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ConvinceMePlease

Integer Change in Number of Chromosomes?

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Hello guys.

 

I have watched few videos in Youtube explaining how evolution works. What I understand so far is that DNA gradually change across generations due to:

1-) Genetic mutation (i.e. imperfect copying of parent DNA?)

2-) Recombination if the creature consist of 2 parents. (i.e. because recombining DNA can result in new traits?)

 

Then they stated that: due to these small changes, if you give it a long time, these changes will add up, eventually changing the creature.

 

It does make sense to me so far. However, There are 2 things that I could not find explanation for:

 

1-) If creatures Had one origin, does evolution explain the integer differences in number of chromosomes? i.e. how did the ancestors of horses and donkeys resulted in a 64-chromosomes horse and 62 chromosomes donkeys? The reason I am asking this is because this is not a small change. This is an integer meaning that it is like a step, or threshold that occurs suddenly. It is not gradual because it is an integer.

 

 

2-) If The original creatures had only one gender, how did the first separation between male and female occur? i.e. lets say there has been a mutation making a certain creature a male that cannot produce by itself; if all the creatures around it are genderless and can reproduce by themselves then how would that male reproduce?

 

Similarly, if a certain creature became a female due to mutation, then how would it be impregnated by other gender-less creatures? If they can reproduce by themselves then they would not need to reproduce with the female.

 

I feel this question is a bit easier to tackle. If I remember correctly from school days, do some creatures reproduce from both one parent and 2 parents? I think I have came accross some kind of bacteria or other creatures that can reproduce both from one parent and 2. However, I want to hear your thoughts regarding this.

 

I hope this is the correct forum to ask. Please feel free to point out any inaccurate or incorrect information.

 

Edit: while I am at it, please do link any other topics that tackle evolution and how it works. I have just came in and I will be lurking around this forum for a while and see if there is anything intresting in regards to evolution.

 

Yours faithfully,

ConvinceMePlease

Edited by ConvinceMePlease

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You're right, mutation is the way DNA changes. But as far as I know recombination is not a major cause of change - it mostly just rearranges what is already there.

 

1. Mutations are not restricted to small changes in base pairs. Sometimes much larger copying errors occur. For example, longer DNA sequences may become deleted or replicated. And the number of chromosomes can also change through mutation. For example, gametes (reproductive cells) normally have half the number of chromosomes compared to normal cells (chromosome are in pairs and they normally separate out in gamete production), but sometimes gametes are produced with the normal number of chromosomes. If two such gametes combine to produce a new individual it will have double the number of chromosomes as the parents.

 

2. I don't know how sexual reproduction first evolved or if it is known, but I suspect it didn't initially involve males and females. It was probably involved 2 similar cells coming together to exchange DNA, and gradually became more complex with a differentiation between cell types.

 

There are others here more expert than me who might correct me, but I think I've got it about right.

 

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A few things that are important:

 

1. While a chromosome mismatch is generally a hinderance to fertility, it is not a guarantee of fertility. It is possible for a mutation to break apart a chromosome. Because of the way DNA combines, this individual may have fertility issues when reproducing with an organism that has a normal chromosome count, but successful reproduction is still possible as they will have all of the same genes capable of matching up with each other, just with a slightly different division.

 

How much this affects fertility is highly dependent upon the specific circumstances, but it is possible for the variant to then spread through the population by chance. Once a high enough proportion of the population has the abnormal chromosome count, they will wind up crossing with each other, and have fewer fertility issues when breeding with an individual with the same abnormal chromosome count. And since they will have an easier time reproducing with one another, and those individuals with the original chromosome number would have an easier time reproducing amongst themselves as well, you've set up a probably future speciation event between those with the original chromosome count and those with the new one.

 

 

2. Yes, you can see what are likely the early stages of sexual development in, for instance, earthworms, which are hermaphroditic and perform the roles of either sex when exchanging genetic material. From there, you can trace out a fairly straightforward hypothetical where some traits make individuals more effective in one role or the other and you wind up creating a sort of sexual dipmorphism that ultimately results in obligate male and female members of the species.

 

 

A lot of the biggest stumbling blocks in conceptualizing processes that seem like big step changes is failing to figure out how they actually can be broken down into smaller intermediate steps instead of the great big leaps it looks like they are at first glance.

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You're right, mutation is the way DNA changes. But as far as I know recombination is not a major cause of change - it mostly just rearranges what is already there.

 

1. Mutations are not restricted to small changes in base pairs. Sometimes much larger copying errors occur. For example, longer DNA sequences may become deleted or replicated. And the number of chromosomes can also change through mutation. For example, gametes (reproductive cells) normally have half the number of chromosomes compared to normal cells (chromosome are in pairs and they normally separate out in gamete production), but sometimes gametes are produced with the normal number of chromosomes. If two such gametes combine to produce a new individual it will have double the number of chromosomes as the parents.

 

2. I don't know how sexual reproduction first evolved or if it is known, but I suspect it didn't initially involve males and females. It was probably involved 2 similar cells coming together to exchange DNA, and gradually became more complex with a differentiation between cell types.

 

There are others here more expert than me who might correct me, but I think I've got it about right.

 

 

1.1 I accept that it may be possible for number of chromosomes change in one offspring of the creatures. However, if the creature consisted of 2 separate genders, what are the chances that this new creature will find a nearby other creature with the same number of chromosome changes? As far as I know, it is usually 1 more chromosome, resulting an ODD number of chromosomes. What are the possibilities of such creatures reproducing? I heard this has been experimented before: that two humans of 1 less chromosomes married and bigot a child, but the result was a disaster. (though I cannot completely validate this as I heard it from one of my friends in school days)
1.2 is it possible for the number of chromosomes to increase or decrease by 2? (i.e. from monkeys to humans or vice versa)
1.3 First time I hear that gemets could double *____*
anyway, assuming this is correct, the same question rises 1.1: what are the chances that 2 creatures on the same environment have this exact mutation simultaneously and then married. It can be done experimentally, but it feels impossible to be nature on the first glance. One way I imagine could make this possible is if the creature frequently produce offsprings with such mutations.
Please note that all of the above applies only to creature that can only reproduce in pairs, i.e. 2 genders.

 

 

A few things that are important:

 

1. While a chromosome mismatch is generally a hinderance to fertility, it is not a guarantee of fertility. It is possible for a mutation to break apart a chromosome. Because of the way DNA combines, this individual may have fertility issues when reproducing with an organism that has a normal chromosome count, but successful reproduction is still possible as they will have all of the same genes capable of matching up with each other, just with a slightly different division.

 

How much this affects fertility is highly dependent upon the specific circumstances, but it is possible for the variant to then spread through the population by chance. Once a high enough proportion of the population has the abnormal chromosome count, they will wind up crossing with each other, and have fewer fertility issues when breeding with an individual with the same abnormal chromosome count. And since they will have an easier time reproducing with one another, and those individuals with the original chromosome number would have an easier time reproducing amongst themselves as well, you've set up a probably future speciation event between those with the original chromosome count and those with the new one.

 

 

2. Yes, you can see what are likely the early stages of sexual development in, for instance, earthworms, which are hermaphroditic and perform the roles of either sex when exchanging genetic material. From there, you can trace out a fairly straightforward hypothetical where some traits make individuals more effective in one role or the other and you wind up creating a sort of sexual dipmorphism that ultimately results in obligate male and female members of the species.

 

 

A lot of the biggest stumbling blocks in conceptualizing processes that seem like big step changes is failing to figure out how they actually can be broken down into smaller intermediate steps instead of the great big leaps it looks like they are at first glance.

 

1.1 I believe you meant to say " it is not a guarantee of infertility" instead?

 

1.2 I believe the issue is the start. once abnormality spreads, then it is a straight forward issue.

 

1.3 I have read in wiki that one example of this is mule. mules usually are not able to reproduce. However, it claimed that one female mule have reproduced from donkey. Is this true? If yes, then how many chromosomes did the off spring have? is it capable of reproducing as well?

 

2.1 hmmm, but why would this no longer be the case in humans? most humans today only have one gender organs. There are rare cases in which a born baby can have organs for both male and female. However, I am not sure if both organs can be fertile and capable of reproduction: are there humans who are able to do both female and male rule in reproduction?

 

2.2 It does make sense that off springs will be specialized just like their parents are (i.e. either more towards male or female). However, this does not mean that the creature will be separated as it has same number of chromosomes, and this specialized can die out.

 

2.3 what happens if more male married with a more female? which sex would the children be more like?

 

2.4 I do not see why the two genders would separate in normal circumstances. The only explanation I see is that it could be possible due to social behaviour. for example, I think it is clear a seemingly man human would want to be pregnant even if he could be.

Edited by ConvinceMePlease

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1. Those chromosome numbers for horse and donkey of 64 and 62 are diploid numbers meaning they received 1/2 from each parent. The hybrid mule receives 32 from its horse parent and 31 from its donkey parent totalling 63.

 

The completely hypothetical evolution of donkeys into horses may be explained in only one mutation event. Imagine one chromosome of a donkey gamete split in 2. The offspring would have 31+32=63 chromosomes. If this offspring was still fertile (this isn't a mule yet) it may supply 31 or 32 chromosomes. But imagine it passed on 32 to a few children, and some of those children passed on 32 to some of their children. Then some of those distant relatives bred and each passed on 32 to form 64. If this new combination was also fertile and fit (more fit than the 63s and even normal 62 donkeys), it would begin to increase in frequency. The breeding population that it belonged to would over time be dominated by 64s to the point that 62s and 63s would be extremely rare. Over enough time to breed on their own, the 64s depart even further from 62s until when they did come in contact again they may no longer be compatible.

 

Ring species are an example of this. There is a salamander in California that lives in the mountains. The mountains form a ring there where there is a coastal range and the inland Sierras. The salamanders have a color gradient along this ring where in one direction they are more uniform reddish, and the other they are darker and blochey. The salamanders are reproductive all along this gradient except for one region. In this one region, the extreme red variety is in contact with the extreme dark blotchey variety and they are not reproductive. It seems to be that the salamander was originally isolated to one area (opposite this ) and radiated outward in each direction. As they migrated in each direction, there were different selective pressures for their color, such as need to hide from predators. Over small distances, these changes are small, but the species radiated out around the mountain ranges until finally the 2 distinct radiations came in contact ( many, many generations later ) and were no longer reproductive. The classification as separate species is very sketchy in this instance because of this gradient. But if there was a natural event that severed contact of populations on this ring, two groups would become fully isolated from one another (the existing cohabitation region without reproduction is the 2nd boundary). Over time these two groups may be completely unique even if the severed region was reconnected. In that case, it would be clear that there are now 2 unique species.

 

2. The "original" creature does not seem to exist in those set boundary terms that you have described. Yes technically there must be an original mutant but this mutant just must continue to live and breed with the existing population. It may be virtually indistinguishable. But if the mutation is lucky or favorable it will increase in frequency and eventually could even be a requirement.

 

Regarding the separation of sexes, there are numerous "stable solutions" to reproduction. They are just that; solutions to a "problem". That is, how to not go extinct. When it comes down to it, that's all that matters. Once you go extinct, it's game over. But before then, whatever works is fine. So populations found numerous solutions to this problem. There is sexual reproduction (separate organisms or selfing hermaphrodites), and a multitude of other mechanisms such as parthenogenesis, fission, sporogenesis, budding. Some organisms seem to be very flexible with this and the number of chromosomes. Quite a few plants have a lot of chromosomes, probably through some kinky sex stuff in their past. Any way, with evolution it important to try not to fall in traps of believing there must be a reason for something. If you flipped a coin and placed a wager on the outcome, technically there was a reason for the outcome (you flipped the coin with this force starting in this position and measured it at this point) but this reason is completely detached in meaning from the outcome that you experienced. So evolution is best thought of as a loose collection of mechanisms with only one punctuation: extinction.

 

Word.

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