Rapid Sympatry Explains Greater Color Pattern Divergence in High Latitude Birds

Abstract

Latitudinal variation in patterns of evolution has fascinated biologists for over a century, but our understanding of latitudinal differences in evolutionary processes-such as selection and drift-remains limited. Here, we test for, and find, accelerated evolution of color patterns in bird taxa that breed at higher latitudes compared with those breeding in the tropics, analyzing data from seven diverse avian families. Most important, we show that the extent of overlap of species' breeding ranges (degree of sympatry) explains the elevated rate of color pattern evolution at higher latitudes. We suggest that the dynamic shifts in breeding ranges that accompanied climatic changes during the last 3 million years (Milankovitch Oscillations) resulted in more rapid and more frequent secondary contact at high latitudes. We argue that sympatry among diverging clades causes greater divergence of color traits in birds at higher latitudes through sexual, social, or ecological character displacement that accelerate rates of evolution, and through the selective elimination of weakly differentiated lineages that hybridize and fuse in sympatry (differential fusion). K E Y W O R D S :Character displacement, color patterns, differential fusion, latitudinal gradients, rates of evolution, reinforcement, speciation, sympatry.

Phenotypic divergence during speciation is inversely associated with differences in seasonal migration

“…In addition, models including only geographical overlap did not provide a better fit to the data than those with separate rates for each migration category (table 2). This finding is in accordance with other comparative analyses that have failed to find an effect of sympatry in explaining phenotypic divergence ( [23,46], but see [21,53]). …”

“…We accounted for this potentially confounding effect by including the extent of geographical overlap between pairs in our models. We downloaded range maps from Bird Life International and Nature Serve [50] and used ArcGIS (Esri) to calculate geographical overlap, as the proportion of the smaller range that occurred within the larger range [21]. Separate range maps were not available for subspecies pairs; for these pairs, we split the ranges and estimated the width and length of hybrid zones using Birds of North America and the literature (see the electronic supplementary material, table S1 for sources).…”

“…Drosophila, plants, and domestic species of birds). New methods to estimate reproductive isolation using phenotypic data from natural populations have recently been employed in the literature [21][22][23]. These methods use divergence in traits related to mate choice and species recognition-morphology, colour and, song-as a proxy for reproductive isolation [17,[24][25][26].…”

Dispersal and the transition to sympatry in vertebrates

“…Overall, our results are consistent with those of previous studies suggesting that rates of sympatry increase with latitude [25,26]. However, even after accounting for this geographical variation, we find that species with contrasting ecologies are characterized by substantial differences in the transition rate to sympatry.…”

A latitudinal gradient in rates of evolution of avian syllable diversity and song length

“…Secondary sympatry in birds evolves about one and a half million years faster on average at high latitudes versus the equator [51]. The faster speed of sympatry at high latitudes has previously been invoked to explain the more rapid divergence of colour patterns in high latitude birds [3]. Our analysis did suggest that sympatric pairs show faster latitudinal increases in rates of complexity (oscines only) and song length than allopatric pairs, but the differences are not significant.…”