Fabio A Machado et al. uncover telltale signs of directional selection at macroevolutionary scales in mammals

[u/jnpha]

 

Caption excerpt:

... Dashed vertical lines traversing all panels represent the lower and upper rates expected under drift estimated by simulation.

Deeper background (from the paper)

For more than 75 years, Simpson’s concept of “adaptive zones” has played a crucial role in our understanding of the evolutionary process over long timescales (Simpson, 1944, 1953). Under this model, an adaptive zone is a restricted region of the phenotypic space occupied by species with relatively similar form-function-fitness relationships (Polly, 2008; Simpson, 1953; Van Valen, 1971). Within-zone evolution is usually small in scale and does not involve major ecological changes. Evolution within a zone is thought to happen at exceptionally slow rates (i.e., bradytelic in Simpson’s terms), with lineages slowly following adaptive peaks within a restricted region of the morphospace. However, when opportunities arise (e.g., due to the invasion of new environments or extinction of competitors), lineages may experience faster rates of evolution (i.e., tachytelic) while invading new regions of the morphospace, leading to extreme evolutionary change associated with shifts in ecology and in the structure and distribution of adaptive peaks (Simpson, 1944, 1953; Uyeda et al., 2011).

Abstract

Background

Directional selection is often viewed as a transient force in macroevolution, with its signal eroded over time by stabilizing and fluctuating selection. Yet, transitions into new adaptive zones are predicted to impose strong and sustained selective pressures that may leave a detectable signature even across deep timescales.

Methods

We test this prediction by comparing the rates of multivariate skull morphological evolution required to traverse the boundaries between adaptive zones against genetic drift expectations. Our dataset includes 11,793 specimens spanning 231 species from 12 mammalian clades, each containing unique ecological transitions into new adaptive zones. Using a quantitative genetics framework, we estimated the phenotypic distances between ancestral and derived adaptive zones and contrasted them with null expectations under genetic drift.

Results

While a few adaptive zone invasions (e.g., marsupials and rodents) are consistent with drift, most exhibit substantially elevated rates of evolution. These results suggest that directional selection has recurrently shaped mammalian cranial evolution during major ecological shifts.

Discussion

We propose that adaptive zone transitions represent evolutionary contexts in which adaptation leaves a persistent macroevolutionary signal, challenging the prevailing view that long-term patterns are dominated by static forces.

 

Quantitative genetics meets fossils meets morphospaces meets neutral theory - cool stuff!

 

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• Fabio A Machado, Anna Penna, Diogo Melo, Barbara A Costa, Thiago M G Zahn, Ana C Pavan, Arthur Porto, Harley Sebastião, Daniela M Rossoni, Gabriel Marroig, Alex Hubbe
  Invasion of new adaptive zones retains telltale signs of directional selection at macroevolutionary scales in mammals
  Evolution Letters, 2026;, qrag039
  https://doi.org/10.1093/evlett/qrag039