Wednesday, March 5, 2008

Phylogenetic methods talk

Today I gave a 10 min presentation on phylogenetic methods and evolutionary model selection. So I talked about how evolutionary trees are made from genetic sequence data. I think I nailed it pretty well.

After the talk, this guy asked me if you could use this stuff to predict where evolution will take a species in the future. I told him no, unfortunately to predict that you need to know what the environmental pressures will be, and there is a huge random component as well. I went on to tell him that the really interesting thing that you can do with phylogenetics is, fairly accurately, determine the genetic sequence of the last common ancestor of the species you test. The lab I am working in actually recently did that to help refute one of the main anti-evolution arguments posed by creationists. Creationists say that complex systems such as hormone-hormone receptor interactions are too specific to be explainable by evolution. That argument makes sense since hormone receptors are often very specific to the hormone that they bind. It seems that to explain the evolution of such interactions you would have to literally produce both hormone and receptor in the same lineage and it would have to immediately serve a purpose and rise to fixation. If that were true, it would be nearly impossible to do such a thing evolutionarily.

People in my lab sequenced estrogen receptors and estrogen related receptors from a very wide range of organisms to try and get at a very ancient common ancestral hormone receptor to examine it and see what it was like. They then took the sequence generated from the ancestor and grafted it onto yeast cells so they could examine which hormones it was sensitive to. They found that the ancestral hormone receptor was able to bind a variety of ligands including estrogen, which hadn't evolved yet. That showed that you in fact didn't need to evolve hormone and hormone receptor at the same time, that you could start out with a really unspecific hormone receptor that worked with completely different hormones which was eventually taken advantage of evolutionarily to develop into several different, specific hormone receptor-hormone interactions.

Evolution of Altruism

Today I learned about the evolution of altruism. Darwin admitted that altruistic behavior seemed to go against the idea of evolution. Recently though people came up with the math to show exactly how, even though it has a negative fitness to the individual who practices it, it can actually have an overall positive fitness effect. Here is the formula: rB > C where C is the negative effect to the individual for practicing altruism measured in # of offspring, B is the positive effect of altruism measured in # of extra grandchildren, and r is the proportion of genetic similarity between the altruistic individual and the individual who benefits directly from the altruism. Here is an example: ground squirrels practice altruism by giving warning cries to alert their fellow squirrels about the approach of predators. The squirrel giving the cries puts itself in danger by being an easier target for predation. However more of the offspring will survive if a squirrel does it. So here is the math, say on average a squirrel who has the gene causing it to give warning cries has a shorter life, and on average produces 2 fewer offspring, however the offspring it does produce has higher rates of survival, say 5 of them survive that would have otherwise been eaten. For a given squirrel since half of the DNA comes from mom and half from dad, the r will be equal to 0.5. doing this math we get 2.5> 2 which is true so you would hypothesize that this altruistic behavior would be genetically selected for. This is the same principle that goes into the evolution of extreme altruism as seen in ant colonies. The purest form of altruism is the worker ant who is completely sterile, it has no chance of having offspring. The reason this evolves though is because ants are so inbred that the value for r is extremely high. Any ant in the ant colony shares about 0.8 of its DNA with any other ant so you will hypothesize that any altruistic behavior will be very beneficial to the fitness of the gene that produced that altruistic behavior. Just for a comparison if you compare the entire human population the likelihood that you share genes with any other individual is about 0.0001, which is why we haven't evolved into having classes of drone workers like the ants have.