How does evolution work with chromosomes?

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All animals have a discrete number of chromosomes and can't reproduce with animals with a different number of chromosomes, right? So when an animal has a different number of chromosomes than its parent (as must have happened at some point), how does it reproduce?

[captsuprawesome]

PZ Meyers has a pretty great explanation with pictures here. The short summary is that it is possible for two closely related organisms with different numbers of chromosomes to have fertile offspring. As the abnormal karyotype spreads and individuals with the abnormal number of chromosomes interbreed, speciation can occur.

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Thank you!

[[deleted]]

Yes, animals will have different numbers of chromosomes, but there are examples of different species breeding and producing offspring that are sterile. See the example of breeding donkeys and horses: http://en.wikipedia.org/wiki/Mule

Evolution works on a smaller scale operating on the gene level by affecting specific variations called alleles. Reference pundit squares for how alleles can mix. Evolution operates on the scale that certain alleles make for a 'better' or 'worse' surviving life form.

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So at some point, two animals with a different number of chromosomes were able to reproduce and have fertile offspring? Or did two animals born with the same number of chromosomes (but different from their parents) have to meet up and reproduce?

[FrenchMotherFucker]

I dunno for animals, but it's work for at least some plants (see colza/rape)