Even if you understand the basics of evolution, the way in which new species evolve can remain a mystery. It’s easy to imagine how the length of a hummingbird’s beak increases through generations. It’s also possible to understand how, over millions of years, apes could turn into humans.
But how could this lead to a diversity of species? It is estimated that more than eight million species live on our planet. Some species are newer than others, so that means they must have “appeared” at some point, right?
What is a species
But first, to understand how new species form, we must understand what species are. In biology, a species is a group of organisms that can interbreed with each other and produce fertile offspring. If two animals can’t produce offspring (or their offspring are sterile), we call them separate species. For example, a male lion would never sire a litter with a female domestic cat. They are distinct species, even though both are cats.
Note that the organisms in question do not necessarily have to be animals. They can be plants or bacteria, for example. If two individuals can’t have offspring, they are distinct species. Complications arise when you look at species that reproduce asexually. However, I won’t go in details here and shall cover the topic in a separate article.
The main point is that a species is a group of organisms that mate exclusively within that group, but not with others. So, the question is: how can an organism give birth to an organism that does not mate with its conspecifics?
How new species evolve through isolation
If you are seeking a quick answer: new species evolve when two groups of one species get separated. Both groups continue to evolve independently from each other, and, if they remain separated for millions of years, they can become distinct species.
For a dramatic example, remember how South America and Africa drifted away from each other over the past 200 million years. It’s how the Atlantic Ocean came into existence and got wider as the years went by. I mean, as the millions of years went by.
The gap between the two continents got larger and larger, and fewer and fewer animals could get from one continent to the other. Land animals can’t cross it today. What happened to animals who lived on either side of the ocean? They continued to evolve, but independently on both sides.
There can be different types of barriers that separate populations and form new species:
- Geographic separation. I already mentioned this in the example above, but there are some notes to add. First, it does not have to be as dramatic as oceans or mountains forming. Quite often, a smaller group can get separated from the larger population. For example, a glacier can cover an entire continent and leave small, isolated refuges where life continues to thrive. A small group of individuals (or plant seeds) can cross an already existing barrier by mere chance and never find their way back. High biodiversity on islands is likely a result of the latter. Most islands are remote, and animals or seeds reach it on rare occasions. But when they do, they can’t go back; they either become established there or die out. Since other animals rarely come, they rapidly become distinct populations. If you are a biology geek, this type of speciation through geographic isolation goes by the name of allopatric speciation. It means when two groups of individuals become distinct because they are physically separated.
- Separation by large habitat. Let’s consider a species that is slow and occupies a massive area. Populations at each end of their habitat may have severely different environmental conditions. The more extensive the differences, the more rapidly the populations become different from each other. However, the groups are connected. Genes can flow from one side to the other, but it can take hundreds of generations for a novel trait to cross the habitat. The question is, do these genes “travel” faster or slower than the populations diverge? If they travel slower, individuals at either end of the habitat will become less and less similar to each other. Given enough time, they may become separate species.
- Separation by the time of breeding. Individuals in a population can differ in the time they prefer to mate; either some may mate in the mornings and others in the evening, or some may prefer to find a mate early in the season and some later. Those who prefer mating early in the season will mate with other individuals with the same preference. If the choice to mate later or earlier is inherited, the differences between the two groups will become more distinct with every generation.
- Genetic separation. Geographic and time barriers are not the only ones. Most species today can’t breed with each other because their genomes are incompatible with each other. For example, we have one fewer chromosome than our closest relatives. During the joining of sperm and ova, chromosomes of both parents must align next to each other. If part of the chromosomes has nothing to align with, the embryo will not form. Genetic isolation usually develops during times of geographic isolation. Once this happens, the geographic barrier may disappear, yet the populations will remain in reproductive isolation. On some occasions, genetic isolation can occur without a geographic one. Speciation without geographic isolation is called sympatric speciation. However, evidence for this actually happening is not easy to come by.
- Behavioral separation. Behavior dictates most animals’ actions and can act as a reproductive barrier even if animals live in the same environment. If an animal chooses not to mate with another animal, they are behaviorally separated. This occurs all over nature. For example, frogs and toads do not leave offspring, and not only because they are genetically incompatible. It’s also because they do not mate. No laws of physics stop them from having sex, yet, for behavioral reasons, toad females do not respond to frog male “songs.” Unfortunately, similar to genetic isolation, it isn’t entirely clear if behavioral isolation can arise without a geographic one.
The above illustrates the most common means of how new species evolve. I must emphasize that geographic isolation is far more common than others. However, it is possible that this just APPEARS to be the case, since evidence of genetic isolation is harder to obtain. Maybe our understanding will improve along with the rapid development of genetic methods.