Do smaller organisms evolve faster?

This week, we examine the old adage - 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. It's 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 - 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 it's 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, it's 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. It's 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 doesn't 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?