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Reevaluating the Mechanisms of Adaptive Evolution in Stick Insects
The quest to comprehend the foundational mechanisms of adaptive evolution has been a pivotal concern in biology since the era of Darwin. A key aspect of contemporary discussions revolves around the question of whether adaptive evolution stems from numerous mutations with minor effects or if it is influenced predominantly by a few mutations that instigate significant phenotypic changes.
Recent research highlights chromosomal rearrangements, which involve substantial alterations such as inversions, deletions, or duplications of chromosome segments, as a possible avenue for substantial evolutionary changes termed “macromutations.” However, accurately characterizing these chromosomal rearrangements using conventional DNA sequencing techniques has posed significant challenges.
Diploidy, a characteristic shared by many organisms including humans and stick insects, adds to the complexity. This dual set of chromosomes—one inherited from each parent—complicates the genome assembly process when identifying chromosomal rearrangements.
According to Zachariah Gompert, an evolutionary biologist at Utah State University, traditional methods have often relied on averaging data across chromosomal sets, which can lead to incomplete understandings. “The limited precision of this method fails to capture the intricate details,” he notes. “Utilizing newer molecular and computational techniques to generate phased genome assemblies—a process where the two chromosome copies are assembled separately—has allowed us to clearly demonstrate how complex chromosomal rearrangements have facilitated the adaptation of stick insects, making them cryptic on various host plants and thus evading predators.”
In an article published in the April 18, 2025 online edition of the journal Science, Gompert and his team explore the phenomenon of adaptive divergence in cryptic color patterns, supported by two unique chromosomal rearrangements. These rearrangements involve extensive segments of DNA that were flipped and relocated within the chromosome structure, occurring independently among populations of stick insects residing on separate mountains. The study includes contributions from Patrik Nosil and researchers from esteemed institutions including the French National Center for Scientific Research, the University of Notre Dame, the University of Nevada, Reno, and The Institute of Cancer Research in the UK, with backing from the National Science Foundation and the European Research Council.
The research focused on the stick insect species Timema cristinae, featuring distinct color patterns collected from two mountains near Santa Barbara, California. These wingless, herbivorous insects exhibit divergent adaptations to different plant species in coastal chaparral ecosystems. One variant possesses a green hue that camouflages it among California lilacs, while another displays a narrow white stripe on its back, rendering it nearly invisible against the needle-like leaves of chamise shrubs.
Gompert and his team found that the adaptive variations in color patterns are largely determined by the presence or absence of these specific chromosomal rearrangements.
“The phased genomic assembly technology adopted in this research was vital for understanding the evolution of color patterns in these insects,” explains Gompert, a professor in USU’s Department of Biology and the USU Ecology Center. “Our findings indicate that chromosomal rearrangements may be far more prevalent and intricate than previously acknowledged.”
He elaborates that despite their size, these large mutations are often overlooked by traditional DNA sequencing methods. “Detecting and characterizing chromosomal rearrangements through standard techniques can be cumbersome,” Gompert states. “We are effectively investigating the ‘dark matter’ of the genome.”
Gompert suggests that structural variations may not be anomalies but rather a common resource driving evolutionary processes. “We are merely beginning to unravel this complexity,” he remarks. “Historically, we’ve lacked the necessary tools to identify structural variations, yet with advancements in technology, we hypothesize that these variations play a significantly more influential role in evolution than has previously been recognized.”
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