Figure 4. Results of colonization modeling.

(a & b) Repeated colonization model. (a) The relationship of the size of the ancestral species pool and the fraction of the aquifers containing sister species after two colonization events (formula 1). An initial niche colonisation probability (p₁) of 0.5 was used as this maximises the probability of sister pairs (see supporting information S3). The last colonisation probability (p₂) was set to 1. The observed fraction of aquifers with sympatric sister species (11/45) is also indicated. (b): The predicted fraction of aquifers containing sympatric sister pairs (blue; formula 2) and triplets (red; formula 3) calculated based on three colonization periods and a niche colonization probability (p₁ = p₂ = 0.4) that maximizes the probability of pairs and triplets (see supporting information S3). Horizontal lines indicate the observed fraction of aquifers with sister pairs and triplets. The last colonisation probability (p₃) was set to 1. (c): Within-aquifer speciation model. The relationship between the initial niche colonization probability and the predicted fraction of aquifers containing sympatric sister pairs (blue) and triplets (red) calculated with single colonizations and subsequent divergence within aquifers. Horizontal lines indicate the observed fraction of aquifers with sister pairs and triplets. The models are calculated using the observed number of aquifers with one (18 aquifers), two (16 aquifers) or three (11 aquifers) species. The shaded areas in (A–C) represent the 5% and 95% percentiles as confidence limits from 10000 randomizations. We assumed that all open niches were filled by speciation (q = 1).