ScienceWise - Jan/Feb 2009

The Editor's Corner

More like a sponge cake than the Queen Mary

Amongst the stories in our first 2009 edition of ScienceWise we take a brief look at some Australian geckos that are capable of parthenogenesis or asexual reproduction. In effect, some members of this species have the built in ability to clone themselves.

Given the enormous efforts most animals go to in order to attract a suitable mate - fancy feathers, elaborate dances, learning to play the guitar - one might wonder what nature was "thinking" when it "invented" sexual reproduction. Why don't we all use parthenogenesis? I guess the answer lies in the fundamental laws of physics and the very nature of life on earth.

The thing is, billions of years ago single celled organisms mostly did reproduce asexually. Sexual reproduction itself evolved, so it must have offered an advantage. Today we believe that advantage comes from the fact that mixing your genes with those of other members of your species frequently introduces what biologists call heterosis or hybrid vigour. Organisms that are a product of both parents have double the genes to choose from and often (though not always) express a batter pick of genes than could be found in either parent.

Another important point is that life on Earth isn't static. The climate changes as do the plants and animals any organism has to share its environment with. Most individuals of a species don't express the less desirable recessive genes buried deep in their DNA. But such genes and the odd individuals that do express them, give a species variability than can be crucial to surviving changes in habitat.

For example, as the world stands; if you were a diminutive tiger only 10cm tall, you would be at a disadvantage because you'd only be able to bite a gazelle on the ankle. But if suddenly all the large prey animals disappeared (maybe because the strange looking primates you've occasionally seen, begin to fill the atmosphere with CO2?) Then it's your lucky day. You can hunt mice and survive whereas the large "normal" tigers can't. This kind of diversity, variability and adaption is the engine room of evolution.

So given that sexual reproduction is great for evolution, why would cloning not be a good selfish strategy for a particular animal? The answer to that comes from physics. The second law of thermodynamics tells us that the entropy of an isolated system always tends to increase. In plain language this means that when you copy something over and over mistakes inevitably occur and the resulting disorder just gets worse and worse over time. Eventually your cloned DNA would become riddled with errors and quite useless. You'd also be competing with other animals around you that were constantly evolving and improving and pretty soon you'd feel very much like last year's model.

Entropy is also one of the major theories to explain human aging. As cells repeatedly replicate errors occur and bad things happen. Sadly despite the claims of cosmetic manufacturers, this isn't an easy thing to fix because the human body isn't just one big porridge of cells. Our bodies also have mineral deposits like bone and connective collagen fibres holding us together. So as structures like the skin sustain and repair damage they slowly drift further and further from their "intended" shape.

One of the key things here is that our DNA isn't a blueprint for our body in the way the plans of the Queen Mary describe the structure of the ship. It's much more like a recipe for a sponge cake. So much flour, so many eggs, mix and bake at this temperature for this time. Your DNA tells a cell how to divide up into an embryo and eventually end up as you. But DNA isn't something cells in your adult body can refer back to see if your nose is still the shape it should be.

I suppose this is why nature doesn't create complex animals and maintain them in perfect repair indefinitely. The process just doesn't work that way. Which from our perspective as individuals is kind of a pity really! Then again, perhaps if nature did work that way, we'd all still be single celled amoebae and wouldn't be reading Sciencewise.

 

Dr Tim Wetherell

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