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.

Hemlocks and Genetic Diversity

It’s nearly impossible to find a more iconic Appalachian wildlife species than the eastern hemlock (Tsuga canadensis). Hemlocks are seemingly everywhere in the southern mountains –  cloaking moist, shaded coves, lining the sides of rivers and streams, and even growing haphazardly out of rock crags and outcrops. The tree represents the region’s most common large conifer, and the year-round shade it provides is thought to keep stream temperatures cool enough to provide ample habitat for native animal species, such as brook trout.

Visit almost any location in southern mountains, however, and you will likely witness not just a forest covered in hemlocks but a forest in which those hemlocks are dying. The cause is a foreign insect pest native to Asia and introduced to the U.S. in the 1950s called the hemlock woolly adelgid (HWA for short). HWA adults are so small that they can barely be seen with the naked eye, but the effects an infestation can have on a single tree or forest of hemlocks can be devastating. Infested trees can be identified by HWA’s characteristic white “puffballs” that form on the trees' branches and serve as the insects’ egg sacs. Adults attach to hemlock branches and feed on nutrients from the trees’ sap. Once a tree becomes heavily infested, it begins to brown and lose needles. If an infestation continues unchecked, the tree will almost certainly die.

In just a decade or so, HWA has moved rapidly down the spine of the Appalachians from Virginia to Georgia. What once were healthy, shaded forests of old growth hemlocks in the Great Smoky Mountains, for example, are now virtual graveyards of bleached, standing dead trees. In the wake of this infestation, foresters and biologists are trying several approaches to managing the pest, but many are beginning to realize that part of their focus should be on protecting some existing trees as a source for the repopulation of forests once the adelgid has done its damage.

 
 HWA egg sacs on a hemlock frond (Photo: Connecticut Agricultural Experiment Station Archive, Connecticut Agricultural Experiment Station / © Bugwood.org / CC-BY-3.0-US)

 Since foresters cannot protect every existing hemlock for this effort (due to the constraints of time and money), several groups are specifically borrowing an evolutionary approach to determine which trees or stands of trees represent the best candidates for seed-gathering once many of our hemlock forests have declined. This approach is based on examining the genetic diversity of hemlock forests across the Appalachians, selecting stands with the highest amounts of genetic variants, called “alleles,” as priorities. 

Genetic diversity relates to evolutionary biology for several reasons, primarily because the pillar of evolutionary theory is that all species change over time and across generations. Genetic diversity relates to this change because diversity is the “stock” through which change can occur; our genes code for the assembly of proteins, which ultimately make up our cells and help direct processes within our cells and our bodies, at large. In a very basic sense, more genetic diversity means that a species can adapt more easily to change – in other words, more “code” exists that is capable of assembling new proteins, and therefore helping organisms survive new environments or threats.

If you’re a hunter or fisherman, you’re likely already familiar with this evolutionary concept, even if you’ve never heard it placed in technical terms. One reason that “bag limits” are placed on wildlife species, for example, is to keep wildlife populations large enough to prevent genetic diversity from crashing and allowing rare genetic defects to predominate. One purpose of wildlife translocations – in which organisms like deer are moved from one forest to another, or when fish are transferred into a stream from a hatchery – is to introduce new alleles (forms of the same gene) into a population and ultimately make those organisms more adaptable to change.

This idea relates to current efforts with the hemlock because biologists want to select seeds from trees with high genetic diversity, or trees which have the capability to better adapt to new conditions when planted in a new environment. The hope is that if HWA does succeed in wiping out many of the Appalachians’ hemlock forests, these seeds could be collected ahead of time, protected until the HWA infestation has lessened, and ultimately replanted in areas where hemlock forests used to exist. In this sense, genetic diversity serves as a form of insurance for the hemlocks’ survival, ensuring that any trees that must be replanted have at least the genetic background to survive their new life.

Biologists have specifically identified several areas within the southern Appalachians where such “hotspots” of genetic diversity exist. Several of these are located near the fringes of the hemlock’s habitat, such as the Blue Ridge Escarpment of South Carolina. Outside of their applied use a conservation tool, these areas may also help illustrate the recent evolutionary history of the species, since the pattern shown by the hemlock suggests a relatively recent expansion of the hemlock’s range across Appalachia since the last ice age. Genetic diversity can therefore act as both a window into the future for conservation efforts, as well as a glimpse into the species’ past. ______________________________________________________________________

 See it for yourself

  Sipsey Wilderness (AL): Hemlock forests untouched (for now) by HWA

  Cataloochee Valley (Great Smoky Mountains NP, NC): Hemlock forest heavily impacted by HWA

  Roaring Branch Cove (VA): Old-growth hemlocks; some over 300 years old - smartphone guide available here
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Relevant articles from the scientific literature:

 Nuckolls, A. et al. 2009. Hemlock Declines Rapidly with Hemlock Woolly Adelgid Infestation: Impacts on the Carbon Cycle of Southern Appalachian Forests. Ecosystems. 12:179-190. 

 Potter, K.M. et al. 2008. Allozyme variation and recent evolutionary history of eastern hemlock (Tsuga canadensis) in the southeastern United States. New Forests. 35:131-145. 

 Zabinski, C. 1992. Isozyme variation in eastern hemlock. 22:1838-1842.