As I explained in a previous post, evolution is simply a change in the genetic makeup of a population, and natural selection is simply a mechanism that causes evolution. Everyone agrees that both evolution and natural selection occur. Even the most fervent and outspoken young earth creationism organizations acknowledge that natural selection is a real thing and causes the genetic makeup of populations to change. For example, everyone agrees that all dogs descended from a common ancestor, and creationists typically have no problems accepting that all of the species of Galapagos finches descended from a single species of finch that got blown out to the islands. So the disagreement between scientists and creationists is not about whether or not evolution occurs. Rather, it is about whether or not evolution has limits. You see, examples like dogs and finches are what creationists refer to as “microevolution,” which they define as small changes within a “kind” (the term “kind” is loosely defined, but they generally state that the scientific classification of “family” is roughly the same as a biblical “kind”). In contrast, they define “macroevolution” as large changes, such as those that would be required for a land mammal to evolve into a whale, and it is these large changes that they say are impossible. As I will demonstrate, however, this distinction is completely arbitrary, and it is neither logically nor scientifically valid.
First, it is important to realize that scientists do not generally accept the creationist definitions of these terms. The use of the terms is not completely standardized, but generally speaking, microevolution refers to either a change from one generation to the next, or a change within a species, whereas macroevolution is simply a large change caused by an accumulation of microevolutionary changes. This is a very important distinction. Creationists act as if micro and macroevolution are two totally separate processes, but in reality macroevolution is simply an accumulation of microevolutionary events. In other words, microevolution inevitably leads to macroevolution. So if microevolution happens, then, ipso facto, macroevolution also happens.
Let me illustrate it this way. Let’s talk about the evolution of land animals (i.e., the evolution of a strictly aquatic species into an amphibian that can live on land). Both creationists and scientists agree that this is an example of macroevolution. There are lots of changes that are necessary for this to take place, but the three biggest ones are:
- The development of limbs for locomotion.
- The ability to balance your body’s water content.
- The ability to breathe oxygen in the air.
There are species of fish alive today (such as mud skippers) which can come on land for short periods of time during low tide in order to forage. All three of the above criteria are more fully developed in them than in most fish. The ancestor of land animals was not a mud skipper, but the image of a mud skipper is useful for this example. So, let’s suppose that the great, great, great, etc. ancestor of land animals was an organism which, like the mud skipper, could come on land for brief periods of time to forage for food. For sake of example, let’s assign this first generation an average value of 1 for each of our three important traits. In other words, there is variation in the population, and some individuals have slightly more developed limbs (limbs = 1.2) and are slightly better at getting oxygen (breathing = 1.2) and regulating water (water balance = 1.2), whereas others are below average (0.8) for all of these traits, and most individuals are in the middle. Let’s further state, for sake of example, that you need a value of 100 for each trait in order to live on land (creationists would, therefore, state that achieving the value of 100 is impossible).
Now, in this first generation, the individuals that rank as 1.2 are going to be able to stay out of the water longer, which means that they have more time to hunt for food. This results in them getting more to eat, which causes them to produce more offspring than the less well developed individuals. As a result, the average value for the next generation will be 1.1. This is a simple example of natural selection causing evolution, and even ardent creationists would agree with me so far.
Now, in Generation 2 there will again be variation, and the individuals with the best limbs, best water balance, and best breathing will be able to forage more, causing them to produce more offspring, resulting in a mean value of 1.2 for Generation 3. This process will continue over and over again until eventually, at generation 1,000 they reach the threshold at which they can actually live on land (i.e., a mean value of 100). For sake of simplistic example, I obviously used artificial numbers, but this is more or less how evolution via natural selection actually occurs in real populations, and we can even use a series of mathematical formulas to calculate exactly how much change occurs from one generation to the next.
Hopefully, at this point, the problem with creationists’ distinction is clear. Creationists would agree with every single individual step of this process. They would agree that Generation 2 evolved from Generation 1, Generation 3 evolved from generation 2, etc. all the way up to Generation 1,000 evolving from Generation 999, but they would simultaneously claim that it is not possible that Generation 1,000 evolved from Generation 1. This is a clear violation of the Law of Transitive Properties. If you acknowledge that each microevolutionary step will occur, then you have just acknowledged that macroevolution will occur, because macroevolution is nothing more than the product of multiple steps of microevolution. So the creationist claim that “microevolution occurs but macroevolution is impossible” is logically inconsistent, and it is totally arbitrary without any scientific reasoning behind it (i.e., it is an ad hoc fallacy). Allow me to illustrate this process as a syllogism to make things more clear.
- Generation 2 evolved from Generation 1 (creationists agree)
- Generation 3 evolved from Generation 2 (creationists agree)
- …
- Generation 1,000 evolved from Generation 999 (creationists agree)
- Therefore, Generation 1,000 evolved from Generation 1 (creationists disagree)
At this point, creationists like John Morris generally state that the problem with my explanation is that some of the steps would require mutations, and “no truly useful mutations have ever been observed.” It is true that you would need mutations to maintain variation and allow this process to happen, but it is completely and totally untrue that no useful mutations have ever been observed. There are numerous studies that have documented extremely beneficial mutations. So this is another great example of creationists not having the foggiest clue what they are talking about. Further, beneficial mutations would also be required for the evolution of some of the beak types found on Galapagos finches (which, remember, creationists have no problems accepting). So this argument is also a case of inconsistent reasoning. Finally, even IF the claim that “no beneficial mutations have been document” was true, it would be an argument from ignorance fallacy to conclude that, “therefore no beneficial mutations exist.”
Another attempt to discredit this argument is simply to state that macroevolution has never been observed. There are two problems with this argument. First, it is another argument from ignorance fallacy. The fact that we haven’t observed it doesn’t mean that it doesn’t occur. Second, of course we haven’t observed it. Large evolutionary changes take thousands, even millions of years to occur! So there is now way that it would even be possible for us to directly observe macroevolution. Therefore, the fact that we haven’t observed it cannot be used as evidence against macroevolution (note: please read this post before claiming that the fact that we haven’t directly observed macroevolution means that it isn’t science).
In summary, the creationist argument that “microevolution happens but macroevolution is impossible” is completely arbitrary, has no scientific support, is logically inconsistent, and violates the Law of Transitive Properties. Ergo, it must be rejected. With this arbitrary distinction now defeated, it is clear that accepting microevolution automatically means accepting macroevolution. Therefore, since creationists fully accept microevolution, they must also accept macroevolution.
The Logic of Science 
One of the dumbest things I have ever read
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May I ask exactly what part of this you disagree with?
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First, let me apologize. I’m having a rather bad day. I disagree with some things you wrote, but that does not make them dumb. You actually seem very intelligent and I would like to discuss things with you. I disagree that accepting microevolution means you must accept macrevolution. I am a physics major and don’t know a lot of chemistry or biology,but doesn’t the dna of an organism prevent major changes?
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Actually, the beautiful thing about DNA is precisely that it does allow for changes. Most traits are polygenic (controlled by multiple genes), which means that any differences in any of those genes will affect the trait. So, without going into too much detail, the process of meiosis (the process that forms eggs and sperm) randomizes an individuals DNA through the processes of crossing over and independent assortment (I can explain how those work if you so desire). The point is that all of an individuals eggs or sperm are different from each other. Their genetic codes are not the identical, even though they all came from the same individual. To produce an offspring, they must combine with another individual’s eggs or sperm. Thus, a tremendous amount of variation can be achieved just by mating (this is why siblings are not identical, with the exception of twins). Still though, the possible genes of the offspring are limited to the genes of the parents. Thus,the larger the population, the more variation you have. So populations have a tremendous ability to change their DNA just by differential survival and selective mating. Everyone agrees with this (even young earth creationists).
More importantly, during meiosis an organism’s DNA is copied. This process is tightly regulated, but mistakes still happen. We call these mistakes mutations, and there are lots of different types of them. Some add new DNA, others rearrange DNA, some remove DNA, etc. All of them are, however, changes in an organism’s DNA. Most mutations are neutral (they don’t do anything useful or harmful), a few are harmful and a few are beneficial. Yes, we have documented many beneficial mutations (there is a link to a bunch of them in the original post). Most mutations, even beneficial and harmful ones, are very minor. For example, a mutation that slightly changes the hue of an organisms color would be afar more common than a mutation that makes a blue individual instead of a red one (though extreme mutations do occasionally happen). Small mutations are important, however, because they add genetic variation to a population. You see, natural selection actualyl eliminates variation because it only selects beneficial traits and it selects against harmful traits. Thus, genetic information gets lost. So without mutations, natural selection will eventually grind to a halt because there is no more variation for it to act on. So the mutation process is absolutely vital. Creationists like to either claim that all mutations are harmful, or that we have never documented a way to gain genetic information, but both of these claims are simply factually incorrect. Further, even the “microevolutionary” processes that creationists acknowledge require mutations. The initial population of finches that colonized the Galapagos islands, for example, wouldn’t have had all of the genes that we see in the Galapagos finches today, they are far too variable for that to be possible. You need mutations to produce the type of variation that we see in those finches. The problem is even more severe when we consider the fact that creationists claim that all of today’s animals descended from pairs of two animals on the ark (e.g., all parrot species descended form two parrots that were on the ark). That would require a tremendous amount of mutation, and it would take millions of years for enough mutations to accumulate to cause all of the parrot species that we see today.
So, to get back to the initial question of whether or not large changes occur, we know that small changes happen, and large changes are just an accumulation of these smaller changes. You don’t get a massive change in one generation, but if each generation changes very slightly from the previous generation, then after enough time, those small changes will add up to a large change. This is the problem with saying that microevolution is possible but macroevolution isn’t. You must provide an explanation for how it is possible to accumulate small changes without ultimately resulting in a large change. To go back to my fish to amphibian example, my question to you would be, what step do you disagree with? Each generation should be able to stay out of the water slightly longer than the previous generation. Even Ken Ham would agree with that. So where is the magical line that they cannot cross? At what point do they hit this invisible wall that prevents them from evolving any further? That is a question that you must answer if you are going to say that macroevolution is impossible.
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I guess I don’t understand how the major changes could occur. Say we pulled all animal life off of Australia, leaving the vegetation and insects(which I know are animals). If we left it alone for millions of years,I think we would see variations of the plants and insects, but no clams, kangaroos, monkeys or people. What mechanism would produce them?
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You are correct that we would not see kangaroos, but that is because we have removed all the genes that are specific to kangaroos, and evolution has no goal in mind (i.e., it is not trying to produce a kangaroo). That is not the same thing as saying that we would not see any new organisms. The organisms that were left behind would slowly accumulate mutations (new genetic information) and change over many generations. Exactly what they would change into is pure speculation, but they would certainly change. In fact, evolution is a mathematical certainty. Anytime that you have variation for a trait and differential reproductive success, natural selection will happen and the genetic makeup of a population will change. So my question to you is, again, where is the gap in this process? In other words, what do you think prevents these small changes from accumulating to form a large change?
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So where did the genes specific to kangaroos come from in the first place? They originally developed “from scratch” according to true evolution and LUCA, why not again? I appreciate your explanations and time. I agree that many small changes could lead to a large change;but a different genus, or family, or order? Not convinced. I would not agree that each generation of your fish could stay out of the water a little longer, why would that be so? I’m sure the current mud puppies are studied, is that the case for them? I also do understand we’re talking about periods of time hard for us to relate to, but I just still see a lot of problems with the theory in its entirety.
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What do you mean by, “from scratch”? The genes for being a kangaroo didn’t simultaneously appear overnight, they slowly accumulated over millions of years. In other words, if we back the clock up several million years, we will find the ancestor of kangaroos. Over time, the environment that it lived in caused it slowly accumulate mutations that gave it a long tail, unusually shaped legs, etc. So the genes didn’t arise “from scratch,” they arose as a result of slight changes to existing genes over millions of years (i.e. small changes accumulated to form large changes). Importantly, evolution is blind, it has no goal or plan in mind, so it wasn’t trying to make a kangaroo. Also, the mutation process is totally random. So, evolution doesn’t give organisms what they “need,” rather, mutations randomly give organisms new traits, and then they are selected for or against based on the current environment. Thus, if we removed kangaroos from Australia, it is unlikely that we would get new kangaroos because that would require them to obtain the same mutations that the first batch of kangaroos got, and since mutations are random and evolution is blind, the odds of that happening are extraordinarily unlikely.
Regarding the fish, they would continue to stay out longer as long as that was beneficial for them. We think that it probably had to do with foraging. In other words, individuals who could stay out longer got more food and, therefore, had more energy and produced more offspring. This is the selection requirement for natural selection. So, as long as there is variation (i.e. not all individuals can stay out for the same amount of time) and a benefit (i.e. food) each generation will slowly evolve. This is a mathematical certainty. Now, if the environment changes such that coming out on land for longer is no longer beneficial, then they would stop evolving in that direction, but as long as there is a benefit to coming out on land, they will keep evolving.
Regarding mud-skippers, I don’t think anyone is studying their current evolution because they are most likely in an equilibrium state where the evolution is too slow to observe in our short lifetimes. Generally speaking, most species are well adapted to their current environment, so there is little or no evolution. Thus, rapid evolution generally happens as a result of some environmental change. There have been a few cases where we have been able to observe this. For example, during a drought several years ago, we were able to observe finches bills evolve as they responded that massive environmental change.
Again, my question to you is why do you think that small changes will not accumulate to form large changes? You need to give some justification for this. The only difference between large changes and small changes is that large changes have had more time to accumulate small changes. So if you accept that small changes happen, then you must accept that large changes happen because there is no difference in the mechanism that causes them.
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I really enjoy talking to you, and hope you will be patient and continue. Most evolutionists cringe when you mention abiogenesis, but ultimately it needs to be addressed. The kangaroo came from mutations of genes, that came from other genes, and so on tracing it back. Eventually there were no genes. I would like your thoughts on that.
You ask me about the small change, big change thing. I’ll use your parrot example. There are parrot fossils dated at 60+ million years old. Today there are over 300 species,many of which cannot mate with the others.(which is what I thought determined a species). Lots of variations and adaptations, but they are still parrots. Based on what I know, they always will be and always were, so where did the first ones come from. I don’t know, but I don’t think there is solid evidence they came from another type of animal or even bird. Lots of small changes over millions of years, without a big change.
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I’m not sure what you mean by, “Most evolutionists cringe when you mention abiogenesis.” Most scientists that I know love to talk about abiogenesis because it is fascinating. It is, however, important to note that abiogenesis has nothing to do with evolution as evolution only deals with what happened after the first cell was formed. Having said that, can you please clarify your question? Abiogenesis is a huge topic and I’m not entirely sure what you are asking. I’m guessing that you are simply asking how the first DNA formed, in which case, my answer is, “we don’t know, but we’re working on it.” We have lots of ideas about how it formed and we are currently in the process of testing those ideas. The most popular hypothesis at the moment is the, “RNA world” hypothesis which proposes that RNA predated DNA. RNA is, of course, single stranded and simpler than DNA, so it makes sense for it to be a precursor.
It’s important to note that the fact that we don’t currently know exactly how DNA formed doesn’t mean that there isn’t an explanation. That would be what’s known as an argument from ignorance fallacy. Science only exists because there are unknowns. If we had all of the answers, there would be nothing left to study, and for everything that science has discovered, there was a point in time when it was unknown. So the fact that we don’t understand how something formed doesn’t mean that it couldn’t have formed, it just means that we don’t know how yet. This is especially true for abigenesis which is such an active area of research, and an area that we have only recently had the molecular tools to really examine. It would be one thing if we had been working on it for centuries and had hit a wall that made it truly seem like the answer just wasn’t out there, but that’s not the case at all. Lots of labs are systematically testing ideas and each year new pieces of the puzzle fall into place. We have a much better understanding of the chemistry now that we did 30 years ago or even just a decade ago. We may not have the answer for a year, a decade, a century, etc., but the fact that we haven’t solved the puzzle yet doesn’t mean that the puzzle is unsolvable.
In regards to parrots, first, a quick side note on the definition of species. There are lots of different ways that scientists define a species. The oldest is the biological species concept which is based on the ability to interbreed, but it has lots of problems. For one thing, it’s very hard to test. Also, it can’t classify asexual species or account for the fact that many species can hybridize. Other species concepts include morphological concepts, geographical concepts, genetic concepts, etc. Today, we tend not to use the biological concept very much, and instead we use a combination of other concepts, and we generally rely heavily on the phylogenic concept which uses genetics to show evolutionary relationships (that’s going to become important later).
With that side topic out of the way, let’s talk about parrots. The fundamental problem with your argument is that there is something of a question begging fallacy inherent in it. Of course today we only see parrots. If they had evolved into something else, we wouldn’t call them parrots. You see, most of our modern groups split long ago. So, for example, if we went back around 70 million years ago, we wouldn’t see parrots, but we would see their ancestor. If we go back even further, we wouldn’t see any birds at all, but we would see their dinosaur ancestors. So parrots clearly had an origin. At one point, there was a species who was not a parrot but became a parrot. In fact, we think that this species split and some of them evolved into our raptors. Part of the problem here is that we think of our current species as the endpoints of evolution, which simply isn’t correct. Today, all parrots are parrots because if they were anything else we wouldn’t call them parrots, but if we could jump forward a few hundred million years, we would almost certainly see species who are not parrots, but evolved from parrots. Evolution is just too slow for us to see that happen in our life time.
As far as evidence, genetics has really helped us out here. First, it’s important to realize that within parrots there are several families and lots of genera. The old world cockatoos are, for example, very different from the new world amazons, which are also different from macaws. So even though they are all parrots, a tremendous amount of evolution has taken place. Genetics are helpful because they let us test those relationships. So, when we test the DNA of macaws, we find that all macaws are more closely related to each other than to other groups. Similarly, amazons share more DNA in common with each other than with other groups. This makes sense since all amazons had a common ancestor, all macaws had a common ancestor, etc. Next, we can compare amongst these groups, and we find, for example, that macaws share more DNA in common with conures than they do with amazons, but macaws, conures, and amazons all share more DNA with each other than they do with cockatoos. This lets us construct a phylogeny or family tree for the parrots. So all macaws have a common ancestor (think of this as a parent), but both macaws (as a group) and conures have a common ancestor (grandparent), and macaws, conures, and amazons all have a common ancestor (great grandparent), and macaws, conures, amazons, and cockatoos all have a common ancestor (great, great grandparent), etc. So the genetics let us map out the course that evolution has taken from a common ancestor of all parrots to the different groups that we see today.
Now, here is where things get fun. Creationists have no problems with what I just said (depending on where they draw the line for microevolution, some draw it at family, in which case they would acknowledge the genetics, but only within a family). They admit that those are true evolutionary histories and the genetics work, BUT when we apply those exact same techniques to broader groups, suddenly they say that the genetics don’t show true relationships. We can, for example, see that parrots share a common ancestor with raptors, and we can see that raptors and parrots as a group share a common ancestor with passerines, and we can see that parrots, raptors and passerines share a common ancestor with shore birds, etc. So if you are going to say the genetics shows a true evolution history within parrots (or even within parrot families), you can’t simultaneously say that those exact same techniques don’t actually show relationships among broader groups. That’s logically inconsistent.
We also have tremendous fossil evidence for many large evolutionary changes which I discussed here: https://thelogicofscience.com/2015/03/09/if-evolution-is-true-where-are-the-missing-links/
Now, with all of that said, you still haven’t taken up my challenge of explaining what part of macro-evolution is impossible, so I am going to ask it a different way. Consider the parrots again. They are extraordinarily diverse, and you seem to have no trouble accepting that they evolved. Now, give them a hundred million more years to evolve. Why do you think that they will never evolve to the point that we would have to consider them to be something other than a parrot? Consider the kakapo. This is a large flightless parrot in New Zealand. It is extremely different from all other parrots (it can’t even fly for crying out loud). So, given a hundred million more years, why do you think that it can’t become so unique that its descendants don’t qualify as parrots? To put this another way, think about the current diversity of parrots. It contains everything from enormous macaws to tiny love-birds. Given enough time, why do you think that those groups won’t accumulate enough small mutations that they become so fundamentally different that we can no longer consider them all to be parrots, and instead have to split them up into different orders? What step of this process do you find to be impossible?
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I am certainly very uninformed about dna and all its capabilities, but here is how I see things. Indulge me for a few moments please. Say we could somehow take a time lapse video of parrots over the last 60 million years. We see them mating with similar birds, laying eggs which hatch into similar birds and so on. Mutations occur and eventually we get parrots that can’t mate with one another. But at the end (now) we still have birds we recognize as parrots. Some may not be able to mate with the originals, but my guess is the dna is still very close(I don’t know how close).
Now in that same time frame , after the dinosaurs went extinct, we follow a little lemur like mammal. It mates over the ages, but somehow it slowly morphs into a human being. The dna of a human is , I assume, very different from that of a rodent. I do see wide varieties of small furry mammals, which is what I would expect. Becoming human SEEMS like too much of a change, but to honestly answer your question I can’t say it’s impossible, nor can I provide hard evidence that it could not happen. But I also maintain that doesn’t prove it did happen.
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The problem with your time laps example is that you started it after parrots had already evolved. Let’s instead start it back 75 million years ago. Now, the first frames have a bird who has a few features of a parrot, but is clearly not a parrot. Eventually, we will see this bird split. One of those lineages is going to become our modern parrots, the other lineage is going to become our modern raptors.
Regarding humans, going from a rodent to a human would certainly be a large change, but going from an ape like ancestor to a human is not that big of a change. Bonobos are more closely related to us than anything else (because we share the most recent common ancestor), and different though we may be, we share roughly 99% of our DNA with Bonobos. That’s pretty dang similar.
“But I also maintain that doesn’t prove it did happen” you are correct that your inability to explain something doesn’t prove anything, but it does raise the interesting question of why would you insist that evolution isn’t true when, by your own admission, you can’t provide any justification for that conclusion?
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Let me add that I did kind of view now as the end, while it certainly isn’t. Changes will continue to occur, but my view is that millions of years down the road , if we don’t blow ourselves up,there will still be recognizable humans. Maybe no longer with appendices or wisdom teeth, or much body hair, but still very genetically similar.
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Regarding humans, you are probably correct, but that is simply because in our industrialized society, natural selection is no longer acting on us (i.e., people with disadvantageous traits are still able to survive and reproduce). Other animals, however, should continue to evolve.
I find it interesting that you seem to accept wisdom teeth, appendices, etc. as true vestigial structures. If you don’t think that humans evolved, then may I ask how you explain the presence of those structures? In other words, if you think that God specially created “kinds” of organisms, then why do whales have pelvic bones? Why do we have wisdom teeth? etc.
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Wow, I started really reading up on DNA today. Pretty humbling how little I know, so I’m gonna take a little break while I read up on it. Telomeres; it seems if we could keep those rascals intact, it could solve a lot of problems. Talk to you soon.
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I applaud and appreciate the fact that you are actually studying up on the issue. On the fascinating topic of telomeres, you should be sure to read about human chromosome #2, its pretty interesting. I look forward to your future questions/comments.
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Somehow I missed your April 7, 12:58 post. I was brought up believing in creation, so I have biases that way, but I’m really trying to find the truth. I don’t think i have insisted evolution isn’t true, I just have doubts and questions. You’ve really helped with the false dichotomy , and it is logical that many small changes can lead to big changes. There are a lot of blogs and forums on evolution, and sadly, the prevailing attitude of the supporters is that if you’re too stupid to understand evolution,then you’re not worth wasting time on. You have not been like that at all, and it is appreciated. I’m a lot of things(you can ask my ex-wife), but I’m not stupid. I enjoy discussing things with you, and hope we can continue.
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I enjoy talking to people who ask good questions, make rational arguments, and are willing to actually consider evidence, and I certainly don’t think that most creationists are stupid. Rather, most of the creationists who I have talked to have simply been misinformed, which is really one of the main purposes of this blog: to try and correct misinformation. It’s also worth noting that there is no reason why rejecting creationism should automatically lead to rejecting your faith. I know plenty of Christians who fully accept the science of evolution, and I have no problems with that.
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Hello, I’m back. More confused than ever. I don’t expect you to tutor me, but hopefully you can explain some things or give me sites I can check on myself. This gets off topic of your post, so I hope you don’t mind. Human DNA contains about 3 billion base pairs. Now, that is split up into the 23 pairs of chromosomes. Would it matter if that same information, same order of ATC and G, was split into ,say, 30 pairs of chromosomes? Is it known why it splits into the 23 pairs? I did look at chromosome number two like you suggested. Now that is interesting, and would sure seem like evidence of evolution( I wouldn’t call it micro). I assume there are more differences, but from what I gather , the same bases are in chimp’s chromosomes labeled 2A and 2B, and just making that information be in one chromosome makes a big difference in the way the organism turns out.
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To put it simply, its what’s on the chromosomes that matters, not how many chromosomes the information is scattered across. So in the case of our chromosome #2, that was a mutation known as a fusion, but it was not the only mutation that occurred during our evolutionary history. There were many other mutations to our DNA and mutations to chimps DNA that caused us to evolve in separate directions (many of these were on chromosomes other than #2). To put this another way, if that fusion was the only mutation that had happened to our ancestors, and no mutations had happened to chimp’s ancestors, then we would probably be nearly identical to chimps, but we still wouldn’t be able to interbreed and produce fertile offspring because our gametes would have 23 chromosomes and the chimp’s would have 24.
This begins to answer your question about why having 23 chromosomes matters. That initial fusion would have resulted in what is known as a prezygotic isolating mechanism (i.e., something that prevents a viable zygote from forming). You see, in order for a population to evolve into two species, there generally has to be something that splits the population into two isolated groups. It can be a behavior, a geographic barrier, etc. The point is simply that as long as the entire population is interbreeding, it tends to remain as one species rather than splitting because any new mutation that arises gets passed throughout the entire population. When the population becomes isolated into two groups, however, the mutation only gets passed among part of the population. Thus, each half of the population begins to slowly accumulate mutations that the other half doesn’t have, and this causes them to evolve in separate directions.
So, when it comes to our evolution, there are some questions that we will never know the answers to for sure, but it seems likely that our chromosomal fusion was important not because it created new information, but rather because it provided an isolating mechanism. Once that fusion happened, our ancestors could no longer interbreed with the chimp’s ancestors. Thus, any mutations that arose in our ancestors stayed with our ancestors and did not get passed to the chimp’s ancestors, and vice versa. That would have caused us to evolve in one direction and chimps to evolve in another direction.
More generally though, you raise an interesting question of why different organisms have different numbers of chromosomes? I don’t think that we fully understand the significance of fusions and fissions other than that they provide isolating mechanisms. They are chance mutations, but different groups have gone in separate directions. In other words, in some groups there is a pattern of reducing chromosomal numbers, and in other groups there is a pattern of increasing chromosomal numbers. I’m not a geneticists, so my knowledge on this topic is admittedly limited, but my understanding is that there is debate among geneticists about the evolutionary significance of these trends.
Did that answer your question?
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Yes, thanks. Genetics, what a field. I wish I was younger. Even though I had heard of the 23 pairs of chromosomes, for some reason when I would hear about human DNA I would think of this one big molecule of 3 billion base pairs. Does it ever appear as such, or is it always split into the chromosomes? I’ve been reading a lot lately, and I hope I can find this one article again. It was about a man that had 22 pairs of chromosomes. Apparently his 14th and 15th had fused. He was normal and healthy. I don’t think the article said, but would that mean he couldn’t reproduce with a woman with the normal 23 pairs?
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The DNA is always separated onto chromosomes, but the chromosomes aren’t always condensed. Cells only actually divide during mitosis and meiosis, so during those two stages, the DNA condenses into what we think of as chromosomes (you can actually see these condensed chromosomes under a microscope). The rest of the time, cells are either in interphase or G0 phase. During those two phases, the DNA is non-condensed in long strands that we call chromatin. So if you use a microscope to look at a cell that is in interphase, you’ll see a cluster of DNA without any distinct chromosomes. The DNA is actually still segregated into different chromosomes, but they aren’t condensed so you can’t actually see the differences.
Regarding someone with 22 chromosomes, sometimes they would be able to reproduce and have a fertile offspring, other times they wouldn’t, it just depends on exactly how the fusion happened. Also, individuals like this may be able to reproduce themselves, but their offspring will often be sterile. This does, however, bring up something of a chicken or the egg paradox. In other words, if fusing a chromosome means that you can only reproduce with other individuals with the same fusion, then how do fusions lead to new species? It seems like they wouldn’t be any good unless many individuals had them, and that is highly improbable. The reality is that my previous explanation was overly simplistic. Fusions (and other genetic rearrangements) don’t always lead to a clean isolation. In other words, its not always a complete break. Sometimes it leads to partial isolation, which still allows species to diverge, but avoids the paradox. How exactly this happens is complicated, not fully understood, and seems to vary among species. So, unfortunately, I don’t really feel qualified to try to explain the details of how it works, but you can find plenty of papers on the topic because fusions, fissions, etc. have happened tons of times in many taxonomic groups.
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Thank you, both of you, for this very interesting thread. It was so refreshing to read a discussion like this that remained so open minded. I personally love the topics relating to evolution, natural selection, and molecular bio, however I have a difficult time discussing these topics with people who disagree or do not understand them. I learned a lot from both sides here!
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