Abstract
Correlation does not imply causation. If two variables, say A and B, are correlated, it could be because A causes B, or that B causes A, or because a third factor affects them both. We suggest that in many cases in biology, the causal link might be bi-directional: A causes B through a fast-acting physiological process, while B causes A through a slowly accumulating evolutionary process. Furthermore, many trained biologists tend to consistently focus at first on the fast-acting direction, and overlook the slower process in the opposite direction. We analyse several examples from modern biology that demonstrate this bias (codon usage optimality and gene expression, gene duplication and genetic dispensability, stem cell division and cancer risk, and the microbiome and host metabolism) and also discuss an example from linguistics. These examples demonstrate mutual effects between the fast physiological processes and the slow evolutionary ones. We believe that building awareness of inference biases among biologists who tend to prefer one causal direction over another could improve scientific reasoning.
Highlights
In each example we describe how a physiological causal link (“A causes B”) could account for an observed correlation, and how the opposite causal direction (“B causes A”), which takes place over an evolutionary time scale, can and perhaps should be invoked
We conclude with a discussion on the interaction between the fast-acting and slow-acting effects in biology and ask why biologists might intuitively focus on one effect over another
Many causal links in biology might be caused by fast-acting physiological processes acting in one direction and slower evolutionary processes acting in the opposite direction
Summary
M ore than 50 years ago Ernst Mayr introduced the distinction between “proximate” and “ultimate” causes in biology (Mayr, 1961). In this classical theory, proximate causes act via short-term physiological processes, and ultimate causes act via long-term evolutionary processes. We follow with a few examples of biological questions in which the direction of a causal link (physiological or evolutionary) has been, or could have been, debated. In each example we describe how a physiological causal link (“A causes B”) could account for an observed correlation, and how the opposite causal direction (“B causes A”), which takes place over an evolutionary time scale, can and perhaps should be invoked. We conclude with a discussion on the interaction between the fast-acting and slow-acting effects in biology and ask why biologists might intuitively focus on one effect over another
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