Waterfalls and Gene Flow

Triple Falls - an incredibly popular destination along western North Carolina's Little River

If you drive through almost any portion of the southern and central Appalachians, you're bound to be bombarded with advertisements pitching the region's natural features as tourism attractions to boost the mountains' economy. Hiking trails through densely-forested wilderness areas are one major draw for visitors, as are long-ranging views from high-elevation outcrops and treeless mountain "balds." Waterfalls, however, are arguably Appalachia's main draw for tourism. Some of the region's most popular hiking trails and sightseeing destinations are built around waterfalls, and some entire regions within the mountains - such as the Highlands Plateau surrounding the towns of Highlands, Cashiers, and Brevard, North Carolina - have made the high abundance of waterfalls in these regions a central point for tourism marketing.

So why is Appalachia, specifically, such a hotspot for waterfalls and cascades? Elevation and slope (the steepness of the mountains' hills) are two major drivers of the region's abundance of falls, as streams that lose more elevation over a shorter distance tend to, by the their very nature, be predisposed to sharp drops. Geology, however, forms another cause for the region's scenic cascades. Much of southern Appalachia - particularly the region's easternmost fringe near the Blue Ridge - are made of "metamorphic" rocks, or rocks that have been changed into highly resistant rock types through the combined effects of heat and pressure.

Metamorphic rocks, such as granite and gneiss, tend to be very resistant to the weathering properties of wind and water, unlike less-resistant rocks elsewhere in the region which tend to easily fragment or wear when exposed to an erosive force. This means that when streams and rivers flow across outcrops and escarpments of metamorphic rock types, the more resistant rocks tend to not wear away easily under the erosive action of moving water - causing steep drops over these deposits to their base below. Some of the best examples of how geology, elevation, and slope can combine to create spectacular falls can be found in the Jocassee Gorges region of North and South Carolina. Waterfalls here tend not only be high but contain the flow from entire rivers as they drop from the Blue Ridge to the foothills below, with Whitewater Falls (pictured here) being one spectacular example.

So why are waterfalls being mentioned on a website about evolution? The answer can be found not in waterfalls themselves but in the organisms that live within the streams that flow throughout the region. For almost all aquatic organisms, waterfalls form a barrier to dispersal - put simply, a "wall" over which organisms cannot move. Fish are one classic example of organisms that are especially susceptible to waterfalls as dispersal barriers. Here in Appalachia, for example, our native brook trout  - a species of high conservation concern - is often restricted to stream reaches above waterfalls that act as barriers to introduced (or "stocked") rainbow or brown trout that would otherwise wipe out populations of native fish.

Dispersal barriers, though, are important not just because they restrict the movement of individuals across a landscape. Instead, these individuals also carry their own unique genotype, or a combination of various forms of genes that can vary from organism to organism and between populations. When organisms move from one place to another - when there is no significant barrier to dispersal - they not only move themselves physically but also transport their genes which can be spread through reproduction, a process called gene flow. Gene flow itself is a major evolutionary force, allowing new combinations of genes to move between populations with the movement of individuals. When a barrier such as a waterfall is present, however, migration and gene flow become limited, often to great evolutionary consequence.

Image in public domain (Wikimedia Commons)

A case study from Appalachia

The aforementioned brook trout (Salvelinus fontinalis) forms a fascinating case study for why gene flow and barriers restricting gene flow are so important to the evolution of taxa worldwide. As already mentioned, the brook trout is a species of conservation concern across Appalachia, largely due to declines caused by a slew of factors from predation by stocked fish species to acid rain, sedimentation in waterways, and climate change. Current focus in the region is on restoring this native species, but bringing lost populations back and saving existing ones first requires understanding how various natural and anthropogenic (manmade) stream features influence the genetic continuity of populations. As a general rule, populations need to have and maintain high genetic diversity - many different forms of varying genes - in order to adapt to changing environmental conditions and be buffered from the effects of population declines. Understanding gene flow and its consequences on individual populations throughout Appalachian streams is therefore one large step in solving the puzzle of bringing brook trout back from the brink.

Within the past several years, biologists working in Connecticut borrowed approaches from evolutionary biology and population genetics (the study of genetic profiles within and among populations of an individual species) to examine patterns of gene flow in native brook trout populations spread across two headwater stream systems. Fish from both stream networks were sampled for tissue, which then had DNA extracted and sequenced in the lab from each individual. From there, biologists created a table of genetic distances (a measure of genetic differentiation between all possible pairs of individuals) and combined these distances with a number of stream features to determine which features specifically drive patterns in genetic distance. Waterfalls were included in this dataset, along with the distance between sites where individuals were sampled, stream temperatures, stream gradient (the "steepness" of the stream), and the number of intersecting tributaries between pairs of sampled fish.

In addition to analyses investigating various molecular aspects of trout in these stream networks, a type of statistical analysis called a Mantel test was then used to determine which stream features best explain patterns of genetic distance between sampled trout. Besides indicating that trout showed a pattern called "isolation by distance" - a common pattern in which individuals spaced father apart tend to be more genetically different - these tests indicated that seasonal waterfall barriers greatly reduced gene flow and led to increased genetic differentiation between sampled fish. Beyond that, biologists found that the presence of a major waterfall in one of their stream networks may be responsible for this stream network having lower overall genetic diversity than the other stream system...all because fish from downstream would be unable to migrate to headwater regions and bring in new combinations of genes when a waterfall barrier is present, ultimately keeping genetic diversity lower than in a waterfall-less stream.

All of this matters for brook trout because understanding how to reintroduce and manage brook trout populations first requires understanding how gene flow and other evolutionary processes might act in regional streams. Streams that are highly fragmented by natural or manmade barriers such as waterfalls or road culverts may keep non-native rainbow and brown trout out, but these same barriers may also restrict gene flow and lower genetic diversity relative to more continuous stream environments. And since maintaining gene flow and keeping genetic diversity high are two keys to evolutionarily "healthy" populations, weighing the influence of such barriers is a key in restoring our native trout, in addition to complicating factors such as stream temperatures, flow rates, and stream network complexity identified by additional studies throughout the region.

Beyond trout and even beyond Appalachia, though, gene flow is still a major influence on evolutionary dynamics of organisms.Waterfalls act as barriers to many species worldwide, as do rivers and mountain ranges to many terrestrial species. Understanding how the landscape itself structures genetic diversity is just one key to uncovering how evolutionary processes act in nature - a process that is especially important here in the southern mountains.

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See it for yourself - major waterfalls in Appalachia

• Whitewater Falls (major waterfall in the Jocassee Gorges of North and South Carolina)
• Fall Creek Falls (dramatic falls in Tennessee)
• Little River Falls (centerpiece of Alabama's Little River Canyon National Preserve)
• Cascades (popular waterfall in Virginia's Ridge and Valley)
• Cumberland Falls (powerful falls on Kentucky's Cumberland River)

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Relevant Sources from the Scientific Literature

Kanno Y, Vokoun JC, & Letcher BH (2011). Fine-scale population structure and riverscape genetics of brook trout (Salvelinus fontinalis) distributed continuously along headwater channel networks. Molecular ecology, 20 (18), 3711-29 PMID: 21819470

Petty, J. T. & Merriam, E. P. (2012) Brook Trout Restoration. Nature Education Knowledge 3(7):17

Pettya, J. Todd, Jeff L. Hansbargerac, Brock M. Huntsman & Patricia M. Mazik (2012). Brook Trout Movement in Response to Temperature, Flow, and Thermal Refugia within a Complex Appalachian Riverscape Transactions of the American Fisheries Society, 141 (4), 1060-1073 DOI: 10.1080/00028487.2012.681102

A plant by any other name...

The odd, spike-like 'inflorescence' (or flowers) of parasitic Squawroot pierces through the leaf litter in an Appalachian hardwood forest.

One of life’s biggest challenges – if not its biggest single challenge – is obtaining the energy required to power the processes of life. Many of a cell’s chemical reactions, the anabolic reactions that make up part of the cell’s metabolism, are centered around the construction of products within the cell such as proteins and nucleic acids that are necessary for life. And since these products cannot be assembled at random, the reactions building these products require an energy input that is often in short supply.

When looking across the breadth of biodiversity from bacteria to plants, animals, and fungi, one of life’s most fascinating evolutionary consequences can be found in the variety of strategies organisms have developed to harvest the energy required to power life. Animals like ourselves, for example, obtain energy through a process known as heterotrophy: ingesting living matter from other organisms and harnessing the energy stored in the chemical bonds of those organisms’ tissues. Other organisms, including plants and many bacteria, use chemical pigments to capture solar energy and shuttle it to reactions in the cell that build energy-storing molecules like glucose (sugar).

Members of the plant kingdom are the classic example of these solar-powered organisms. Nearly all plants contain a cocktail of pigments within their cells  - most famously green-colored chlorophyll – that serve to capture incoming light energy from the sun. This energy is then used to build glucose within the cell, which serves to power the necessary reactions the plant needs to sustain itself and to reproduce. This strategy, called photosynthesis, is one of the most successful processes found in living organisms and forms an energy base for most of the food webs found in the world’s ecosystems.

However, not all plants follow this rule. A much smaller number of plants siphon energy from other living plants as heterotrophs. Some of these plants are direct parasites, obtaining energy  from other living plant species through specialized structures. The remainder of these plants, called mycotrophic plants, actually harness energy by tapping into fungi that live in association with another plant’s roots.

Here in Appalachia, two plants in particular stand out as commonly-seen organisms that nonetheless belong to this more uncommon group of plant strategies. The first, Squawroot (also called Bear Corn or Cancer Root), follows a parasitic strategy. This plant attaches to the roots of oaks and beech trees, siphoning off energy and nutrients from host trees’ tissues. Squawroot is easily distinguishable by its flowers, which appear as corn cob-like spikes projecting from the soil. Black bears infamously flock to Squawroot in the spring as a high-energy food source (hence the colloquial name “Bear Corn”), but most of squawroot itself remains hidden belowground, as its spiked flowers only appear during the reproductive season.

Unlike most other plants, which use leaves and photosynthetic pigments as solar panels, Squawroot is both leafless and lacks chlorophyll altogether, a fact that produces its odd shape and yellow-brown coloration. The reason behind these differences is simple enough: why waste energy producing chemical pigments and leaf tissue when you can obtain nutrition from another organism? Biologists have further delved into this odd strategy and found that not only does Squawroot lack chlorophyll – it also lacks the genes that code for the production of photosynthetic pigments altogether! This information suggests that Squawroot’s odd lifestyle is not a one-time fluke but rather an evolutionary adaptation that has been produced over time and is passed down across generations. Genetic comparisons across several parasitic species, in fact, suggest that the type of parasitic strategy employed by Squawroot may have evolved several separate times throughout the plant world.

Ghost-like Indian Pipe grows out of fungal association with a host plant on the forest floor.
Photo Attribution:Tim Pierce [Public domain], via Wikimedia Commons

Squawroot, however, is far from the only heterotrophic plant in Appalachia. Another species, called Indian Pipe (Monotropa uniflora) also lacks chlorophyll and is commonly mistaken for a fungus by many at first glance. Instead of simply attaching to a host plant’s roots, though, Indian Pipe takes a more roundabout route to accessing nutrients and energy from another plant’s tissues. To understand this approach, it is best to first consider what occurs with many plants belowground.

           

Found across most members of the plant kingdom are a number of mutualistic relationships in which individual plants pair with fungi, called mycorrhizae, which typically grow in association with a plant’s roots. Being a mutualism, both members of this pair benefit from their partnership: plants provide the mycorrhizae with sugars, while the mycorrhizae return the favor by helping the plant obtain nutrients, such as phosphorus. Indian Pipe has found a way to game this system by linking its roots with the mycorrhizae growing in association with a host plant. Indian Pipe then siphons off energy from the fungi, which in turn are pulling energy from the other plant, a chain-reaction type of parasitic relationship called mycotrophy.

Just like Squawroot, Indian Pipe grows close to the ground layer and looks very little like other land plants, appearing as a narrow stalk topped with a single flower, colored white or (more rarely) a pink or red hue. And also like Squawroot, studies of Indian Pipe’s genetic material have uncovered a fascinating evolutionary past. Researchers have specifically sequenced the DNA of Indian Pipe and other close relatives and compared these sequences with the DNA of many different fungal mycorrhizae, finding that most species in Indian Pipe’s genus prefer very specific groups of fungi when siphoning off energy. This information suggests that mycotrophy is not a “one size fits all” relationship; rather, the evolution of this unique energetic strategy appears to occur in close tandem with belowground fungal hosts.

Both Squawroot and Indian Pipe can be incredibly common in Appalachia....if you know when and where to look. Squawroot tends to appear in spring, while Indian Pipe is most common in summer months, both growing close to the ground layer in mature forests.

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See it for yourself

(Note: Although both Indian Pipe and Squawroot are common in Appalachia, it can be next to impossible to pinpoint any single spot where these plants may be visible, and either plant may be present in almost any healthy, moist hardwood forest. However, the below destinations all encompass hikes through hardwood forests where either plant may appear during the flowering season.)

Standing Indian Basin, North Carolina (Appalachian Trail)

High Knob, Virginia (In particular the Chief Benge Trail below the mountain's summit)

Blood Mountain Wilderness, Georgia (Appalachian Trail)

Cucumber Gap Loop, Great Smoky Mountains National Park

Pipestem State Resort Park, West Virginia

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Relevant Journal Articles

Cullings, K. W., T. M. Szaro, and T. D. Bruns. 1996. Evolution of extreme specialization within a lineage of ectomycorrhizal epiparasites. Nature 379:63-66.

dePamphilis, C.W., N.D. Young, and A.D. Wolfe. 1997. Evolution of plastid gene rps2 in a lineage of hemiparasitic and holoparasitic plants: Many losses of photosynthesis and complex patterns of ratevariation. PNAS. 94:7367-7372.

McNeal, J.R., J.R. Bennett, A.D. Wolfe, and S. Matthews. 2013. Phylogeny and origins of holoparasitism in Orobanchaceae. American Journal of Botany. 100:971-983.

Islands in the Sky

High-elevation evergreen forests (dark trees) on Wilburn Ridge and Whitetop Mountain (far background) in Virginia.

Consider, for a moment, taking a drive from a low-lying Appalachian valley to the high country of the region's highest peaks. A classic example of such a drive would be taking U.S. Highway 441 from Gatlinburg, TN or Cherokee, NC to the summit of Newfound Gap in the Great Smoky Mountains, or perhaps a trip on the Blue Ridge Parkway in western North Carolina. These incredibly steep (and scenic) drives can easily climb three or four-thousand feet in 20 miles of highway and traverse an incredible array of habitat types, from low-elevation bogs and cove forests to dense, evergreen thickets of spruce-fir forests on the uppermost peaks above 4,500 feet. That 20 miles of driving, in fact, can in many places cover the ecological equivalent of traveling from the southeastern U.S. to Quebec or Newfoundland!

What causes this change from warm, lowland forests to evergreen-dominated woods found more in New England or Canada? The answer lies in climate. As the steep slopes of the Appalachians rise abruptly from their foothills, higher elevations lead to cooler temperatures and, in many cases, higher amounts of precipitation as moist air cools and drops its moisture as rainfall or snow. Some of the East's highest annual precipitation actually occurs in the steep high mountains of extreme western North Carolina for this very reason. While small in size, these high-elevation "islands" have a climate vastly different than their surrounding ridges and thus harbor a unique array of habitats and species, such as the dense spruce-fir forests mentioned in the paragraph above.

The southern Appalachians, however, did not always appear this way. That 20-mile drive through an Appalachian valley, in fact, would have appeared vastly different to the naked eye 10,000-15,000 years ago than it does today. Instead, Appalachian forests at the end of Pleistocene Epoch - the geological period occurring from roughly 2.5 million to 11,000 years before present - were vast, park-like forests dominated by spruce and fir, even at the lowest elevations of Appalachian valleys. Rather than seeing black bears and deer serve as the mountains' larger mammals, massive "megafauna" such as mammoths, mastodons, and musk ox roamed the ridges and valleys. In short, this was a region largely unrecognizable relative to the mountains we see today.

Clues from the past 

How do we know all of this? After all, we can't simply hop in a time machine and travel back 15,000 years to view these ecosystems for ourselves. We do, however, have clues that can give us a window into this wildly-varying past. The biggest of these clues come from a small, isolated valley in southwest Virginia. Within this valley - named Saltville for its large salt marshes - lie a series of ancient Appalachian bogs, many of which have been present since long before the end of the Pleistocene. At some point during this period,  a huge collection of now-extinct megafauna died and were preserved within the area now covering the valley floor. A sampling of mammoths, mastodons, and even giant bears have been pulled from these bogs and older geological deposits nearby in Tennessee during scientific excavations, providing an incredible look into the fauna of Appalachia's past.

One of Saltville's most fascinating specimens from an evolutionary perspective was a single musk ox that perished in the valley's bogs during this time period over 100 centuries ago. This musk ox skeleton was preserved until the 1960s, when paleontologists unearthed the creature and took samples from inside its cranial cavity, or the open space within its skull. Inside, they found pollen from spruce and fir trees that now occur nowhere near the Saltville valley, instead occupying mountain summits over 4,000 feet higher. This evidence, along with similar findings in sediment samples from the same region, tell us that today's high-elevation forests were much more widespread in the Pleisotcene than they are currently.

More broadly, though, these clues into Appalachia's past serve as examples of one of our most common methods for studying evolutionary patterns and processes. Although evolution has been observed directly both in the field and in the laboratory, many evolutionary studies must rely on evidence such as that found in Saltville to understand a species' past. This reliance on evidence from the present as a window into processes from the past touches on a larger topics called uniformitarianism and is one of our most powerful methods for understanding how evolutionary processes have shaped our world. Here in Appalachia, uniformitarianism has allowed us to make inferences into what our forests and mountains would have looked like thousands of years into the past.

Change to the present

Even with this knowledge, though, a major question still remains: how did these one-vast spruce-fir forests become reduced to small, isolated islands restricted to our highest mountain peaks? The answer again lies in climate. During the Pleistocene, much of North America was covered by vast ice sheets up to 2 miles thick and formed by a much cooler climate than we experience today. Although major glacial activity did not extend into southern Appalachia, the mountains' climate was instead much more like modern-day New England or Canada than it is currently - a fact that allowed cold-adapted species like spruce and fir to persist even in our lower valleys.

As Earth warmed and the Pleistocene ended, however, things began to change. Appalachian valleys were no longer climatically-suitable for these evergreen forests, and their component tree species had nowhere to go...but up! The range of spruce and fir across the mountains gradually retracted until these forests were reduced to their locations today, exiled on the Appalachians' highest peaks. Once again, evidence from the present provides us with a window into this process, including growth data from tree rings and genetic material. This evidence, in fact, also tells us that our mountaintop islands are still retracting, retreating ever higher onto summits that don't have much room left as our climate continues warming. There is concern that with the combined effects of climate change and pollution, these unique forests may not last for long.

...which brings us back to that drive from an Appalachian valley to a high summit. The habitats and organisms our mountainsides harbor are not only varied but everchanging, structured by a menagerie of forces from climate and pollution to competition with other species. The high-elevation forests mentioned here are but one example of these changes, and these forests themselves harbor a staggering number of species that have evolved within our present mountaintop islands and will be detailed in later pieces on this website. _________________________________________________________________

  See it for yourself

  Clingmans Dome (Great Smoky Mountains NP, NC and TN): Spruce-fir ecosystems

  Grandfather Mountain (NC): Spruce-fir ecosystems

  Blue Ridge Parkway (Milepost 443 to 423): Roadside spruce-fir

  Mount Rogers (VA): Spruce-fir ecosystem - see smartphone guide here

  Museum of the Middle Appalachians (Saltville, VA): Pleistocene megafauna __________________________________________________________________

Relevant journal articles

 McLaughlin, S.B., D.J. Downing, T.J. Blasing, E.R. Cook, and H.S. Adams. 1987. An analysis of climate and competition as contributors to decline of red spruce in high elevation Appalachian forests of the Eastern United states. Oecologica. 72:487-501.

Potter, K.M., J. Frampton, S.A. Josserand, and C.D. Nelson. 2010. Evolutionary history of two endemic Appalachian conifers revealed using microsatellite markers. Conservation Genetics. 11:1499-1513.

Ray, C.E., B.N. Cooper, and W.S. Benninghoff. 1967. Fossil mammals and pollen in a late Pleistocene deposit at Saltville, Virginia. Journal of Paleontology. 41:608-22.