Sands in an hourglass

The title comes from my husband, a lovely electrical engineer who has managed to pick up some biology in the almost decade we've known each other.

Imagine each evolutionary change as being like a single grain of sand in an hourglass. Each individual grain seems insignificant, but when they are all amassed together you can start to do amazing things with them, like tell time in the case of the hourglass. In fact, it is possible to tell time using evolutionary changes (the concept of a molecular clock), but that's for another post.

Let's start with a mathematical simulation of natural selection.

http://www.cs.laurentian.ca/badams/evolution/EvolutionApplet102.html

If you have a constant selective pressure, you can infer that over time any small change that improves one's fitness can build on previous changes in order to eventually result in something that seems perfectly adapted to that environment.

So where are all those missing links? All the intermediate organisms between one major type and another? Well, first of all, the fossil record is incredibly incomplete. Everyday paleontologists are discovering new fossils and reinterpreting past fossil discoveries. Also, organisms with similar morphologies can be very different at the molecular level. DNA evidence in many cases has required re-examination of how we thought organisms were related to each other based on morphology (shape, form). So it is unlikely that we will ever be able to completely trace the evolution of a particular lineage. However, with molecular evolution combined with genetics, we can begin to hypothesize what genetic changes may have happened that resulted in the emergence of a group of organisms like flowering plants from organisms that were closely related to some modern day freshwater pond scum.

I personally think many of the best explanations of how radically different organisms evolve from common ancestors is through work in the field of evolutionary developmental biology, also known as evo-devo. There's a lot of cool work being done in evo-devo. Most of the research in evo-devo focuses on regulatory genes - the on/off switches in the genome. A single gene can control entire networks of genes that are responsible for complex machinery such as eyes, wings, and flowers. Mutations in these genes or the stretches of DNA that control when these genes are active can have radical results. These type of mutations are called homeotic mutations. And there are tons of examples. So I'm going to end this post with some examples of homeotic mutations - some are familiar, like domesticated roses with their multiple layers of petals compared to their wild cousins, and some are a little more strange, like fruit flies with additional pairs of wings or legs where antenna should be and various eye mutations (the same gene is implicated in the blind cavefish I talked about last time).

I'd like to think I'm not typing into thin air, so if you have a question about something, feel free to ask it and I'll try to answer it.