assaulted by a variety of pathogens. Thus, vertebrates have evolved a contingency plan in the form of immune systems and barriers to invasion, such as skin and cell membranes. These systems combine early-warning

that we cannot hope to predict their form or timing.

To a large extent, evolution is about preparing for the unknown, because the scope of possible changes in our environments is so immense

Both are dynamic systems in which the selfish actions of countless individuals—whether they be cells or investors—lead to unpredictable consequences at the system level. In turn, these collective actions and consequences feed back to influence individual actions in endless cycles of adaptation and evolution.

with striking similarities.

The biosphere and the “financiosphere” are both dazzling in their complexity,

At all levels, the competitive interactions give rise to counterparts of the familiar interactions of population biology—symbiosis, parasitism, competitive exclusion, and the like.

The value (“fitness”) of a given combination of building blocks often cannot be predicted by a summing up of values assigned to the component blocks. This nonlinearity (commonly called epistasis in genetics) leads to co-adapted sets of blocks (alleles) that serve to bias sampling and add additional layers to the hierarchy.