Biology for Dan and Sophie: Part 2 - Evolution (II)
This is another long post with a lot of complicated words. I'm sorry about that, but this will hopefully be the last post like this for a while. In the next one we'll start looking at the different types of animals.
What is natural selection?
Here’s an experiment for you to try. It will take both of you and your mum to watch and time you (and maybe help with the maths).
You will need:
- four sheets of coloured paper; 2 of one colour, 2 of another. Make sure the colours are very different (i.e. green and yellow, not light green and dark green). I’m going to refer to them as black and white from now on but you don’t have to use these colours.
- scissors
- a pair of tweezers
- a stop-clock
What you need to do:
- Take one sheet of black paper and one sheet of white paper and carefully cut each sheet into squares. You need about 100 squares of each colour so cut them quite small (around 1cm square should be good).
- Take the spare sheet of white paper and put it on the table.
- Now put 50 black squares and 50 white squares onto the white sheet. They don’t have to be neat and they should be all mixed up.
- You need to chose who will go first, I’m going to chose Dan this time but you need to decide for yourselves.
- Dan has 30 seconds to pick as many pieces of paper as possible from the sheet. But you must only use the tweezers and each square has to be removed one at a time (no trying to scoop up several pieces in one go!).
- At the end of the 30 seconds you need to count how many squares of each colour you have collected. If you have an odd number of one colour then you need to remove one more to make it an even number. Then you need to do a bit of maths. It hopefully isn’t too complicated for you but your mum can help. You are going to work out how many squares to replace for the next round and then work out the percentages of each colour left. Here’s a table you need to fill in. I’ve made up some numbers for generation 1 and 2 so you can see how the numbers are calculated:
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| Click on me to make me bigger! |
- Do this 10 times.
- Look at how the percentage of each colour has changed.
You should find that the percentage of white squares goes up and the number of black squares goes down. This is because it’s easier to find the black squares against the white background than it is to find the white squares.
Now keep the pieces of paper but change the background to the black paper and do the whole thing again.
What do you find?
You should find that there starts to be more black squares.
What does all this have to do with natural selection?
This is natural selection, or at least a very simplified example of it. Natural selection is the process by which organisms that are well suited – biologists say well “adapted” - to their environment survive better than those organisms which are not.
In the experiment the paper squares were a population which had variation in its colour. You hopefully remember 'variation' from last time. It just means differences. There were differences in the colour of your paper. 'Population' is a new word. It just means a group. But in biology it has a special meaning: it doesn't just mean a group, it means a group of a particular organism (for example blackbirds) who all live in the same area and can breed together. This means that the blackbirds that live in Swindon are a population and the blackbirds that live in Dublin are a population but they are separate populations because they don't fly and visit each other and have baby blackbirds together.
The paper squares were pretending to be prey and in the experiment you were acting as a predator, catching as many prey as you could. You went for the easy prey - the ones which were easy to see. These easy prey were poorly adapted to their environment and so they got caught by the predator (you) more often that the ones who were better adapted to their environment. In the first part of the experiment the black paper prey were poorly adapted to their white environment so their numbers decreased. But when the environment changed - when you turned the background from white to black - the black paper prey were better adapted and their numbers began to increase while the white prey began to decrease.
But what is "evolution by natural selection"?
The first thing to do is to go through each word and explain what they mean, then I'll explain what the idea of "evolution by natural selection" means. So first, the words:
"Evolution", you hopefully remember from last time, just means "change".
"Natural" means that there's no-one guiding the change. It means that there was no one choosing which organisms will have babies and which ones won't. They got eaten or died from lack of food or just never found a mate so they could have babies, but there was no-one saying "You, little fly, you will die now!", and more importantly there was no-one forcing them to have babies with someone: they could chose their own mate. There is another type of evolution which has humans guiding the change by choosing which animals or plants will have babies and who they will mate with to make those babies. This type of evolution is by "artificial selection" because we control it. It's how we have made all the different types of cows and dogs and cats. We may discuss it a bit later but it's important to understand that while humans have made some new types of organisms, most of them evolved naturally, without anyone controlling it.
"Selection" is a really important word. It means choosing something that is best or most suitable. You select the ripest peach or the freshest egg. Now, this may seem to be a strange word to use seeing as I've just explained how there is no one choosing, but I lied (a little). There is something selecting but that thing is the environment. You may ask how an environment can select but something can chose even if it doesn't think. A boulder falling down a hill will "chose" to squish the people who can't get out of its way.
But Sarah, the boulder doesn't chose to squish people, it just does!
Well done! You're absolutely right. And it's the same with the environment. It doesn't chose to kill an animal, but sometimes it just does.
And now we're ready to explain "evolution by natural selection". The first thing you need to understand is that for evolution by natural selection to happen we need a few things:
- a population (a group of organisms who live together and can have babies together, as I explained above)
- variation in the population (in your paper experiment the paper population had variation in its colour)
- variation in the environment (in your paper experiment you changed the colour of the background)
- time
To properly understand evolution you must know that it happens to a population. You can't say that "this particular blackbird evolved" but you can say "this population of blackbirds evolved". The reason for this is because evolution doesn't work on an individual. You don't evolve, at least in terms of your genetics, but your children will be different from you at a genetic level and that's what matters.
The reason we need variation is so that your genes will be different from your mum and dad's. If you mum and dad were identical and had identical genes then you would be identical to them and there would be no evolution. It's a similar idea for the environment. If the environment never changed (if it was always 16 degrees and rained in the afternoon for 1 hour between 2 and 3pm and it always got dark at 8pm and the sun came up at 6am) then there wouldn't be any need for evolution. Once organisms had adapted to that environment any attempt to change would mean they would end up worse off.
Finally, the reason we need time is so that organisms can reproduce. For humans it takes about 25 years for a 'generation': that's the time from someone being born to them having children of their own. This means that evolution is slow in humans: to have four generations takes about 100 years. In contrast, some bacteria have a generation time of just hours, meaning they can have as many generations in a day as humans have had in 1000 years! This means that bacteria evolve much more quickly and it's why people catch colds so often - the cold bacteria evolves so quickly it doesn't get recognised by the immune system and can make them sick again and again.
Now we can put all these pieces together and see what evolution really is. It is just change in response to the environment. The "environment" means everything that affects an organism: the climate (the earth has been much hotter and much colder than it is today), predators (many prey have evolved defences to try and stop being eaten), food (many organisms have evolved different ways of getting food, something I will come back to in just a minute), and other things I can't think of right now. See if you can think of any other things - you can write them in the comments and I'll let you know if you're right.
What does all this mean?
Ultimately, it means that all the different animals and plants that you see have evolved to fit their environment. Nothing is any better than anything else - you are no more "evolved" than an ant. You are just as well suited to your environment as an ant is to theirs.
When life first began on this planet around 3.6 billion years ago it was very simple. It stayed simple for a very long time. But around 550 million years ago many of the different groups of animals that we know today evolved. My next post will start looking at these animals but I just want to finish off the subject of evolution by briefly explaining how evolution created the diversity of life we see.
How does evolution make diversity?
550 million years ago the earth was very different. The continents were different and the land was lifeless. The only life was in the sea and it was very basic. But there were different environments - open sea, shallow sea, deep sea, sea beds and so on, and these very basic animals started to adapt to these different environments. Once this started, competition began and this meant that the animals not only had to adapt to their environment but each other. Predators evolved to eat prey so the prey had to evolve defences and the predators had to evolve ways to beat those defences. Different prey evolved different defences and so predators evolved different ways to beat them. Over time (lots and lots and lots of time) these animals were able to live in different environments and eventually were able to live on the land. All these different environments meant more and more species evolved and that is how we got all the diversity we see today.
A nice example of this can be seen in the bills of birds. There are lots of different types of bill and they all do different things. The bill of a bullfinch is tough and able to crack seeds while the bill of a hawk is sharp and can rip meat. Have a look at this picture and see if you can work out how each of the bills helps each bird:
One final thing. Species will evolve and live for a long time but they all die in the end. We call this dying "extinction". You may have heard that the dinosaurs went extinct. This happened 65 million years ago when an asteroid hit the earth and caused a sudden and massive change in the climate. There are lots of animals and plants that lived a long time ago that don't live any more. This is because something changed in their environment which they couldn't adapt to so they all died. We know about these plants and animals because some left fossils. I'm sure I'll talk about fossils in later posts as they're very interesting.
So that's hopefully explained a bit about how evolution by natural selection works. I know it's quite complicated if you don't understand everything don't worry. If I haven't explained something very well please let me know and I'll try and explain more clearly.
Summary
Evolution just means "change" but in biology it means "change in response to the environment". Biologists call this "evolution by natural selection". For evolution to occur you need:
1) a population of animals
2) variation within that population
3) variation in the environment
4) time, and lots of it
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Evolution is the reason we have so many different plants and animals on the earth. Over millions of years new species have evolved, lived, and gone extinct. The animals we see today are not the "best" but simply the ones that have survived.
Next time . . .
Sponges! The simplest animal but very interesting all the same!
New words used:
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