One of the lines of evidence from nature that supports an evolutionary origin for species is the fact that species often hybridize with each other and there are weak reproductive barriers between many species. Different species have different names and the most common way to define them is by reproductive isolation. Species are different because they don't reproduce with each other. However, some clearly distinct species can interbreed with each other.
For example, tigers and lions are considered different species, different types of big cats, but they can mate with each other to to produce offspring. A male tiger and a female lion can produce a tigon, while a female tiger and a male lion produce a liger. Tigons and ligers are not fertile themselves, but they can be healthy and viable. If there are such completely different species, why can they produce offspring with one another?
Different species can produce offspring because different species share a history. If you think about it, different individuals within a species are separated by a number of generations from each other, and they can still mate with each other. Different species are just separated by a larger number of generations and are often unable to meet with each other.
But where is the black and white line? At what point is the number of generations too large to allow reproduction? There isn't a hard and fast black and white line. Reproductive isolation arises gradually over time as groups of organisms diverge from one another.
Different species didn't form separately and independently, they became different gradually over time. Hybrids between different species are quite common. Mules are the result of a mating between a male donkey and a female horse.
The offspring of a female donkey and a male horse is called a henny, but they are not created as often as mules because they are not as useful. Mules exhibit the best of both parent species. They are healthier than horses and more trainable than donkeys. However, because the parent species have a different number of chromosomes, most, but not all, mules are sterile due to problems during meiosis. The different number of chromosomes causes fertility problems, but it doesn't cause physiological problems.
Plants can also be hybridized to create individuals with the best traits of each parent species. Wheat provides large grains suitable for harvesting, but is not as hardy and able to grow in poor soil as rye. The F2 generation of the cross between wheat and rye produces triticale, which has large grains and is able to grow in poor soil.
Hybrids between different species are not always artificial as in the previous two cases. They occur in nature too. In 2014, a female common black hawk mated with a male red-shouldered hawk.
These are two very different looking species, and the offspring exhibited phenotypes that were intermediate. Another phenomenon showing how the line between different species is not as black and white as we may think comes from ring species. These are cases in which some populations that appear somewhat distinct can hybridize, but not in all combinations. The most well-known example is the Larus genus, in which the gulls in Eastern Europe were reported to hybridize with gulls in Western Asia, who hybridize with gulls in Eastern Asia, who hybridize with gulls in North America, who hybridize with gulls in Western Europe.
However, the gulls in Western Europe and Eastern Europe don't appear to mate. More recent studies indicate that the true situation is more complex, but this only speaks to the lack of hard and fast barriers between species. A more unambiguous ring species situation in birds is seen in the greenish warbler, which has populations throughout Asia, encircling the Himalaya mountain range. They interbreed to the south, as shown by the orange to yellow to green coloration, but these populations that are reproductively connected to the south do not interbreed to the north where the red and blue regions are indicated. And it's not just birds that do this.
Many other species do too. An example is the salamander Ensatina escholtzi, which lives in California. This species has a set of subspecies that hybridize all around a central valley they don't inhabit, but not all of the subspecies can hybridize directly. Again, the details are complex, but this just illustrates the point that species are not discrete, separate units of life.
Species are populations that are related to one another, and differing degrees of that relatedness can influence their similarity and propensity to interbreed. A final example comes from a pair of macaque species that have been observed to hybridize often, 28 times, at a facility that houses them both. and two of those hybrids were they themselves fertile.
What makes this example so interesting is that these two species appear to have diverged from one another approximately 4.9 million years ago, which is almost identical to the divergence time estimated for humans and chimpanzees. Based on this, it would not be a surprise if humans and chimps were capable of producing a viable hybrid. To test this would be unethical, so we may never know whether humans and chimpanzees are as different as we have always assumed that these two species are. The world is full of various species that appear similar and distinct from one another, but the black and white lines that we think we see between species are not as absolute as we may think.
This is because every species on earth is the product of divergence from ancestor species. The time since that divergence is what contributes to those differences, but we should never lose sight of the fact that this history is the explanation for the diversity and similarity that we see. Feel free to like, subscribe, share, or some hybrid combination of these options.