By John P. Roche
The order Orthoptera — which includes the familiar crickets, katydids, and grasshoppers — is a huge and diverse group of winged insects with more than 25,000 species, many of which are scientifically and economically important. Because of its size, understanding the evolutionary relationships in the Orthoptera is important, but up until recently, the hypothesized taxonomy of the group was disorganized and inconsistent. A giant step in correcting this problem was made with a recent study of the evolutionary relationships of the Orthoptera, published in the journal Cladistics by Hojun Song of Texas A&M University and his colleagues.
Previous analyses of the taxonomy of the Orthoptera were based on physical traits, or on genetic evidence from relatively small numbers of samples. The problem with using physical traits for inferring evolutionary relationships is that organisms can have profound physical similarities shaped not by shared ancestry but by similar selective pressures arising from similar environments — a process known as convergent evolution. By looking at DNA sequences, on the other hand, scientists can infer evolutionary relationships much more accurately because similarities in the genetic code more reliably reflect evolutionary relationships. With DNA studies, the larger the number of samples, the more comprehensive the analysis.
The most extensive molecular study of the phylogeny of the Orthoptera prior to Song’s study was performed by Paul Flook and colleagues in 1999. Flook’s study looked at only 31 orthopteran taxa and only three genetic loci. Song’s study used DNA samples taken from 254 different taxa within the Orthoptera. For his genetic evidence, Song and his colleagues used the entire mitochondrial genome as well as some DNA from the nucleus. Mitochondria are organelles within eukaroytic cells that contain their own DNA, separate from the DNA found in the nucleus. Mitochondrial DNA is hypothesized to be a remnant of a past endosymbiosis between bacteria and eukaryotes. The advantage of mitochondrial DNA is that, being vestigial, it is less subject to stabilizing selective pressures that resist genetic change, so mitochondrial DNA changes more rapidly, thus providing a finer-grained yardstick for inferring branchings on an evolutionary tree. By using the entire mitochondrial genome, Song and colleagues were able to resolve evolutionary relationships in the Orthoptera over broad time scales.
With their DNA samples, Song and colleagues reconstructed the phylogeny of the Orthoptera using the science of cladistics, which infers evolutionary relationships based on the number of shared characters. Song used two different cladistics techniques: parsimony and maximum likelihood. Parsimony is a tool that looks for the simplest possible taxonomic hypothesis; maximum likelihood is a statistical tool that computes the probability of particular evolutionary trees and comes up with the highest probability tree. Song added all of the data together into a “total evidence phylogeny.” The resulting phylogeny is much more robust than previous trees constructed for the Orthoptera. The results from the DNA analysis were then calibrated with fossil information to determine the dates at which subgroups on the tree branched off from one another.
Song’s study found that there are two main evolutionary branches in the order Orthoptera: suborder Ensifera and suborder Caelifera. The figure below (click for larger image) illustrates these suborders as the two main branches of the Orthopteran tree. The column of names to the right of the tree lists the families in these suborders. (The family Mantodea — the mantids — shown at the top is what evolutionary biologists call an outgroup; outgroups are used for comparison to refine the phylogenetic analysis.) The Ensifera includes the crickets and katydids and their relatives; the Caelifera includes the grasshoppers and locusts and their relatives. The members within each of these groups descended from a common ancestor unique to that suborder — what evolutionary biologists call a monophyletic group, or clade.
Analysis of the genetic and fossil data revealed that the order Orthoptera originated in the Carboniferous period (~350–300 million years ago) and the two suborders diverged in the Permian (~300–250 million years ago). The analysis found six superfamilies within the Ensifera and nine superfamilies within the Caelifera (superfamilies are a taxonomic group above the level of a family and below the level of a suborder). Song’s study provides what is by far the most rigorous and comprehensive phylogeny of the Orthoptera to date. Song made numerous important discoveries about specific patterns of relationship and divergence in subgroups of the Orthoptera. I summarize some of the key findings relating to the most diverse of the superfamilies below.
The crickets (superfamily Grylloidea) diverged from other Orthopterans in the Triassic period (~250–200 million years ago) and continued diverging throughout the Triassic and Jurassic periods (~200–145 million years ago). With more than 4,800 species alive today, they are the third most diverse group in the order. A key characteristic of crickets — the trait by which we know and love them — is their music. Crickets use acoustic communication to find mates, and thus their songs are shaped by sexual selection. Traits under sexual selection tend to diverge rapidly, and thus acoustic communication might be a factor in the rapid divergence observed in crickets.
The katydids (superfamily Tettigonioidea) evolved in the Cretaceous period (~145–65 million years ago) and diversified at the same time that flowering plants were diversifying. They are the second most diverse Orthopteran group, with more than 7,000 species. The wings of many katydids look like the leaves of flowering plants, which provides camouflage against predators. Therefore, natural selection may have shaped katydid wings to blend in with the leaves of flowering plants. We know that the leaf-shaped wings of katydids can provide a selective advantage via camouflage in the present, but we do not know if leaf-shaped wings evolved for this reason in the past. However, leaf-shaped wings evolved independently several times in the katydids, and this provides support for the evolved-for-camouflage hypothesis.
The grasshoppers (superfamily Acridoidea) are the most diverse group within the Orthoptera, with about 8,000 current species. Grasshoppers diverged in the mid to late Cenozoic Era (~65 million years ago to the present), and are therefore the most recent of the orthopteran superfamilies. Their evolution coincided with the origin and radiation of grasslands. All grasshoppers are herbivores, and grasshoppers are major consumers of grassland biomass.
Another intriguing finding of the study was that whereas there was a relatively steady and slow background rate of diversification in the Orthoptera over time, there were three instances where there was a high rate of evolutionary divergence. One was a rapid diversification in the Cenozoic that occurred in the branch containing the families Acrididae, Romaleidae, and Ommexechidae (the node of this diversification event is indicated by the lowest of the three black dots indicated by vertical black arrows in the figure above). There was also an instance of an increased rate of diversification in the family Pamphagidae (indicated by the middle of the three black dots in the figure). In addition, there was an instance of rapid diversification in the branch containing the families Tettigoniidae, Rhaphidophoridae, Prophalangopsiadae, Anostostomatidae, Gryllacrididae, and Stenopelmatidae (indicated by the highest of the three black dots in the figure).
In their study, Song and his colleagues made valuable discoveries that refined our understanding of orthopteran phylogeny. But many questions remain for further investigation. What are potential next steps in the investigation of orthopteran evolution? One practical step would be to obtain more DNA samples from the Gryllidea and run phylogenetic analyses on that group, as it was under-sampled in comparison with other groups in this study. Another question relates to the three nodes where rapid diversification was observed: Why was there rapid divergence in these instances? On a big-question level, one thing that particularly intrigues Dr. Song is the evolution of acoustic communication in Orthopterans.
“Crickets sing, katydids sing, and even grasshoppers sing,” Dr. Song said in an interview. “They use different mechanisms for generating sound and receiving sound. How did the acoustic communication evolve and diversify and in what context?”
All of these questions will provide fertile ground for future work on the evolutionary ecology of this group. Given the rapid diversification driven by sexual selection on their songs, we can wonder what the next newly-evolved group of Orthopterans will look and sound like, and how its behavior and its morphology will differ from its relatives — and why?
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John P. Roche is a science writer and author with a PhD and a postdoctoral fellowship in the biological sciences. He has served as editor-in-chief of university research periodicals at Indiana University and Boston College, has published more than 150 articles, and has written and taught extensively about science and science writing. Dr. Roche also directs Science View Productions™, which provides technical writing and developmental editing for clients in academia and industry.