Saturday, August 16, 2008

Natural Selection

Natural selection is one of those terms which is thrown around a lot by people in the scientific and lay community alike, however many people are not aware of exactly what natural selection is or how it works. Not only this, but there are some people who believe that natural selection is something which happens ‘out there’ in nature, and doesn’t really affect us directly. I will show you some of the ways in which natural selection does affect our everyday life and of course, talk to you about natural selection in general.

Natural selection was first accredited to have been ‘discovered’ by Charles Darwin, the man who made the link between humans and primates, telling the world that we evolved from our hairy cousins. Darwin studied finches on the Galapagos Islands and was able to come up with the theory of natural selection. However, I will get to the finches later.

If you have a population of rabbits (a population is a group of the same species of animals, such as a herd of cows or school of salmon), within that population you will find white rabbits, brown rabbits and black rabbits. If we assume that all the rabbits are the same, they are all the same size, they run at the same speed and all have perfect eye sight, then the only thing which is different is pure luck and fur colour. During spring and summer, the surroundings of the rabbits are green, which means that all the rabbits stand out equally, no matter the colour of their fur. It is just as easy to see and eat a white rabbit (you know, if you are a hawk) as it is to see and eat a brown rabbit.

However, during winter, the ground is covered with snow; this means that the white rabbits are much harder to see than the black or brown rabbits. Therefore, the black and brown rabbits are much more likely to get eaten by the hawk, and thus, we have what is called a Selective Pressure. A selective pressure is a change in the environment which causes one organism (or rabbit) to be ‘fitter’ than the other. The phrase ‘survival of the fittest’ is one which many people associate with natural selection. It means that not only those organisms which are strong and fit survive, but those who ‘fit’ their environment survive, those with camouflage, or able to dig in the sand (if they live in the dessert). So during winter, the white rabbits are the ‘fittest’, resulting in more of the black and brown rabbits being eaten.

After winter is over, the population of rabbits looks a little more like this:

During winter, less of the white rabbits got eaten, so more of the white rabbits were able to breed and have babies. While more of the black and brown rabbits got eaten, and a few survived due to pure luck. As you can see, the overall fur colour of the population has changed dramatically. This is natural selection at work.

Natural selection is not stationary, just because a population changed in some way, doesn’t mean that it cannot change again. If there were a fire for example, the black rabbits would camouflage very well in the soot covered environment – thus leading to an increase in black rabbits.

Now to the finches I mentioned earlier.

The Galapagos Islands were once one large land mass, many years ago, the land mass broke up into islands, and those islands began to float away from each other. When the land mass broke up, some animals were trapped on the islands, and as the distance between the islands grew, travel between them became harder and harder for flying and swimming animals. The type of plants found on these islands began to change due to changes in position, which led to changes in climate (a selective pressure!). Because of the change in vegetation, some finches were better suited to eating seeds, while others insects, depending on their beak shape. Just as the white rabbits were more suited to winter. Over time, the shape of the beaks of the finches changed dramatically between the island species. This can be seen well in the picture below.

Picture from here:

Darwin observed these differences, as well as the differences in vegetation, and came up with the theory of natural selection. One of the most interesting parts about this is that one species of finch actually uses sticks to reach the grubs within tree bark – it will even break the stick into small lengths if needed. While we accept that for a primate to use a tool is pretty ordinary – for a bird to use a stick, considering their small brain size - it’s amazing!

This video talks about Darwin's Finches and it also shows the Tool Using Finch at work at time 1:54. Clicking on the YouTube logo will take you back to the original video.

How is natural selection all around us?

When you become sick, sometimes that sickness is caused by bacteria(germs). To kill those bacteria, you take a course of antibiotics. If you do not finish that course of antibiotics, even if you feel better, you haven’t killed all the bacteria; you have left behind the lucky and more ‘fit’ bacteria. This is why it is important to finish a full course of antibiotics, because you need to make sure that you kill all of the bacteria, otherwise they may just come back stronger. I will address exactly how bacteria can come back stronger in my next post, but now, on to fish.

When you go fishing, you must throw back fish which are not of a certain length. This means that more large fish of one species are taken out of their population, leaving behind the smaller fish. Over time this means that the maximum size of fish has become smaller. Why is this problem? Because fish need to breed, and when they do, they need space inside themselves to store eggs. If fish in general are smaller than they used to be, then they cannot store as many eggs as normal. The result of this is that fish populations cannot become as large as they used to be, because they cannot breed in the numbers which they did before.

For example:

I go out and I hunt only the big fish in a population of 100. This leaves 50 smaller fish, some fish as babies and some are just small adults. If I stop hunting, then the population will eventually return to 100 fish. If I then hunt the big fish, the population once again goes down to 50. However, this time, there were more small fish breeding then there were big fish breeding. This means that more of the 50 remaining fish are small fish (whether baby or adult). Thus, when I stop hunting, the population only goes up to 75, because the fish physically cannot have as many baby fish. If I were to hunt again, then the population would decrease even more.

This is a sad example of how a law to protect a species, has actually worked against it. So if you are just fishing for fun, please, throw back all your fish.

Well there you have it, natural selection at work.

Happy readings!

- Bethany

1 comment:

putty182 said...

amazing what you learn on the internet :p