One of the basic things evolution has taught us is that as time goes by, the body shapes of living beings tend to change. However, this is not the case with two species of annelid worms, both belonging to the genus Stygocapitella, which can be found in all the sandy beaches of the world.
Over the course of 275 million years, these two species have evolved separately but both have shown no sign of morphological change since the two species have undergone bifurcation and differentiation. A team of researchers led by the biologists of the Natural History Museum of the University of Oslo has analyzed precisely these species of worms in which the evolutionary change seems to be completely halted.
The two Stygocapitella species examined were divided into two species when there were dinosaurs such as the stegosaur or the brachiosaurus, about 275 million years ago. Since then, these two species (because they are two species, as researchers have also analyzed them on a genetic level) do not show distinct morphological differences.
However, this aspect may be misleading: through molecular investigations, the researchers have in fact confirmed that they are genetically distinct and can be considered two isolated species altogether. What is disconcerting is that the taxa more closely related to these two worms have instead evolved, differentiating themselves significantly on a morphological level.
The researchers have come to the conclusion that the formation of several species from one parent is not necessarily accompanied by morphological changes other than genetic or molecular changes. According to the researchers themselves, the lack of morphological changes in these two species, a phenomenon that is not reserved only for these two worms but that has also been found in other species usually called “cryptic species,” is most probably due to the environment that the two worms frequent, that of the sand, and that substantially has not changed at all during all these millions of years.
The study was published in the journal Evolution.