Size Related Bias in planktonic Foraminifera

Following the finding that the lineages were likely founded by small individuals and the fact that they had already eliminated several hypotheses, Arnold et al. (1995) were forced to fall back on the hypothesis that the phyletic increase in mean was due to stochastic diversification after periods of extinction. By assuming this was the case, it was necessary to determine why smaller species tended to survive mass extinction more so than larger species. The above hypothesis actually

Globigerina bulloides (image by Natalia Vazquez Riveros)
suggests that there is a short-lived disadvantage to being large during periods of high stress (mass extinctions) and that is the reason for small founding populations in subsequent generations (Arnold et al., 1995). To test this, the authors first compared speciation and extinction rates between large and small species at short time intervals throughout the era and found no pattern which would lead to increase in mean size (Arnold et al., 1995). They then examined extinction by breaking into two separate groups; pseudoextinctions and true lineage extinctions. Pseudoextinctions are those which leave a extant descendant (Arnold et al., 1995). The data found suggests that small-bodied species with short generation times are better able to adapt through times of stress and will therefore found new lineages (Arnold et al., 1995). Arnold et al. (1995) concluded their paper by stating that their results suggest that Cope’s rule could actually be due to a microevolutionary mechanism (the enhanced ability of smaller animals to speciate in times of stress), but that this mechanism is not driven by size, and that the apparent size change is only a byproduct of Cope’s rule.