I have been working on this post since 2013. ” I’ll just go to Encyclopedia of Life and count the genera in each of the beetle families”, I said to myself. If my previous post on the perching birds represented one neighbourhood in a tidy little town of 10,000 occupants, the order of beetles is more like a large section of a teeming shanty town of one million people! Not only are there several more layers of taxonomy (sub orders, super families, sub families and tribes), there are several different accepted classification systems. As a result, my beetles project took a little longer than I had anticipated.
The four most diverse families in the order Coleoptera are the weevils (long snouts), the rove beetles (long abdomens), the click beetles (flexible middles parts) and the jewel beetles ( rainbow colours!). If it were not for the great age of the beetles (almost twice as old as the birds and flowering plants), the branching events in these four families could easily claim to be the four most intense speciation incidents in the history of life. It is just as likely, that these families have been splitting off genera at a constant rate for all of their 200 million years of existence.Photo: Umo Schmidt
Our question for today is whether the frenetic branching and successful survival of so many kinds of beetles is just a continuation of the pattern set by the class of insects branching into orders or is it a wholly new phenomenon confined to the beetles. The question matters as it goes to the heart of the nature of evolution.Previously, I identified three broad generalizations for biodiversity, 1) Species numbers increase over time, 2) species are more abundant where ecosystems have abundant energy, and 3) select groups of organisms contain the lion’s share of species. If there are characteristics of a class of species that allow it to branch more quickly – a speciation gene -then we would expect successive time periods and taxonomic levels to be equally productive for the group of organisms possessing those characteristics.
In the case of the bird orders splitting into families, the maximum number of branches was consistent with the pattern of branching in the perching birds but the average number of families in each bird order was significantly lower than you would expect based on the perching birds. Since lopsidedness is a combination of how long a tail a distribution has and where its most typical members occur, there appears to be something new in the perching birds that was not found in the branching of the earliest birds. The insects, however, tell a more consistent story.
The early insects’ average of 28.2 branches per order was exceeded 29 times by the beetle model. The real distribution has an average that ranks 30th out of 100 distributions. This translates into a 30% chance of coming from the same distribution as the beetles. The maximum number of 182 branches for the insect orders comes 15th out of 100 beetle model simulations. That is , there is a 15% chance of coming from the same distribution. Though these numbers suggest some distinction between branching patterns in the early insects and among the beetles, there is no significant difference. The insects are just as lopsided as the beetles.
Something happened among the first terrestrial arthropods (joint legged animals) that led to unprecedented speciation. Natural selection offers a number of possibilities, including superior design and the availability of numerous new terrestrestrial niches or livelihoods. What I am suggesting here is that, beginning with the insects and continuing to quite recent speciation among the beetles (and other insect groups), these populations are much more likely to split into new species even without great geological events to separate the populations into different sub-groups. Speciation occurs when individuals are content to exclude “outsiders” on the smallest of pretexts. They keep to their own in a way that ensures the formation of a genetically distinct species.
Next time, we will look at the third group involved in the Cretaceous triple bang, the flowering plants. Just how common is lopsidedness and why are some groups more lopsided than others?