Scientists determined body size evolution can be predicted, and it looks like many species in Earth's oceans are getting bigger.
In what was considered to be one of the most comprehensive study of the evolution of body size ever conducted, researchers found support for Cope's rule, which states that animal lineages evolve to be larger over time, Stanford University reported.
"We've known for some time now that the largest organisms alive today are larger than the largest organisms that were alive when life originated or even when animals first evolved," said Jonathan Payne, a paleobiologist at Stanford School of Earth, Energy & Environmental Sciences.
What researchers were not sure about was whether or not the average size of animals has been growing over time. The recent study shows over the past 542 million years, the mean size of marine animals has increased by whopping 150-fold.
"For reasons that we don't completely understand, the classes with large body size appear to be the ones that over time have become differentially more diverse," Payne said.
Paleontologist Edward Cope's rule has been tested many times since it was first proposed in the late 19th century, but the results have been mixed. Scientists have been torn over whether observed pattern was a true evolutionary phenomenon, or if it was a result of random, non-selective evolution known as "neutral drift."
To test this, the research team compiled a dataset including more than 17,000 groups of marine mammals spanning over 542 million years. The information in this dataset relied heavily on the detailed fossil records outlined in the "Treatise on Invertebrate Paleontology." This information allowed the scientists to calculate body size and volume for 17,208 marine genera.
The data revealed not all classes, but groups of related species and genera of animals tended to trend towards larger sizes, and those that were bigger became the most diverse over time. The researchers believe this occurs because larger animals have more advantages, such as the ability to move faster and capture larger prey.
"It's really a story of the survival and diversification of big things relative to small things," said Noel Heim, a postdoctoral researcher in Payne's lab.
In further investigations, the research team used a computer model to simulate body size evolution in the species. When a new virtual species was created, the computer model assigned it to be of either a larger or smaller size than its ancestor. The team ran a number of different scenarios involving varied assumptions, such as neutral drift or natural selection. The model determined neutral drift could not explain the body size trends observed in the fossil record.
"The degree of increase in both mean and maximum body size just aren't well explained by neutral drift," Heim said. "It appears that you actually need some active evolutionary process that promotes larger sizes."
The findings could inspire scientists to look for and identify evolutionary trends in other traits.
"The discovery that body size often does evolve in a directional way makes it at least worth asking whether we're going to find directionality in other traits if we measure them carefully and systematically," Payne concluded.
The findings were published in a recent edition of the journal Science.
