Two decades of suspect evidence for adaptive DNA-sequence evolution - Failure in consistent detection of positive selection

A recent study suggests that the evidence of adaptive DNA sequence evolution accumulated in the last 20 years may be suspect. The suspicion thus calls for a re-examination of the reported evidence. The two main lines of evidence are from the McDonald-Kreitman (MK) test, which compares divergence and polymorphism data, and the PAML test, which analyzes multi-species divergence data. Here, we apply these two tests concurrently on the genomic data of Drosophila and Arabidopsis. To our surprise, the >100 genes identified by the two tests do not overlap beyond random expectations. The results could mean i) high false positives by either test or ii) high false-negatives by both tests due to low powers. To rule out the latter, we merge every 20 - 30 genes into a "supergene". At the supergene level, the power of detection is high, with 8% - 56% yielding adaptive signals. Nevertheless, the calls still do not overlap. Since it is unlikely that one test is largely correct and the other is mostly wrong (see Discussion), the total evidence of adaptive DNA sequence evolution should be deemed unreliable. As suggested by Chen et al., the reported evidence for positive selection may in fact be signals of fluctuating negative selection, which are handled differently by the two tests. Possible paths forward on this central evolutionary issue are discussed.