Reproduction and Human evolution

  1. Hey, I recently watched a video on YouTube by Kenneth Miller, The collapse of Intelligent Design.

    One of the things he brought up in the lecture was a comparison between. Human and chimpanzee genome. What was specifically said was that we had 23 pairs of chromosomes while the chimp and all other apes had 24.

    What must have happened is that over the course of our evolution, two chromosomes in our common ancestors with the chimp must have fused.
    This was found to be the case, our chromosome #2 is this fused chromosome.

    What puzzles me is that before this mutation spread throughout the population that lead to us, it must have occurred in one individual to start.

    Now we get half our chromosomes from mum and half from dad. How did this Individual with 23 pairs reproduce given that everyone else in the population had 24?

    From what I remember from bio class, the chromosomes need to pair up. So how did this produce a viable offspring?

    What am I missing?

  2. jcsd
  3. Evolution does not occur with just one individual's genotype changing. Who knows the first individual (with genetic change) would have died off without producing offspring. But this would have occurred many times in a particular,perhaps isolated population and this particular transformation would have a slight advantage in the environment they lived in and thus would have become common in the population.

    Evolution is change over time (and its thousands if not millions of years) occurring in a population or group of individuals in a particular environment. So changes occur in generations not in individuals.

    Of course I am not an expert in the field, maybe experts can comment on it.
    Last edited: Feb 19, 2012
  4. Yes i realize that evolution occurs in populations not individuals but it seems unreasonable to me to simply assume that the same mutation (same chromosome fusing) occurred in several members of a population. Perhaps im completely wrong and similar/same mutations can occur but i thought that these things get passed down through the generations until they become prevalent in the given population.

    My problem was that you'd have a population with mostly 24 pairs of chromosomes with a handful (or one in the scenario i described in the OP) of individuals with 23. I'm have to assume that the 23's were breeding with the 24's so did all the offspring they had simply die? did the 23's get really lucky and keep finding each other?

    Am i wrong to think that our ancestors with 23 pairs cannot reproduce with those who had 24 pairs?
  5. Well Routaran, I too am no expert, and I’m sure the regular experts on this forum will respond eventually. But my understanding is that in its essentials, you are entirely right that any individual genetic mutation happens to an individual organism, but that organism has to have a sufficiently similar chromosomal layout to be able to breed with another member of the same species. This business of the fused chromosome two is something I have encountered before, and the particular issue you raise was not discussed when I encountered it. My strong speculation is that, where you are going wrong is in assuming that the fusion of chromosome two was itself a single mutation event. Exactly as thorium1010 suggested, significant evolutionary change is built up of many tiny incremental changes, and so too, would be my guess, was the fusion of chromosome two.
  6. I'm not an expert on this either, but you might check this out.

    Also it appears that there has been a similar discussion on this PF thread.
  7. Have you read about high prevalence of sickle cell anemia in certain african population where there is high incidence of malaria. Using this as a analogy populations change over time where a particular trait (IE sickle cell or fused chromosome) would be more prevalent in a particular population because it gives a slight advantage over rest of them that allows them survive in a environment. This does not mean those without a particular trait would not survive or reproduce.

    This is again speculation on what could have occurred, but whether they could have interbred or not is something we do not have evidence.

    This was originally posted bY bobze, so all credits go to him.

    Last edited: Feb 20, 2012
  8. I think that it has something to do with the fact that as long as all the genes are there in the correct proportions, then the organism will come out fine.
    You can see in polyploidy that some organisms are haploid and have one set of chromosomes, some have 2 sets, some 3 and some 4. Generally the more sets they have the bigger they come out!

    But the point being, that if one gamete had 24 chromosomes and the other 23, as long as all the relevant genetic material was there in the correct proportions then the result is viable.
  9. phyzguy

    phyzguy 2,489
    Science Advisor

    Your assumption that two organisms with different chromosome counts cannot produce fertile offspring is incorrect. See the link below - there are at least two documented cases (one in sheep (Ref 7), one in horses (Ref 9)) where organisms with different chromosome counts produced fertile offspring.
  10. "How did this Individual with 23 pairs reproduce given that everyone else in the population had 24?"

    The following is from Monica Rodriguez of Stanford University:

    "As long as the information is all there, it mostly doesn't matter how it is packaged. The information that the ape has on the two 'split' chromosomes is the same as in our large chromosome 2. So even though we have one less chromosome, all of the information is still there.

    As I said before, this sort of chromosomal rearrangement actually happens in about 1 in every 1000 babies. It's called a Robertsonian translocation. A part of one chromosome joins together with another chromosome and just like with the ape chromosomes, no information is usually lost. So this person is completely normal despite having the translocation and one less chromosome.

    But this can be a problem when the person tries to have a baby. Half the time their children are fine. These kids are either normal or carry the translocation.

    Half of the time, though, a fertilized egg will inherit a missing or extra part of a chromosome. Most commonly, this results in a miscarriage."


    "For example, wild horses have 33 pairs of chromosomes and domesticated ones have 32. This was a relatively recent change that happened because of the sort of translocation we are talking about. In fact, this happened recently enough that these animals can mate and have fertile horses.

    So there is plenty of evidence that this kind of thing (i.e., viable offspring from parents with different numbers of chromosomes due to translocation) happens.

    Read more and see some helpful diagrams from Dr. Barry Starr, also from Stanford, explaining how this works.
  11. Kenneth Miller, The collapse of Intelligent Design.

    While I am not an advocate of Intelligent Design, neither am I swayed by Biological Evolution.

    The trokia of Biological Evolution is

    1. Common descent
    2. Random mutation
    3. Natural selection

    Your conceptual experiment focuses on the weakest link of Biological Evolution - random mutation.

    How random can a mutation be if at a min a mating pair is required? This eludes to the same "random" mutation happening at least twice in a very short period of time.

    Obiter dictum - Biological Evolution works quite well when considering asexual reproduction. A lone "random" mutation can be perpetuated. However, with sexual reproduction a much great constraint is placed.
  12. Please explain this . What are you exactly talking about?
  13. phyzguy

    phyzguy 2,489
    Science Advisor

    (1) I think you mean 'alludes', not 'eludes'.

    (2) As I said above, this is simply not true. A single mutation can occur resulting in fusing two chromosomes to create 23 pairs, and this individual can then mate with another individual with 24 chromosome pairs and produce fertile offspring. See the references in my earlier post.
  14. Thoruim - I am not a biological wiz and perhaps I have it wrong.

    With asexual reproduction - I actually mean perpetuation by cell division - a mutation happens and is moved forward without the requirement of a mating pair. This makes the perpetuation of a random mutation almost effortless.

    I am assuming, and perhaps this is wrong, that a mating pair is required to make a mutation viable. If my assumption is correct, and now I suspect it might not be, then a mating pair with the same "random" mutation" is needed to fit the same requirement. This is much more constraining. Not only is it much more constraining but most would not consider this event "random".
  15. Phyzguy - I am familiar with Robertsonian Translocation. Just because it occurs doesn't mean that in the case in question it happened but I am not debating that.

    Again, I might be wrong, but I am assuming that for the mutation itself to be viable, it requires a mating pair with the same mutation.

    And yes you caught me out, it should be alludes, no thinking about it.
  16. bobze

    bobze 651
    Science Advisor
    Gold Member

    Incorrect. Evolution doesn't happen to individuals, it happens to populations. It is the change in allele frequency over time that matters. Individuals with mutations that wouldn't allow them to reproduce without another "mutant" individual would simply die off. Mutations don't change the genome so much that it requires a similarly mutated mate. Evolution works through small vertical variation (between parents in a population and their offspring in a population).

    A point mutation which changes an amino acid and alters the function of hemoglobin then, isn't going to inhibit mating by that "mutant" individual and another member of the population.
  17. bobze

    bobze 651
    Science Advisor
    Gold Member

    Think of chromosomes as filing cabinets. The cabinets themselves aren't important, its the documents in them that matter (genes). So long as all the genetic information is there, how they are arranged matters little (note, this is a simplification for you biology buffs, it can in fact matter but for the novice of biology its probably better they don't think of it that way--Learn the "rules" first, then worry about exceptions).

    Each chromosome has a centromere, which as a nucleation point for the kinetochore, who's job it is to aid in chromosome segregation during division. This centromere can be found in different places on the chromosome.


    In the case of acrocentric chromosomes they can fall in such a way that little, if any, necessary information lays on the short arm (if there even is one) of the chromosome. This can result then, in two acrocentric chromosomes fusing at the centromere with no loss of genetic material.

    Humans have a number of acrocentric chromosomes this could happen too;


    as did our ancestors (more on that later).

    For example, a fusion between chromosome 14 and 21 could occur;


    resulting in an offspring with karyotype on the right. While we consider this a "chromosomal abnormality", you'd never know it. The individual has all the necessary genetic information. If you met them on the street (and you very well could have as its estimated to occur in around 1.2% of births) you wouldn't know they were "chromosomally abnormal".

    What's important though again, is that the individual has all the necessary genetic information. And their offspring can potentially have it as well. How fused chromosomes arrange on the meiotic plate can get a little complex, so we'll leave that out for now. What's more important is the gametes that can be produced;


    I want you to focus on the second individual on the right there; with the gamete "t14/21". This gamete would produce an offspring who is a carrier also of the translocation, but like the parent remains unaffected by it (necessary genetic information again). Meaning the 45 chromosome parent can produce offspring with other members of the population ("normal individuals", or better "wild type") who also have 45 chromosomes.

    If any of these descendents happen to mate (two with 45 chromosomes) their offspring could possibly get the t14/21 from each parent, meaning the offspring will now have 44 chromosomes. Yet, still having all the necessary genetic information--Meaning they are "normal".

    Edit for an important note; You should also notice here that 2 of these individuals because of excessive genetic information (either the equivalent of 3 chromosome 21's or 14's) would be "non-viable" in some kind of historic world. Which means that if offspring were produced that would survive into adulthood then there was a 50% chance they were carriers or 50% chance they had the "normal" karyotype.

    How could this happen? Low population numbers with lots of inbreeding for one. If you had a small isolated population, it would be relatively easy for "chance" (better genetic drift) to fix this new chromosome number in the population. Populations go through these bottlenecks from time to time, humans possibly went through one about 80,000 years ago that reduced our population down to possibly around 10,000 individuals (some estimates put it as low as 1,000 individuals possibly).

    Does this really happen though? And the answer is yes. We know it happened once in our recent (geologically speaking) history with our second chromosome.

    When we look at human chromosome 2 and chimp 2p and 2q (note we've named them here in reference to our [strike]ego[/strike] selves) we see they are beyond just "similar", and rather almost identical;


    Like Ken Miller (as others have referenced) is so eloquent at explaining, the "smoking gun" comes from genetics. Specifically the sequences. When we sequence our 2nd chromosome we find something peculiar. 4 telomeres and 2 centromeres! Normally there should be 2 and 1, respectively;


    At some point in our common ape ancestor past, we reduced the number of chromosomes from 48 (like all other greater apes) to 46 through a similar scenario as described above (though it is possible this new fused chromosome also entailed some kind of selective benefit, that is a discussion for another day).
    Last edited: Feb 21, 2012
  18. bobze, that was an amazing post! Thank you for sharing that, it helped tremendously.
  19. @bobze: I think I understand where i went wrong. I was getting hung up on the number of chromosomes and not realizing that the actual genetic information was all still there. The illustrations helped big time. Thank you very much for the education.
  20. The number of chromosomes is the equivalent to the number of characters that you can send in an SMS. With the same space, you can transfer different kind of messages depending on the words that you use. That is the reason why you can have different apperances from the same number of chromosomes. The DNA contained in the chromosomes of a person is different from one person to another.

    Tigers and lions can interbreed.
  21. You are incorrect in assuming that the chromosomes have to pair up one on one. In a case of balanced chromosome fusion, the fused chromosome can pair up with corresponding unfused chromosomes.

    There was probably a time when our ape-like ancestors were a population of individuals with varying chromosome number. This situation would be similar to the deer in the link that I posted below. The individuals with a fused chromosome freely crossed with the individuals with the unfused chromosomes.

    The consequences of crossing with a dissimilar individual were very small. Hybrids were just as healthy and fertile as the homozygous individuals. There may have been a small penalty in the grandchildren of the hybrid. However, most of the grandchildren were probably healthy. The likelihood that an individual would be maladapted due to the fusion mutation was very small.

    Hybrids of parents with different numbers of chromosome pairs can be viable even in species which with obligatory sexual reproduction. Meiosis does not "count" chromosomes. Meiosis "counts" genes and centromeres.

    A lot depends on the structure of the chromosomes that make up the difference in chromosome number. If there is a chromosome fusion that preserves both the genome and the centromeres, then the hybrid will be healthy and fertile.

    Here is an example where these different varieties (species?) of deer were studied. One of the varieties had a different species number, yet could cross productively with the other varieties.
    “It was shown that compared with conventional cattle the Bos frontalis has a homozygous, species specific 2/27 centric fusion which reduced the diploid chromosome number from 60-58. This provided further proof that Robertson translocation-type rearrangements have been the major source of interspecies karyotype differences in the evolution of the Bovidae. There was also a report on the cytogenetics of twin offsprings from an interspecies cross in marmosets (Callitrichinae, Platyrrhini), resulting from a pairing between a female Common marmoset (Callithrix jacchus, 2n = 46) and a male Pygmy marmoset (Cebuella pygmaea, 2n = 44). Both hybrid individuals had a karyotype with a diploid chromosome number of 2n = 45. These genomic imbalances were confined to centromeric and telomeric heterochromatin, while euchromatic chromosome regions appeared balanced in all species investigated[3]”

    There are other examples that I can find in the literature involving populations of organisms with varying chromosome number. Horse species vary quite a lot in chromosome number. There are other horse species with differing chromosome number that freely cross.

    Even the sterility of the mule and hinny isn't strictly true all the time. However, the occasionally fertile mule crosses with either a horse or a donkey. Thus, the grandchildren end up looking like either a horse or a donkey. There are herds of horses with varying chromosome numbers in the population.

    There are also a population of extant gophers in New Mexico with varying chromosome number.

    Note that the balanced fusion of two chromosomes actually has very little effect on the phenotype of the mutant. If the fusion occurred in a balanced way, then the mutant can cross freely with other individuals with very small fitness penalty. Hybrids are not always sterile, even if the parents have different number of chromosome pairs.

    When biologists say that only small mutations contribute to evolution, they are talking about mutations with a small effect on the phenotype. Admittedly, chromosome fusion has to occur in one step. However, the effect on the body of the mutant is sometimes very small. This balanced type of mutation is improbable on the time scale of decades, but is highly probable on the time scale of megayears.

    Most polyploidy hybrids are sterile. However, there is occasionally a mutation that changes the number of chromosomes without changing the genes in the total sum of chromosomes. This mutant can cross with an unchanged individual and have offspring that has nearly the same fecundity as the unchanged offspring. The phenotype in such an individual is almost unchanged. Such a mutation is rare but it still happens.

    This type of mutation is small in the sense of having a small effect. There is no intermediate step in any individual when a change in chromosome number occurs. However, there is an intermediate population that has individuals with differing chromosome number. Eventually, even a small difference in fecundity can split such a population into two or more varieties.

    Here is a simple article with pictures. It shows how parents with different chromosome numbers can have fertile offspring if the genes and centromeres of the parental chromosomes are the same.
    “This happens with a low frequency, too, and has been observed many times (hint: look up Robertsonian fusions on the web.) I think the key issue to understand here is that chromosome number changes are typically going to represent nothing but reorganizations of the genes — the same genes are just being moved around to different filing cabinets. This has some consequences, of course — you increase the chances of losing some important file folders in the process, and making it more difficult to sort out important information — but it’s not as drastic as some seem to think, and chromosome numbers can change dramatically with no obvious effect on the phenotype of the organism. These really are “small adaptations over time”, or more accurately, “small changes over time”, since there is no necessary presumption that these are adaptive at all.”
    Last edited: Oct 24, 2012
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