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Wednesday, 23 November 2011

What is the point of evolution in bacteria?

This is yet another question I was asked. The idea was that bacteria aren’t becoming complex organisms “on the way”, so what is the point?

This is like another question often asked by sceptics “If humans evolved from monkeys, how come there are no monkeys becoming humans nowadays?” The Earth is a rich place, rich with resources, rich with diversity. Humans did not evolve from monkeys, it’s not like monkeys are living in the past and we are living in the present. We all live in the present, and monkeys have spent time evolving like we have. Monkeys are adapted to their environment, and we are to ours, and bacteria are to theirs. Could you live off bananas, in a tree? Could you turn water, carbon dioxide and light into food? No. There are many different niches on Earth, each of which is inhabited by different organisms. Hot springs, dry deserts, deep oceans, high mountains and an airplane are all very different places. Why would there only be one species?? We are complex for our environment, but we are not universally adaptable to anything. We CAN live in deep oceans, but that’s not our home address.

Back to the question. The point of evolution in bacteria is to have bacteria live. If that so happens to increase their complexity due to their environment and the selection pressures present (which are due to more basic, chemical or physical factors and properties), then that’s just a side effect. Food for thought.

Is evolution in bacteria different to that in primates?

I got asked a question yesterday on Facebook on whether evolution in bacteria and primates differs. The short answer is no.

The long answer is that the process of evolution itself is so simple in a way that it really doesn’t matter what its object is, i.e. what it operates on. It’s easier to understand this if you think about atoms. The forces governing their behaviour are equal properties in different measures. Mass is mass, whether it’s zero or one hundred units. The slight differences in these properties can lead to huge noticeable differences in final products, just look around.

Natural selection isn’t something that can change depending on what it acts on, be it bacteria or primates. Natural selection is there, and any difference in products is just that. Bacteria and primates are different because of the different evolutionary paths their ancestors took, the different environments and selection pressures that led to their present day evolution.

Another question was whether increasing complexity of species was an inevitable result of evolution. I suppose this is a bit like the question “Can you return a fried egg back to a raw egg?” According to our current knowledge, the answer is no, we can’t undo the burning of an egg or piece of paper. It’s hard to imagine evolution working backwards, because it’s hard to imagine undoing evolution. The matter of fried egg can’t work its way to become a raw egg, it’s an irreversible reaction. In a sense, ancient species had the resources to evolve in certain ways, but that doesn’t mean the subsequent versions of themselves necessarily have the potential to reverse it. If anything, any further evolution that leads to a similarity to previous species would not be a reversal, but merely a continuation of the same evolutionary pathway. Strictly speaking, there is no reason why natural selection on its own couldn’t lead to future species looking more like ancient species rather than more recent past species on the same evolutionary pathway.
An example that springs to mind, although not on an interspecific level, but on an intraspecific level (that is, within one species), is one insect. This insect has switched its wings on and off repeatedly over a long period of time, depending on its environment. Would you call that backwards evolution? I wouldn’t. Evolution evolves one way, regardless of similarity between past and future species. A bit like time.

Saturday, 19 November 2011

The place of intelligence in evolution

Without maybe realising, we think about intelligence as being the ability to perform changes to one's environment. We know dolphins have relatively sophisticated communication, and we refer to them as intelligent. However, they are excellent in their given aquatic environment without trying to change it, or colonise other places (land, air, space; yes, it sounds silly).

This distinction is important because changing one's environment has an impact on the selection pressures themselves, and hence natural selection and evolution. How could inability to breathe underwater be selected against if someone has an oxygen tank to breathe out of? How could inability to find a suitable nesting spot be selected against if a bird can build its own nest? The more a natural environment is manipulated into an artificial environment by organisms, the less its evolution is left to chance, as inheritance shifts from strictly molecular level and DNA, to other levels (cognition, artificial storage and memory, learning). Essentially, natural selection acting on a genetic level (reproduction, genes passed on) interplays with artificial forms of selection which act on the other levels of inheritance.

In a sense, it is possible to use alternative methods of inheritance such as learning, to control an unlimited number of people's minds. Genetic make-up is no longer enough to guarantee certain advantageous traits in an artificial environment. Children are born without knowing a certain language. It takes only 1 Albert Einstein to spread insightful ideas to countless others, countless other who may use that knowledge to one day change the universe. The likelihood of that happening due to a certain group of people at any given point in time is a lot higher than it happening due to Einstein's imaginary children.

Directional, stabilising and disruptive natural selection

Natural selection leaves a certain gene pool’s allele make-up quite different depending on selection pressures. The results are quite easy to understand. Take a population of toads. Directional selection would happen if smaller toads avoided predators better, hence larger toads would be gobbled up more frequently. The average toad size in the following generations would shift in the DIRECTION of the smaller extreme.

Stabilising selection, on the other hand, shifts extremes towards the middle values. Say small toads have less reproductive success, and large toads are more appealing to predators. Average-sized toads will be best adapted to that situation (of course, unless they too happen to have some “issue”, in which case, bad news for the toad empire). 

Finally, perhaps the most bizarre of these is disruptive selection. It occurs as a result of extreme phenotypes (noticeable traits) being better adapted to the environment than mid-range values. Say smaller toads survive better due to weather conditions during a certain period of time, while larger toads survive better during other times, but average-sized toads have no advantage at any given time.

This stuff becomes so much more interesting when applied to humans. Where you live, which type of selection do you think is taking place?

…if any.

Friday, 18 November 2011

Variety is the spice of life II

Imagine a bacterium whose only mode of reproduction is binary fission (it splits itself into two). And then the process goes on and on. All the offspring are clones. You’d expect zero variation in their DNA, and hence in their appearance and function.

Pretty dull huh? Not only that, but this population would be doomed. With only one allele of every gene, ALL of them will either be resistant to certain antibiotics, or NONE will. So, antibiotic comes around, and there is a 50% probability the population will survive. Throw in 100 antibiotics and 10 different environmental pressures, and the population is good as dead.
Good thing for them that that’s not the case, that is, not all individual bacteria have identical DNA. How is that possible, since they reproduce by binary fission?? In fact, it is mind-opening just how many different ways they have of achieving just that: variation. Some are:

1.       Conjugation. One bacterium produces a mating bridge through which a plasmid (circular piece of DNA) can pass to another bacterium.
2.       Transformation. Bacteria can take up DNA from dead bacteria and use it as their own.
3.       Transduction. Viruses which enter bacteria may pick up some of their DNA and pass it to the next bacteria they infect.
But none of these actually explain the CAUSE of DNA variation itself. By far the biggest cause of DNA variation in bacteria is, of course, mutation. One might say mutation is the inevitable effect of the very way DNA replicates, perhaps not a mistake, as it is often perceived, but as crucial a part of the overall process as the multitude of enzymes which take part. Just because it defies our (designer?) expectations of rigid rules that are never broken, doesn’t mean it is not integrated in nature as obviously as the pairing of DNA nucleotides is.

In fact, imagine a world without mutation and variation. Can you?

Variety is the spice of life

Natural selection acts on variation between organisms. In fact, evolution itself would be impossible without variation. If you have a population where most or all individuals are not varied, then there is no way the population is going to be able to adapt over time, so potential for change doesn’t exist. Adaptation can’t arise spontaneously just because it is needed.

For example, the case of the black and white moths during a time when tree trunks were painted white is a good illustration of this principle. You start with an even variation of about 50-50 between black moths and white moths. The tree trunks get painted white, so clearly the black moths will contrast highly with the white trunks. They are visible to predators, and so most die out, don’t get a chance to reproduce, therefore don’t pass on the gene (allele) that codes for their black colour. As a result, future generations have few black individuals, and a lot more white ones (as the white moths were well camouflaged so were not that visible to predators).

The point is, what would have happened to that moth population had there been no white moths at all? It’s not far-fetched to speculate that that specific population might well have died out. But imagine this: think about our current, mostly white moth population. What if the tree trunk dye gets washed away, and the white moths now appear obvious on the trunks? Does that mean the population is bound to die off anyway at some point in the future? It becomes obvious that, despite directional natural selection (directional means favouring one extreme attribute rather than another), some variation must be maintained. Indeed, some variation is always maintained by many very different processes, from the molecular level to the ecosystems. Find out about these in the next post.

Wednesday, 9 November 2011

hmmm, and DNA

You know what, I've been getting a ridiculous amount of visitors lately, and I don't know why, cos I haven't been writing much lately. Uni is good, doing DNA stuff at the moment, and I was reading about something quite interesting.

Until relatively recently, species and their ancestry was determined mostly by things such as physical appearance and function, i.e. two species look similar, so they must be closely related. Funnily enough, after DNA technology took off and it was possible to determine how closely related species are to one another, a lot of these associations and assumptions that were wrong could be rectified. That similarly looking species may not actually have a recent common ancestor, and different looking members of the same species share most of their genetic make-up. Look for yourself.
Flying squirrels on different continents developed their flying wings in parallel with each other, and don't in fact have a relatively recent common ancestor. 

These dogs however look so different yet they are both part of the same species and likely have a very recent common ancestor i.e. great great great great grandparent.

DNA is key to determining evolution in this sense, much more so than simple physical similarity that we as humans are likely to be biased in.