What is a Species?
What is a species? At first, it seems obvious. The difference between a lion and a tiger is clear. However, when you explore the concept, it gets a lot blurrier. And along the way, we can learn a lot about evolution.
The most common descriptor of a species is the pool of individuals that can create fertile offspring and have some common characteristics. When populations of animals evolve apart for a number of reasons including geography and changing environmental circumstances, they often become unable to effectively breed with those other parts of what was once the same species, separating into different species. Over time, this creates many kinds of life that are distinct from each other.
However, this doesn’t apply to all life. To start, natural hybridization that produces fertile offspring can occur. While many physical variations within a species are waved away because all the species’ organisms can reproduce together, this does not always work. You probably wouldn’t say that a Neanderthal and a human are the same species.
There are also other outliers. For example, sometimes groups of animals evolve into ring species, where the organisms occupy a roughly ring-shaped region and each variation can hybridize with its neighbors except for the two groups on the ends.
Some organisms, like bacteria, reproduce asexually. In these cases, species type is usually defined by other observations like genetic similarity.
Similarly, fossils are typically organized by morphology (the shape/characteristics of the organism). This gets especially interesting since some extinct species that humans discovered fossils of don’t have a full fossilized animal that could serve as a type specimen. In this situation, we know the species existed, but we don’t have a complete example to define the species. Another way species classification is harder to do over time is that since species evolve slowly, there is no first organism of one species that separates it from the one it evolved from. From this perspective, species are snapshots of moments meant to convey the changes life takes over time.
While it may seem counterintuitive, this is reflective of how evolution really works. Evolution is an extremely gradual process, where random mutations and natural selection slowly change species over many generations. Meanwhile, species is a category humans made up.
Still, species is a useful term scientifically if not always an entirely correct one. The largest available gene pool criteria is helpful. While evolution can occasionally be helped along by hybridization, analyzing it through species can still be mostly accurate if it describes what is usually the largest available gene pool. Species differentiation can also help with conservation, archaeology, and many other subjects of research.
Ultimately, these nuances and attempts to set rules help biology be an interesting endeavor. Life is not always clear-cut, but that makes it beautiful.
Sources
https://www.britannica.com/science/species-taxon
https://en.wikipedia.org/wiki/Species
https://evolution.berkeley.edu/evolibrary/article/0_0_0/evo_40