Are the spatial and temporal scales biologically meaningful?
Millette et al . (2020) examined IGD temporal trends across 909
animal species where COI sequences were available for ≥4 years.
Sequences were grouped across ≤1,000 km to avoid “conflating
spatial and temporal effects” . This scale far exceeds the dispersal
capabilities of many included species, the scale of habitat change
affecting them, and thus the scale at which population genetic processes
influencing IGD operate. Additionally, the clustering algorithm used can
‘daisy chain’ locations together so that sites >1,000 km
apart are grouped together (Appendix A1). Grouping sequences into
biologically-implausible “populations” likely obscures any
anthropogenic effects on IGD, especially when combined with the small
sample sizes (<10 sequences/year for 77% of time-series
overall) and the large number of locations sampled yearly (e.g. mean of
3 locations sampled per year for fish; Appendix A1). We
reexamined 104 Inland and Coastal Bony Fish time-series from
Millette et al. (2020), and found that the sequences included were
sampled in inland freshwaters for most time-series (96/104), with
sequences from multiple demographically-independent locations (i.e. from
disconnected drainage basins; median 3 [range 0-15], see Figure 1
and
Appendix
A1
for examples) erroneously pooled into “populations”. By pooling
sequences from independent locations, changes in IGD attributable to
anthropogenic pressures would be lost in the noise, with uneven sampling
across space and time compounding the issue.
Additionally, the median span of included time-series is only seven
years overall, and represents an average of just 2.2 generations for
fish (Appendix A1). Thus for many taxa, the data covers an insufficient
time-span for most measurable evolutionary changes in IGD. Of course,
such analysis also neglects any pre-1980s impacts. DNA from museums,
herbaria and fossil archives are needed for this.