Understanding the material basis of adaptive evolution has long been a pivotal aim in the field of biology, tracing back to the foundational ideas proposed by Charles Darwin. A key area of ongoing debate among scientists revolves around whether adaptive evolution is primarily driven by a multitude of mutations with minor and comparable effects or if it is largely influenced by one or a few significant mutations that can lead to substantial changes in organisms' traits.

One intriguing possibility regarding these major mutations lies in chromosomal rearrangements, wherein large segments of chromosomes undergo inversion, displacement, deletion, or duplication. Such macromutations may provide a substantial driving force for evolutionary change. However, characterizing these chromosomal rearrangements has proven particularly challenging with conventional DNA sequencing techniques, as many organismsincluding humansare diploid. This means they possess two sets of chromosomes, one inherited from each parent, a characteristic shared by stick insects as well.

According to Zachariah Gompert, an evolutionary biologist at Utah State University, traditional methods that average data across chromosome sets often lack the necessary accuracy to convey a complete understanding of chromosomal arrangements. Gompert explains, In the past, we've averaged data from each chromosome set, but the limited accuracy of this method doesn't tell the whole story. Recent advancements in molecular and computational biology have introduced phased genome assemblies, which allow for the separate assembly of the two chromosome copies. This innovative approach has enabled researchers to reveal how intricate chromosomal rearrangements have facilitated the adaptive evolution of stick insects, allowing them to become cryptic and blend in with various host plants, thus evading predators.

In a study published in the April 18, 2025 online edition of the journal Science, Gompert and his collaborators demonstrate that adaptive divergence in the cryptic color patterns of stick insects is underpinned by two distinct and complex chromosomal rearrangements. These rearrangements involve millions of base pairs of DNA being reversed and relocated within chromosomes independently in different populations of stick insects inhabiting separate mountains.

The research team focused their investigation on the Timema cristinae species, known for its diverse color patterns, and collected samples from two mountains near Santa Barbara, California. These wingless, plant-feeding insects exhibit divergent adaptations to two distinct plant species thriving in the coastal chaparral regions. One phenotype of the stick insect features a green coloration, allowing it to seamlessly blend into the environment of California lilac, while another exhibits a white stripe on its back, rendering it almost invisible amongst the needle-like foliage of chamise shrubs.

The findings reveal that the adaptive differences in color patterns between these stick insects can almost entirely be attributed to the presence or absence of specific complex chromosomal rearrangements. Gompert emphasizes the significance of the new phased genomic assembly technology employed in the study, stating, This was a critical piece in helping us examine how color patterns evolved in these insects. He adds, Our findings suggest chromosomal rearrangements might be more widespread and more complex than we previously thought.

Despite their size, Gompert notes, these large mutations are often overlooked using traditional DNA sequencing techniques. Chromosomal rearrangements can be difficult to detect and characterize using standard approaches, he explains. We're essentially exploring the 'dark matter' of the genome.

Gompert further posits that structural variations, rather than being rare occurrences, may be readily available as potential catalysts for evolutionary processes. Were just scratching the surface, he asserts. We've lacked the tools to detect structural variation, but with improved technology, we hypothesize it plays a more significant role in evolution than previously recognized.

Story Source: Materials provided by Utah State University. Original written by Mary-Ann Muffoletto.

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