Please forgive my hiatus, in the past few weeks I had both a midterm exam and a major project due at once.
In our last discussion about flu, I briefly touched on the evolution of flu strains via mutations. As you may recall we discussed how a mutation can create variations that can change a flu from one subtype to another. Deciding when a mutated form of, say, H1 stops being just another H1 variant and starts becoming a completely different subtype of H is more complicated than it sounds. My research on the Flu post reminded me of material I learned in my undergraduate ecology class that I personally found fascinating-the process of evolution. In spite of its importance to all of the life sciences, from the microscopic like viruses to the mighty like elephants and humans, evolution is often not well understood by the public. So, I figured talking about speciation, the process by which new species emerge, would be an interesting post and provide a good way to visualize the mechanism of evolution by natural selection. Please note that while speciation does play out with viruses and bacteria, I will focus this explanation on speciation in animals and plants. Because viruses and bacteria evolve so quickly and are able to exchange DNA with one another, differentiating their species is much more complicated and would require a more difficult explanation.
Populations and Gene Flow
A common (and often bad faith) question asked about evolution by natural selection is, “If humans evolved from monkeys, why are there still monkeys?” The simple answer is because not all monkeys (technically apes) are part of the same population. In biology, a population is an interbreeding group of organisms of the same species. If there is a large group of animals who are all able to breed with each other, they are all part of one population. But if there were some kind of obstacle that stopped two organisms from breeding, they would be part of two different populations. This obstacle can be anything from a physical barrier like a river or mountain range, to a large distance (we’ll talk about that later), to even something like time. Scientists have observed plants that always flower at the same time of day, so they can’t breed with plants flowering at different times, even if they’re right next to each other.
As a way to visualize this process, let’s imagine an island that’s home to a population of weasels. These weasels primarily eat small birds and are in turn hunted by falcons. The north of this island gets extremely snowy in the winter, so the weasels and the birds they prey on migrate to the south of the island and spread back out in the summer. One day, a large earthquake strikes the island and causes the valley spanning the island’s middle to flood. The island is split in two, so the weasel population is also split into two distinct populations. For the weasels on the southern island, this isn’t a huge issue; most of them live here year-round and their lifestyles are not significantly affected. For the weasels on the northern island, this is a bigger problem. They are stuck in the snowy region during the winter and have to deal with the cold and with seasonal food scarcity (the birds can fly, so they continue to migrate).
Mutation
DNA is the medium in which all the instructions needed to build an organism are written, from how to build proteins to how many proteins to build and when and where they should be built. Every time a new cell is made, these instructions have to be recopied. And whenever something so large is recopied so often, mistakes are inevitable; a letter gets changed, added, or removed by accident. These small changes in the code of life are more or less random, though environmental factors can make them more common. They occur in every cell of every organism, including the weasels from both the northern and southern island.
The vast majority of mutations are neutral; they don’t affect an organism’s ability to survive at all, either because it doesn’t change any of the cell’s function or it changes it too slightly to notice. Perhaps a mutation occurs in an ova cell of a northern weasel which slightly changes the shape of the offspring’s ear, in a way that doesn’t impact its hearing. This weasel doesn’t have a great chance of spreading this mutation; she could die before it’s old enough to breed, any offspring she has will only have a one-in-two chance of inheriting the mutant gene, and those offspring might not survive to adulthood either. Statistically, any neutral mutation has a 94% chance of dying out within a few generations. The 6% of mutations that do survive will eventually spread throughout the population and become common traits.
The next most common mutations are harmful ones; mutations which break an important function of the organism or otherwise weakens it. Perhaps a mutation occurs in an ova cell of a northern weasel which worsens the offspring’s vision or makes it slightly less able to digest a certain protein. This weasel will be even less likely to survive to adulthood and spread this mutant gene, so this mutation will likely die out. Now, it’s not impossible for this weasel to get lucky and survive long enough to make this mutation a common fixture of their population, as with some genetic disorders, but the odds are very much against her. Keep in mind though that not all of these breaking mutations are negative; if a mutation broke a gene responsible for keeping a weasel cool in excessively hot days, these northern weasels might never notice this gene had broken. This broken gene would count as a neutral mutation and could spread throughout the population with less difficulty.
Finally, there are mutations which are actively beneficial to the organism. Perhaps a mutation occurs in an ova cell of a northern weasel which gives the offspring a slightly thicker coat, or slightly larger fat deposits, or slightly restructures its brain to make it slightly better at hunting burrowing rodents which live in this region year-round. These mutations will make this weasel ever-so-slightly better at thriving in her new environment, so she is more likely to survive to adulthood and pass this mutant gene on to her offspring. Again, the odds of any new mutation becoming a fixture in a population are never great; a gene which can ensure three surviving offspring for each successive generation will only have a 24% chance of becoming a fixture. But these new mutations are happening all the time. Every sperm and ova cell carries a few of these mutations, so given enough time and a large enough population, new traits do emerge.
Differentiation
Over generations, the northern weasels will accumulate numerous new adaptations to their wintery environment. Fur and fat will slowly become thicker while the color of their fur will slowly become lighter to improve their camouflage against the snow. Front claws will slowly become longer to make burrowing through snow to find hibernating prey easier. Their brains will slowly rewire so rodent-hunting instincts will strengthen while bird-hunting instincts become obsolete. Their metabolisms change in a thousand little ways to become better at subsisting off their new diets. Even small things like what time of day their most active can change as generations of trial and error hone the population to survive in this new environment. Keep in mind the southern weasels have the exact same rate of new mutation and may produce many of the same mutations. (fur color, metabolism, etc.) The difference is the factors that determine whether these new variants are useful; a slightly whiter coat is at best a neutral mutation to a southern weasel, but could save the life of a northern weasel. Not to mention which neutral mutations occur and survive come down to random chance, which might not be the same for each population. The southern weasels might wind up with slightly rounded ears while the same trait died out in the northern weasels, for no reason other than a different roll of the dice.
Now let’s imagine that after a few thousand generations, another earthquake thrusts the sunken valley upward, causing it to drain. The two islands are now one island again and the two weasel populations are able to recombine. But...they don’t. They’re habitats are now different, so they’re rarely in the same place together. They look different from each other, so they’re less likely to see the other as a potential mate. And should a northern and southern weasel mate, the numerous small differences in their DNA causes all sorts of health problems in their offspring. At best, the resulting weasel is sterile, like mules and other hybrid animals. At worst, the incompatibilities in the parent’s DNA are so severe, the offspring can’t survive long enough to be born. These two populations are now too different to ever be able to interbreed again. New genes that arise in the northern weasel population will never spread into the southern weasel population, and vice versa. Even with the islands reunited, the two populations will remain distinct. They are now two different species.
The official definition of a species is “the largest group of organisms in which any two organisms can produce fertile offspring.” But like everything in biology, this isn’t black and white. For example, let’s imagine a long chain of islands that loops around in a big circle. Let’s say a species of small bird gets introduced to the southernmost island (the 6 o’clock position) and soon there’s a whole population of birds. As time goes on, birds start to fly to the islands on either side. These islands are slightly different, so these birds develop adaptations to their new home and become a bit different from the 6 o’clock birds. But they can still fly back to the 6 o’clock island and mate with those birds, so they’re still part of the same population. Over time, birds travel up the ring of islands, going both clockwise and counterclockwise, always adapting to their new island. They can fly one island over to mate, but these birds will never see the 6 o’clock island. One day, the northernmost island (12 o’clock) will be colonized by birds, both coming in from the neighboring 11o’clock and 1o’clock islands. But these two groups of birds can’t mate with each other; they’ve been separated for too long and are now too different. But the 11 o’clock birds could still mate with birds from the next island over, who could mate with the next island over, so on and so on until you reach the 1 o’clock birds. These two groups of birds would be called ring species, named so for the ring shape of the islands. So...should the 11 o’clock and 1 o’clock birds count as separate species? On the one hand, there is some gene flow between them; it is technically possible for a new gene variant to mutate into existence inside a 1 o’clock bird and for that variant to spread to an 11 o’clock bird through generations of mating with birds from the next island over. On the other hand, this gene flow is entirely dependent on other birds; if a volcanic eruption caused the extinction of the 6 o’clock birds or any other island of birds, that gene flow would be gone forever. Is our definition of species useful if it’s dependent on a third group’s existence to work?
Ultimately, any definition of species we come up with will have a ton of exceptions and gray areas, just like our definition of anything else. Organisms change incrementally, so saying when two groups of organisms stop being the same species can be a bit like saying at what point does the color below stop being blue and start being green. Any hard line you draw is going to leave out some complexity. But that is how evolution works; imperceptibly small changes over inconceivably long time periods. It’s understandable that humans don’t always appreciate these processes because they occur on time scales that we have trouble wrapping our heads around. But we are capable of understanding it.
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