Expert: Florence M Rollwagen - 6/17/2008

Question
QUESTION: At A level i was led to believe that a defining point of a species was the ability to have fertile young, and 2 organisms of different species could not do this. however the Scottish wildcat for example, Felis silvestris, and the domestic cat, felis catus, can have fertile "hybrid" young, despite being different species. What relationship does there need to be between two animals or species, or how recent the common ancestor, for viable offspring to be produced?

ANSWER: Hi James:  Thanks for your question.

When we define a species, one of the considerations is whether or not they can produce fertile offspring under normal conditions.  The key phrase here is "under normal conditions".  Your example, the Scottish wildcat does not "normally" breed with the domestic cat (believed to have been domesticated in the Middle East) since they originally lived in different parts of the world.  Nowadays, with jet travel, some animals are put together in ways that wouldn't have happened "normally"

That's the simple answer.  Naturally, there are complexities.

There are pre-zygotic and post-zygotic barriers to interspecies mating, and that's what I think you are referring to.

As an example:

In pre-zygotic barriers, there are mechanisms that prevent a zygote from being formed, such as: Ecological isolation – physically separated,  Behavioral isolation – mating calls,  Mechanical isolation – physically unable,  Gametic isolation – gametes do not fuse.

In post-zygotic barriers, the mechanisms prevent or impair the offspring, such as: zygote death – offspring is never born, hybrid performance – not as fit as parents, or sterile.

Sometimes species cannot interbreed because they have different numbers of chromosomes.  This is true when a horse and a donkey mate, the offspring (mule or hinny) is usually sterile.

So the long answer to your question is "it depends".  On which particular pairs we are talking about, what are the pre- and post-zygotic barriers, and what other unknown variables that may exist.

I hope this answer has helped you.  Please write back if you have more questions.

FM Rollwagen, PhD



---------- FOLLOW-UP ----------

QUESTION: Thanks very much, that was very helpful. one question i do now have is if two different species are able to interbreed and produced fertile offspring. How would the two species be defined as different species, when they are morphologically very similar, and share many traits. Also if the hybrid young of two populations could start a population of there own, would they be considered a species in their own right.

Thanks in advance.
James

Answer
Hi again, James:

First you have to consider the nature of hybrids.  The software of this site doesn't let me put tables into the text, but here's a website that will help.  http://users.adelphia.net/~lubehawk/BioHELP!/psquare.htm

When considering the nature of hybrids it's important to remember that every gene is inherited as an allele (version) of the gene.  For example, eye color can have brown alleles or blue alleles.  So each of your hypothetical hybrids would inherit one allele from each parental species.

To go back to your cat example, each offspring will inherit a version of each gene unique to either the Scottish wildcat (sc) or the domestic cat (dc).

So let's say a gene for fur length is homozygous in each parent (that is, each parent has only it's own species specific genes) so the Scottish fur length will be F(sc)F(sc).  The domestic cat will be F(dc)F(dc).  Now during reduction division (meiosis) the chromosome numbers are halved so that each gamete (sperm or egg) receives only half the number of chromosomes.  Each parent has only one choice F(sc) or F(dc).  So all of the offspring will be F(sc)F(dc), having inherited one allele from each parent.

In the second generation is where the fun begins.  The possibilities are F(dc) or F(sc) alleles becoming gametes.  So the offspring have a chance of being F(dc)F(dc), F(sc)F(sc), F(dc)F(sc).  

So for fur length (at least) there is a chance that some of the offspring wil "revert" to the parental type.  Only after many, many, many generations will a gene become "fixed" in the population, so that revertants will not occur (but they still do).

This is a simplified explanation for how inheritance works in outbred populations.  It's really much more complex, given how many genes there are, how many variations of each gene exist in each species, and the possible combinatorial situations that will arise.

One of my students found a nice book called "Genetics for Dummies" which you might look into.  You can also search the internet for genetics lectures and problems.

Hope this helps1

FMR