Environment, but not genetic divergence, influences geographic variation in colour morph frequencies in a lizard

Abstract

Background: Identifying the causes of intraspecific phenotypic variation is essential for understanding evolutionary processes that maintain diversity and promote speciation. In polymorphic species, the relative frequencies of discrete morphs often vary geographically; yet the drivers of spatial variation in morph frequencies are seldom known. Here, we test the relative importance of gene flow and natural selection to identify the causes of geographic variation in colour morph frequencies in the Australian tawny dragon lizard, Ctenophorus decresii.

Results: Populations of C. decresii are polymorphic for male throat coloration and all populations surveyed shared the same four morphs but differed in the relative frequencies of morphs. Despite genetic structure among populations, there was no relationship between genetic similarity or geographic proximity and similarity in morph frequencies. However, we detected remarkably strong associations between morph frequencies and two environmental variables (mean annual aridity index and vegetation cover), which together explained approximately 45 % of the total variance in morph frequencies.

Conclusions: Spatial variation in selection appears to play an important role in shaping morph frequency patterns in C. decresii. Selection associated with differences in local environmental conditions, combined with relatively low levels of gene flow, is expected to favour population divergence in morph composition, but may be counteracted by negative frequency-dependent selection favouring rare morphs.

Fig. 1

Relative frequencies of orange, orange-yellow, yellow and grey male throat colour morphs in eight populations of C. decresii: Aroona (N = 22), Wilpena (N = 48), Yourambulla Caves (N = 29), Warren gorge (N = 22), Devil’s Peak (N = 26), Bimbowrie Station (N = 20), Mt Remarkable (N = 21) and Telowie Gorge (N = 20). Grey shading represents elevation according to the scale bar

Fig. 2

STRUCTURE analysis figures showing hierarchical levels of population structure. Individual assignment probabilities are for a K = 2 b K = 3 c K = 3 and d K = 2. Populations are: Yourambulla Caves (YC), Aroona (A), Warren Gorge (WG), Devil’s Peak (DP), Mt Remarkable (MR), Telowie Gorge (TG), Wilpena (W), and Bimbowrie Station (BS)

Fig. 3

Relationships detected between colour morph frequencies and mean annual aridity index (aridity) and the proportion cover of vegetation <1 m in height (vegetation). Here we plotted the raw variables for which we detected significant relationships in the multiple regression analyses. The analysis for orange morph frequencies was performed with (black) and without (grey) Bimbowrie Station (BS) as this population was identified as an outlier