# How does evolution explain differing numbers of chromosomes? **URL:** \ **Category:** Factual Questions\ **Created:** [May 29, 2010, 4:04am UTC](https://boards.straightdope.com/t/how-does-evolution-explain-differing-numbers-of-chromosomes/541145 "2010-05-29T04:04:33Z")\ **Posts on this page:** 9\ **Page:** 1
**Author:** ![jebert](https://avatars.discourse-cdn.com/v4/letter/j/ec9cab/32.png) [@jebert](https://boards.straightdope.com/u/jebert)\ **Post date:** [May 29, 2010, 4:04am UTC](https://boards.straightdope.com/t/how-does-evolution-explain-differing-numbers-of-chromosomes/541145/1 "2010-05-29T04:04:33Z")
My limited understanding of evolution is based on the notion of genetic mutations where those mutations which confer some survival advantage are naturally selected over time. But species have different numbers of chromosomes. A frequently cited example is that humans have 46 and chimpanzees have 48. To me, a change in the number of chromosomes is hugely different than a change in an individual gene. How does the theory of evolution explain the descent of a species with a different chromosome number than its ancestors? ---
**Author:** ![engineer\_comp\_geek](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/engineer_comp_geek/32/504_2.png) [@engineer\_comp\_geek](https://boards.straightdope.com/u/engineer_comp_geek)\ **Post date:** [May 29, 2010, 4:11am UTC](https://boards.straightdope.com/t/how-does-evolution-explain-differing-numbers-of-chromosomes/541145/2 "2010-05-29T04:11:10Z")
Basically, two chromosome pairs didn’t copy correctly and ended up fusing into one. > [@](#): > > Humans have 23 pairs of chromosomes and other great apes have 24 pairs of chromosomes. In the human evolutionary lineage, two ancestral ape chromosomes fused at their telomeres producing human chromosome 2.[1] There are nine other major chromosomal differences between chimpanzees and humans: chromosome segment inversions on human chromosomes 1, 4, 5, 9, 12, 15, 16, 17, and 18. From here: > **[Chimpanzee genome project](https://en.wikipedia.org/wiki/Chimpanzee_genome_project)** > > The Chimpanzee Genome Project was an effort to determine the DNA sequence of the chimpanzee genome. Sequencing began in 2005 and by 2013 twenty-four individual chimpanzees had been sequenced. This project was folded into the Great Ape Genome Project. > In 2013 high resolution sequences were published from each of the four recognized chimpanzee subspecies: Central chimpanzee, Pan troglodytes troglodytes, 10 sequences; Western chimpanzee, Pan troglodytes verus, 6 sequences; Nigeria-Cameroon chimpan... Animals with different numbers of chromosomes can sometimes reproduce and produce fertile offspring, if the remaining chromosomes are similar enough. You aren’t going to get something with 12 chromosome pairs breeding with something that has 32, but 23 and 24 are pretty close. ETA: This article has more details, and even has a nice pic showing the evidence that two fused into one. > **[Chromosome 2](https://en.wikipedia.org/wiki/Chromosome_2_(human))** > > Chromosome 2 is one of the twenty-three pairs of chromosomes in humans. People normally have two copies of this chromosome. Chromosome 2 is the second-largest human chromosome, spanning more than 242 million base pairs and representing almost eight percent of the total DNA in human cells. > Chromosome 2 contains the HOXD homeobox gene cluster. > Humans have only twenty-three pairs of chromosomes, while all other extant members of Hominidae have twenty-four pairs. It is believed that Neanderthals ... ---
**Author:** ![Triskadecamus](https://avatars.discourse-cdn.com/v4/letter/t/b19c9b/32.png) [@Triskadecamus](https://boards.straightdope.com/u/Triskadecamus)\ **Post date:** [May 29, 2010, 4:14am UTC](https://boards.straightdope.com/t/how-does-evolution-explain-differing-numbers-of-chromosomes/541145/3 "2010-05-29T04:14:46Z")
In the specific example cited, Humans and Chimps, oddly enough there are two chromosomes in humans which are quite similar to one in Chimps, but seem to be joined into a single one, that one having a region in the middle very similar to the termination sequences of the Chimp chromosomes. Species gain and loose whole chromosomes by individual members experiencing genetic accidents, like non-disjunctions, or breaks. Mostly those individuals are never born. Of those that are born the overwhelming majority die very young. Of that tiny surviving percentage, almost all fail to have offspring. That extraordinarily small number of individual members pass on their abnormal genes, and almost all of their progeny also die. Among the survivors, many do not pass on the abnormal genetic information. Another extremely small percentage of such cases actually have multi-generational descendants who have a small chance of finding a niche in the habitat they live in that allows them to survive. Wait a thousand years, and in most cases, the particular resultant mutation is less common in that area, and if it is isolated from members of the original species, it has begun the long process of becoming a separate species. It will probably die out sooner or later, most species do. Tris ---
**Author:** ![Blake](https://avatars.discourse-cdn.com/v4/letter/b/a9a28c/32.png) [@Blake](https://boards.straightdope.com/u/Blake)\ **Post date:** [May 29, 2010, 5:35am UTC](https://boards.straightdope.com/t/how-does-evolution-explain-differing-numbers-of-chromosomes/541145/4 "2010-05-29T05:35:28Z")
> [@engineer\_comp\_geek](#): > > Animals with different numbers of chromosomes can sometimes reproduce and produce fertile offspring, if the remaining chromosomes are similar enough. You aren’t going to get something with 12 chromosome pairs breeding with something that has 32… That’s not really true. There are numerous triploid or tetraploid animals that retain fertility with the diploid form. In plants the case is even more extreme, and organisms with anything up to 16 copies of each chromosome retain fertility. Chromosome count doesn’t actually appear to be a major barrier to fertility provided that the chromosomes all retain some homology in both partners. ---
**Author:** ![Colibri](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/colibri/32/1841_2.png) [@Colibri](https://boards.straightdope.com/u/Colibri)\ **Post date:** [May 29, 2010, 2:01pm UTC](https://boards.straightdope.com/t/how-does-evolution-explain-differing-numbers-of-chromosomes/541145/5 "2010-05-29T14:01:01Z")
As to the question of _why_ differences in chromosome number occur, the activity and action of genes can vary depending on where they are on a chromosome, and which other genes they are near. So that once a chromosomal division or fusion occurs, it can be selected for or against just like other genetic variations. ---
**Author:** ![Buck\_Godot](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/buck_godot/32/6573_2.png) [@Buck\_Godot](https://boards.straightdope.com/u/Buck_Godot)\ **Post date:** [May 29, 2010, 6:16pm UTC](https://boards.straightdope.com/t/how-does-evolution-explain-differing-numbers-of-chromosomes/541145/6 "2010-05-29T18:16:36Z")
> [@Triskadecamus](#): > > In the specific example cited, Humans and Chimps, oddly enough there are two chromosomes in humans which are quite similar to one in Chimps, but seem to be joined into a single one, that one having a region in the middle very similar to the termination sequences of the Chimp chromosomes. > > Species gain and loose whole chromosomes by individual members experiencing genetic accidents, like non-disjunctions, or breaks. Mostly those individuals are never born. Of those that are born the overwhelming majority die very young. Of that tiny surviving percentage, almost all fail to have offspring. That extraordinarily small number of individual members pass on their abnormal genes, and almost all of their progeny also die. Among the survivors, many do not pass on the abnormal genetic information. Another extremely small percentage of such cases actually have multi-generational descendants who have a small chance of finding a niche in the habitat they live in that allows them to survive. Wait a thousand years, and in most cases, the particular resultant mutation is less common in that area, and if it is isolated from members of the original species, it has begun the long process of becoming a separate species. It will probably die out sooner or later, most species do. > > Tris Even though he’s right, I hereby nominate Triskadecamus for the Eeyore memorial buzzkill of the year award. ---
**Author:** ![bup](https://avatars.discourse-cdn.com/v4/letter/b/6bbea6/32.png) [@bup](https://boards.straightdope.com/u/bup)\ **Post date:** [December 29, 2012, 4:04pm UTC](https://boards.straightdope.com/t/how-does-evolution-explain-differing-numbers-of-chromosomes/541145/7 "2012-12-29T16:04:03Z")
I was about to start a new thread, but searched first, and this is pretty close. However, to broaden the question: Is it common for speciation to involve differing numbers of chromosomes? It doesn’t seem like it would be. What about increasing number of chromosomes? Two chromosomes fusing kind of makes sense to me, and I could see how it would happen even if only one parent had the lesser number of chromosomes (with one twice as big). How would it go in the other direction? How could one parent with a chromosome that ‘broke in two’ produce offspring like that? ---
**Author:** ![Der\_Trihs](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/der_trihs/32/233_2.png) [@Der\_Trihs](https://boards.straightdope.com/u/Der_Trihs)\ **Post date:** [December 29, 2012, 4:07pm UTC](https://boards.straightdope.com/t/how-does-evolution-explain-differing-numbers-of-chromosomes/541145/8 "2012-12-29T16:07:25Z")
> [@bup](#): > > What about increasing number of chromosomes? Sometimes an error causes the production of an extra pair of chromosomes. Usually the effect is just a genetic illness, but sometimes it presumably helps the organism and like any other successful mutation it begins to spread. ---
**Author:** ![Iggy](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/iggy/32/3364_2.png) [@Iggy](https://boards.straightdope.com/u/Iggy)\ **Post date:** [December 29, 2012, 4:51pm UTC](https://boards.straightdope.com/t/how-does-evolution-explain-differing-numbers-of-chromosomes/541145/9 "2012-12-29T16:51:05Z")
> [@bup](#): > > I was about to start a new thread, but searched first, and this is pretty close. > > However, to broaden the question: > > Is it common for speciation to involve differing numbers of chromosomes? It doesn’t seem like it would be. > > What about increasing number of chromosomes? Two chromosomes fusing kind of makes sense to me, and I could see how it would happen even if only one parent had the lesser number of chromosomes (with one twice as big). How would it go in the other direction? How could one parent with a chromosome that ‘broke in two’ produce offspring like that? **Column A** Usually if a chromosome breaks the fragment not attached to a centromere is lost. Sometimes that centromere-lacking fragment attaches itself to a completely different chromosome instead. **Column B** Sometimes non-disjunction results in an extra copy of a chromosome. Take **Column A** + **Column B** and _hypothetically_ a trisomic individual could have one of the extra chromosomes break, leaving a small fragment with little more than a centromere. Over generations that centromere fragment gets passed down and could accumulate translocations (per **Column A** process). It works on paper. I have not had the time to research to see if there is evolutionary evidence of this process.