  A central feature of vertebrate immune response is affinity maturation,
wherein antibody-producing B cells undergo evolutionary selection in
microanatomical structures called germinal centers, which form in secondary
lymphoid organs upon antigen exposure. While it has been shown that the median
B cell affinity dependably increases over the course of maturation, the exact
logic behind this evolution remains vague. Three potential selection methods
include encouraging the reproduction of high affinity cells (``birth/positive
selection''), encouraging cell death in low affinity cells (``death/negative
selection''), and adjusting the mutation rate based on cell affinity
(``mutational selection''). While all three forms of selection would lead to a
net increase in affinity, different selection methods may lead to distinct
statistical dynamics. We present a tractable model of selection, and analyze
proposed signatures of negative selection. Given the simplicity of the model,
such signatures should be stronger here than in real systems. However, we find
a number of intuitively appealing metrics -- such as preferential ancestry
ratios, terminal node counts, and mutation count skewness -- are all ill-suited
for detecting selection method.
