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beezlebub33 t1_j4zurfq wrote

Your premise is incorrect. During reproduction, the chromosomes have to line up in order for them to produce offspring. The number of chromosomes is important but it's not clear exactly how difficult the chromosome number makes for reproduction, relative to other factors, and it depends on where and how the chromosome number changed. It is not the barrier to evolution that it is portrayed in anti-evolution literature.

People with Down's syndrome can and do reproduce and they have an extra chromosome. Consider Robertsonian translocations. which can reduce the number of chromosomes. See: https://en.wikipedia.org/wiki/Robertsonian_translocation

Of course, the most famous 'cross' is a male donkey and a female horse to produce a mule, which is sterile. However, there are a large number of equine species, and they have wildly different numbers of chromosomes. See: https://en.wikipedia.org/wiki/Equid_hybrid and https://en.wikipedia.org/wiki/Zebroid . Some of the crosses are fertile; for example, Przewalski's horse (66 chromosomes) and domestic horses (64 chromosomes) can and do produce fertile offspring.

Scientists can study the changes that have occurred in the number chromosomes, their shape (lengths of arms for example), banding patterns, etc. (this is called the karyotype of the organism) in related to help understand the evolutionary history of them. See, for example: https://pubmed.ncbi.nlm.nih.gov/23532666/ which focuses on equines. However, the same thing can be done for much more distant species. See this which reconstructs different chromosomes a wide diversity of animal: https://www.pnas.org/doi/10.1073/pnas.2209139119 Figure 2 in particular shows how the chromosomes line up, and what happened as they split, merged, grew, and shrank.

Summary: reproduction between individuals with different numbers of chromosomes can and does happen. The history of related (both near and far) animals provides evidence for what changes occurred in chromosome number (and shape, and banding, etc.).

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yerfukkinbaws t1_j514tql wrote

> During reproduction, the chromosomes have to line up in order for them to produce offspring.

Just to be clear, chromosomes do not line up during reproduction. They only pair during meiosis, which is the production of sperm or eggs, but not during fertilization or embryonic development (mitosis). So chromosome number has absolutely zero effect on whether two individuals can produce offspring together, but may affect the fertility of their offspring.

This confusion over when chromosome pairing happens leads to a lot of the misconceptions around chromosome number.

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CoolFreeze23 t1_j51rx41 wrote

I think your misunderstanding OP's full question. They're asking, if the reproduction is difficult and the offspring are most likely sterile, how did species come to have different chromosomes at all? If we all have a common ancestor when you go back far enough, that must mean a mutation happened that caused one of them to have a different number of chromosomes. Most of the comments are saying how the actual producing offspring isnt difficult, but the fertility of that offspring is rare. But then how could those mutations in the number of chromosomes have become persistent enough that the offspring of them were fertile and able to even pass that down themselves?

Summary: If a member of a species was born with an extra chromosome, or two chromosomes fused, their offspring have a high change of being sterile. How could the increase of decrease of a chromosome become wide spread in a species if that happens?

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AdeepinAmerica t1_j51xd67 wrote

People with 45 chromosomes (or to generalize to other species: individuals with mismatched chromosome counts as a result of evolutionarily recent chromosome fusion or splitting) do not generally have a high chance of being sterile. They may have a high chance of some kind of reduced fertility, though even that is not clear. There's a major detection bias here since almost no one ever gets karyotyped unless they believe they have fertility problems in the first place. This inflates estimates of how often these chromosome mismatches cause fertility problems.

The answer to the question of why any mutation that has any negative effect on fertility would spread is, as others have said, random success. In evolutionary terms, this is called genetic drift. Genetic drift is sometimes thought of as affecting neutral variation that doesn't have either positive or negative effects. However, it's been well understood from the beginning of genetic drift research that what really matters is the "strength" of genetic drift versus the "strength" of natural selection. Many things can make genetic drift stronger, like a small population or an expanding population or pops where some individuals reproduce more than others, etc. If enough of these drift exagerating factors are found in a population (as they often have to humans), then even variation with pretty strong negative effects can still spread. Beneficial mutations can also be lost in the same way.

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beezlebub33 t1_j52d84h wrote

>If a member of a species was born with an extra chromosome, or two chromosomes fused, their offspring have a high change of being sterile. How could the increase of decrease of a chromosome become wide spread in a species if that happens?

I think I understood the question and answered it. 1. The sterility of an offspring with an additional / fused chromosome isn't that high as shown by examples, it can be neutral; and 2. neutral mutations can become fixed.

The argument is quite similar to mutations in general. There is the general opinion that mutations are bad and overwhelmingly deleterious. They aren't. Most are neutral; the result is most people have mutations, often quite a few. Those mutations can become fixed simply because there are so many of them and they are not selected out. There are certainly bad mutations, which cause developmental or functional problems. They are sometimes really bad and really obvious, and people remember those. Sometimes they are good and increase selection.

Similarly, sometimes chromosomes fuse or split, and it doesn't make a difference. Sure, sometimes, in fact more often than not, they are bad and get selected out. But sometimes they are neutral, and sometimes the different number gets fixed. This is not unexpected.

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reedmore t1_j50ha2w wrote

I'm learning so much through this, thank you!

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