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Once upon a time there was a pop song with the line 'Everyone is beautiful in their own way'.
A nice thought I'm sure you'll agree. I guess it means that we're all different and have a beauty in the way we're put together.
Some differences, or variations, cover a whole range of things and are called continuous variations.
Others differences have only a very few possible options, these are called discontinuous variations.
Some of these differences vary over a whole range, for example the height, skin colour and weight of people. We call this type continuous variation.
So if you lined up a hundred people you would find a whole range of heights - within sensible limits! People don't just come in set heights like shoe sizes.
However there are other differences where there are only a few possible forms. For example, I can't roll my tongue lengthways. Can you?
Try it now in a mirror. Don't try it if there are any big, strong aggressive people about! The only options are that you can or cannot roll your tongue, you can't half roll your tongue.
Other examples of discontinuous variation are blood groups and eye colour in humans. An example that could apply to plants and animals would be resistance to a particular disease.
But how do all the various differences arise?
Where do they come from in the first place?
The answers have been found only during the last one hundred years. Differences between animals or plants come about through either genetic variation or environmental variation.
We all know that we get our features and characteristics from our parents through their genes.
Even with the same parents, brothers and sisters can be very different. For one thing they might be a different sex! Each one of us receives a unique combination of genes.
An interesting exception to this is where you get identical twins who have developed from the same fertilised egg. Both will be the same sex and share identical genes.
Identical twins are similar but can still have differences. One might end up being much stronger than the other, this due to environmental variation.
This is where the environment that you grow up in or live in has an effecton you. For example if you had an identical twin who was brought up from childhood in a much poorer environment than you, where they might not get fed as much as you.
What effect might the environment have on their development?
You might find that when you met up later they were perhaps lighter in weight than you, perhaps shorter in height, and they might have performed less well at school.
To easily and clearly see the effects of environmental variation look at plants, as they are often susceptible to environmental effects.
For example: Imagine two plants grown in the same soil but one is in the shade of a large tree. The plant that is in the shade will not grow as large as the one that is not shaded.