Animal Hands Different Functions Of Homologous Structures: Fill & Download for Free

Definition of homologous. 1a : having the same relative position, value, or structure: such as. (1) biology : exhibiting biological homology. (2) biology : having the same or allelic genes with genetic loci usually arranged in the same order homologous chromosomes.Homologous Organs:Developmental biology refers to homologous organs in the males and females of a species. In terms of sexual differentiation, organs are considered homologous if they develop from the same embryonic tissue. For example, the ovaries and testes both develop from the embryonic gonad.Following are some examples of homology: The arm of a human, the wing of a bird or a bat, the leg of a dog and the flipper of a dolphin or whale are homologous structures. They are different and have a different purpose, but they are similar and share common traits.Homologous organs may be defined as the organs of different animals which have similar basic structure but different functions. For example, the flippers of a whale, the forelimbs of a frog and man have the same basic structures but they perform different functions, hence these are called homologous organs.EtymologyA homophone is a word that sounds the same as another but has a different meaning. If a group is homogenous, all of its members are uniform or consistent. A homosexual is a person sexually attracted to a member of the same sex. Noticing a pattern? Words with the prefix 'homo' all have the idea of sameness in their meaning. We will be continuing with this etymology as we discuss the various definitions of 'homologous' that are found in biology.The term homologous comes from the Greek 'homos' meaning same and 'logos' meaning relation. So, literally, it means having the same relation. To make that a little clearer, we can say it means having a similar relative position, structure, function or origin. However, that definition is still somewhat broad and ambiguous. The reason for this is because the term 'homologous' is used in several different fashions in biology. The individual definitions are more specific, but they are all derived from the idea of 'having the same relation.' In this lesson, we will discuss homologous structures, homologous chromosomes and homologous organs in males and females.Homologous StructuresA homologous structure is an organ, system, or body part that shares a common ancestry in multiple organisms. This definition is found in evolutionary biology and uses the meaning of having a similar structure or origin.Common examples of homologous structures are the bones in the forelimbs of various vertebrates, such as humans, dogs, birds and whales. Even though the functions of these limbs are different (a human hand is for grasping, a dog's foot is for walking, a bird's wing is for flying, and a whale's flipper is for swimming), they all have similar bone structures. In evolutionary biology, this is evidence for descent with modification from a common ancestor. For example, check out the forelimbs of four modern vertebrates. Compare these to the flipper bones of the three ancient, lobe-finned fishes, one of which could be the common ancestor of the modern vertebrates.Ancient fishAnother example of homologous structures can be found in plants. Pine needles in fir trees, spines on a cactus, the petals of the poinsettia and the trap of the Venus fly trap all have different functions. However, they are all modifications of a leaf derived from a common ancestor.Homologous ChromosomesIn genetics, you will hear reference to the term homologous chromosomes. Humans have 46 chromosomes; we'll get half of them from our mother and half from our father. Homologous chromosomes refer to a pair of chromosomes that have genes for the same traits at corresponding chromosomal locations. One member of the pair is maternal and one is paternal. This uses homologous in the sense of having a similar position and function.Human karyotypeHomologous: Definition, Characteristics & Structure Examples - Video & Lesson Transcript | Study.com

IT doesn’t, that is an alternative explanation, from Aristotle.By his observations, Aristotle realized the importance of structural homology, basically similar organs in different animals, and functional analogy, different structures that serve somewhat the same function—e.g., the hand, the claw, and the hoof are analogous structures. So if I recall correctly from way back when I was studying Airistotle’s Biological thought, he said, What is a fin? It is foot for walking in the water. Something like that.

We start with the claims of “irreducible complexity”. Here’s the “inventor” of IC, Michael Behe describing the why IC systems “prove” an intelligent designer:"Darwin knew that his theory of gradual evolution by natural selection carried a heavy burden: 'If it could be demonstrated that any complex organ existed which could not possibly have been formed by numerous, successive, slight modifications, my theory would absolutely break down.' (24) ... What type of biological system could not be formed by 'numerous, successive, slight modifications"? Well, for starters, a system that is irreducibly complex. ... An irreducibly complex system cannot be produced directly (that is, by continuously improving the initial function, which continues to work by the same mechanism) by slight, successive modifications of a precursor system, because any precursor to an irreducibly complex system that is missing a part is, by definition non fucntional. An irreducibly complex biological system, if there is such a thing, would be a powerful challenge to Darwinian evolution. Since natural selection can only choose systems that are already working, then if a biological system cannot be produced gradually it would have to arise as an integrated unit, in one fell swoop, for natural selection to have anything to act on. ... When we descend from the level of a whole animal ... or whole organ ... to the molecular level, then in many cases we can make a judgement on evolution because all of the parts of many discrete molecular systems are known. In the next five chapters we will meet a number of such systems -- and render our judgement." Behe, Darwin's Black Box, pp 39-41Behe’s claim is that there are 2 choices to make designs in living organisms: natural selection (unintelligent process that produces design) and an intelligent designer. If natural selection cannot make the design, then the designer must be an “intelligent being” and the design is a manufactured artifact.A problem here is that Behe has made a Straw Man definition of natural selection. Behe constrains natural selection to a method of only making incremental, improving changes to a system that already works. More on that later.There are 3 refutations to Behe’s claim. One refutation comes from religion, particularly Christianity. We can infer aspects of the character of a designer from the design. If we look at all those “IC” designs, the inescapable inference of the “intelligent designer” is that it is stupid, sadistic, and suffering from Alzheimer’s. Such a being cannot be worshiped as deity.The second refutation comes from Behe. Behe claims that IC structures can arise by chance."Let me inject a note of caution: some systems require several pieces but not ones that need to be closely matched. For example, suppose you were walking in the woods and came across an old log, where the wind had blown a tree branch onto it, and the branch was perpendicular to the log. Here you have an irreducibly complex system -- a lever and a fulcrum. If there were a boulder nearby, you possibly could use the lever and fulcrum to move it. So some systems require several parts but not closely matched ones." Michael Behe, Intelligent design theory as a tool for analyzing biochemical systems in Mere Creation, Science, Faith, and Intelligent Design edited by William A. Dembski, 1998, page 179 [quote function not working]So, if we can get a crude IC system by chance, that gives us a function by which Behe’s Straw Man version of natural selection can change to improve, add components, and make the parts well-matched. This kills Behe’s either-or claim. Now Behe says there is a third way to get designs: chance.However, the most comprehensive scientific refutation of Behe can be found here:http://www.cbs.dtu.dk/CBS/courses/genomics_course/2000_ClassDarwin.pdf Many parts of that argument are found in Kenneth Miller’s Finding Darwin’s God in the chapter on ID and IC.Let me go over the paper:"1. IntroductionIt is generally assumed that Darwinian evolution must occur in a gradual, step-by-step manner, with natural selection acting at each step. A common argument used by anti-Darwinists involves the difficulty of explaining the origin of complex structures by such a process. However, there are several different mechanisms by which Darwinian evolution can occur. It is the purpose of this article to classify the different possible routes of Darwinian evolution. It is important to define four terms clearly before further discussion."This paper is directed specifically at Behe and his "irreducible complexity" hypothesis. Remember Behe's specific claims: there is NO POSSIBLE Darwinian explanation for IC systems. Note that Thornhill and Ussery have already exposed one flaw of IC (and thus of ID), namely, that IDers make a strawman argument of Darwinian evolution to attack. This will become plainer in a bit.The authors start out with definitions, the first being IC lifted right from Darwin's Black Box. They then define a term Behe missed:"Functional IndivisibilityThe quality of a component of a structure such that there is at least one alteration to it which would render the whole structure absolutely non-functional. This term was implied but not used by Behe (1996a, pp. 45, 142)."The authors then define Darwinian evolution."Darwinian EvolutionDescent of organisms in which the following criteria are met: (i) intergenerational differences are very much smaller than inter-specific ones; (ii) no intervention by conscious agent(s) occurs; (iii) the frequency of mutations or other heritable modifications is unrelated to functional utility; and (iv) selection is the sole means by which heritable modifications are accumulated to form functional structures."I disagree that (ii) is part of Darwinian evolution. It certainly is not part of Origin of the Species. But I'll leave that until later."Accessibility by Darwinian EvolutionThe quality of a biological structure such that it could be generated by a sequence of very small changes, each of which is selectively neutral or advantageous (Darwin, 1859, p. 189; Dawkins, 1986, p. 91).""BACKGROUNDIt was recently suggested that many biological structures are irreducibly complex, and therefore inaccessible by Darwinian evolution. Thus far, this is merely a restatement of the (fallacious) popular creationist argument about organs such as the eye. However, the new departure was to argue that the components of biochemical systems, unlike those of supramolecular structures, are single molecules, which are often functionally indivisible. The conclusion was that irreducibly complex structures of functionally indivisible components are inaccessible by Darwinian evolution. Eukaryotic undulipodia (cilia and flagella), bacterial flagella, intracellular vesicular transport, and the mammalian immune response and blood-clotting systems were given as examples (Behe, 1996a).The above thesis is unsound, as it is not certain either that any biological structures are irreducibly complex, or that theircomponent molecules are functionally indivisible (Coyne, 1996; Doolittle, 1997; Fulton, 1997; Ussery, 1999). However, the more theoretical question about the accessibility by Darwinian evolution of irreducibly complex structures of functionally indivisible components, if such exist, has not been thoroughly examined. ... One factor hampering examination of the accessibility of biological structures by Darwinian evolution is the absence of a classification of possible routes. A suggested classification is presented here."So, let's get to the different routes of Darwinian evolution,and the authors list 4."2.1 SERIAL DIRECT DARWINIAN EVOLUTIONThis means change along a single axis. Although it can generate complicated structures, it cannot generate irreducibly complex structures. The components added may be functionally indivisible, having originated by either mutation or adoption (see below), with a probable example being the steps in an A -* B -* C -* D metabolic pathway, such as the TCA cycle (Behe, 1996a,b). On the other hand, they may be functionally divisible, with an example being increments of giraffe neck length. A molecular example of the latter is the gradual change in enzyme specificity and activity resulting from single amino acid substitutions. ""2.2 PARALLEL DIRECT DARWINIAN EVOLUTIONThis means approximately synchronous changes in more than one component, so that modification to other components always occurs before the total modification to any one component has become significant. For example, in the evolution of the eye of Mautilus, and of the vertebrate eye if this passed through a Nautilus-like stage (Land & Fernald, 1992), it would be necessary for the evolution of the retina to be approximately synchronous with that of the pinhole eye. The retina is accessible via small steps from a single photosensitive cell, with increments of photosensitivity, and the pinhole eye is likewise accessible from a minor concavity, with incremental advantages initially in physical protection and then in focusing (Nilsson & Pelger, 1994). However, neither component would function without the other, and, furthermore, the retina would be exposed to damage if not enclosed. Parallel direct Darwinian evolution can generate irreducibly complex structures, but not irreducibly complex structures of functionally indivisible components (Fig. 1), and this is the valid conclusion to draw from Behe's thesis."So, Behe was correct about this, but the strawman argument is that parallel direct Darwinian evolution is the ONLY route."As with serial direct Darwinian evolution, single steps in any of the parallel routes may be functionally either divisible or indivisible. Most complex supramolecular biological structures have primarily this type of accessibility by Darwinian evolution, with examples being bat echolocation, spiders' web construction, honeybee waggle dances, and insect mimicry by orchids (Dawkins, 1986, 1995). Some complex (but not irreducibly complex) molecular systems, such as the globin proteins (Ptitsyn, 1999; Satoh et al., 1999), could also have evolved in this manner.""2.3 ELIMINATION OF FUNCTIONAL REDUNDANCYFor example, it is difficult to hypothesize a direct route by Darwinian evolution from mammalian to reptilian jaws, as they consist of different pairs of bones. However, the fossil intermediates Morganucodon and Kuehneotherium had both quadrate-articular and dentarysquamosal articulation. The following postulated evolutionary sequence from reptilian to mammalian jaws, for which there is considerable fossil evidence, involves selective advantage at each step (Kermack & Kermack, 1984): ...Redundancy elimination can generate irreducibly complex structures of functionally indivisiblecomponents, and a Darwinian evolutionary route of this type has been suggested for biochemical cascades, such as the blood-clotting system (Robison, 1996)."Oops. Generation of just those systems that Behe says cannot be generated. Not generated by parallel direct evolution, but by elimination of functional redundancy."2.4 ADOPTION FROM A DIFFERENT FUNCTIONFor example, scale-feather intermediates would offer no aerodynamic advantage, but one can hypothesize a sequence from scales to primitive but airworthy feathers in which each step offers anincreased advantage as insulation. Their use for proto-flight motility would therefore only begin after this sequence. Recently discovered fossil evidence suggests that feather evolution did indeed follow such a sequence, with protofeathers, composed of the same proteins as feathers, in Sinosauropteryx (Chen et al., 1998; K. Padian, pers. comm., 1999), probably marginally airworthy feathers in the non-flying Caudipteryx and Protarchaeopteryx (Ji et al., 1998), and feathers in the flying Archaeopteryx (Padian, 1998). The proto-feathers and feathers probably also possessed functions in display, camouflage, recognition, etc. and it is possible that the actual sequence was more complicated than the above hypothetical one, with evolution at some stages being driven primarily by selection for such functions (Padian & Chiappe, 1998). However, the proto-feathers in Sinosauropteryx were so thickly distributed that they almost certainly did function as insulation (K. Padian, pers. comm., 1999). Adoption from other functions, whether generating an irreducibly complex structure or otherwise, appears to be widespread at the molecular level. The following are a few examples: (I) Many bacteria and yeasts contain chimeric flavohaemoglobins, consisting of a haem domain which is homologous to non-chimeric haem proteins, and a flavin-binding domain which is homologous to NADPH sulphite reductase, toluate 1,2 dioxygenase, cytochrome P450 reductase, and nitric oxide synthase (Moens et al., 1996). (ii) Antifreeze glycoprotein in the blood of Antarctic notothenioid fishes, which enables them to survive in icy seas, is considered to have evolved from a functionally unrelated pancreatic trypsinogen-like protease, and the recent discovery of chimenc genes which encode both the protease and an antifreeze glycoprotein polyprotein strongly supports this theory (Cheng & Chen, 1999). (iii) Crystallins (proteins with refractive functions in the eye lens) are closely related or identical to stress-protective proteins in non-ocular tissues (e.g. Drosophila alpha-crystallins and small heat-shock proteins are homologous). Piatigorsky uses the term "gene-sharing" for the encoding in a single gene of a protein with two or more functions, and suggests that this may be a widespread evolutionary "strategy" (Piatigorsky, 1998)."Thornhill and Ussery go on to demonstrate that one of Behe's examples of IC could rise this way:"There are several apparent instances of adoption in one of Behe's examples, the blood-clotting system. One is the kringle domain, a structure of 90 amino acids with three characteristic disulphide bonds, which is present in various proteins of the blood-clotting cascade, and also in hepatocyte growth factor, which is not involved in blood clotting (Gerhart & Kirschner, 1997, pp. 220-222). A second example is epidermal growth factor, a 53 amino acid peptide with a characteristic motif of six cysteines, which is present in several blood-clotting proteins, and also in the epidermal growth factor precursor, the low-density lipoprotein receptor, laminin (an extracellular matrix protein), and several transmembrane receptors (Davis, 1990).""There are two ways by which irreducibly complex structures of functionally indivisible components could result from adoption:(i) Generation of an irreducibly complex structure by the joining of two or more non-irreducibly complex structures of functionally indivisible components. A possible example is the V(D)J joining mechanism in the immune systems of jawed vertebrates, ...(ii) Supply of an existing irreducibly complex structure of functionally indivisible components. The structure would have evolved previously by either redundancy elimination or the joining of two or more non-irreducibly complex structures of functionally indivisible components. Undulipodia may be accessible by Darwinian evolution in this manner, as their two main hypothesized origins are from ectosymbionts (Szathm ry, 1987) and spindle tubules (McQuade, 1977; Cavalier-Smith, 1978, 1982). However, the most detailed published hypothetical pathway for the transformation of ectosymbionts into undulipodia was actually one of parallel direct Darwinian evolution."Again, remember Behe's claim. NO POSSIBLE Darwinian route. To refute this claim, it is not necessary to show the actual route but only to have a plausible route.Thornhill and Ussery end up by comparing Dawkin's "brittle" structures to IC systems:"Dawkins uses "brittleness" to mean the quality of a structure such that it must be perfect if it is to work at all, and "brittle" is therefore close or identical in meaning to irreducibly complex and composed of functionally indivisible components. He argues that no biological, and very few artificial, structures are "brittle", and gives the arch as his sole example of one (Dawkins, 1995, pp. 82-83). “The authors then go on to show TWO routes of Darwinian evolution by which the arch could have evolved. The arch is one of, if not the, most difficult of design problems. (Tongs is another difficult design)"the arch would be accessible from a single cuboid by two routes of Darwinian evolution: (i) via a heap of stones, which is then removed (i.e. redundancy elimination); and (ii) from a lintel, by two lintels being positioned diagonally and end to end, followed by the insertion of a key stone, and then by the diagonals being replaced by stones increasingly trapezoidal along one axis (i.e. parallel direct Darwinian evolution). The latter is probably analogous to the actual Roman route of invention,"So, the whole concept of IC as evidence of ID is refuted by showing the ways and combinations of ways an IC system can arise by Darwinian evolution.