New research from the University of Reading
overturns conventional views on the nature of evolution, arguing that mammals
did not develop into their many different forms in one early and rapid burst of
evolution, but rather found many different evolutionary routes.
It is widely assumed that species often
diverge rapidly early in their evolution, and that this is followed by a
longer, drawn-out period of slower evolutionary fine tuning.
Explanations for this pattern suppose
that mammals moved into a largely unoccupied niche and geographical space as
they came to be the dominant vertebrate group on Earth. Then, as time went on,
niche space and unexplored geographical regions became scarce, reducing
opportunities for diversification.
However, the research led by Professor
Mark Pagel, in conjunction with Dr. Chris Venditti and Dr. Andrew Meade, shows
that there is no necessary connection between the rates at which new species
emerge and the underlying rates of evolution of their features. Thus, the
majority of mammal species, including two of the most prevalent orders of
mammals (the rodents and bats), have no history of substantial and sustained
increases in the rates at which they evolve.
Instead, these species achieved high
rates of ‘speciation’ or the production of new species, even though their rates
of evolution were close to normal for mammals.
By comparison, some of the highest
average rates of change occur in one of the least numerous groups. The
Proboscidea, including elephants and sea cows, evolve on average 4.6-fold
faster than the mammalian norm. The results highlight natural selection’s role
as a precise sculptor of mammalian size diversity, able to produce rapid body
size changes seemingly at will.
Pagel says: “It has long been
believed mammals underwent a burst of body-size evolution that occurred early
in their history and that this was followed by a gradual slowdown towards the
present. However, we find that the processes that give rise to the
morphological diversity of this class of animals are far more free to vary than
previously considered. Niches do not seem to fill up, and diversity seems to
arise whenever, wherever and at whatever rate it is advantageous. We find that
natural selection has found multiple different routes to producing the current
diversity of sizes.”
‘Multiple routes to mammalian
diversity’, by Chris Venditti (now at University of Hull),
Andrew Meade, and Mark Pagel, is published in Nature.
