Conservatism of Ecological Niches in Evolutionary Time

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Science  20 Aug 1999:
Vol. 285, Issue 5431, pp. 1265-1267
DOI: 10.1126/science.285.5431.1265


Theory predicts low niche differentiation between species over evolutionary time scales, but little empirical evidence is available. Reciprocal geographic predictions based on ecological niche models of sister taxon pairs of birds, mammals, and butterflies in southern Mexico indicate niche conservatism over several million years of independent evolution (between putative sister taxon pairs) but little conservatism at the level of families. Niche conservatism over such time scales indicates that speciation takes place in geographic, not ecological, dimensions and that ecological differences evolve later.

Critical characteristics of species' biology, such as physiology, feeding ecology, and reproductive behavior, define their fundamental ecological niches (1). In the early 1990s, several theoretical community ecologists independently predicted that fundamental niches of species under natural selection could change, but slowly. Based on diverse models that coupled population and genetic dynamics in heterogeneous environments, niche conservatism was predicted, because rates of adaptation in environments outside of the fundamental niche would often be slower than the extinction process (2).

However, little empirical evidence has been assembled to address these theoretical predictions (3). One study (4) that compared population response surfaces to climatic conditions in two closely related species of beeches (Fagus spp.) showed that limiting conditions for the presence of populations were coincident. Another study (5) documented conservatism in geographic range size in disjunct Asian and North American plant taxa but focused principally on distributional area as opposed to ecological niche characteristics. Other recent studies, however, have revealed rapid (over about 100 years) niche evolution that may be linked to speciation (6). These two contrasting views remain to be tested in broad samples of taxa to assess the generality of niche conservatism on evolutionary time scales.

We now apply new tools and approaches to examine this question in birds, mammals, and butterflies in an arena of active speciation and population differentiation—the Isthmus of Tehuantepec in southern Mexico (7). Drawing on extensive databases that summarize scientific specimen holdings, we examined 21 sister taxon pairs of birds, 11 sister taxon pairs of mammals, and 5 sister taxon pairs of butterflies and tested the degree to which ecological characteristics of one taxon were able to predict (with an artificial intelligence algorithm) the geographic distribution of its putative sister taxon and vice versa. To provide comparisons over longer time scales, we also analyzed randomly chosen confamilial, nonsister taxa (8–11).

Based on large-scale ecological dimensions, the approach uses a genetic algorithm to produce a set of decision rules in ecological space (a model of the fundamental niche) that can be projected onto maps to predict potential geographic distributions. Modeling each member of the putative sister taxon pairs in this study yielded not just a predicted geographic range approximating its own geographic distribution but also a predicted range mirroring the geographic distribution of its allopatric sister taxon. For example, for the hummingbird species pair Atthis heloisa (north and west of the isthmus) and Atthis ellioti (south and east of the isthmus), the model for A. heloisa successfully predicted all six occurrence points available for A. ellioti, and the model for A. ellioti predicted 66 of 79 occurrence points for A. heloisa (Fig. 1A). Statistical significance of theAtthis comparisons was clear, with probabilities at about 0.03 for the first comparison and at about 10−22 for the second. Across the 37 pairs, 32 eastern taxa predicted distributions of western taxa significantly, and 26 western taxa predicted distributions of eastern taxa significantly (Fig. 1). When we examined significant and nonsignificant predictive models, we noted a strong relationship with sample size: models were nonsignificant only at sample sizes of <15 points for the predicted taxon (Fig. 2). For all taxon pairs, at least one of the reciprocal predictions was statistically significant. When we compared the ability of each taxon to predict its sister taxon's distribution with its ability to predict the distributions of all other taxa on the opposite side of the Isthmus of Tehuantepec, the difference was marked and statistically significant (12). In contrast, only a few predictions of distributions of confamilial taxa were statistically significant regardless of sample size (Fig. 2).

Figure 1

Geographic distributions of A. heloisa (circles) and A. ellioti (squares) (A), P. melanocarpus (circles) and P. zarhynchus (squares) (B), and P. c. charops(circles) and P. c. nigricans (squares) (C). Occurrence points for each taxon are overlaid on geographic predictions based on the ecological characteristics of occurrence points of its sister taxon (dark gray for east of Isthmus of Tehuantepec predicting west, light gray for converse). Dashed lines show approximate position of the Isthmus of Tehuantepec.

Figure 2

Graph of average departure in reciprocal predictions among putative sister taxon pairs (open symbols) and confamilial species pairs (filled symbols), illustrating the significant interpredictiveness among sister taxa and the nonsignificant interpredictiveness among distantly related, confamilial taxa. Vertical axis represents the average of χ2 values for interpredicting taxon pairs, taking into account direction of departure from expectation (that is, predictions worse than expectation are assigned negative values). Solid and dashed lines represent linear regressions of departure values on sample size for sister-taxon and confamilial comparisons, respectively.

The above analyses show conservative evolution in ecological niches of 37 sister taxon pairs of birds, mammals, and butterflies isolated on either side of the lowland barrier Isthmus of Tehuantepec. The forested habitats on either side of the isthmus have been isolated for 2.4 to 10 × 106 years (13); hence, the observed conservatism has held for effectively twice that time of independent evolution in the pairs of lineages involved. Although ages of “families” are disputed (14), this expanded time scale (perhaps 10 to 50 × 106 years) has been sufficient to permit evolutionary diversification in niche characteristics. Hence, our results broadly confirm theoretical predictions of relative conservatism in ecological characteristics of species.

Conservatism of ecological niches across moderate periods of evolutionary time also reflects the modes of speciation involved. Strict vicariant speciation depends simply on geographic isolation, whereas other scenarios, such as the peripheral isolates model of speciation and many models of sympatric speciation (15), invoke invasion of novel ecological situations as part of the speciation process. The taxa and ecological dimensions treated here support the vicariant hypothesis, with ecological differences building up later, well after the speciation event. An untested question is whether the observed conservatism results from active constraint (stabilizing selection) or whether it reflects the absence of additive genetic variation in niche-related traits (16). Similarly, our analysis does not eliminate the possibility of niches of both members of species pairs responding in parallel to broad-scale environmental changes.

To the extent that our geographic scenario is representative, our results suggest that ecological niches evolve little at or around the time of the speciation event. Rather, ecological niche differences appear to accumulate later, over the time scale of familial relationships. Finding general conservatism in ecological niches opens the door to phylogenetic studies of niche evolution, comparative evaluations of conditions under which niche conservatism breaks down, construction of predictive distributional models, and numerous other applications to questions in biogeography (estimates of α and β diversity, centers of endemism), biodiversity (foci of species diversity), and conservation biology (development of conservation prioritizations).

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