Evolutionary mode routinely varies among morphological traits within fossil species lineages

Abstract

Recent studies have revitalized interest in methods for detecting evolutionary modes in both fossil sequences and phylogenies. Most of these studies examine single size or shape traits, often implicitly assuming that single phenotypic traits are adequate representations of species-level change. We test the validity of this assumption by tallying the frequency with which traits vary in evolutionary mode within fossil species lineages. After fitting models of directional change, unbiased random walk, and stasis to a dataset of 635 traits across 153 species lineages, we find that within the majority of lineages, evolutionary mode varies across traits and the likelihood of conflicting within-lineage patterns increases with the number of traits analyzed. In addition, single traits may show variation in evolutionary mode even in situations where the overall morphological evolution of the lineage is dominated by one type of mode. These quantified, stratigraphically based findings validate the idea that morphological patterns of mosaic evolution are pervasive across groups of organisms throughout Earth's history.

Fig. 1.

Distribution of AICc weights. (A) Frequency of AICc weights for directional change (GRW), unbiased random walk (URW), and stasis. (B) Ternary diagram showing AICc weights for each model for each analyzed trait. Strongly supported traits shown in gray.

Fig. 2.

Stacked histograms showing distribution of species lineages where all measured traits show same evolutionary mode (black) and where evolutionary mode varies across measured traits (gray). (A) All single traits, (B) size traits only, (C) shape traits only, and (D) strongly supported traits only.

Fig. 3.

Boxplots showing proportion of traits within species lineages that show directional change (GRW), unbiased random walk (URW), and stasis. Tallied for sequences where at least four traits were measured.

Fig. 4.

Results from simulated trilobite cranidial shape data. (A) Landmarks representing overall shape of cranidium (required only from half because of bilateral symmetry) and all possible length measurements between landmarks. (B) Time series of selected PC axes scores. (Upper Left) Image shows example of typical morphology from oldest sample; (Upper Right) image shows typical morphology from stratigraphically youngest sample (scale bars = 1 mm). Change in morphology is dominated by an expansion and rotation of palpebral lobes relative to rest of the cranidium. (C) Time series of individual length:length measurements, divided up into four panels showing those that were best characterized by directional change (Upper Left) (black), unbiased random walk (Upper Right) (gray), stasis (Lower Left) (white), and strongly supported trends of all types (Lower Right). Bars on right show range of means for each mode of evolution at the end of the sequence. (D) Stacked histograms showing distribution of samples of randomly selected length:length measurements where all traits in the sample show the same evolutionary mode (black) and where evolutionary mode varies across traits (gray). Number of samples taken at each sample size = 1,000.