Humans have an inherent drive to classify the environment around them, perhaps due to the need to simplify into groups the many objects they come into contact with and to keep their world organized. Over time there is evidence of this in the array of classification systems man has arbitrarily created, as Jones points out (276), but in this chapter the author explains why there is only one true “arrangement” for life on earth, a classification based on a hierarchy of shared descent.
The classifying system Jones writes about was first created by Carl Linnaeus (1707-1778), a Swedish zoologist and botanist. His adoption of a binomial nomenclature (the scientific naming of a species) was the genesis of the taxonomic filing cabinet we use to sort living creatures today. The Linnaean system is a hierarchical ordering based on observable physical characteristics in plants and animals. Animals that share traits are within the same unit. From an overarching Kingdom down to a more specific Species, every bird, wolf, pine tree, and fish can be identified and placed into categories. Though his initial plan has been altered over time, Linnaeus is still credited with the concept.
Today the Linnaean taxonomic system is extremely important for understanding evolution. “It depends on a single idea: that groups sharing traits not present in others must descend from a common ancestor (280).” We are able to see by looking at shared characteristics just how far back the ancestral line may go and who is related when. We are able to determine relatedness, what evolved from what, by which traits we have in common. Humans are related to fish, because they both have backbones, but we are closer to other mammals because we possess hair and produce live births. These traits are evidence of evolution in the past. Some traits are stronger determinants of this. How do we decide what traits count and which ones don’t? In other words, why aren’t we a closer cousin to the iguana (we have five fingers, right?) than to the horse (with no fingers)?
Common descent can be more easily ascertained today. Where there used to be disagreement on where to place specific plants and animals, it can now be determined by analyzing the genetic landscape of a given species. Studying DNA patterns reveal we, as humans, have much in common with bats and rabbits; dogs share many of the same genes with whales and elephants. Known as cladistics, this approach focuses on classifying groups, or clades, according to shared characteristics and discovering just how far back our common ancestors go. For instance, all vertebrates are in one clade, and tetrapods (vertebrates with four limbs) form another clade within the vertebrate clade. These groupings can be derived from common characteristics, behaviors, or anatomical resemblances. The important point here is that all living creatures are related and have a shared ancestry which can be traced. From this chapter and what we have previously read, how is the study of cladistics more reliable than the fossil record in determining who our common ancestors are? Also, what is the difference between cladistics and traditional Linnaean taxonomy?
For further reading:
-Cladistics: The Theory and Practice of Parsimony Analysis; Ian Kitching, Peter L. Forey, David Williams, Christopher Humphries
-Cladistics: A Practical Course in Systematics; Peter L. Forey, Christopher Humphries, Ian Kitching
-Transformed Cladistics, Taxonomy and Evolution; N. R. Scott-Ram
-Cladistics and Archaeology; Michael J. O'Brien, Daniel S. Glover, John Darwent, R. Lee Lyman
Morphology refers to the study of an organism’s structure and anatomy. In many cases looking at internal bones and organs, and external body parts, can shed light on how closely related different species are. Why, for example, do most mammals have similar extensions such as arms and flippers? Jones argues that duplication is key. Repetition in molecule formation allows the body to form and mutate while keeping “the stamp of its shared ancestry (293).” Genes determine what is to be copied, when and where. Having a multitude of teeth is an example of duplication through natural selection. One mutation, one wrong order in the wrong place, can cause a fly to have multiple wings, or a flower to have several petals. Though a body may create separate modules and copy itself, there also evolves individual characteristics in these groupings. How, then, does one module end up a brain and the other a tail?
Embryology is basically the study of life in the stages before birth, hatching, or germination. Something as complex as the human brain, or the structure of a deer or oak tree, always begins life as an embryo. Analyzing the make-up of a fetus or an egg may give us answers about our ancestors, as “each animal relives its ancient history” in these early stages of life (298). The likenesses between animals, known as homology, which we now know to be evidence of a shared ancestor and not similar environments, can be clearly seen at the embryonic stage. Before an organism matures it showcases the same early cell and gene structures as other organisms at the same stage. It is only when it grows to adulthood that the specific changes which makes it unique emerges. Thus the genes of a mouse in the first week can be almost exact to that of a fruit fly. Evolutionary developmental biology can reveal how similar we are to other species thus demarcating the family tree we all belong to, showing “how universal is the history revealed before birth (301).” The embryo gives us an image of what conditions were like for our earlier, less altered, ancestors. How else does embryology help fit in and bolster Darwin’s three principles of natural selection, heredity, and variation?
Indiana University’s human embryo animations:
http://www.indiana.edu/~anat550/embryo_main/index.html
Rudimentary Organs are structural parts and organs which have lost their original function and are no longer useful to the organism. We see lots of evidence of these leftover parts in the embryonic stage and usually they disappear, but there are some that develop into adulthood. Why, for instance, do humans have appendixes, and why do male mammals have nipples if they do not lactate for their children? Jones maintains that these leftover organs once had utility for our ancestors in the past. Some organs and bones, such as those in the ear canal, may have had duel purposes, and evolution has selected for the elimination of one purpose over the other. “Natural selection does not hesitate to pick up and use whatever becomes available (304).” It also won’t hesitate to lose organs that are too burdensome or costly. It would seem troublesome to find so many errors in repetition in a system that seems geared toward removing them for the sake of efficiency. But it is within these leftover parts that we see greater relatedness among species as it retains traces of the past. What typically happens to vestigial structures over time, and why else are these organs “evidence of descent with modification?” Also, why might it be dangerous to assume that all “vestiges” have no function at all?
Additional links concerning rudimentary organs:
http://www.talkorigins.org/faqs/comdesc/section2.html#vestiges
http://www.livescience.com/animals/top10_vestigial_organs.html http://news.nationalgeographic.com/news/2009/07/090730-spleen-vestigial-organs.html
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Morphology of an animal is determined by homeobox genes. Essentially, they are switches that turn on that know the what, when, where, and how often to create specific parts of a body form. Homeoboxes lead from front to back as a result of the sequence that is responsible for the organs. Although Jones makes this sound very succinct, is it really just a sequence of triggers that cascades into increasingly higher organization? Creationists would argue that this is God’s plan, while evolutionists would call it part of the “process” of evolution. Is this just part of the natural laws of the universe that are continuously trying to bring order from the chaos?
ReplyDeleteI think that embryology can help bolster Darwin’s principles in a few ways. Early embryos are a basic blueprint for life that retains traits from an earlier shared past. Natural selection then acts upon the embryo and the result is modification. Hereditary plays a role in which genes were selected for an embryo. All of the variables fit together and interweave to give a clear picture of the fundamental processes at work. It is evidence of a common origin not seen so clearly through the fossil record, natural selection can be seen in the individual adaptations retained and passed down to offspring, as well as the mutations that determined how a species adapted. In addition, certain genes produce different resulting appendages, structures, and organs. All rolled into a little tailed embryo with gills and flippers.
Jones mentions that the mammalian male nipple still does retain its function under certain circumstances even though they have no function because of evolution. He bases the argument on an economic principle. There is no market for the milk. As sexual selection can alter certain traits, could a similar force other than natural selection be acting to repress the male’s ability to lactate? Can we argue that the principle female/infant unit in some animals is a result of natural selection or just a behavior modification?
Vestigial organs and structures typically disappear over time. That is because it has detrimental value if a structure or organ does not serve a function that outweighs its burden. This is essentially a cost/benefit relationship on biological terms. Instead of retaining a vestige, evolutionary process dictates that it shall be extinguished to never again return. Why doesn’t selection go in reverse? It may just be that it is biologically cheaper to create a new one through selection and mutation. Maybe it is because no two selective pressures are ever identical?
It is dangerous to assume that there is no apparent function just because we are not aware of it. Just as in the problem of the lack of intermediate forms, just because they haven’t been found doesn’t mean they don’t exist. Given our knowledge of the complexity of living systems, it would be safe to assume that they are more likely to serve a specific purpose.
In response to Emily's question concerning the reliability of cladistics over the fossil record (or Linnaean Taxonomy), cladistics let the genes speak for themselves rather than our interpretation of important characteristics. So, rather than human interpretation being that having hair is a more important characteristic than having 5 fingers, cladistics offers us the genetic evidence of common ancestry and modification that help broaden our picture of evolution and important shared genetic characteristics. To me, the end goal of cladistics is much like a genetic database that helps define commonality and fill in the holes that endlessly looking through the fossil record can not do. Rather than finding the organism in the fossil record in which all life is descended from, cladistics offers the basic genetic attributes that let species evolve to complex forms while maintaining those simple genes.
ReplyDelete- What is the difference between cladistics and traditional Linnaean taxonomy?
ReplyDeleteCladistics is based on shared ancestry, which allows the discipline to group organisms impartially with regard to the similarity of their characters. The more shared characters that are being examined, the further back relationships go. It differs from Linnaean taxonomy primarily in that it does not assign fixed levels of classification because evolutionary history is too big and complex for everything to adhere to them.
-What typically happens to vestigial structures over time, and why else are these organs “evidence of descent with modification?” Also, why might it be dangerous to assume that all “vestiges” have no function at all?
The analogy Jones uses of vestigial structures and words that contain letters which are not pronounced works well. The spelling that retains all forms previously used, no matter how briefly, gives a fuller history of the word. This is more applicable to DNA because the code does retain parts that are no longer used, whereas structures are eliminated by selection once they become too expensive to carry along. By definition vestiges have no current use except to illustrate evolutionary history. However, because they have no use currently they are still retained because they were needed relatively recently. Therefore, one has to assume that they might have use provided the environmental conditions change especially if conditions revert to a similar to the state it was in when the vestiges were operational.