What makes it that if two species crossbreed, the baby is infertile? What makes it possible for the parents to mate in the first place?
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The first reason is chromosomal.
Your body has 2 copies of each chromosome (one from your mother and father). Chromosomes each contain an assortment of genes (protein recipes) and they sort of work like chapters in a cookbook. Mechanically it doesn’t really matter how the chapters are organized, it’s all the same information, but different species may sort things differently.
So a first-generation hybrid of close species is often fine, because they still have two copies of each “recipe” like a normal animal, just each side is organized a bit differently.
Cells that become eggs or sperm go through a process known as meiosis. One of the important steps in meiosis involves chromosomes (chapters) recognizing their partners and lining up together to be sorted.
If a hybrid species comes from two different parents of different species, this process can suddenly get very messy. Parent A’s genes are organized into chapters (chromosomes) 1-8, and Parent B’s genes may be all the same needed contents but instead labeled chapters 1-12. Your sperm-and-egg-making cells can no longer figure out which chapters are “partnered” with each other, so attempts to randomize new eggs/sperm fail.
The second reason is general health issues. There are any number of developmental disorders that can arise in hybrid animals. For example, ligers suffer from gigantism, which puts stress on the body. Even hybrid animals that are fairly healthy, might struggle to be seen as attractive mates by females of either species.
It should, however, be noted that not all inter-species hybrids are sterile. Speciesism is a broad spectrum rather than an absolute; it is up to humans to decide where one classified species ends and another begins. Seagulls and ducks will hybridize with closely related species quite happily, and the offspring can be fertile. And when scientists start looking at non-animals, like plants, fungi, or bacteria, the conversation changes quite a lot as the idea of “mating” stops being a constant.
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