Wednesday, October 26, 2011

Darwin and Hitler

We've talked a bit in class about how social Darwinism is a misinterpretation of what the theory of evolution actually says. A common claim by anti-Darwinists is that Hitler was motivated by the theory of evolution by natural selection. Here's an example:
"If everything is a product of chance - purposeless - which is widespread in biology textbooks . . . then I don't think you have any grounds to criticize Hitler." - Historian Richard Weikart
At her blog, Planet of the Apes, science journalist Faye Flam has collected a nice series of scientist responses to this claim. What do you think? Do you think that Darwin could have motivated Hitler? Do you think it matters if he did? Do you think that the theory of evolution encourages racism or immorality?

Tuesday, October 18, 2011

Malaria vaccine

We've been talking in class about overdominance, or heterozygote advantage. One of the classic examples of overdominance is the case of sickle-cell anemia. A brief synopsis is given here. Basically, the sickle-cell mutation results in clumped hemoglobin. If you are homozygous for this mutation, you have the disease and have a low probability of survival. However, heterozygotes do not show the disease, and are more resistant to malaria than homozygotes without the sickle-cell allele. Therefore, in malaria-ridden areas, the fittest genotype will be the heterozygote. Because of this fact, we see an equilibrium that maximizes the fitness of the population by balancing the number of heterozygotes with high fitness against the number of homozygous individuals with sickle-cell anemia, resulting in an intermediate frequency of the sickle-cell allele. This is natural selection's solution to the problem, and it isn't an ideal one. Again, natural selection doesn't lead to perfection.

Perhaps we won't have to rely on this cheesy evolutionary fix much longer though, as there is great news on the fight against malaria. A new study of an experimental vaccine has shown that it reduces incidence of malaria in children by about 55% [STORY]. This is the first vaccine to be used against a eukaryotic parasite. In general, fighting eukaryotic diseases can be tricky. For example, it is often easier to clear a bacterial infection than a fungal infection. Why? Evolutionary principles! Eukaryotes are more closely related to us, so they share more of our molecular machinery. Consequently, it's easy to kill a fungus, but it may be hard to kill a fungus without killing us in the process.

Monday, October 17, 2011

Chromosome fun

Someone asked me the other day how our ancestors made the transition from having 24 pairs of chromosomes to 23 pairs of chromosomes. I didn't have a great answer then. What I presume would have happened is this sequence of events: One individual in the population has a fusion between two chromosomes (just one of the two pairs probably) in a germ line cell. This single mutation then spreads through the entire population. Either by drift, or by selection, if it was favored. I would guess that while this was happening, the progenitor chromosomes 2p and 2q, were probably present in most individuals initially as well. My guess is that the fused chromosome 2 rose in frequency in the population to fairly high levels, and that 2p and 2q dropped out of the population after that.

Note that chromosome fusion is not unprecedented in other species. The poster child of chromosomal fusion is the muntjac, particularly the Indian Muntjac (Muntiacus muntjak). The female of this species has the lowest chromosome number of any mammal (6) while the male has 7. This extreme reduction in chromosome numbers has resulted from several serial fusions of chromosomes. Take a look at this phylogeny by Wang and Lan (2000), which shows the diploid number of several Muntjac species, going from the probable ancestral state of 2n = 46 to the current number of 2n = 6 in female Indian Muntjacs:

Amazingly, Indian Muntjacs can produce viable hybrids with Chinese Muntjacs, which have the ancestral 46 chromosomes, and that partial spermatogenesis followed, indicating that they may be partially fertile. In my brief foray into the internet, I was unable to find anyone who had posited why these species have had such dramatic reductions in chromosome number.

Tree of Life

Here is a presentation I made that uses the tree of life phylogeny made by David Hillis' lab group at the University of Texas using the free presentation software, Prezi. Just hit play and it will take you on a tour of the tree of life.