Science Questions

Do smaller organisms evolve faster?

Tue, 17th Dec 2013

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Question

Thorp Paul asked:

We know evolution happens over the course of generations, and smaller creatures tend to have many more generations than larger creatures over a given time period, that suggests they should be able to evolve faster. Is this the case and is there evidence of any organisms using this to their advantage ?

 

We are seeing things like MRSA that's evolving obvious survival characteristics over the course of a few human generations. How have human immune systems kept up with the many generations of various things that infect us ?

 

I love the podcast. As a lapsed physicist that went into a non-science field of employment, it is brilliantly pitched at an informative level for somebody with a bit of a science background, whilst still being accessible to those with far less background than I have.

 

Thanks

 

Paul

 

Answer

This week, we examine the old adage Ė is size important?

Paul -   Hello.  My name is Paul Thorpe, from Wigan in the northwest of England, and my question is: do smaller organisms evolve faster than larger organisms?

Hannah -   So, does a fly evolve faster than a toad, a whale slower than a barnacle, or the plague faster than people? Over to Robert Foley, professor of human evolution at Cambridge University.

Robert -   The short answer is yes.  Itís not so much that smallness makes you evolve faster, but it's what' correlated with small size that matters.  Small animals on the whole reproduce faster than larger animals.  They grow up, reach maturity, have their babies and die over a much shorter period of time.  So, the number of generations of a small animal is going to be much, much greater.  This means that there will be more mutations and more exposure to selection in each generation, so that the potential for evolutionary change is just that much greater.

Hannah -   So, size is important in evolution Ė the smaller, the better.  In which case, not to sound all doom and gloom, but how have us, long-living and relatively gargantuan human beings managed to survive against the deadly invading bugs which are out there?  Is the end of the world nigh?  John Trowsdale, professor of immunology at Cambridge University, reassures us.

John -   First, we must remember that itís not advantageous in fact for a microbe to kill its host.  If it did that, it would then lose its livelihood.  So, most bugs are adapted to live in harmony with us.  In fact, itís been calculated that we each carry around in our gut and on our skin, maybe 10 times more bacteria than we have cells in our body.  Second, we have many, many lines of defence and any pathogen has to overcome all of them to gain an advantage.  Itís been calculated that well over 15% of the genes in our genome are involved with the immune system.  Third, if an infection does take hold, there's rapid selection within the individual by the adaptive immune system by a process of rearrangement and mutation.  We each make many trillions of different antibodies and the best is selected at which time to respond to a specific infection.  Finally, there is great variation in the immune systems of different individuals.  This is to our advantage as if a virus mutates to overcome the defences of an individual, other people will be resistant.  So, spread of the infection is limited.

Hannah -   Thanks, Paul, Robert and John.  Smaller organisms evolve faster, but our large bodies have clever immune systems evolved to help keep us humans one step ahead and bugs at bay.  Next, we get ourselves in a bit of a tizz, trying to answer this...

John -   Hello.  My name is John and I live in Melbourne, Australia.  My question is, are electrons orbiting atoms?  I've always wondered how come electrons seem to perpetually spin around a nucleus?  What forces are involved and how come friction doesnít play a part in stopping their movement?  Thanks guys.

Hannah -   So, you might remember from school that electrons whizz around the centre of your atoms.  What keeps them whizzing?  Don't they ever get tired?  What do you think?

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My thoughts as a non-scientist...

I would have thought lifespan would have more impact than size. Specifically, the time taken for an organsim to reach sexual maturity. If something could go through fifty generations in the time it takes for us to go through one, it increases the amount of beneficial mutations possible.

Also, if a species has a shorter lifespan and is quicker to die, that species will be faster at weeding out the genetic material that doesn't do so well, and the ones that breed will drive the evolutionary process. bizerl, Mon, 16th Dec 2013

Evolution being a result of individual change expressed in a herd, it's much easier to detect among species that reproduce quickly, reproduce in large numbers, and are relatively simple so that a small genetic variation produces a significant change in appearance or behaviour.

A genetic variant that makes a bacterium resistant to an antibiotic will express itself very quickly (in days if not hours) as the descendants of "normal" bacteria are killed off in vast numbers by their bactericidal environment whilst the daughters of the variant are free from competition for nutrition. But a variant that renders humans immune to, say, peanut allergy, will take millennia to express as a change in herd characteristics because (1) very few humans are allergic to peanuts (2) we are not regularly attacked by hordes of peanuts and (3) it takes around 20 years to produce a new generation of humans. There is evidence of human evolution to cope with dairy products and alcohol, but these traits are not universal throughout the species as they are only essential for survival in a primitive, cold, urban society - environmental conditions that only lasted for a few generations before the gene pool became stirred by travel.          alancalverd, Tue, 24th Dec 2013

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