Why Do Some Younger Animal Families Have More Species Tha Older Families?
August 29, 2012

Why Do Some Younger Animal Families Have More Species Than Older Families?

Brett Smith for redOrbit.com - Your Universe Online

Basic logic would suggest that the longer a species is around–the more time it has to adapt and evolve, eventually sprouting another whole branch on the tree of life.

However, that may not necessarily be the case according to a team of American biologists who just completed an evolutionary survey that accounted for around 1.2 million species.

"When we look across the tree of life, the age of the group tells us almost nothing about how many species we would expect to find. In most groups, it tells us nothing,” said Michael Alfaro, a UCLA associate professor of ecology and evolutionary biology who co-authored the team´s report that was published this week in PLoS Biology.

For example, crocodiles and alligators are nearly 250 million years old, yet have diversified into only 23 species, while there are over 25 species of dolphins that have evolved in the past 50 million years alone.

The stark contrasts in diversification lead the team to investigate why some animals are so genetically adaptive and others maintain strict lineages.

"We know that some groups, like flowering plants or cichlid fishes, have been exceptionally good at producing species during certain periods of their evolutionary history," Alfaro said. "However, when we look at the ages of all of the major groups of plants and animals, these differences in speciation rate are not sufficient to explain the differences in species number that exist in extant groups."

Alfaro and his colleagues studied almost 1,400 major groups of multi-cellular animals, plants and fungi, which accounted for 1.2 million total species. To categorize each of the groups, they gave each one a "species richness score" using novel statistical and computational methods that were applied to a species database. With the exception of beetles, they found little or no correlation between group age and species richness.

According to Alfaro, one theory that could explain how a lineage will produce species is the “adaptive zone hypothesis” that was introduced by George Gaylord Simpson in the mid-20th century. The theory says the lineage will diversify until the point that it fills an "adaptive zone". This hypothetical zone allows for a maximum number of species that is determined by habitat requirements and competition.

Once an adaptive zone is full, the speciation rate will level off until existing species may go extinct, or a species within the adaptive zone evolves a new trait that confers a significant ecological advantage. In the latter case, the adaptation moves the new species into its own adaptive zone creating an entire new lineage and set of opportunities for new species to emerge.

However, the results of the study point to a decoupling of the species richness-age relationship–and do not support the adaptive zone theory.

"Adaptive zones are an old idea in evolutionary biology, but there is little understanding of whether speciation rates or adaptive zones are more important in explaining species richness across the tree of life," Alfaro said.

"If adaptive zones control biodiversity at the broadest scales, then the rate of species growth will be a good explanation of species richness only right after a lineage has entered into a new adaptive zone. Once the adaptive zone has filled up, then, no matter how much time goes by, the number of species will not change much."