The improbability of detecting trade-offs and some practical solutions

Trade-offs are a fundamental concept in evolutionary biology because they
are thought to explain much of nature’s biological diversity, from
variation in life-histories to differences in metabolism. Despite the
predicted importance of trade-offs, they are notoriously difficult to
detect. Here we contribute to the existing rich theoretical literature on
trade-offs by examining how the shape of the distribution of resources or
metabolites acquired in an allocation pathway influences the strength of
trade-offs between traits. We further explore how variation in resource
distribution interacts with two aspects of pathway complexity (i.e., the
number of branches and hierarchical structure) affects tradeoffs. We
simulate variation in the shape of the distribution of a resource by
sampling 106 individuals from a beta distribution with varying parameters
to alter the resource shape. In a simple “Y-model” allocation of resources
to two traits, any variation in a resource leads to slopes less than -1,
with left skewed and symmetrical distributions leading to negative
relationships between traits, and highly right skewed distributions
associated with positive relationships between traits. Adding more
branches further weakens negative and positive relationships between
traits, and the hierarchical structure of pathways typically weakens
relationships between traits, although in some contexts hierarchical
complexity can strengthen positive relationships between traits. Our
results further illuminate how variation in the acquisition and allocation
of resources, and particularly the shape of a resource distribution and
how it interacts with pathway complexity, makes it challenging to detect
trade-offs. We offer several practical suggestions on how to detect
trade-offs given these challenges.