Compare and contrast M. leprae with S. kudriavzevii, the only yeast that cannot utilize glucose. What does the pattern of the genes of the S. kudriavzevii reveal about natural selection? One could relate their response to fermentation. How would fermentation (if fermentation changes) differ in the S. kudriavzevii when compared to other yeasts that can utilize galactose? (What would happen if the glucose supply ran out?)
What do the accumulation of fossil genes in both of the organisms reveal about the environment of the organisms? One should remember to address the themes of biology: for example, heritable information and evolution.
As you said, M. leprae and M. tuberculosis are closely related organisms. The difference lies in their respective habitats and niches. M. leprae is dependent upon a host to live; as such, it has become specialized to be apt at doing so. As a parasite, "the host cell genes [do] much of the work," allowing for relaxed selection on the genes of M. leprae (Carroll 132). So, as we've seen before, relaxed selection allows unnecesary genes to become fossilized. In contrast, M. tuberculosis can and does live independently and is thus under selective pressure to maintain the genes that M. leprae has lost. Though they are closely related and emerged from a common ancestor, divergence in lifestyle led to divergence of DNA.
ReplyDeleteM. leprae and S. kudriavzevii are similar in that both have lost functions that have become useless to their lifestyle; M. leprae has lost many metabolic functions, S. kudriavzevii has lost the ability to convert galactose into glucose. This shows us how quickly and decisively changes in lifestyles can lead to degeneration and loss of genetic information.
Using S. kudriavzevii as an example; S. kudriavzevii lives on decaying leaves in Japan, a niche where utilizing galactose is unnecessary due to the availability of other sugar sources. Thus natural selection on this gene was relaxed, and the gene became fossilized. Unfortunately, fossilization is "generally a one-way street"; once lost, these genes cannot simply be brought back (Carroll 132). S. kudriavzevii and its descendents will never be able to utilize galactose unless the ability is independently reevolved, an unlikely occurence. So, should the other sources of sugar run out and galactose be the only source available, S. kudriavzevii would be in a whole lot of trouble.
Darwin's theory of evolution makes the inference that individuals with traits best suited to an environment will be able to survive and reproduce most efficiently and successfully. In the cases discussed above, the traits best suited for the environments did not include galactose conversion or myriad metabolic processes, respectively. So, by Darwin's theory, evolution did not naturally select for these traits, and they were lost. Thus we can see that natural selection is not a progressive force, but rather one that acts only in the present to select those organisms best suited for the current environment, and nothing else.