When people think about evolution, they tend to think about it as positive traits increasing over time. Basically, living things with an advantage do better and so are more likely to pass on their genes. Over time, those "good" genes become more common.
When trees got dirty,
being light colored became a
disadvantage for these moths.
The classic example of this is peppered moths in England. Before the industrial revolution, the trees in a certain region were all light colored. And so were most of the moths.
These lighter colored moths blended in better with their surroundings which meant birds had a harder time finding them. Darker moths stood out and so were quickly gobbled up.
Once smokestacks started belching out smoke, things changed. Now the trees were all darker so that the lighter moths stood out. In a short time, most of the moths were dark.
These kinds of examples make sense. And they are somehow comforting because the better genes for a particular environment prevail it seems like a species improves over time. But this isn't how it always works.
As the Quebec study showed, sometimes it just helps being in the right place at the right time. The most extreme example of this is something called the founder effect
If two people move to an island and found a population there, everyone is going to have a combination of those two people's genes (barring the occasional mutation). So if both had red hair, everyone on the island will have red hair too. (Click here
for why this is the case for red hair and not necessarily all traits.) The red hair version of the MC1R gene will be very common here even if it is a disadvantage.
This is also how certain diseases become more common in some groups. For example, Ashkenazi Jews suffer from more Tay Sachs and some Amish groups have extra fingers and toes because the founding ancestors were carriers for these diseases. Even though these traits are a disadvantage, they end up being more common that you might predict because of the founder effect.
By chance the blue bugs win out.
Another way a trait can become common is through something called genetic drift
. This is where random chance leads to certain traits winning out in a population.
Imagine 10 bugs, 5 green and 5 blue. By chance, the green ones are clustered mostly in the north part of a field and the blue ones in the south.
Now imagine a fire sweeps across the northern part of the field, sparing the south. In a few generations, blue will become more common not because it is better, but because blue ancestors were in the right place at the right time.
This is kind of what happened on an island in the Pacific called Pingelap. In 1775, a huge typhoon wiped out 90% of the people on the island. The remaining 20 survivors stayed and repopulated the island.
One of the 20 was a carrier for a rare form of colorblindness where people see everything in grays. Now, 10% of the people have this form of colorblindness and 30% are carriers. All because of the typhoon that spared the carrier.
And these are just some of the ways that gene versions can become common whether they are an advantage or not. Evolution is mostly survival of the fit. But sometimes it is survival of the lucky too.