Wednesday, December 27, 2006

Paul sent us this...

My dad is having trouble totally accepting evolution. He brings up complex chemical reactions such as the Kreb Cycle and how half a Kreb cycle would be useless. I've tried to explain that for things to evolve, each intermediate step must be useful. I've managed to convince him of the eye but microbiology still stymies my attempts to get him to completely accept evolution. Any suggestions?

It is always difficult to explain why evolution makes sense, and of course we can't alwasys explain everything. That does not mean that things have not evolved, but just because physicists cannot (yet) explain how black holes operate it does not mean that they do notobey the laws of physics. In the same way evolution has occured and does occur, although we may not always be able to see what has happened in the case of some things.

The trap your father has fallen into is assuming that the origin of the Krebs cycle was somehow 'aimed' at producing energy for a cell. But the basis of the cycle probably had a function beyond this. Organisms billions of years ago may have made different uses of the chemical pathways to produce useful products. Evolution added new steps to this (each of whichwould have been advantagous to the organism at the time) and this eventually resulted in the Krebs cycle as we see it now.

It is often said that the components of a structure, for example the eye, or the flagellum of a bacterium, are only worth having if they are part of an eye, or flagellum. There is no point evolving a lens, if you don't have the rest of the eye to go with it. However, the eye has been shown to have been built up from a simple light sensitive spot seen in all animals. It is throught the addition of structures like a lens which makes it into a really specialised organ that can allow the brain to differentiate the tiniest variation in colour and shape of a structure. We see (no pun intended) that the complex camera eye that we have, and also the Cephalopod molluscs, has evolved independently in both of these groups. The animals in between have a variety of simpler eyes from which our sort can be seen to have evolved.

It is interesting that all eyes whatever their morphology, be they camera eyes, or compound eyes seen in the arthropods, are all patterned by the same set of genes, the Pax6 genes.

Likewise the individual components of the flagellum have been shown to have an individual function, before they came together to form the Bacterium's propulsion system.

The is a series of links to various books and papers here and some more information on the origins of complex pathways here (scroll down a bit in both cases!).

Friday, December 08, 2006

Anonymous said...

Where can I find more information on the role of adaptation in evolution? Many adaptations are explained away in what seem to be 'just so' stories, when in fact they could just as easily be explained by a phenotype being predominant purely by association at the genome level to some sexually selected trait - and then the organisms in the new population exploit the phenotype in such a way that afterwards it looks like adaptation drove the evolution. It just seems to me that, for example, 'Longer fingers for tree climbing' is hardly the kind of thing that would affect reproductive success - which HAS TO happen for a change in a population's genome. My 'gut feeling' is that evolution just randomly speciates, with crazy variety, and THEN niches are exploited; then in retrospect paleontologists infer reproductive success for adaptations -when in reality nothing of the sort happened.
So where can i go to validate/debunk this gut feeling? who is doing work to test the mechanisms of adaptation and selection and writing books for the layperson? is convergent evolution 'proof' of the selective power of adaptation, or just statistical certitudes given enough time, in a pretty much random process exploring all morphological spaces?

That’s quite a question, but you have picked up a few misconceptions there. I’ll try to pack it all into a reasonable reply post, but do send us another comment if you want more (and leave us your contact details).

Reproductive success is by no means the be-all and end-all of success in terms of passing on your genes. Animals must survive long enough to reproduce, and few survival traits are linked to sexual selection or to sexual characteristics. Quite the reverse in fact if you look at how sexual selection operates. Look at this recent paper (abstract here) on the changes in limbs in lizards. Limb length changed twice in less than a year in response to the introduction of a new predator. A change in limbs and behaviours allows only some lizards to survive to reproduce, so their mating success is directly tied to this, if not their ability to compete successfully for mates.

You have also used a very strict definition of 'evolution', assuming that it must be an active trend. Traits may appear that have no obvious function (like red hair in humans, or a 6th finger), but if they do not affect the survival of an animal, or its reproductive success then (all other factors being equal) there will be nothing to stop it from passing on its genes and this trait may spread through a population by genetic drift. Reproductive success does not ‘have to happen’ to change the genoype of a population. However, these traits have still evolved, as they did not exist in earlier generations. They may subsequently become 'functional' (red hair might be deemed more attractive, or a 6th finger might help climbing) but this does mean that evolution has not occured.

Novel adaptations can certainly drive evolution however, see some examples here. Obviously the success of groups like birds and bats is in response to their ability to fly and the evolution of flight has allowed them to exploit new niches and speciate and diversify as a result. With much of this work being done by the Gants on Darwin’s finches. At times of great change (after mass extinctions say, or the initial colonization of land) then niches are available to be exploited, and otherwise unsuccessful or uncompetitive adaptations may (temporarily) become established (evolution may go a little nuts) but in times of stability, the better adapted organisms will come to dominate (e.g. the rise of the dinosaurs in the Jurassic). Niches are plastic things and as environments change (over space and time) and organisms, evolve, adapt and compete, they will shape their own niches and those of other species around them.

As for convergent evolution. There are a limited number of ways of solving certain problems (e.g. feeding on termites) with certain body plans and so it is somewhat of an inevitability. In some cases (e.g. birds, bats and pterosaurs) a remarkable problem (flight) can be solved in very different ways, but in other, conservatism of shape is clearly the best solution (whales, penguins, icthyosaurs, tuna) and so evolution will eventually stumble on the ‘best’ solution and shapes will converge.

You also asked about books, take a look around. A quick search on Amazon for “evolution” and “adaptation” found a few dozen popular and more technical science books. A good start would be the Dawkins books The Blind Watchmaker and Climbing Mount Improbable.

Monday, December 04, 2006

Welcome to Biology & Palaeontology Questions and Answers!

This is a new service aimed at school children across the UK to help them get involved and interested in science. Collectively we are a group of professional biologists and biology workers (ecologists, museum curators, science writers and the like) who have got together to answer your questions about biology. Everything that you read here will be provided by real, working scientists who are experts in all aspects of biology. If you look below at our older posts (and back into the archives on the right) you will see some mini-biographies of many of the people who will be answering your questions (such as out latest 'capture', Professor PZ Myers).

Together, we are in a great position to answer your questions about biology, evolution, palaeontology, fossils, conservation and the environment. All you have to is click on the 'comments' button that appears at the bottom of this post (or any of the more recent posts), put in your first name, your age and your question, and then submit it. One of us will get back to you as soon as possible with an answer to your question. We will post up your question and our answer as a new post (like this one) on the main board so it is easy to find!

There is a bit more info about us in our July archives: but for right now you can look at the profiles of our contributers and bloggers below, look through the links or submit a question. We look forward to answering your questions, so give us some to answer!