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Understanding Evolution and are there Several Species of Salvelinus?

Evolution is probably one of the best known, yet most poorly understood scientific idea ever. There is nothing all that controversial about evolution. Species change over time - that is evolution. Today we recognize that those changes occur at the genetic level but before that, we recognized changes in morphology, coloration, and other ways because we did not know about DNA, genes, and alleles. The simplest, most common definition of evolution today is a change in allele frequency. Alleles are simply a region on a gene that codes for a particular protein for a trait or character. It is our definition of evolution because it is testable and falsifiable - two of the pillars of science. We have to be able to test our ideas and we have to be able to show them to be false. If an idea can not be proven to be false, it is not scientific. When over time and many, many failures to falsify ideas, we call those ideas theories - such as evolution by natural selection or by a number of other mechanisms. There may be no idea in science that is more tested than is evolution.

Charles Darwin - one of the most well known people in all of history. Image from British Heritage.


The definition of evolution has changed over time as we have learned more about how life operates. We knew that species change over time well before we knew that DNA was the "code of life". Before Mendel, before Lamarck, before Darwin and Wallace (skip the rest but PLEASE read about Wallace, the poor "other guy"), we had some idea that life changes and somehow features were passed down from generation to generation. Now we know that evolution shapes the world around us - including us - we just did not have a word for it before Darwin and Wallace. That life changes should not be controversial. That humans change over time - we are animals after all - should not be surprising. Hell, Homo sapiens used to mate with H. neanderthalensis, in fact, we still have the remnants of those matings in our DNA.


How I want to attack this is to give a very short description of each of the mechanisms of evolution first. Then - because this is an angling blog - relate the mechanisms to the aquatic world in which we fish. I will mostly use the fairly recent discovery of 3 different species of Salvelinus - the genus that Brook and Lake Trout (char) belong to - that have been described from the Great Smoky Mountains as examples of the mechanisms of evolution.


The mechanisms of evolution are:

  1. Mutation

  2. Natural Selection

  3. Non-Random Mating / Sexual Selection

  4. Gene Flow

  5. Genetic Drift

To keep this whole discussion a bit more simple, I am only going to write about sexually reproducing animals - like us and fishes. In the biotic world, there is a ton of variation of how species reproduce and even alternate between sexual and asexual reproduction. Reproduction is a messy, complicated thing - we are going to talk about it in a simple, "males make sperm, females make eggs, and those come together to produce what will become a new offspring" approach. The world is really not that simple but humor me.


Mutation


I wanted to start with mutations because they are the ultimate reason there is variation in the world. Mutations are changes to the DNA and for mutations to be passed on, they need to happen in genetic material that can be passed on to the next generation. In single celled organisms that reproduce asexually (make clones of themselves) any DNA changes are passed down to the next generation as the next generation. In organisms that reproduce sexually, those changes need to occur in cells that are passed on to the next generation. In animals the germ cells that produce our gametes, sperm and egg, must be mutated for those mutations to be passed on. Not all mutations are passed down to future generations. For example, skin cancer is caused by DNA mutations from the sun - but those mutations do not get passed down to future generations (thankfully). Progeny of sexually reproducing animals - like YOU - are the result of one haploid sperm and one haploid egg to produce a diploid fertilized egg, the first state of an embryo.


Mutation alone does not have much effect on the genetics of a population. However, it is the ultimate source of variation in a species. One or more of the other mechanisms of evolution may make that mutation become much more common over time in a population. For example, we have Methicillin-resistant Staphylococcus aureus (MRSA) which is a bacteria that is resistant to a family of commonly used antibiotic drugs which makes these infections particularly difficult to treat. There had to have been a mutation (or series of mutations) in an individual bacteria that then became more common because of the strong selection pressures we put on the species by trying to kill it with antibiotics. Over the next number of generations, MRSA became more common because it survives while other S. aureus strains do not. That is how mutation and natural selection can work together.


Natural Selection


This is the mechanism that you almost certainly first thought about when seeing the title and reading the first paragraph. Darwin's idea is really rather simple. It states that there has to variation within a population with some individuals being more or less suited for the current environment. Those that fit the environment well are more likely to survive, reproduce, and pass on those adaptive traits. Individuals that do not fit the environment as well are less likely to survive, reproduce, and pass on those traits. Over time, our population's genetics change due to these differences in survival and reproduction. We can measure these differences in reproductive success - evolution really is all about passing on genes - and we call this idea fitness. You can think of fitness as the measure of the number of grand-offspring produced by an individual because it is not enough to simply reproduce but you have to produce offspring that also survive and reproduce for your genetic material to be passed on.


Many want to make this idea much more controversial than it really is but evolution is a fact - that is, it is observable that species change over time. One - but just one - of the ways species do this is through natural selection.


Non-Random Mating / Sexual Selection


This simply says that mating is not random and that has an affect on the combinations of genetics within individuals in a population. If you are a loyal reader, the post "Eggs are Expensive, Sperm is Cheap" may come to mind. For species that reproduce sexually, females almost always get to do the choosing and males typically have ways that they show off to impress her. Think of White-tailed Deer and their antlers, bright red male Northern Cardinals, or the mating dances of any number of insects, birds, or other taxa. Males are trying to "prove their worth"; trying to show how effectively they are able to acquire resources and even squander those resources on things that do not help them survive but are meant to help them reproduce. In fact, a male Ring-necked Pheasant's tail and its bright feathers may make them less likely to survive. Sexual selection can be a very powerful force driving males to some crazy behaviors and ornamentations - all in the name of love (or sex).


Gene Flow


The name gene flow comes from the idea that genetic material is moved - flows - between two or more populations. This mechanism is often called migration but that word does not fully capture what is occurring in gene flow. In this movement of genetic material, the two populations become more similar. Let's say we have one population of beetles where 70% are green and 30% are tan and another population where things are reversed and 30% are green and 70% are tan. If individuals from once population move to the other population, now those populations will become more genetically similar.

Barrier on Hermosa Creek, Colorado
From Hermosa Creek in Colorado, a barrier designed to prevent non-native trout from moving into native Colorado River Cutthroat Trout habitat. Barriers prevent movement of fishes which also prevents gene flow.

What becomes really important in an evolutionary sense is that the elimination or at least great reduction in gene flow allows one population to have evolved differently than another population of the same species. At some point in time, when these populations become "different enough", we may elect to call them different species. Species are a bit of a human construct but we like to be able to give things names, even if they do not necessarily fit into nice, neat species boxes. (oh, you'll see later...)


Genetic Drift


Genetic drift is probably the idea that most people have the most difficult time with, in large part because it is random and we struggle with thinking or accepting the randomness of the world around us. Genetic drift is random changes to the genetics of a population and we often refer to genetic drift as sampling error. As an example, founder effect is where a small portion of a larger population moves and creates a new population and by chance. The genetic makeup of the new population is not the same as the population from which it came and that is random. Another example of genetic drift is a bottleneck where a larger population is reduced, often through a drastic event, and the remaining population does not have the same genetic makeup as the original population. What is important to see here is that the surviving individuals did not have genes that allowed them to survive better than those that died - that would be natural selection. Instead, they survived due to luck - their survival was random. Have I mentioned that genetic drift is random?


Mechanisms of Evolution

It is important to understand that these five mechanisms of evolution do not occur in a vacuum; they work with and against one another. A barrier for fishes such as a waterfall limits gene flow and different predators (natural selection) or sexual selection (non-random mating) pressures may allow the two populations to become genetically different. Genetic drift is more pronounced in smaller populations (random chance affects small populations more than larger populations). Mutations do little at the population level by themselves but coupled with natural or sexual selection or random chance (genetic drift), mutations become much more prevalent in a population.


New Salvelinus Species from the Great Smoky Mountains


Brook Trout (Salvelinus fontinalis) - "little salmon of the springs" - are the native char of streams of much of eastern North America, as far west as Minnesota and east to the Atlantic Ocean drainages; as far south as northern Georgia and north to the Hudson Bay drainages in northern Canada. Fishes of the genus Salvelinus are native to much of the northern hemisphere and in North America include Lake Trout (S. namaycush), Bull Trout (S. confluentus), and Arctic Char (S. alpinus) and Dolly Varden (S. malma) which were once thought to be a single species. Outside of North America, Salvelinus species are found in Europe and Asia and although their taxonomy is disputed, there are thought to be at least fifty (50) different species within the genus Salvelinus.

Appalachian Trail view in the GSMNP
View from the Appalachian Trail in the Great Smoky Mountains National Park.

The story for this section is that there are (may be?) three different species of Salvelinus that are closely related to Brook Trout that are found within a small geographic area within the Great Smoky Mountains National Park (download the source journal article). Of course for their entire existence until these new species were discovered and described by "science", they were considered to be Brook Trout. The new species are S. kingi for Tim King, a fisheries geneticist who worked with the author to study these species; S. multidentatus, named for having more teeth; and S. angustus referring to their narrow jaw compared to the other species. Most interesting about these "new" species is that they occur within a really small geographic area.


Intuitively we think we know what a species is. Take a second and think about a definition of a species. I'll wait...


Does it work for the huge number of organisms that reproduce asexually? Does your definition contains something about genetics? How different two populations need to be to be different species is exceedingly poorly understood and is highly variable across taxa. Biologist do not have a great definition of what a species is or more accurately, they have dozens of definitions. Experts in different areas have different species concepts which are basically their definition of what a species is. For many vertebrate taxa, we may use the biological species concept which says that members of a species can reproduce and produce sexually viable offspring. Donkeys and horses are different species because while they can interbreed and create viable offspring, those mules they create are unable to mate. We talk about this as post-zygotic isolation; a zygote is produced but it is not reproductively viable.

View from the Appalachian Trail, GSMNP
View from a break in the trees on our hike along the Appalachian Trail in Great Smoky Mountains National Park.

And some of our thinking goes to whether we are "lumpers" that put similar taxa into a single species because they are hard to differentiate or "splitters" that are apt to split populations we may call subspecies, strains, or races into different species. Here is the mind-blowing thing about species, they do not really exist. I mean, they exist - Brook Trout are different species than Lake Trout - but it is not as if there is a great mechanisms to say when two things "different enough" and now they are species. Species are largely a human construct as we need to have names for things.


Back to our Salvelinus species from the Great Smoky Mountains. At first blush, many of us, myself included, are apt to say that they are not really different species because if four species occur in an area that is a tiny fraction of Brook Trout's range, how many species of "Brook Trout" are there? That was certainly my first inclination but then I read the paper and the most intriguing thing was that these three new species each are known from just one native stream (they may exist elsewhere but it is not known). When these three species from three different streams were stocked into Le Conte Creek to restore "Brook Trout" in that creek; over a decade later we saw very little gene flow between the four species in Le Conte Creek. To me, this is pretty vital. They are for whatever reason(s), not interbreeding with the other Salvelinus species despite ample chances to do so. This lack of gene flow could be that they have evolved to spawn at different times or temperatures, sexual selection may keep them separate, or there may be some pre-zygotic isolation factor that prevents individuals of different species from successfully producing offspring. It is also important to note that the differences among Salvelinus species are morphological and meristic (counts of things like the number of lateral line scales, vertebrae, and other physical characters) and not just genetic. Though it would be difficult for most people to tell them apart from one another or Brook Trout but all three new species do have a subterminal mouth that is different from the terminal mouth of Brook Trout.

GSMNP waterfall
A small waterfall on a Great Smoky Mountains stream we encountered during a hike. This may be enough to reduce gene flow - is it enough to create new species of Salvelinus?

Why we have three new species of Salvelinus known from only three different streams in fairly small part of the Great Smoky Mountains is not fully known. How and why they became separate species and others did not is not known. Of course, more studies like this one might find many more species of "Brook Trout" that have evolved in isolation (lack of gene flow) and may have developed post-zygotic isolation factors through natural and sexual selection, an accumulation of mutations, and/or genetic drift that has (randomly) shaped the species genetics over time.


What is a species? I am not sure we will ever have a great answer to that question.


Go read the paper for yourself and tell me what you think...

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