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Twins

September 9, 2005 02:57 PM

Twins was a funny Arnold Schwarzennegger/Danny De Vito movie about twins who were nothing alike. They were different in height and shape and temperament. The puzzle of the movie was how could twins be so different from one another? They couldn't be as different as Arnold and Danny were if they really were twins. Everyone knows this about identical twins, but it helped make the movie that they differed as much in their appearances as in their evil/good aspects.

Here are two identical twins that look almost as different from one another as Danny and Arnold.

Twins.tiff

This photo of identical 23 year old twins is from M. Rennie, Exp. Physiol 90.4 pp 427-436. Otto trained in distance running and Ewald trained in the field events. They are the same height and have similar facial structure, but they look very different otherwise.

Their bodies adapted to the demands they made on them; these epigenetic or environmental factors have a large influence on physiology. They have to because the human body is designed to efficiently adapt to what enables survival. Not that their survival was a risk, but the body doesn't know this. It just adapts to demands.

This is a good example of the difference between a proximate or mechanistic explanation and an adaptive one. They differ because they imposed different demands on their bodies. But, that is merely the mechanism. The deeper explanation is that this kind of physiological adaptability ensured survival and reproduction.

Genes are not destiny; they are if/then off/on switches capable of enormous variation in output (proteins) when combined in signal cascades; yet they are stable too. So, Otto and Ewald are similar in many respects. But they are so different too.

It turns out that Otto's more low intensity stimulation decreased ATP concentrations and activated AMP kinase. This inhibited stimulation of TSC2 so that mTOR-mediated myofibrillar stimulation did not occur. In Ewald's case, the genes got another signal: high intensity contraction stimulated PKB activity, increasing TSC2 and activating the mTOR signal, resulting in markedly increased myofibrillar protein synthesis.

So, a low intensity signal turns on different genes and signal cascades than a high intensity signal. Low intensity -- no muscle protein synthesis. High intensity -- markedly increased muscle protein synthesis. Same genes, different signals, different bodies.

· Evolutionary Fitness

Comments

Posted by: Flower Online [TypeKey Profile Page] at September 11, 2006 2:06 PM

That is an amazing illustration of something that, up to now, I've never fully believed. Your description of genes being if/then off/on switches is somehow motivating, in that it means we can make choices in influencing our genetic expression. This post reminds me of your "All you need to know about Zen" post where you state that although outcome is not guaranteed, we can choose paths that increase the odds of certain desired outcomes.

Posted by: Scott [TypeKey Profile Page] at September 9, 2005 11:25 PM

Interestingly enough, their calves seem to be roughly the same size.

Posted by: A. McGowan [TypeKey Profile Page] at September 9, 2005 6:40 PM

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