Creationists will make a distinction between microevolution and macroevolution, accepting that the former happens but denying that the latter happens. One example of this is the article What Is The Difference Between Macroevolution And Microevolution? by John D. Morris from the Institute for Creation Research. Microevolution is basically understood as evolutionary change within a species. One example of this would be the transformation of wolves into dogs and the breeding of various dog breeds. Despite the differences between wolves and various dog breeds, wolves and dogs all have 78 chromosomes, and they may mate together and have Wolfdog offspring. So they may still be regarded as a single species. Creationists can accept that this happens. But they have a problem with the idea that one species can evolve into another. This is called macroevolution.
I was wondering about the mechanics of this last night. I knew that humans have 46 chromosomes and our nearest cousins, chimpanzees, have 48. So I was considering the idea that humans could not have evolved unless two of our ancestors picked up the same mutation for 46 chromosomes and started mating. This scenario would seem very unlikely, and I was wondering if it might have happened differently. So I did some research. My research led me to an article by PZ Myers called Basics: How can chromosome numbers change?. What I learned from this article is that having different numbers of chromosomes does not always prevent mating. Chromosomes are made by separating genetic code into divisions. A chromosome can be thought of as equivalent to a paragraph.
You can increase the number of paragraphs in a piece of writing just by making more divisions in the text. I could have written the text in this paragraph as part of the previous paragraph. But I started a new paragraph to make a point. Breaking something up into more paragraphs does not change what is written. Likewise, splitting genetic material into multiple chromosomes does not change the genetic code. Chromosomes can also fuse together without losing genetic material. This works like running two paragraphs together instead of splitting them apart. That’s what I’m doing here. I could have started a new paragraph to explain this idea, but I’m explaining it in the same paragraph I illustrated fission in.
Chromosomes change in number through mutations that add or remove barriers between strings of genetic code. In computer code, we might distinguish strings with a separator character such as the newline character. In written text, we separate paragraphs by beginning a new paragraph on a new line, usually with blank space or an extra blank line before the beginning of the paragraph. So changing the number of paragraphs just requires inserting or deleting a few characters. In the same manner, changing the number of chromosomes requires very little in the way of mutation. A mutation just has to remove or add a barrier between pieces of genetic code. This rearranges the genetic code without increasing, deleting, or otherwise modifying it.
So, when a mutation changes the number of chromosomes in an animal, there is still a chance that its genetic code can line up with that of a potential mate with the usual number of chromosomes and produce offspring. PZ Myers does note that differences in chromosome number can decrease the chances of two animals successfully reproducing together. But in trying to combine together, genetic code from two different animals doesn’t always follow the strict rule of pairing up chromosome by chromosome. If the genetic code is otherwise roughly the same on both sides, the right genes may pair up despite differences in chromosome number.
Initially, changes in chromosome number might have no genetic advantage. Since they do decrease fertility with the main population, they tend to be selected against. In fact, there is some chance that the offspring of parents with different numbers of chromosomes will be infertile. This normally happens when a horse, which has 64 chromosomes, and a donkey, which has 62, mate and produce a mule. But it doesn’t have to happen all the time. And it is less likely to happen with close relatives than it is to happen with the horse and donkey, which are already separate species. Since differences in chromosome number do not totally prevent fertility, they can continue in the population despite selective pressures against them. This gives them the opportunity to spread in the population, such that eventually related animals who share the new number of chromosomes may mate together. And when they do mate together, they will be more fertile than they would be with members of the larger population. If other factors favor them, they may separate off from the larger population and grow in number. In time, they could grow different enough genetically that they could no longer breed with their cousins whose chromosome numbers remained the same.
This is basically what happened with the our ancestors. Some time in the past, two pairs of chromosomes in the 48-chromosome primates fused into a single pair, so that this primate had 46 instead of 48 chromosomes. This might have happened once, or it might have happened multiple times. Regardless of how many times it happened, this 46-chromosome mutation spread in the primate population, and circumstances then separated out a group of 46-chromosome primates from the others. They evolved down a different path, becoming humans, while their 48-chromosome cousins evolved into modern apes.
The main thing to take away from this is that the ability of a male and female to reproduce offspring together is affected by two things: chromosome number and genetic similarity. Back when our ancestors’ chromosome numbers changed, there was enough genetic similarity to allow mating with 48-chromosome relatives. Since that time, enough genetic differences have evolved to prevent humans and apes from mating, thereby splitting them off into separate species. So speciation happens through two kinds of mutations — mutations that reorganize genetic material and mutations that change it — and when enough of both kinds distinguish two separated populations, they can split into separate species. That’s one way macroevolution is able to happen.
But it should be noted that both are not required. Two separate species can share the same number of chromosomes. For example, Reeve’s Muntjac and the Sable Antelope both have 46 chromosomes without being humans. Likewise, potatoes, hares, and chimpanzees all have 48 chromosomes, but they are different species. So, sometimes genetic differences might be all that distinguishes two species from each other. These come about through the same kinds of mutations that creationists admit happen when they accept microevolution. One final thing is that species is not really an absolute, inviolable category. A species is generally a population of lifeforms capable of producing offspring together. But the boundaries between species are fuzzy rather than clear cut. At the boundaries, what we might regard as members of two different species can sometimes mate. This fuzziness at the boundaries between species allows the transition from one species to another. It’s never the case that one species suddenly has offspring of an entirely new species. Rather, it is that differences in separate populations eventually build up enough that reproduction between them becomes impossible or problematic. These populations may be separated by space (as the human and ape lines were) or by time (as modern humans are from our ancient ancestors).
One last thought here is that a species is analogous to a language. While members of the same species can reproduce together, members of the same language group can communicate with each other. There are languages that are distinct from each other, such as English and French, yet the boundaries between languages remain fuzzy, and over time a single language may change so much that it effectively becomes a new language, such as Old English changing to Modern English or Latin changing into Italian, Spanish, and French. For more on the evolution of language, I refer you to my earlier post called Linguistic Creationism in the Tower of Babel.