Implications of the last glacial maximum on the genetic diversity of six co-distributed taxa in the Baja California Peninsula

Aim: During the Last Glacial Maximum, the temperature in the Baja California Peninsula decreased and the precipitation increased compared to present climatic conditions. These changes influenced the geographic distribution and the demographic processes of plants and animals. This article aims to determine how Pleistocene and current climate impacted the geographic distribution and the genetic diversity patterns of six species co-distributed in the Baja California Peninsula.

Location: Northwest Mexico.

Taxon: A group of six species with desert affinities, two cacti ( Pachycereus pringlei and Stenocereus gummosus), one spider ( Pardosa sierra), one reptile ( Dipsosaurus dorsalis) and two birds ( Melanerpes uropygialis and Basilinna xantusii).

Methods: Meta-analysis of published microsatellite data was carried out for the previously mentioned species. Hierarchical Generalized Additive Models (HGAMs) were used to assess the relationship between genetic diversity values and six abiotic predictors (latitude, elevation, current climate, habitat suitability, climatic stability, and habitat suitability stability from the Last Glacial Maximum to the present). The change in the distribution range from the Last Glacial Maximum to the present was found using Potential Distribution Models (PDMs).

Results: HGAMs revealed that Melanerpes uropygialis showed a significant relationship between genetic diversity and climatic stability, while the genetic diversity of Dipsosaurus dorsalis responded to the current climate. The genetic diversity of Melanerpes uropygialis and Pachycereus pringlei was affected by habitat suitability change while in Basilinna xantusii, genetic diversity changed with current habitat suitability. According to potential distribution models, four areas of northwest Mexico functioned as a refuge during the Last Glacial Maximum: the south of the peninsula, the region between the western tip of the Vizcaíno peninsula and Cedros Island, the upper Gulf of California and the coasts of Sonora. However, only the spatial patterns of predicted genetic diversity of Melanerpes uropygialis coincide with the location of Pleistocene refugia.

Main conclusions: No evidence was found of a concerted response of species to climate change during the Pleistocene, only one species showed evidence of discrete Pleistocene refugia, and climatic stability and habitat suitability are not always conducive to higher genetic diversity values.

Methods
Gam_data: Expected heterozygosity corrected for the number of samples (He) was determined in GenAlEx version 6.5 (Peakall & Smouse, 2006). Allelic richness (Ar) was calculated in the R package hierfstat (Goudet, 2005) and the rarefaction method was applied to avoid biases due to differences in the number of samples (the sample size was adjusted to two in all species). Latitude and longitude were retrieved from the coordinates of each locality. Elevation data were obtained from the NASADEM Digital Elevation Model (NASA JPL, 2020). Current climate data (B1, B5, and B12) were obtained from the 19 climate layers of WorldClimm 2.1 (Fick & Hijmans, 2017) while for climate change data (the absolute difference between current and Last Glacial Maximum climatic variables; B1_c, B5_c, B6_c, and B12_c) the Last Glacial Maximum climate data was obtained by averaging the raster of the three global circulation models available in WorldClimm 1.4 (Hijmans et al., 2005), the Community Climate System Model (CCSM4; Gent et al., 2011), the Model for Interdisciplinary Research on Climate (MIROC-ESM; Hasumi & Emori, 2004) and the Max Planck Institute for Meteorology Earth System Model (MPI-ESM-P; Giorgetta et al., 2013). Habitat suitability (HS) and habitat suitability stability (HS_c) values were obtained from Potential Distribution Models.

Species_OccurrenceData: Occurrence records for P. pringlei (Pp) and P. sierra (Ps) were obtained from previous work (González-Trujillo et al., 2016; Gutiérrez-Flores et al., 2016). For B. xantusii (Bx) were obtained from the geographic metadata catalog of the Comisión Nacional para el Conocimiento y Uso de la Biodiversidad (CONABIO; Navarro-Sigüenza et al., 2018). S. gummosus (Sg), D. dorsalis (Dd) and M. uropygialis (Mu) occurrence records were obtained from the Global Biodiversity Information Facility (GBIF; GBIF.org, 2022, 2024a, 2024b).