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

Patchiness and Plutons: The Origins of Some of the South's Rarest Plants

Big Green Mountain,  a pluton in Western North Carolina's Panthertown Valley

No, the title of this post is not a reference to some obscure, science fiction B-movie, nor is it a gripe about the naming controversy surrounding our solar system's outermost planet. This post is instead about something far more fascinating and a bit closer to home.

Scattered across the southern Appalachian Mountains and their surrounding foothills are a number of mountains which seem to stand out from the rest of their more pointed, forest-covered neighboring peaks. These summits, most famously Georgia's Stone Mountain or North Carolina's Looking Glass Rock, instead appear as squat, rounded domes of granite, most of which shows through vegetation as steep slopes of bare, exposed rock. Asheville's Looking Glass Rock, in fact, received its name due to the reflective nature of sunlight hitting the ice sheets that form on its bare granite surface in the winter.

These unique mountains, though, are far from just an oddly-shaped summit scattered haphazardly across the region. These peaks are instead referred to geologically as "plutons," or rock formations with their own curious past. Unlike most of the Appalachians, which were built from continental collisions eons ago and have been gradually worn down, plutons have actually appeared as a result of surrounding rock being slowly eroded away - literally the mountain-building process acting in reverse!

Pluton formation results initially from deposits of molten magma below the Earth's surface, typically during periods of intense geologic activity such as that found during the Appalachians' infancy. As this activity slows, magma deposits eventually cool and crystallize to form deep "bubbles" of resistant granite belowground. Then, as the eroding action of wind and water act over millennia, the less-resistant layers of rock surrounding the granite weather away, leaving portions of the hard granite projecting outward from the surface. To better understand how this process might work in an oversimplified sense, consider burying several hard, fist-size pebbles of rock in loose sand beside the ocean. As waves gradually eat away at the loose, pliable sand, you would slowly begin to the see the denser, more resistant pebbles remaining in place. This process, acted out on a much grander scale and with more complex geologic forces, helps to explain why we see huge granite monoliths like Stone Mountain projecting from the surrounding landscape today.

Rock outcrop plant communities

Plutons, however, are far from just a geological wonder. A number of wildly unique living organisms can also be found on the rock formations' surfaces. Georgia's Stone Mountain, for example, contains potholes on its summit that harbor fairy shrimp...at nearly 1,700 feet above sea level! Other plutons serve as habitat for rare plant and lichen species, some found few other places on Earth outside of these and other nearby rock outcrops in the Appalachian foothills region. To see some fantastic pictures of plant communities from plutons and other significant rock formations in southern Appalachia, check out photographer Alan Cressler's images from the region.

The plant communities on the Appalachians' rock outcrops specifically show a pattern of high endemism, a term meaning that these habitats contain a number of plant species found nowhere else other than the immediate Appalachian region. Some of these species occur in shallow depressions on the rock surfaces that slowly fill with soil, while others occur in or near pools of collected rainfall in similar depressions and surface irregularities. This unique collection of plants has led many biologists to ask an important question: why do plutons and similar outcrops contain such a high number of rare plants?

Several hypotheses have been proposed to explain this high level of endemism in Appalachian rock outcrop ecosystems, and these discussions have hinged around a central question: do rock outcrops possess a special characteristic that allows endemic plants to survive there and nowhere else, or does competition with other plants - those found outside of outcrops - serve to restrict endemics solely to their rocky islands? Answering this question is key to understanding how these plants may have evolved, since a mechanism must exist to "trap" these species on outcrops, allowing for increased rates of adaptation to the rock surfaces' unique environments.

A granitic dome plant community, all housed in a shallow pocket of soil atop a pluton in Western North Carolina

Explaining outcrop endemism

In the late 1980s, biologists decided to test this question using several lines of evidence from Appalachian outcrop endemics. Specifically, they identified a single observation of these endemic plants that opened the door to more questions: plants that occur only on outcrops can also grow if planted in captivity in soil obtained far away from the outcrops themselves. This observation suggested that the physical characteristics of the outcrops themselves was not likely the culprit for why these plants are found nowhere else. So what other factors could be driving their isolation?

Further observation of rock outcrop plants showed that endemics tended to favor areas of outcrops exposed to high levels of light, while avoiding shaded portions of outcrops and surrounding forests. A closer observation of these plants yielded an even stronger relationship to light. The growth rates and rates of photosynthesis - how fast these endemic plants grow and convert light energy to sugar - were highest in high-light conditions and extraordinarily reduced in shady conditions with little direct light. Coupled with earlier observations about a lack of preference for soil, this solidified a major piece of the rock outcrop puzzle: endemic outcrop plants possess an adaptation for high-light environments, rather than a specific preference for physical conditions found only on the rocks' surface.

Even with this information, one question still remained. What process was keeping these plants from moving outward from their rocky habitats? Follow-up work examined the results of studies performed in the field and in greenhouses, in which the ability of rock outcrop endemics to compete against other native plants were examined. Once again, these results were striking. Endemic plants tended to be less tolerant of shade due to the fact that they are poor competitors for resources compared to taller plants found in surrounding forests, which quickly shade out and eliminate endemics in forested areas outside of outcrops.

Completing the puzzle

When viewed together, the above evidence creates a rough picture of how plutons and rock outcrops may have acquired their highly unique flora. Exposed rock surfaces tend to be extreme environments relative to surrounding forests, containing little to no soil depth and direct, almost continuous exposure to sunlight. Plants preferring high-light conditions could easily colonize these sites if and when sufficient soil and moisture occur, yet poor competitive abilities with neighboring forest plants would "trap" these species on their outcrop islands, in a refuge from competition.

Once trapped within these patchy habitats, evolutionary processes are free to run wild. With little migration beyond their isolated, island-like habitat, genetic frequencies can shift at random, and new mutations can arise within these outcrops and further drive selection, producing adaptations more finely-honed against the extreme pressures of outcrop life. Although there is much more to the evolution of these endemic plants than light and competition - complex relationships with pollinators and habitat variability within outcrops also exist - these basic factors show how simple adaptations and environmental patchiness can drive runaway selection. These same processes, in fact, also have a dark side. Rock outcrop plants in the Appalachians tend to be not only highly adapted but also highly threatened, all a consequence of being restricted to habitats that are small in number and even smaller in size.


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 See it for yourself 

• Stone Mountain Park, Georgia
• Looking Glass Rock, North Carolina
• Panthertown Valley Plutons, North Carolina (multiple peaks)
• Table Rock State Park, South Carolina

Note: Rock outcrop flora only occur in a few places on the planet, and the communities they live in can take centuries to form due to the painstakingly slow collection of soil and other resources on rock surfaces. Many species are even protected at various legislative levels across southern Appalachia. Be sure to care for these species if visiting public lands by not trampling flora on plutons and other rock outcrops across the region, never picking or disturbing native plants, and staying only on established trails atop overlooks and across outcrop hiking routes.

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 Relevant sources from the scientific literature

Jerry M. Baskin, & Carol C. Baskin (1988). Endemism in Rock Outcrop Plant Communities of Unglaciated Eastern United States: An Evaluation of the Role of the Edaphic, Genetic, and Light Factors Journal of Biogeography, 15 (5/6), 829-840

Shure, D.J. 1999. Granite Outcrops of the Southeastern United States. Pp.99-116 in Savannas, Barrens, and Rock Outcrop Plant Communities of North America, R.C. Anderson, J.S. Fralish, and J.M. Baskins, eds. Cambridge University Press, New York.

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.