Sunday, 5 February 2012

Evolution: From mouse to elephant size, and backwards

Evolución: Del tamaño de un ratón al tamaño de un elefante, y viceversa.

The theory of evolution is a topic that captures the attention of many, including me. And the source of such fascination can be traced back to the interest that as a kid we had on the mystery that surrounds the dinosaurs. I guess the roots of the interest of the team of Alister Evans could also be traced to that point. Evans is an evolutionary biologist at the Monash University in Australia, and his work on mammals evolution (published Dec.29th 2011, PNAS) is the topic of an article published few days ago on the National Geographic News (online). The question that Evans aims to answer is: ¿How fast can a mammal evolve from the size of a mouse to the size of an elephant?

As mentioned on Evans´s team paper, microevolution (from large to small) occurs over very fast rates over short timescales (less than 100 generations), and hypotehtically macroevolution (from small to large) could take a mouse to grow to the size of an elephant in about 200 000 generations. However (still citing the paper of Evans et al.), fossil records of mammals do not show a continuous evolution because of several external constraints. To overcome such obstacle (the external variables), the methods applied for the inferences presented in the paper are based on a relatively simple parameter: the body mass, a fundamental animal trait. The results show that using this scaling relationship, the 100,000-fold transformation from mouse to elephant would take 24 million generations; which is substantially longer than 200,000–2 million generations suggested by microevolutionary rates, this is a shrink rate that is 30 times slower than the growth rate. 

What about primates?
According to Evans et al. (2011), contrary to what occurs with cetaceans, which are the fastest to evolve (3 million generations to grow 1 000 times their size), primates are the slowest.

The findings support the conclusions of other researchers, e.g., Gingerich et al. 2001, who conclude that evolution on a generational scale, by itself, cannot explain microevolutionary change over multiple generations, nor can it explain macroevolutionary change over geological time. Each must be studied on its own time scale, remembering that the scale itself is important when relating one to another. I wonder if this body mass measure could provide us a hint on how big our successors will be...

I am aware that the theory of evolution is only one among many possible theories, and I do not want to offense anybody with my post as I apparently did with my friend (a scientist) when I commented him on the topic I now write about. I try to respect the position of all, but, please since I do not explicitly write on religion, I ask you to respect my right to write on any topic.

- The maximum rate of mammal evolution, Evans et al., 2011. Proceedings of the National Academy of Sciences of the USA, PNAS 1120774109.
- Rates of evolution on the time scale of the evolutionary process, Gingerich et al., 2001. Genetica 112-113:127–144.
- Elephants took 24 million generations to evolve from mouse size, By Ker Than, Feb 3rd 2012. National Geographic News.

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