What causes changes in DNA? Do accumulative changes in DNA lead to evolution? Could you explain that process to me. How does the DNA know that it needs to adapt? It is my understanding that the germ cells that pass traits to the next generation are isolated from the cells that are affected by the environment. How does that knowledge and or adaptation get passed to the next generation?
-An undergraduate student from Colorado
December 06, 2007
Your questions really get at the heart of what evolution is all about! Evolution starts out with DNA differences that are passed on to the next generation. And to be passed on, these differences have to be in sperm or egg DNA.
For it to be evolution, the DNA difference has to lead to some sort of change that has an advantage. Something like being able to drink milk as an adult. Or having the right kind of tongue to allow speech.
Our DNA has a buildup of changes that have happened in our past and are continuing to happen right now. All of these DNA changes happened randomly and that is the same way they are happening today. The changes only look directed because sometimes we get lucky and a DNA change happens that helps us out.
DNA is a set of instructions for making a living creature. The instructions are written in a simple genetic code that has just four letters. Just like letters that come together to make words in a sentence, DNA "letters" (or nucleotides) come together to make a gene.
Genes are the instructions for making a protein. And proteins perform most of the important tasks for the cell. A big part of who we are is the proteins from our 20,000 or 25,000 genes working together in the right way. Think about a gene as a specific set of instructions for one small part of you.
As humans, our DNA is pretty much the same as the next person's. In fact any two people's DNA sequence is 99.5% the same. Or 0.5% different. The differences are what lead to various eye colors, blood types, and all the other variation that exists between each of us.
Everyone gets more changes to their DNA during the course of their life. We can think of these like typing errors. It may seem surprising that an occasional typo can allow for evolution, but they can. Let's look at a set of instructions and see what a simple letter change can do.
Here are some instructions for making a glass of chocolate milk:
Fill a glass with milk. Add 2 tbsp of chocolate powder. Stir.
Now here is the same set of instructions with various typos:
Fill a glass with silk. Add 2 tbsp. of chocolate powder. Stir.
Fill a glass with milk. Add 2 tsp. of chocolate powder. Stir.
Fill a glass with milk. Add 2 tbsp of chocolate powder. Add 2 tbsp of chocolate powder. Stir.
Instead of getting a glass of chocolate milk, you get a glass of chocolate silk, a very weak chocolate milk or a very strong one. As you can see, a small change can have a huge effect on your glass of milk!
This is how DNA works too. A small change in a gene can change how that gene works. This is because we've changed the instructions for making that protein. DNA changes in a gene are called mutations.
Mutations happen in many ways. Every cell in your body has a complete set of DNA, which copies itself to create new cells in the body. Sometimes there are simple copying errors or "typos" that are made when DNA makes copies of itself. These can happen at any "letter" or nucleotide and are random.
The environment can also cause DNA mutations. Sunlight, cigarette smoke, and radiation are all known to cause changes to our DNA. These are also random and can happen anywhere in the DNA sequence. Sometimes these mutations don't change a gene at all and the protein stays the same.
Other times they can change the gene's instructions and we get a different protein. Think of how a typo can change the meaning of a word. These kinds of mutations can be harmful, especially if that gene has the instructions for a very important protein. And by harmful, I mean that the mutation could cause a disease or even death.
But not all mutations are harmful. Some are even helpful! We say that a mutation is beneficial or helpful if it gives us some advantage to an organism. In other words, that change in the DNA actually helps us live or adapt to our environment.
Plants or animals may get a mutation that changes their skin color to blend into their surroundings. People have single changes in their DNA that allows them to drink milk as adults. Or to have the lighter skin they need to survive in Northern Europe.
Some of these mutations happen in DNA that is in the cells of our body other than sperm or eggs. These are called "somatic mutations". For example, we can get mutations in our skin cells from sunlight and get skin cancer.
But these kinds of mutations won't be passed on. For a change to be seen in the next generation, mutations need to occur in the DNA of cells that produce our eggs and sperm. These are called "germline mutations."
Germline mutations can also come from mistakes when DNA is copied and from the environment. For example, radiation can cause major changes to DNA in sperm and eggs. This is why some people that get radiation to treat cancer have their eggs or sperm frozen before the treatment.
Germline mutations can be passed from parent to child. If a child inherits a germline mutation from their parents, every cell in their body will have this change in their DNA.
This is how new traits get passed along to a new generation - by germline mutations. In this way, both harmful and beneficial mutations get passed from parent to child.
If a good mutation gets inherited, those with this DNA change usually survive longer and are able to have more kids. And those with harmful mutations often die before they can have kids. So over time, more and more will have the beneficial mutation and fewer will have the harmful mutation. This process is called "natural selection."
Combining these two processes, mutation and natural selection, is how evolution ultimately works! Evolution is how species change over time due to DNA differences (some of which are new) and selection.
So, is evolution "directed"? This gets to your other question - how does the DNA know to adapt? And the answer is that it doesn't.
Remember that mutations happen randomly. For example, the sun doesn't know where to make the typos. And the radiation doesn't know which DNA sequence to change.
Mutations just happen. And if they are beneficial, they stick around. If they are harmful, they get lost because the individual will die or not have any kids.
Evolution works because of DNA mutations in our germ cells and having these get passed down. Over time mutations have helped some animals and plants to adapt to changing climates and environments. And without random mutations, this never would have happened.
Monica Rodriguez, Stanford University