Mechanisms of Evolution
Teenage Mutation Ninja TurtlesMutations often get a bad rap, but who doesn't want to mutate from a sweet little pond turtle to a massive crime-fighting ninja reptile? Are mutations good or bad? Eh…it depends. We hate to be so indecisive, but the details make a difference when it comes to mutations.
Organisms, including turtles, are constantly having their DNA mutated by the harmful UV rays in sunlight (if they forget their SPF 50, that is) or by treading through chemicals in a sewer. Occasionally, these mutations can help a member of the population survive. Or fight Shredder and the Foot Clan. Mutations can also be harmful, such as those that cause cancer.
A mutation is simply a change in the original DNA sequence of an organism. This can be a big change or a small one. Examples of mutations include:
1. A single base pair change
2. An addition of a base pair or larger piece of DNA
3. A deletion of a base pair or larger piece of DNA
4. A rearrangement of DNA sequences
Most mutations are small, happen randomly, and don't cause organisms to become mean green fighting machines. In fact, most mutations do nothing at all. One human cell contains over 3 billion base pairs. However, in that ridiculously large amount of DNA, there are only 20,000 or 25,000 protein-coding genes. That means the rest of the DNA could be regulatory regions, other types of RNAs, or it could be simply "junk." In fact, current estimates are that there is way more "junk" DNA than protein-coding DNA. No, the cell does not collect junk DNA because it likes the look of it on its bookcase. This is actually an extremely good thing for a cell, and for you. It means that a random mutation is more likely to hit a region that is unimportant to you and your cells' functions.
When a mutation does occur within a protein-coding region of a gene, one of the following might happen:
1. The mutation might not change the protein sequence
2. The mutation might change the protein, but it has no consequence
3. The mutation might change the protein, and change its function (positively or negatively)
If a mutation has a negative outcome, the organism probably won't do much reproducing and the population will remain largely unaffected. It's true that most random mutations are deleterious, and are largely selected against in a population. Conversely, an adaptive mutation gives an organism a selective advantage. It drives evolution, albeit like a tortoise. That's okay. Slow and steady wins the race.
Take a pretty little species of butterflies with buttercup colored wings. A female butterfly gets shot with a ray of sunshine that mutates one copy of a gene that codes for a protein involved in wing color. The resulting wings have a greenish hue. These new greener wings help this butterfly blend in with its favorite leafy treat, making it more difficult for predating birds to detect it. This mutated allele is passed down to the butterfly's offspring. Because this new phenotype is advantageous to the organism, the members of the population that carry the mutation live longer and are able to produce more offspring. The allele frequency of the adaptive mutation continues to increase, and microevolution is a result. Before you know it, the world's full of green butterflies and silly love songs.
It's simpler to see how adaptive mutations cause evolution in bacteria. Because of their small size, they replicate quickly, allowing us to see evolution in a shorter amount of time.
Bacteria reproduce much quicker than eukaryotes; some can duplicate themselves by the time you're out of the shower. Gross. For that reason, it's much easier to see adaptive mutations causing evolution in bacteria. In a real world example, an insertion mutation in a species of bacteria gave a few individuals a new way to take up delicious copper cobalt. Having this adaptive mutation meant better growth in environments with large amounts of metal. Over multiple generations, these mutants were selected for their new adaptation, becoming a bigger part of the population.