The Normal Distribution

September 30, 2006

Evolution is all about populations. And genetics. And selection pressure and genetic drift and speciation.

But it’s mostly about the normal distribution.

So what is the normal distribution?

Ok, imagine you measured the height of 10,000 people. You can easily get the average height by adding all the heights together and dividing by 10,000. Say this came out as 5’8″.

What proportion of the group were exactly 5’8″ tall? How many were 5’7″ or 6’2″?

So, you divide all the heights into groups. Say from 0″ to 1″, from 1″ to 2″, etc, all the way up to 12’6″ to 12’7″

Then you counted the number pf people who fell into each group, and plotted it onto a chart. The chart would look something like this:

… but probably not as nice and smooth because reality is never as neat as theory.

The shape of that chart is a normal distribution. It also gets called ‘The Bell Curve’ because it looks like the cross-section of a bell.

One thing to notice is that it’s easy to get the average from a normal distribution. It’s the value under the highest point of the distribution.

Most naturally occurring measurements (like height, IQ, length of tongue, etc) follow a normal distribution. There are lots of statistical measurements that you can use to check whether measurements are normally distributed (like ‘skewness’ and ‘kurtosis’) but we won’t bother with them just now, they just complicate things.

The Oxford English Dictionary defines evolution as ‘any net change in the genetics of a population’.

To all intents and purposes we can take this as ‘any change to the normal distribution of a characteristic within a population’.

That’s why evolution is all about the normal distribution. Except when it isn’t, but I’m not going to worry about that.


If you’re interested in writing software, check out my other blog: Coding at The Coal Face

Written while listening to:

Butterflies and Hurricanes by Muse from the album Absolution


Plug In Baby by Muse from the album Origin Of Symmetry


Stay Loose by Belle & Sebastian from the album Dear Catastrophe Waitress

Currently Listening …

September 6, 2006

I have found a plug-in for Live Writer called ‘Currently Listening’ (trackback) that puts Amazon links to the music currently playing in media player as I type.  Like this:


Butterflies and Hurricanes by Muse from the album Absolution


Unfortunately, it links to, and not and there’s no way to change it without manually editing the HTML.  Never mind, eh?

I tried leaving a comment about this on the blog entry, but kept getting a CGI error.

Evolution for Geeks

September 5, 2006

I’ve decided that I should write something to explain evolution to people. I’m not a biologist any more, but I’d hate to think that 15 years of study was for nothing.

Given that I used to be an evolutionary biologist, and I now write software, I though I should bridge these two camps. Hence, Evolution for Geeks.

Strangely, evolution crystallised for me after reading about genetic algorithms. The ability to simplify the while deal down to a few dimensions makes everything far more clear.

I’ll have a go at explaining evolutionary mechanisms using analogies that geeks will understand, maybe even some example simulations (probably written in C#), and lots of visualisations (hopefully).

I’ve been thinking about doing this for quite a while now. I think it’s time to make a start.

Blog post 1 will introduce the Normal Distribution, since it’s central to many of the visualisations that I’ll want to use later. Normal distributions are hard to draw, so I’ll need to sort out some sort of graphics package.  I’ll start with Inkscape, because it does vectors, and maybe look at Blender for the more complex, 3D stuff.


If you’re interested in writing software, check out my other blog: Coding at The Coal Face

Using LiveWriter Beta

September 5, 2006

I’ve decided to use Microsoft’s beta for LiveWriter to do this blog.

I don’t know why I prefer desktop application to web-based ones.  Maybe I’m just old fashioned.  Or emotionally scarred from the loss of data files on the server when I was a student.

Why bother blogging?

September 5, 2006

OK, so we don’t really need yet another blog. Especially from a nobody like me. So why bother blogging?

Really, it’s just to get stuff out of my head. All this stuff floats around until I write it down and I want it out, dammit! It’s my head, I wan’t some ‘me time’.

So I don’t really care if no-one ever reads this, it not for reading, its for writing.