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Evolution Basics: The Basis of Heritable Variation, Part 1

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May 2, 2013 Tags: Genetics

Today's entry was written by Dennis Venema. You can read more about what we believe here.

Evolution Basics: The Basis of Heritable Variation, Part 1

Note: This series of posts is intended as a basic introduction to the science of evolution for non-specialists. You can see the introduction to this series here. In this post we examine how variation arises and is passed on in populations through DNA copying errors.

How organisms reproduce “after their own kind” (to borrow the language from Genesis) is a longstanding question in biology. A closely related question arises from the observation that within a “kind,” not all individuals are the same—variation exists within populations of the same species. For many years, the mechanism that could explain both the observed constancy of a species (faithful reproduction of the form of an organism) and variation (not all members of a species are identical) remained a mystery. In order to shed some light on these important issues for evolutionary biology, we need to take some time to explore the “nuts and bolts” of how two important biological molecules work, and how they relate to one another: deoxyribonucleic acid (DNA) and proteins.

Molecular Genetics 101: Proteins and DNA

You might be surprised to learn that early work in exploring the molecular basis for genetics favored proteins as the hereditary molecule instead of DNA. It was suspected that whatever was acting as a hereditary molecule would be large and complex, and proteins were both. Proteins can be very long, since they are a polymer of smaller, repeating components (monomers). We can use children’s interlocking bricks to illustrate what we mean. For bricks, each individual piece is a monomer, and when they’re snapped together, they form a polymer:

Proteins are built pretty much in the same way. For proteins, the monomers are a group of compounds called amino acids (each amino acid is one monomer). Like the bricks in our analogy, they have features in common that allow them to be “snapped together” into a long chain. They also have significant differences, analogous to the different colors in the diagram: some amino acids are hydrophobic (i.e. they are repelled by water), others are hydrophilic (i.e. attracted to water). Some are large and bulky, others are comparatively small, and so on. Unlike the rigid bricks in our analogy, proteins are marvelously flexible, and fold up into a three-dimensional shape, as directed by the properties of the monomers.

There are 20 different amino acids that are used to make proteins, and they can be combined in any sequence in order to produce a protein with specific properties—properties that arise from the combination and specific order of amino acids, and the final shape they give to the protein. This diversity in monomers means that there are many, many different possibilities for protein sequences (and thus shapes, and functions)—even a polymer only two monomers in length has 400 possible sequences (i.e. 202, or 20x20), and proteins can be thousands of amino acids long. It was this possibility for large-scale complexity that suggested that proteins might have enough “storage capacity” to hold hereditary information and pass it on to the next generation.

Beginning in the late 1920s, however, research began to point away from proteins and towards DNA as the hereditary molecule. DNA, like proteins, is a polymer formed from a set of monomers (in this case, nucleic acids). In contrast to the 20 monomers found in proteins, DNA has only four monomers: compounds abbreviated as A, C, G and T. It was for this reason that researchers were initially skeptical that such a “simple” polymer could act as a source of hereditary information.

Despite this skepticism, evidence continued to mount that DNA was in fact the physical basis for hereditary information. Once this evidence convinced the majority of scientists, the race was on to understand exactly how DNA accomplished this remarkable task. Soon, it became clear that understanding the structure of DNA was crucial to understanding its function, and several research groups famously competed to be the first to decipher it.

Determining the structure of DNA did indeed shed light on its function. Though it has only four monomers, the structure of DNA revealed how it can easily replicate and pass information on: not only is DNA a long polymer, it is a polymer that can specify its own replication through interactions between its monomers. Perhaps a picture would help explain. Imagine bricks that now have “partners” they are attracted to. We’ll represent that attraction, which is a type of chemical bond called a hydrogen bond, with a black dot. The “A” and “T” monomers are attracted with two hydrogen bonds, and the “C” and “G” monomers with three:

These “attraction pairings” between monomers are important: they allow one DNA polymer to act as a template for a second, “complimentary” DNA polymer. Imagine a DNA sequence as follows:

As the second DNA polymer is made, monomers are selected, one at a time, to match their “partners” in the first polymer:

These two polymers are held together by the alignment of many hydrogen bonds, and you are likely familiar with them as the “two strands” of the DNA double helix:


Source: https://en.wikipedia.org/wiki/File:DNA_Structure%2BKey%2BLabelled.pn_NoBB.png

While this more realistic model of DNA shows the precise details of its molecular structure, the important features are summarized by our simple “toy brick” model. DNA is a pair of long polymers that can be separated and used to make new copies that are faithful to the original.

While these features of DNA readily explain how it is faithfully copied, recall that we also need to explain variation. Variation, in the most basic terms, means there is sometimes imperfection in the copying process. If DNA is indeed the hereditary molecule, and if DNA copying was 100% accurate, then variation would never arise, and all offspring would be genetically identical to their parents. Without variation, recombination would have no effect (since there would be no variation to mix into new combinations).

There are many ways that variation can enter during the DNA copying process, and in a future post we will examine several of them. One way that we will consider now is simple “mispairing” of monomers during replication. At a certain (very low) frequency, inappropriate monomers are paired together. The arrow in the figure below shows one such mismatched pair, where a red monomer (G) on the bottom strand was incorrectly paired with a yellow monomer (T) when the top strand was made. When this set is replicated, both the top and bottom strands are copied, but now the correct partners for each monomer are found. The result is two different outcomes: one copy now has the original, correct C:G pair (on the left), and the other has a new variant, with an A:T pair (on the right). This change will be faithfully copied from here on, since later copies don’t “know” what the original sequence was. The result is a new variant in the population.

Taken together, the properties of DNA match what we observe in nature: faithful reproduction of form, but not perfect reproduction of form. At its base, constancy and heritable variation in biological populations trace back to how DNA functions.  

What about proteins?

While the properties of DNA make it a great hereditary molecule (that nonetheless allows for variation to arise), DNA itself is not capable of doing the day-to-day functions that organisms need (enzyme functions, structural functions, and so on). For these functions, the vast structural diversity of proteins is required. In the next post in this series, we’ll discuss how the hereditary information in DNA is transferred to protein structure and function, and how variation in DNA can cause variation at the protein level.

 


Dennis Venema is professor of biology at Trinity Western University in Langley, British Columbia. He holds a B.Sc. (with Honors) from the University of British Columbia (1996), and received his Ph.D. from the University of British Columbia in 2003. His research is focused on the genetics of pattern formation and signaling using the common fruit fly Drosophila melanogaster as a model organism. Dennis is a gifted thinker and writer on matters of science and faith, but also an award-winning biology teacher—he won the 2008 College Biology Teaching Award from the National Association of Biology Teachers. He and his family enjoy numerous outdoor activities that the Canadian Pacific coast region has to offer. Dennis writes regularly for the BioLogos Forum about the biological evidence for evolution.

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Seenoevo - #79437

May 3rd 2013

Isn’t there a “Which came first, the chicken or the egg?” issue with proteins and DNA?

I thought that DNA consisted of proteins but also that DNA is the producer of proteins.

 


Dennis Venema - #79589

May 6th 2013

We’ll cover this issue more in the future posts that deal with abiogenesis, but the answer is bascially the same as for chickens and eggs - one predates the other, and it’s not an issue. Eggs greatly predate chickens, and hereditary functions predate other functions. It’s likely that the first replicating entities were some sort of nucleic acid that could catalyze its own replication (i.e. it had hereditary and enzymatic functions all in one). We have some lines of evidence for this, but it’s comparatively speculative when compared to evolutionary biology.


Seenoevo - #79594

May 6th 2013

Dennis,

“…the answer is bascially the same as for chickens and eggs - one predates the other, and it’s not an issue. Eggs greatly predate chickens…”

I honestly hadn’t heard this before. It’s a wonder people still use that old line. Ignorance and fear are to blame, I suppose. [I’ll plead guilty to the former on this one.]

But this is exciting news.

In advance of the future article on this, could you provide a link or just identify which animal or animals laid the first eggs? I want to read up on what the birthing process was for what would become the original egg-layer(s). Also, how its reproductive system changed so dramatically in one generation.

Thanks in advance.


Dennis Venema - #79595

May 6th 2013

Nothing changes dramatically in one generation - you’re working with a faulty view of evolution there (albiet a common one). Changes are slow - think of a child’s flip book, where you flip pages rapidly to see the image change. Evolution is like that - each generation (page) is pretty much identical to the one before and after it. The changes add up over succesive generations.

As for eggs, egg-laying was the reproductive mode for all tetrapods (and fish, the group tetrapods arise within) until the advent of marsupial and placental mammals. So, egg laying goes back to at least the origin of fishes.


Dennis Venema - #79596

May 6th 2013

.. and of course much further back than that. The point being that chickens, as highly derived tetrapods, are relative newcomers compared to eggs.


Seenoevo - #79604

May 6th 2013

Dennis,

Thanks for the rapid response, and for the additional scientific information.

But you’re not answering the fundamental question. I’ll restate it, with the additional scientific information:

Which came first, the egg-laying tetrapod or the tetrapod egg?

And if, as you imply, an organism can’t go from non-egg-laying to egg-laying in one generation, but needs many generations, what does the reproductive system look like for the in-between generations? (If you’d like, you can just direct me to the proper pages of that evolution flip-page book. If there’s more than one version, the “child’s flip book” might be best for me.)


Dennis Venema - #79607

May 6th 2013

For tetrapod eggs (with a shell) the starting point would be something amphibian-like, with a gradual shift towards eggs that are more and more resistant to dessication.

If you’re talking about eggs in general, they’re very ancient, and we know less about their evolution (it’s wrapped up in the origins of sexual reproduction and meiosis).


Seenoevo - #79613

May 7th 2013

Dennis,

“For tetrapod eggs (with a shell) the starting point would be something amphibian-like, with a gradual shift towards eggs … If you’re talking about eggs in general, they’re very ancient, and we know less about their evolution (it’s wrapped up in the origins of sexual reproduction and meiosis).”

I see.

Allow me to try a third time to express the fundamental question:

Which came first, the first egg-laying THING, or the first egg?

 

No answer today, I guess. Not until we unwrap that “meiosis”, which freedictionary defines as

“1. Genetics The process of cell division in sexually reproducing organisms that reduces the number of chromosomes in reproductive cells from diploid to haploid, leading to the production of gametes in animals and spores in plants.”

And surprisingly,

“2. Rhetorical understatement.”


Dennis Venema - #79624

May 7th 2013

“Allow me to try a third time to express the fundamental question:

Which came first, the first egg-laying THING, or the first egg?”


I’m not sure how to help you get past the misconception of evolution that you have, but I’ll try. Features don’t arise independently of the organism that bears them. Your question is similar to asking “what arose first - the placenta, or placental mammals?” or “what arose first, birds or the wing?” The point is that all of these structures (eggs, placentas, and wings) arose gradually, over thousands of generations or more, along with the organisms that exhibit those features. At no point in the process was any given generation radically different from the one that preceded it.



Chip - #79633

May 7th 2013

“what arose first, birds or the wing?” The point is that all of these structures (eggs, placentas, and wings) arose gradually, over thousands of generations or more, along with the organisms that exhibit those features. At no point in the process was any given generation radically different from the one that preceded it.

While such stories sound nice, it’s hard to see how this works in practical terms, given standard evolutionary assumptions. Take the wing.  If it took thousands of generations or more for a wing to develop and if each mutation along the way had to be beneficial as it happened in order to be persisted by natural selection (which is not capable of planning or foresight), what you unavoidably end up with is a whole series of beasts (birds, bats, insects…) with partial wings: too underdeveloped to be supportive for flight, while still big enough to be a liability to the owner. Having to flee from a predator under any circumstances is unappealing enough; which of us would like to do so while strapped to a parachute—even if it’s only a small one? And this state would have had to have been persisted for generations and generations by a process whose fundamental raison d’etre is to filter out such mal-adaptive structures and the creatures saddled with them. Take any modern bird, clip it’s wings and drop it in a neighborhood of cat owners. Who’d bet on the bird?


Dennis Venema - #79636

May 7th 2013

Chip, by that reasoning, flightless birds should not exist - and yet they do. Do you think penguins (for example) are descended from flying birds?

We’ll cover more of this when we do some posts on exaptation - how a structure can come under selection for a modified function over time. Wings are exapted tetrapod forelimbs, and there is good evidence that they were used for other things before flight (balance, gliding, courtship displays, and so on).


Seenoevo - #79649

May 7th 2013

“The point is that all of these structures (eggs, placentas, and wings) arose gradually, over thousands of generations… At no point in the process was any given generation radically different from the one that preceded it.”

Thanks for your responses, Dennis.

No need to respond to me further here. You can use the time instead to work on the next articles in this educational series.

Maybe I’m just having a bad day.

Has anyone else out there ever enjoyed some Monty Python skits, and the theater of the absurd? I have. But there’s a time and place for everything, I suppose. (cf. Ecclesiastes).

For anyone else out there – here are what I hope would be undeniable, common-ground points:

1) There was a first egg on earth.

2) The first egg was laid by some organism.

3) This egg, and every egg, is part of a reproductive process.

4) That organism in 2), which laid that first egg as part of the reproductive process, was itself  NOT the result of an egg-laying reproductive process. It couldn’t be. There were no previous eggs.

5) Point 4) necessitates a change from non-egg laying reproduction to egg-laying reproduction in exactly one generation (i.e. not gradually). (It would not be unreasonable to call such a one-generation change “radical” or “drastic”.)

 

For those who agree that these are indeed undeniable, common-ground points,

 do they not conflict with Dennis’ quote directly above?

 

Does anyone out there (besides Dennis) have a problem with any of the above points? If so, would you please make explicit the reasoning for your misgivings?

Anyone?

 

Anyone?

http://www.youtube.com/watch?v=uhiCFdWeQfA


Merv - #79658

May 7th 2013

Seeno, you have a committment (that many of us share in one way or another to some degree) to a world of “Dedekindian cuts”.  What I mean by that is that we often want to divide things into precisely defined (often binary) sets.  Was that an egg?  Was it the first egg?  Does life begin at conception?  Is it a member of this species or that species?  We liked the old Linnaean classification system because it aspired to a taxonomy of sets, even with all the difficulties of defining them.  Cladistics blurs (or rather recognizes the blur in) such a system and tries to provide a more realistic organization.

What all this means is that your eagerness to identify (even in principle) some “first egg” may be comparable to trying to identify the first day you knew how to read.  It is quite possible that the question itself is meaningless because you learned to read, not in one day, but over months and years.  There was no one day where your reading abilities were significantly different than the day before.  And yet here you are, able to read whereas you clearly couldn’t at only one year of age.  The nonexistence of some clearly defined “first something” does not preclude that something eventually coming to be.

I think Dennis has given you very patient answers, and the rest of us appreciate his clarity and willingness to respond even in the face of sarcasm because we all learn how such misunderstandings are well-answered.  I also understand your yearning for those clear-cut divisions because I also share in those tendencies.  Sometimes they seem so Christian—- is that person saved or not saved?  Is an action good or evil?  Was neanderthal human or not?  Is a 6-year old morally responsible or not?   I maintain that this binary approach to life has a well-grounded place in our thoughts and theology, but there are so many examples where we mistakenly try to force reality through this binary filter (even on theological questions!) and end up looking for something that probably doesn’t exist in the way we want to imagine; like that clear-cut first egg.


PNG - #79768

May 9th 2013

Merv, I think your answer is great. I would add that Seeno’s approach is what I would expect from a scholastic philosopher 500 years ago. It assumes that you can deduce from words what the world must be like. Words make absolute qualitative distinctions based on their definition. Thus, “egg” has a definition, and all the world can be divided into “eggs” and “not-eggs,” based on whether the thing fits the definition or not. (I once had a cookbook written by a college student that divided all beverages into “beer” and “not-beer.” Needless to say, the author was a philosophy major.) Thus, there must have been a first egg. Logically true, but trivial.

If you want to find out about the world, you have to leave word games behind and go out and study the world. In the world you find that it is only when you look at “snapshots” that you see strictly qualitative differences. You need “video” and if it covers enough time, it shows continuous gradual change.


beaglelady - #79773

May 9th 2013

If you want to find out about the world, you have to leave word games behind and go out and study the world.


That would make a lot of people break out in hives.


Merv - #79790

May 9th 2013

I’m not even sure that the “first egg” concept can even be defended in a ‘logical truth’ context.  In an evolutionary scenario what would likely be found if we could rewind and watch the movie would be egg-like things over which we would get into arguments about whether or not they qualified as true eggs.  

Your beer example may also be pressed into service to challenge our ‘firsts’ obsessions  as well as your friend’s binary appraisal of beverages.  Did there have to be a first beer?  Well, there probably was a first time that the word “beer” (or a similar sounding word that morphed into our modern word ‘beer’  —-etymology certainly won’t deliver us from gradualism!)  was applied to a fermented beverage.  But fermented beverages would have long predated any such labels.  So as with so many things it may be logical nonsense to be expecting a clear “first” of such a thing as beer.  The only way we achieve knowledge of true firsts is by our careful definitions of what can qualify.  Seeno alluded to birthdays in a post that disappeared.  Those are easy because traveling the birth canal is a fairly definable event.  So this isn’t to say that we can’t ever find examples of clear firsts.  One just needs to keep in mind that distinctions made may often be more reflective of human convention and convenience than of reality.


PNG - #79765

May 9th 2013

I just wrote a long response to this and then lost it through sheer copy and paste stupidity. So, again…

Seeno, you have posted “undeniable” points here and elsewhere, and it always seemed to me that they were mostly all too deniable. Any biologist would have trouble with your points, not just Dennis. Sometimes I think you might really want some information. Other times I think you just want to mock the nasty old scientists (I assume that’s what your screen name implies.) Maybe you could candidly tell us what your intentions are. Anyway, maybe someone will find this useful. 

Eggs are differentiated cells specialized for sexual reproduction (as are sperm, of course.) Single celled eukaryotes commonly have sexual reproduction, but with cells of 2 (or more!) mating types that look and act alike, other than mating with the other (or a different) type. Yeast has ‘a’ and ‘alpha’ cells, which differ in the expression of a few genes, but look alike under the microscope. They fuse, like egg and sperm, to make a diploid (2 sets of chromosomes) cell, which in yeast is bigger but otherwise pretty much the same as haploids (except for undergoing sporulation to make haploids. ) In some unicellular eukaryotes, the two types of mating cells are more different, of 2 different sizes, etc. This is the simplest morphological differentiation of gametes.

In organisms a little more complicated, very small multicellular organisms, some have egg-like and sperm-like cells, and sometime these cells are released into the surrounding water, so I guess this could be early “egg-laying.” You could look at something like this for the origin of “eggs,” but it seems like basically a matter of terminology. You could pick mating cells with lower degrees of observable difference like those described above and say that eggs began there. A molecular biologist would probably say, “what’s the simplest organism in which specific egg genes of higher organisms appear?” (To go the other way, some of the egg genes that were useful to egg-laying reptiles and monotremes are still present as non-functional fragments in our own genomes.)

The point is, if you only look at widely separated organisms, you see very clear qualitative differences in many specific features like laying eggs or not, but if you look at a lot of organisms, including unicellular organisms, where there are huge numbers of species, you see all kinds of variations and gradations of things, so that it becomes a matter of definition where to draw a line. 

It happens that I was looking at some material on the early egg/sperm business before I looked at Seeno’s comment. For early development of sex, see the section Unicellular Protists and the Origins of Sexual Reproduction in this link - http://www.ncbi.nlm.nih.gov/books/NBK10066/

For early differentiation of mating cells see this and search for “heterogamy.”  http://www.ncbi.nlm.nih.gov/books/NBK10066/ 

So as to your points, 1) Not umambiguously. Eggs have multiple traits - they can appear at different times and each trait in different quantitative amounts. Where you designate the first egg is a matter of definition. 2) No. The first egg could be in an organism that couldn’t “lay” anything. 3) Yes. 4) & 5) See 1.


glsi - #79745

May 8th 2013

“Changes are slow - think of a child’s flip book, where you flip pages rapidly to see the image change. Evolution is like that - each generation (page) is pretty much identical to the one before and after it. The changes add up over succesive generations.”

 

Trouble is the fossil record doesn’t show that.  Not by a long shot.  It shows stasis.  The fossils don’t look anything like your child’s flip book.  If they did you’d have a lot more people who believed in evolution because then the evidence would actually fit the theory.


Dennis Venema - #79748

May 9th 2013

Trouble is the fossil record doesn’t show that.  Not by a long shot.  It shows stasis.  The fossils don’t look anything like your child’s flip book.  If they did you’d have a lot more people who believed in evolution because then the evidence would actually fit the theory.

Two things you have to keep in mind about the fossil record are (a) fossilization is an infrequent event, and (b) only common species that persist for a long time have a reasonable chance of fossilization, unless their lifestyle predisposes them to fossilization in the first place. So, to continue with our flip-book analogy, fossils are pages torn from the book every 100,000 pages or so.

Think back to the example we’ve just gone over - the change between wolves and domestic dogs. Wolves and most domestic dogs are morphologically distinct enough that they would be recognized as separate species in the fossil record. Yet there is no fossil record to speak of that documents the wolf-to-dachshund transition. What one observes in the fossil record is wolves, and dogs, with nary a transition to be found. The sampling frequency of fossilization was too infrequent to “capture” the event. Other events were captured, because they took longer (such as the transition to land in tetrapods, or the evolution of whales, etc).

It’s also important to note that fossils are not, by any stretch, the only evidence we have for evolution. I think that Christians, for many years, put off dealing with evolution because they felt that there was “reasonable doubt” about the fossil record. Now that we have full genome sequences, and can see shared mutations with many other species, there is a realization that the evidence cannot be evaded any longer.

Here is a link to a blog that covers some of this evidence. Even if we had no fossil record at all, and biologists had never thought of common ancestry or evolution until now, genome sequencing would have been enough to firmly establish evolution as a theory (in the scientific sense).




glsi - #79751

May 9th 2013

There are untold scores of fossils which have been found and are being found all the time.  By and large, they show long periods of stasis rather than the slow, gradual change predicted by Darwinism. The fact that fossilization is itself a rare event only makes your position more desperate.


melanogaster - #79792

May 9th 2013

“It’s also important to note that fossils are not, by any stretch, the only evidence we have for evolution. I think that Christians, for many years, put off dealing with evolution because they felt that there was “reasonable doubt” about the fossil record. Now that we have full genome sequences, and can see shared mutations with many other species, there is a realization that the evidence cannot be evaded any longer.”

And yet Gisi evades in a response to your comment.

Dennis, I think the only approach that will effectively expose such evasiveness is one in which the theme is a challenge to “predict the differences.” Creationists have easily evaded the sequence evidence for years by pretending that it is simple, vague similarity and you’ve given them no reason to stop doing so.

I have an idea: how about hosting a discussion about sequence differences in which any form of the word “similar” and its synonyms prevents a comment from being posted?


Darwin Guy Dan - #80042

May 14th 2013

seenoevo #79649, #79613, #79594, #79437 

In my view, the inexcusable muddle regards “evolution” is bad for Science and bad for Natural History studies.  You see no Evolution because there has been no Evolution.   Sorry to see your light has turned red.

The interesting information provided by PNG correlates well with some speculations I have been considering in regards to global abiogenesis, global biogenesis, and Naturalistic Parallelism theory.  (Merv’s explication of Richard? Dedekin? Cuts is interesting, but curious.)  The color spectrum and very broad visible and invisible full spectrum of electromagnetic radiation is of interest and might also be seen as an analogy.  E.g., when exactly does red become purple, and blue become green, etc?  In some sense, one might see a particular linked series such as electromagnetic field frequencies as being analogue in nature. But quantum physics tells us that at very fine detail all analogues eventually become digital in nature. 

Hypothesis:  Metaphorical “chickens” came first.  (They are metaphorical in that, obviously, we all agree that eons ago there were no such organic beings as the chickens we know today.)  These first “chickens” were the precursors to beings that are still with us, essentially unchanged, and are known as cyanobacteria, etc.  Their morphological form at global origins appears to have been that of a torus (doughnut).  A further speculation is that genomes came about as EMF reflections of these toroidal cell walls.  Various EMFs are seen as possible organizing fields for the relevant atoms and molecules of various organelles and internal structures.  Microtubules, cellulose microfibrils, other symmetries and the origins thereof certainly are interesting and support the hypothesis. 

The word “species,” while useful in any one contemporaneous horizontal slice of time, such as our current time, is clearly problematical when thinking in terms of the vertical discovery and study of Natural History.  As Merv and others have indicated, a modern species / kind can only be carried back just so far before it is seen as some other species / kind.  This is why the theory of Naturalistic Parallelism prefers to think in terms of lineages.  All organic beings of today have lineages extending back in time to abiogenesis.  (Most species represent the symbiotic integration of many lineages.)  In this view, the aim of Natural Historians is to discern, to the extent possible, what the nature of these lineages, and combinations thereof, are and how they may have connected over time. 

With high regards for your skepticism,

DGD, a.k.a. NaturalHistoryGuy, a.k.a. thereisnoevo    <DarwinGuyDan  at Gmail dot com>

 

 

Papalinton - #80579

May 30th 2013

Which came first?  The cjicken or the egg?

The egg of course.  Dinosaurs laid eggs for millions millions of of years even before chickens ever began to evolve as a species.  


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