Sunday, April 12, 2009
Why so much homology ?
Homology is a fundamental theme of evolution and is ubiquitous in nature. Explain how homology relates to taxonomy, the evolutionary tree, an Darwin's "descent with modification". Many homologous structures exist as vestigial organs, non-functional or unimportant structures. If vestigial organs are essentially useless and thus a waste of energy why don't they disappear through evolution? Essentially, why is homology maintained? (Hint: chapter 22 of the AP Bio text book.)
Subscribe to:
Post Comments (Atom)
Organisms with the most homologies share a recent common ancestor and are therefore close together in the taxonomic hierarchy. However, organisms that lack homologies are separated by the taxonomic hierarchy and have a common ancestor that dates back into the far past. This can be seen by the lack of anatomical resemblance between plants and animals. The similarity between plants and animals is seen molecularly in the genetic code which is the same in plants and animals as well as all organisms known to man. This means that even the organisms that seem to have no similarities share a genetic code and therefore a common ancestor. Cats, humans, whales, and bats all have different ways moving. However, they are homologous in the fact that they all share the same “five-digit limb structure” seen on page 439 of the Campbell textbook. This closer resemblance that is not seen between plants and animals exists because they have a more recent common ancestor and are therefore closer taxonomically. They are all tetrapods which is a vertebrate branch containing amphibians, reptiles, bird, and mammals.
ReplyDeleteBecause all organisms share a common ancestor it makes sense for organisms to have vestigial organs. However, I believe that vestigial organs are present today because enough time has not passed for them to disappear. At one point there probably was vestigial organs that existed between plants and animals. These can not be seen today. This will most likely be the case in the future. The pelvic/ leg skeletal structure that is seen in snakes is a vestigial organ. It comes from their close walking relatives however given enough time the similarity will no longer exist.
Homology essentially refers to the similarities in characteristics due to a common ancestry. An example would be the beaks of bird species: each species' beak is unique, but it is still a "beak".
ReplyDeleteTaxonomy is the science of classification, which has improved with the use of DNA in creating the phylogenetic tree. Taxonomy groups together species based on similar characteristics (homologous characteristics). After examining many species, living and extinct, a tree is made that shows the differentiation of species over time. Ancestral species evolved in different ways to varied environments and branched out, resulting in many similar (but unique) species with varying traits. This is the essence of Darwin's idea of descent with modification, the idea that "the descendants of that ancestral organism spilled into various habitats over millions of years, accumulated diverse modifications, or adaptations, that fit them to specific ways of life." (Campbell) Since all species originate from some ancestor, homologous traits are very common.
Vestigial organs occur when a species with an organ that originated from its ancestor loses its usefulness. This is usually due to changes in the environment. It's worth noting that vestigial organs are not useless, and may still be insignificantly beneficial/harmful.
The human appendix is small and serves no beneficial use for the body. In primates (and koalas, too- we learned about the koala's large caecum for digesting cellulose), the appendix is much larger. In the very old days, the appendix may have also served our ancestors in digesting cellulose. However, over times, we began eating meatier diets, rendering the appendix less useful.
Why do we still have an appendix? First of all, evolution takes time: the appendix is already very small compared to other primates. Also, the appendix doesn't use very much energy, and with our excessively large diets, the body doesn't exactly feel great selective pressure to get rid of such a small energy-sap.
Lastly, this article:
http://www.newscientist.com/article/mg19826562.100-vestigial-organs-remnants-of-evolution.html
claims that natural selection selects larger appendices, because smaller ones are more prone to infection. This may be the reason why we still haven't evolved our appendices away.
Like Becca said, organisms have common traits (homologies) when the two organisms have a common ancestor. In essence, all organisms are related from a last universal common ancestor (LUCA). A phylogenic tree such as the one on pg. 636 of Campbell shows how the different phylum of invertebrates and chordates are all related. For example, one of the first differences was Parazoa vs. Eumetazoa (no true tissues vs. true tissues, respectively).
ReplyDeleteAs Becca began to describe, the genetic code between organisms is almost continuous with small changes. This is because most of an organism's DNA is "junk DNA". Junk DNA doesn't code for anything and isn't changed by selective pressures. I like Carroll's analogy for junk DNA as "genomes as archipelagoes of islands (genes) separated by vast areas of open sea (junk DNA)" (76). Therefore, the majority of one's DNA has been the same since the beginning of time.
I would just like to point out that vestigial organs/structures are not all worthless. True, all of those structures have lost their PRIMARY function, but that doesn't mean that the structures don't have a SECONDARY function. As an example, recent studies have fuond that the appendix actually acts as a "safe house" for beneficial bacteria. The beneficial bacteria can be released into the large intestine in order to clean the large intestine after a case of dysentery or cholera. (http://www.abc.net.au/news/stories/2007/10/10/2055374.htm) Charles Darwin, in his novel "The Descent of Man", claimed that the tailbone was a vestigial organ. However, the tailbone serves a function to support the pelvic floor and has a slight role in balancing while sitting.
I applaud you, Vikram, for such a thoughtful and provocative question. It shows your sharp wit and penchant for biological brilliance that will make you a truly great biologist (or NASA astrophysicist) someday.
ReplyDeleteHomology relates to taxonomy because groups of species, in various degrees of specificity, are grouped together according to their similarities. For example, vertebrates are grouped under the term "vertebrates" because of the underlying homolgy in the possession of vertebrae in all members of the group. Homology relates to the evolutionary tree because the members of each group on the tree have underlying homology in the traits they share. As one moves back in time in the tree, the previous ancestral groups had homologous traits as well. While evolution splits up the original group, all of the different groups coming from the ancestral group still share the same triats that were present in the ancestral group. For example, all tetrapods share a five-digit limb structure. The tetrapod group split up into amphibians, reptiles, birds, and mammals, based on evolutionary differences. However, all of these different groups still share the five-digit structure present in the tetrapod group. This process leads to the conclusion that there exists a LUCA (Last Universal Common Ancestor), that had traits that ALL living things today share. Darwin's "descent from modification" implies homology. His idea was that species evolved by altering things that ALREADY existed. This implies homology because the group of species must have shared a common set of traits before the evolution, and one or more of those traits was changed to lead to a new species (or group of species). Therefore, homology is an essential part of Darwin's theory of "descent from modification."
Vestigial organs remain as symbols of homologous traits from previous ancestors. At one time, they served a purpose, but are now useless. A good example is that of the appendix, tonsils, and adenoids in humans. These are not essential organs, as they can be removed without harming the chances of survival of the individual. Their position in the immune system implies that in an ancestor, they once played a role in the immune system. The reason they are not eliminated is similar to the reason why owl monkeys became colorblind after they became nocturnal. since these organs play no purpose, they do not confer a selective advantage or disadvantage. Therefore, having non-functional versions of these organs is not a disadvantage, so natural selection does not eliminate the genes for non-functional versions of these organs. Thus, over a long period of time, mutations are allowed to remain in the population, and the amount of individuals with non-fnctional organs becomes more than those with functional organs, and the genes for functional versions of the vestigial organs become fossilized. Homology is maintained because the homologous traits shared by the LUCA remain in all the descendants. While there is a large degree of alteration leading to the great diversity of life, there is also a great unity (the 7th theme of biology). Darwin's "descent with modification," as discussed before, requires that there is some homologous trait in a universal ancestor, and that alterations of that trait create diversity. Certain traits, those possessed by the LUCA, are present in all living things, and so they are preserved, essentially, because those triats cannot be completely eliminated, but rather, can only be changed.