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Evolution is most usually defined as a change in a population of a species over time through the accumulation of adaptations that are acted upon by natural selection. That may be a mouth full and nearly impossible to truly understand if there is not a full grasp on what a species actually is or how one changes over time. Sure, things change, but what makes them change? How does that affect other species? How long does it all take? Here we will shed some light on these questions and others like them about how evolution and speciation works.
Definition of "Species"
Perhaps the most important thing to be understood before truly grasping the idea of speciation and evolution is correctly defining the word species. Most books and reference materials will define the word species as a group of individual organisms that can interbreed in nature and produce viable offspring. While this definition is a good starting place, let's examine why it may not be quite as accurate as it should be.
First of all, there are many species out there that are asexual. This means there is no real "interbreeding" happening within those species. Any unicellular organism would be asexual. Some other types of fungi also produce their own spores for asexual reproduction. Some plants can also self-pollinate meaning they also do not interbreed.
Do these species undergo speciation and ultimately evolution? The short answer to this question is yes, they do. However, while evolution is usually driven by natural selection, natural selection cannot work on a gene pool that does not have any variation. Offspring of an asexual organism are essentially clones and have no traits that are different within the entire population. However, some changes at the microevolutionary level may occur. Spontaneous DNA mutations are one way new genes can enter the picture and natural selection then has diversity to work on within that species. Eventually, those mutations and adaptations add up if they are favorable and the species changes.
Another problem with the basic definition of a species is the existence of what are known as hybrids. Hybrids are offspring of two different species, like how mating a horse with a donkey gives a mule. Some hybrids are sterile, which is sort of taken care of with the "viable offspring" part of the original species definition. However, many other hybrids are capable of producing their own offspring. This is especially true in plants.
Biologists do not agree on a single definition of the term species. Depending on the context, the word species may be defined in more than a dozen different ways. Scientists often choose a definition that fits their needs or combines several to take care of that problem. For the majority of evolution biologists, the general definition above usually suits their purposes, although alternate definitions may be used to explain various parts of the Theory of Evolution.
Definition of "Speciation"
Now that a basic definition of "species" has been decided, it is possible to define the term speciation. Much like a family tree, the tree of life has several branches that show where species change and become new species. The point on the tree where a species change is called speciation. Using the definition of "species" above, it is when the new organisms can no longer interbreed with the original organisms in nature and produce viable offspring. At that point, they are now a new species and speciation has occurred.
On a phylogenetic tree, speciation is the point on the tree where the branches diverge from one another. The farther back on the tree the branches diverge, the less closely they are related to one another. Points, where the branches are closer together, means those species recently diverged from each other.
How Does Speciation Occur?
Most of the time, speciation occurs through divergent evolution. Divergent evolution is when a species becomes less similar and changes into new species. The original species that branches off is then known as the most recent common ancestor of the new species. That is the process that causes speciation, but what triggers divergent evolution?
Charles Darwin described the mechanism of evolution which he called natural selection. The basic idea behind natural selection is that species undergo changes and accumulate adaptations that are favorable for their environments. After enough adaptations have built up, the species is no longer the same as it was and speciation has occurred.
Where do these changes come from? Microevolution is the changing of the species on a molecular level like with DNA mutations. If they are significant mutations, they will cause adaptations that may or may not be favorable for their environment. Natural selection will work on these individuals and the ones with the most favorable adaptations survive to create the new species.
Changes in species can also happen on a larger scale. Macroevolution examines those changes. One of the most common causes of speciation is called geographic isolation. This is when a population of a species is separated from the original population and over time, the two populations accumulate different adaptations and undergo speciation. If they were brought back together after the speciation has happened, they will no longer be able to interbreed and are therefore not the same species anymore.
Sometimes speciation happens because of reproductive isolation. Unlike geographic isolation, the population is still together in the same area, but something causes some of the individuals to no longer be able to mate and produce offspring with the original species. This could be something along the lines of a change in mating season or a different mating ritual. In some cases, males and females of the species have special colors or distinct markings. If these mating indicators were to change, the original species may no longer recognize the new individuals as potential mates.
There are four types of speciation. Allopatric speciation and peripatric speciation are caused by geographic isolation. Parapatric speciation and sympatic speciation are the other two types and are generally due to reproductive isolation.
How Speciation Affects Other Species
Speciation of one species can affect the evolution of other species if they have a close relationship in an ecosystem. When populations of different species come together to form a community, they often depend on each other in some way for survival or to make life easier. This is especially apparent in food webs and food chains and in particular predator and prey relationships. If one of these species were to change, other species may also need to change.
An example of this coevolution or cospeciation could be the speed of a prey species. The prey may accumulate adaptations that create larger leg muscles to help them run faster. If the predator does not adapt, it may starve. Therefore, only faster predators, or perhaps stealthier predators, will survive to pass down their favorable adaptations to their offspring. That means since the prey evolved or became a new species, the predator had to also evolve or change.