Author: Chris Colby (colby@bu-bio.bu.edu) Title: Some Evidence of Common Ancestry 1.] Comp

---
Master Index Current Directory Index Go to SkepticTank Go to Human Rights activist Keith Henson Go to Scientology cult

Skeptic Tank!

=============================================================================== Author: Chris Colby (colby@bu-bio.bu.edu) Title: Some Evidence of Common Ancestry =============================================================================== 1.] Comparative genetic and biochemical data DNA or protein sequence comparisons of closely related species (as determined by morphologists) yeild similar sequences. The genetic code is read in blocks of three nucleotides. A nucleotide is a single "letter" of the genetic code, each block of three is called a codon. Each codon signifies an amino acid. Amino acids are the sub- units of proteins just as nucleotides are the subunits of DNA. When a protein is needed by a cell, the DNA of the gene is transcribed into RNA in the nucleus of the cell. The RNA is then transported out into the cytoplasm where it is needed and proteins are synthesized by cellular organelles (ribosomes) that "decode" the RNA and make the appropriate protein. All living organisms use DNA as their genetic material(*). In addition the three letter code is the same for all organisms, in fact it is called the universal genetic code. (*) some viruses use RNA The universal genetic code is a redundant code; that is there are more possible codons than are needed. So, most amino acids are coded for by several codons. Gene sequences from closely related species show that at homologous positions within a given gene the same codon is most often used for amino acids. If, however, there are differences they are usually in these "silent" sites -- nucleotide positions that can be changed without changing the amino acid they code for. In addition, the genome is loaded with 'dead genes' called pseudo- genes (of which there are several classes). Pseudogenes occupy the same location in the genome in closely related species. The same can be said for introns, intervening sequences of DNA that do not code for anything. Introns are spliced out of the RNA prior to translation so they do not contribute information needed to make the protein. They are sometimes, however, involved in regulation transcription of the gene from DNA to RNA. Evolutionary biologists take the above mentioned patterns of similarity as evidence of common descent. All organisms use DNA (and the universal genetic code) because they are descendants of an ancestral organism that used these. The same redundant codons are used by closely related species because their common ancestor had it. Pseudogenes and introns are located in the same positions in closely related species because both inherited their gene/intron placement from their common ancestor. This pattern of genetic variation is what one would expect if lineages accumulated genetic changes (mutations) and occasionally split (i.e. speciated). 2.] Comparative developmental biology -- Closely related species share similar patterns of ontogeny (development). The first stages (for ex.) of mammal development are extremely similar for all mammals -- differentiation occurs in later stages. In addition, the ontogeny of an individual organism yeilds clues about the ancestral species it evolved from. For instance, why should mammals (including humans) develop gill slits as a stage in their development? Evolutionary biologists would say this is because mammals evolved from organisms that had gills. In order for modern day mammals to develop, they must pass through a stage where the gills begin to form in order to get to their present form. Present day organisms are modified descendents of their ancestors. Traces of their ancestry cannot be erased from ontogeny because there is no evolutionary mechanism for starting an organism from scratch. Natural selection can only modify existing traits of an organism. So, modern species adapt to their environment (if neccessary) by modifying their ancestral species ontogney. Since changes early on are more likely to be deleterious because all subsequent development is dependent on early growth, organisms pass through stages of development that are reminiscent of the stages of develop ment of their ancestors. Thus, early development is highly conserved so it appears as if mammals (for example) share an early developmental pathway then diverge from it. Remnants of an organisms phylogenetic history that serve no present purpose are called vestigal structures. A good example of a vestigal structure are the rudiments of legs still seen in whale skeletons. 3.] Comparative Morphology/Anatomy -- Groups of related organisms are 'variations on a theme' -- the same set of bones/tissues/whatever are used to construct all mammals. The bones of the human hand grow out of the same tissue the bones of a bat's wing or a whale's flipper does and they share many identifying features (muscle insertion points, ridges). The only difference is that they are scaled differently. Evolutionary biologists say this indicates that all mammals are modified descendents of a common ancestor. 4.] The heirarchical organization of biologic diversity For evidence of common descent, look at the heirarchical distribution of various biological traits. DNA sequence data is extremely good for this sort of thing, but all the above examples also fit. Why, for instance do only plants with other angiosperm characteristics have flowers? If flowers are a useful character- istic, why don't we see an occasional fern or moss with a flower? Why don't we see any eubacteria with histone proteins? The grouping of biological traits only makes sense when you assume all organisms share a common ancestor, and subgroups of organisms share common ancestors and so on. Traits are thus "linked" via common ancestry. It's not just that similar species share similar traits (although that is evidence in and of itself); when you look at large groups of organisms, biological traits are heirarchically distributed. for example: KINGDOM ANIMALIA / \ Parazoa Eumetazoa | | \ | Radiata Bilateria | | |\ \ | | | \ \ | | | \ \ | | | \ \ | | Acoelomates Pseudocoelomates Coelomates | | | | | \ | | | | Protostomes Deuterostomes | | | | | | sponges cnidarinas flatworms nemotodes arthropods echinoderms ctenophores probiscids rotifers annelids chordates mollusks ________________________________________________________________ (all the above animals have a more complex devlopmental plan than sponges) ____________________________________________________ (all the above animals develop three germ layers in early development, they are also (with a few derived exceptions) bilaterally symmetric) ________________________________________ (all the above animals have a body cavity) ________________________ (all the above animals have a body cavity lined with mesoderm) ________ _________ blastopore blastopore becomes becomes mouth anus ..... ...... spiral, radial, determinate indeterminate cleavage cleavage Note: not all animal phyla are represented above, just the more well known ones. Heirarchical distributions of biological traits is seen at all levels you care to look at (even within species). It is due to common ancestry of groups and groups within groups and groups within groups within groups and etc. Another way of looking this is, if you see an organism with one trait -- you can specify several others traits it will (probably) have. From the diagram above you can see that if an organism undergoes spiral cleavage, it will also be determinate; the animal will follow the deuterostome developmental pathway and it will have a true coelom (mesoderm lined body cavity) and so on. I should point out that a heirarchical pattern to biological traits does not neccessarily mean the mechanisms of evolution need to be heirarchical to produce them. This pattern of diversity (which is seen both within and among species) is predicted if species share a common ancestor and lineages split and diverge with time. 5.] Evolution is a successful predictive theory. The hierarchy explained above is not only an observation, but a prediction. Any new traits surveyed should fall with previously established hierarchies. Recent advances in molecular biology have shown what a solid, predictive theory evolution is. Phylogenies used to be based primarily on morphological data. With the advent of molecular biology (esp DNA sequencing) another character was available to systematists. Phylogenies drawn from DNA sequences match morphological trees in almost all cases. In the studies where they do not, usually there has been some argument about classification before the sequencing was done. [A later clarification:] > when you look at large groups >of organisms, biological traits are hierarchically distributed. > >sponges cnidarinas flatworms nemotodes arthropods echinoderms > ctenophores probiscids rotifers annelids chordates > mollusks > ________________________________________________________________ > (all the above animals have a more complex devlopmental plan > than sponges) > ---------------------------------------------------- > _______________________________________ > ________________________ [deletions] In my response to McGrath, I included the above (now edited) diagram intended to show a hierarchy. In it I only followed the subdivisions down one "path". The end result was protostomes (arthropods and etc.) and deuterostomes (echinoderms and etc.) had the most subdivisions. I should point out that this is only a consequence of following that particular path -- I did not mean to imply that outgroups (where I did not fill in any further information) were more primitive or less evolved. I could have followed any (or all) lineages and showed different subdivisions (and therefore a hierarchy), but I singled out the path leading to more familiar organisms because I thought the characteristics might be a bit more, well... familiar. I could have just as easily (if I knew anything about sponge taxonomy) started out with "sponges" and "other animals" and continued to divide up the sponge phylum (Porifera). All lineages of organisms have equally long evolutionary histories. Within any phylum you can find species with highly derived characters along with less derived characters. In fact, all species are a mix of derived and ancestral characters, that's where the hierarchy bit comes from. In the diagram I simply chose to mention increasingly derived characters in the path leading to chordates. So, in any case, I didn't mean to imply an _overall_ more/less specialized direction to the diagram. If I was a sponge or a jellyfish, or writing for an audience of sponges or jellyfish, I would have filled in the diagram with different information. (I got some email telling me I was implying "higher" and "lower" so I thought I'd try to clear that up.)

---

E-Mail Fredric L. Rice / The Skeptic Tank