4.1 | Effects of geological changes on the evolution ofSinocyclocheilus in the central Yunnan Basin
The Yunnan-Guizhou Plateau in China has a high concentration of plateau lakes, with most forming as a result of stratigraphic fault subsidence in the late Cenozoic. Located in the hinterland of the Yunnan-Guizhou Plateau, Dianchi Lake is the largest freshwater lake in Yunnan Province. The basin of Dianchi Lake has experienced extensive geological and climatic changes over the past 3.4 million years. Specifically, with the uplift and subsidence of the lake basin and the associated expansion and contraction of the lake area, Dianchi Lake which was previously a part of the Nanpanjiang River system, has transformed into a tributary source for the Jinsha River in a “river capture event” (Zhu et al., 1991). Such changes in the landscape have made it possible for aquatic organisms that were once restricted to the Nanpanjiang River to colonize the Yangtze River Basin, thereby expanding their geographic ranges.The structural evolution of Dianchi Lake is a notable feature of the geomorphology and neotectonics of the Yunnan Plateau. The population structure of Sinocyclocheilus fishes inhabiting the region also reflects the complex geological development of Dianchi Lake.
Dianchi Lake is a fault subsidence lake of plateau limestone formed by the Tertiary Himalayan Orogeny. During the Yanshan Movement, the Yunnan Plateau experienced stratigraphic uplift, folding, and faulting. These processes led to the formation of high mountains and basins and shaped the present landform. As a result of strong neotectonic movement that occurred up to the Himalayan movement in the late Cenozoic, a large north-south trending fault spanning was formed in the Xishan Mountains. When the fault collapsed, Dianchi Lake was formed by the accumulation of water during the initial formation of the resulting graben. The formation of Dianchi Lake comprised three main stages, which also influenced the population structure of Sinocyclocheilus fishes in the region.
The first stage of lake formation occurred approximately 3.4 Ma. According to the divergence times of Sinocyclocheilus in the Central Yunnan Basin, we speculate that the common ancestor ofSinocyclocheilus in the Central Yunnan Basin, the “Central Yunnan Group,” colonized the lake during this period.
The second stage of lake formation saw the expansion of the lake basin during the early and middle Pleistocene (2.59~0.13 Ma). The early Pleistocene (2.59~0.78 Ma) saw a gradual intensification of the uplifting and depression of the Dianchi Lake Basin relative to the surrounding fault block mountains. As the northern section of the basin continued to subside, the lake deepened and its area expanded. During this time, the “Central Yunnan Group” differentiated into the “Mile Group” and the other groups. During the middle Pleistocene (0.78~0.13) Ma, the lake basin accumulated water, forming the main lake. This period corresponded with the differentiation of Sinocyclocheilus fish populations in central Yunnan, a process that may have been driven by the specialized ecology of Sinocyclocheilus and the distinct geological conditions of Dianchi Lake. Notably, populations ofSinocyclocheilus (C and F) in Dianchi Lake and in Fuxian Lake differentiated 0.84~0.49 Ma, which is related to the original Nanpan River system in Dianchi Lake. This is further supported by the fact that the capture of Tanglang River occurred after this period.
The third stage of lake formation occurred during the period in which the Tanglang River was captured (0.126~0.01 Ma), leading to a transformation of the water system of the ancient Dianchi Lake. The Dianchi Lake, originally belonging to the Nanpan River water system, was transformed into a tributary source lake of the Jinsha River water system. Bailong vauclusian spring (FM) and Long vauclusian spring (JW) happened to be at the intersection of the Nanpan River water system and the Jinsha River water system. Due to the contraction of lake with decline of water level, differentiation (0.12~0.05 Ma) occurred (Fig. 3) .