orwant@athena.mit.edu (Jonathan L Orwant) (06/28/88)
I'm a bit confused about how special-purpose anatomical parts can evolve, e.g. the evolution of a bird's wings from an unwinged predecessor. If it happens gradually, then why isn't the 2% of a wing along the way an evolutionary detriment? I've heard that it acts as a heat insulator; maybe so, but I would think that there would be much more efficient ways of retaining heat that would give competing organisms an advantage in the meantime. Another explanation I have heard is that specialized master-genes turn on entire sequences of genes so that once a mutation triggers the proper master gene, an entire wing appears. If this is the case, where is the information of how to grow a wing stored in an animal whose ancestors were wingless? -Jon Orwant (orwant@wheaties.ai.mit.edu) Pity me. I'm a computer scientist.
majka@grads.cs.ubc.ca (Marc Majka) (06/29/88)
Jonathan L Orwant writes: > I'm a bit confused about how special-purpose anatomical parts can evolve, > e.g. the evolution of a bird's wings from an unwinged predecessor. There is an ENORMOUS body of literature on evolution, which you can find in your local library (MIT has one, doesn't it? :-). Read any introductory book on evolution. Stephen Jay Gould is a popular writer who loves making evolution simple. See his "The Panda's Thumb", "Ever Since Darwin", and etc. Special - purpose structures such as eyes and wings often have precursors. A common example of a course that the evolution might take in making wings is that of tree dwelling squirrels. Squirrels with the ability to leap more deftly from branch to branch would enjoy advantages in food gathering and preditor evasion. An advantagous adaptation like flaps of skin around the legs which enable the squirrel to stay aloft (glide) longer would be selected for. Now, just add some time for the selecive pressure to do its work, and voila! wings. I don't think this discussion should be overworked on the net. It is a topic that is too well covered, and in simple terms, in easily available references. Enjoy reading and learning from them! > -Jon Orwant (orwant@wheaties.ai.mit.edu) > Pity me. I'm a computer scientist. Pity me too - I'm trying to be one! --- Marc Majka
doug@feedme.UUCP (Doug Salot) (06/29/88)
In article <5944@bloom-beacon.MIT.EDU> orwant@athena.mit.edu (Jonathan L Orwant) writes: >I'm a bit confused about how special-purpose anatomical parts can evolve, e.g. >the evolution of a bird's wings from an unwinged predecessor. > >If it happens gradually, then why isn't the 2% of a wing along the way an >evolutionary detriment? > >-Jon Orwant (orwant@wheaties.ai.mit.edu) >Pity me. I'm a computer scientist. As a fellow pitiful CS-type, I present my own uninformed opinion (all sentences have an implied "I think" prefix): The selection aspect of evolution is highly overrated. I'd bet that neutral mutations (both [un]expressed) dominate [dis]advantageous ones. Consider a bird-like creature that can't fly but has some vestigial arms that neither help nor hinder it. At some point, a descendent develops the ability to fly. A new phenotype enters the pool. Going from a non-flying species to a flying one seems especially independent of selection because a new niche is found rather than better success in an old domain. As the new niche becomes populated, selective forces would probably act to increase speed, distance, etc. What I want to know is how did the cell and its organelles evolve? I can imagine cell populations evolving into multicelled organisms, but its quite a feat to go from primordial soup to an autonomous factory. There appear to be no selective forces, so an entire functional cell has to organize pretty much by happenstance, right? -- Doug Salot || doug@feedme.UUCP || ...{trwrb,hplabs}!felix!dhw68k!feedme!doug "Thinking: The Thinking Man's Sport"
hes@ecsvax.uncecs.edu (Henry Schaffer) (07/01/88)
In article <112@feedme.UUCP>, doug@feedme.UUCP (Doug Salot) writes: > In article <5944@bloom-beacon.MIT.EDU> orwant@athena.mit.edu (Jonathan L Orwant) writes: > >I'm a bit confused about how special-purpose anatomical parts can evolve, ... > >-Jon Orwant (orwant@wheaties.ai.mit.edu) > >Pity me. I'm a computer scientist. > > As a fellow pitiful CS-type, I present my own uninformed opinion (all > sentences have an implied "I think" prefix): > > The selection aspect of evolution is highly overrated. I'd bet > that neutral mutations ... > > What I want to know is how did the cell and its organelles evolve? So do I. (Seriously.) However the whole story is not known, even though there is much research in this area, and we may never know all of the details. The thing to keep in mind is that the cell and its organelles is a *very* complicated entity. Its organization and functions are orders of magnitude beyond that of, e.g. a NOR gate. It is not easy to explain to a layman how an expert system is made of NOR gates - but at least we do know *all* of the steps in between. > I can imagine cell populations evolving into multicelled organisms, > but its quite a feat to go from primordial soup to an autonomous > factory. There appear to be no selective forces, so an entire functional ^^^^^^ Appearances can be deceiving - but this is an area in which there has been much research (and speculation.) There is an old book by Oparin, "Origin of Life" which goes into some of this. > cell has to organize pretty much by happenstance, right? > -- > Doug Salot || doug@feedme.UUCP || ...{trwrb,hplabs}!felix!dhw68k!feedme!doug In order to understand evolution, one must also understand biology. People can spend whole careers in just one small area of evolutionary biology. I have been personally interested in the population aspects of evolution. A book in this area which covers a broad sweep of the population/speciation subject is "Population Genetics and Evolution, 2nd ed. by Mettler, Gregg and Schaffer" and it should be readable by people who have had a course in biology and one in genetics. Without the genetics much should be understandable, but you would be one step further away from understanding the mechanisms. --henry schaffer n c state univ
jyamato@cory.Berkeley.EDU (YAMATO JON AYAO) (07/04/88)
In article <5944@bloom-beacon.MIT.EDU> orwant@athena.mit.edu (Jonathan L Orwant) writes: >I'm a bit confused about how special-purpose anatomical parts can evolve, e.g. >the evolution of a bird's wings from an unwinged predecessor. >If it happens gradually, then why isn't the 2% of a wing along the way an >evolutionary detriment? I've heard that it acts as a heat insulator; maybe >so, but I would think that there would be much more efficient ways of >retaining heat that would give competing organisms an advantage in the >meantime. The general feeling is that there is often some function served by the precursor organ, often one which is not apparent when examining the final evolutionary result. For example, flaps of skin along the limbs and sides (the apparent precursor of wings in mammals, at least) can be useful for gliding, keeping infants warm, storing fat during hibernation, lots of things. Finding out what the ancestors of birds used their modified limbs for could be tricky, but the answer may be in the fossil record. It is also possible for moderately complex structures to occur as a more or less random result of changes elsewhere in the genome. Consider the human big toe. We may have well-developed big toes because we once needed them, but it is also reasonable to suspect that the same programming used to do hands is being used to do feet. A poster to talk.origins recently brought up the suggestive if not conclusive finding that insect wings (harder to explain than bird wings, since it can be shown genetically that they are *not* modified legs) are functional as heat exchangers, and are in fact used this way by butterflies. >Another explanation I have heard is that specialized master-genes turn on >entire sequences of genes so that once a mutation triggers the proper master >gene, an entire wing appears. If this is the case, where is the information >of how to grow a wing stored in an animal whose ancestors were wingless? >-Jon Orwant (orwant@wheaties.ai.mit.edu) >Pity me. I'm a computer scientist. You're right, this is not a plausible explanation for the appearence of a whole, specialized wing *de novo*. What I think is being got at here is that a program normally used for one purpose can be accidentally triggered at a different time or place or under different circumstances, resulting in a fairly complex new structure (because of its interactions with its surroundings). For example, fruit flies can go from having two wings to four in a single mutation, or from having antennae to having legs on their heads--the information is there already. In theory perhaps the leg-headed flies could eventually develop some use for those legs (in fact, this could be the origin of antennae--a misplaced leg subroutine). Computer science is not a bad starting point for understanding genetics. The development of the nematode is my favorite example-- the entire pattern of cell division, differentiation and death to produce an adult nematode (1100 cells) is known, and there are some subroutines clearly visible--patterns which repeat in many parts of the program. There are also some nice switches, where one minor variable controls expression of large blocks of code. Mary Kuhner graduate student, genetics, UC Berkeley (but my opinions are my own)
doug@feedme.UUCP (Doug Salot) (07/05/88)
Thanks to all who threw books at me for my question on what
selective forces may have contributed to prebiotic cellular
formation. I was hoping for a quick summary of current
theory such as what I was able to generate from chapter 1 of
Molecular Biology of the Cell (1983, Alberts, Bray, Lewis, Raff,
Roberts and Watson):
1) assume the ready availability of CO2, CH4, NH3 and H2
2) amino acids, nucleotides, sugars, and fatty acids will
be produced in aqueous solution when energy is supplied
in the form of heat or u.v.
3) polynucleotides and polypeptides will be formed by heating
the dry organics or in the presence of polyphosphate catalysts
4) polynucleotides "reproduce" by acting as templates for the
polymerization reactions of their complements. poly-
merization can be sped up by the presence of minerals and
metal ions (clay?)
5) errors in replication will lead to new sequences
Enter selection:
6) for suitably long polynucleotides, bases will be paired with
with other bases within the polymer itself forming various
folds and 3-d conformations
7) some 3-d conformations will be unstable or lead to replication
difficulties. these will not survive (ie, reproduce)
8) when the raw materials become limited, the nucleotide sequences
which can be replicated with the greatest speed and accuracy
will dominate
That's about as far as you can go with linear complexity. After
this, you have to think about interactions within pools of polynucleotides,
polypeptides and other molecules big and small. For products
formed by multiple-step processes to benefit the originating structures
by selection would require localization of everything involved along
all of the paths leading to the beneficial products. Cell membranes
provide one level of localization and are formed pretty much spontaneously
from lipids in aqueous sol'n.
It seems clear to me that for evolution to continue on this scale
would require increasing orders of exponential time for each new
improvement. I find it fascinating that "modules" with varying
degrees of functional independence evolved from these relatively
simple systems and that the modules can then compete on a new
level. I wonder what the next level of organization beyond societies
will be.
Thinking about self-similarity and parallels between different
levels of evolution makes me wonder if anyone has proposed war-like
mechanisms at the sub-cellular scale. If you consider the
development of certain protein-RNA complexes used for reproduction
catalysis as analogous to the development of tools for human
survival, is it likely that certain cells won primordial wars
by producing nucleases and proteases which digested competitors
much as humans use their tools to hunt other species?
--
Doug Salot || doug@feedme.UUCP || ...{trwrb,hplabs}!felix!dhw68k!feedme!doug
"Thinking: The Thinking Man's Sport"bs_wab@ux63.bath.ac.uk (Bains) (07/05/88)
In article <5944@bloom-beacon.MIT.EDU> orwant@athena.mit.edu (Jonathan L Orwant) writes: >I'm a bit confused about how special-purpose anatomical parts can evolve, e.g. >the evolution of a bird's wings from an unwinged predecessor. > >If it happens gradually, then why isn't the 2% of a wing along the way an >evolutionary detriment? This example has been studied in some depth. The answer is that 2% of a vertebrate wing is a major advantage. If a pre-bird ancestor is insectivorous, and wants to feed on flying insects (as the ground-hugging ones are already spoken for) , it has to catch the insects while they are on the wing. To do so it can chase them and hope to jump up and catch them in its mouth. Any specialisation which helps it to do this will therefore be advantageous. (Few animals nowadays use this route, as the birds and bats have cornered the market in airborn insects). Aerodynamic studies show that, for a lizard which runs on its hind legs (itself very useful as it gets your head further off the ground) any minor modification of the fore-limbs which makes them able to give some lift increases the volume of air from which the lizard is able to sweep its food, because it increases the height and duration of short hops. This is an additive effect all the way from a slightly wing-shaped alteration of the scales on the arm to full wings. >Another explanation I have heard is that specialized master-genes turn on >entire sequences of genes so that once a mutation triggers the proper master >gene, an entire wing appears. If this is the case, where is the information >of how to grow a wing stored in an animal whose ancestors were wingless? >-Jon Orwant (orwant@wheaties.ai.mit.edu) This is an insuperable objection to developing wings. However it might be a way of developing multiple wings, legs or whatever. Ask a passing geneticist about the Drosophile (fruit-fly) Bithorax complex of genes, which show just this sort of behaviour. By mutating the key Bithorax gene you can turn a fly with one pair of wings into a fly with two, a radical change involving many body structures. William Bains, Department of Biochemistry (although not for long) University of Bath, Bath, UK bs_wab@uk.ac.bath.ux63
ayermish@athena.mit.edu (Aimee Yermish) (07/15/88)
No one seems to have mentioned the idea about organelles having previously been independent prokaryotes that entered into symbiotic relationships. Several organelles (for example, mitochondria, which do cellular respiration (for you CS-types, a serious win when it comes to using glucose efficiently), and chloroplasts, which do photosynthesis (another big win, 'cause it making food is easier than finding it, and CO2 is cheap)) have their own DNA. I'm not sure about the chloroplasts, but the triplet code for amino acids is slightly different in mitochondrial DNA. What exactly that means is still somewhat up in the air, though. --Aimee ------------------------------------------------------------------ Aimee Yermish ayermish@athena.mit.edu MIT couldn't care less about anything I say. (as long as I finish that last paper...)
elwood@cfa250.harvard.edu (Elwood) (07/19/88)
In fact I just read something (unfortunately it was
sci fi) which talked about mitochondria being
independant organisms originally, which gradually
became imersed in the body-It was called-The Leaves
of October-I cant remember the author-good story
though, deals with genetics.
--
"Just call me Honig......" (EIT)
hugh@chook.adelaide.edu.au (Hugh Garsden) (12/15/90)
Well, what I really want is a book on evolution. Can someone recommend a book that details the current thinking, in reasonable detail. Or if there isn't a book you can recommend, perhaps the work of a particular researcher, or papers he/she has published. Come to think of it, is there a good journal devoted to evolution? Please reply via e-mail. ----- Hugh Garsden "I'm walking backwards for Christmas" University of Adelaide - Spike Milligan hugh@cs.adelaide.edu.au