For those of us who have a basic understanding of biological evolution (and I hope that’s all of you), one of the easiest and most straightforward ways of conceptualizing the process of natural selection as it impacts evolutionary change is imagining an environmental pressure that eventually results in a genetically-based trait alteration in populations of living organisms. For example, a bunch of short-haired hamsters move into a cold area. The shortest haired individuals die off more readily than their more shaggy counterparts, selection favors longer hair, the trait increases in overall percentage in the population, bada boom you have evolution. But in reality, there is a lot more going on in nature than non-living factors impacting living things; other living things are very much a part of the environment, and are a very large component of the evolutionary trajectory of other living things. Since both co-influencing factors are of a biologic construction, reinforcing evolutionary change can occur as a response to the consequences of a relationship in both member species…this is known as coevolution. Coevolution allows the emergence of ecologically complex interdependent relationships, sometimes with many different species involved. Coevolution often results in predator-prey evolutionary dynamics, host-symbiont relationships (as well as host-parasite relationships), and a whole mess of interactions based on nothing but exploitation of an unwitting victim species.
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Consider a flower. Most people enjoy flowers; they can be brightly colored, intricately shaped, often smell nice, and for those of us living in the seasonal latitudes, they are a symbol of the warm days of spring and summer. However, the pleasantries of a flower are not for you. Flowers do not care about you. The plant that makes the flower is doing it for reproduction, and this reproduction is almost never dependent on your swooning suspirations and giddy entrancement with color. For angiosperm (the division of plants that produce seeds and flowers) plants, flowers are the embodiment of sex. They are there to both disperse genetic material and grow fertilized offspring. This is where something called pollination syndrome comes in, and no, it’s not contagious and no it’s not what is killing all the honeybees. Pollination syndromes are essentially a suite of traits of flowers that have evolved as a response to a certain pollination vector (which could be abiotic, like wind or water, or biotic, like through living animals). The relationships with animals entirely for reproductive means allow for rampant coevolution. The difference between the biological vectors is significant because it determines many characteristics of the targeted flower. The plant has to do a good job of making a flower that attracts a certain group of pollinators, so the evolution of pollinator-flower relationships tends to drive towards the tailor-making of flowers to maximize the appeal to the vector. Most flowers we know and love are like fast-food billboard advertisements along a freeway. The sultry Carl’s Jr. billboard appeals to our desire for fatty, greasy, starchy goodness; a flower’s coloration pattern and scent tap into the sensory batteries of the bee brain.
If Carl’s Jr. is successful, we end up making a detour, eating ourselves into a bloated and ashamed mess, and they get our money. If the flower is successful, the vector drops in for a sample of nectar, and leaves with a dusting of pollen to transmit to the next flower.
Bees get a lot of attention for their role in pollination. And they should, as they tend to pollinate many of our most important food crops…hence the anxiety about Colony Collapse Disorder. But often overlooked are the bigger, non-stinging vertebrate pollinators like birds. Flowers pollinated by birds, due to a different set of parameters for coevolution, tend to be different from insect-pollinated flowers. Bee-pollinated flowers tend to be yellow or blue with nectar guides, which are basically landing strip lights that show up as stripes only visible in the UV spectrum (which bees conveniently can see in) that say HEY BEE, THERE’S FOOD AT THE CENTER OF THIS FLOWER AND THIS IS HOW YOU GET THERE. Nectar production by the flower is moderate, as bees aren’t all that big, and they tend to be scented, as bees have finely tuned chemical sensors. With flowers that are bird-pollinated (or “ornithophilous”), a different strategy is taken. Red or orange is far more common color used to attract birds, since these stand out more readily in the vision of birds. The flowers are usually unscented, because birds have a shitty sense of smell, and making sweet-smelling compounds for no reason is counterproductive to overall fitness. Ornithophilous flowers also tend to make a LOT of nectar to keep the bird well-fed and coming back for more, hence keeping the relationship stable. Since nectar, being pure sugar, is energetically expensive, there is an evolutionary push towards trying to maximize benefit from each encounter, while minimizing the loss. Because of this, these flowers have evolved shapes that tend to be long and tubular. This forces the bird to force its face deep into the flower to get at the nectar in the back, potentially minimizing the amount it can lap up, and increasing the amount of pollen that gets all over the bird’s head. Many groups of birds have coevolved alongside this floral effort, combating this attempt to fleece them in the name of sex by evolving longer and thinner beaks and longer tongues. The most specialized nectar-feeding birds in the worlds (hummingbirds, honeycreepers, and sunbirds) all have long, curved beaks with insanely long tongues. This coevolution goes back and forth, selecting for ever longer bird beaks and tongues and ever deeper flowers. Hummingbirds and flowers have been engaged in a beautiful game of attempting to screw each other over for tens of millions of years.
Some plants have evolved to exploit the other flying vertebrates; bats. Bat-pollinated flowers tend to, of course, open at night. They are also large, white, incredibly odiferous, bell-shaped, and produce loads of nectar. The downcast bell uniquely suits the upside-down clinging of bats, the large size makes them easy to distinguish with echolocation, and seeing as how bats have an awesome sense of smell, all that sweet and sticky odor works out perfectly.
Sometimes there is another level of complexity in these pollination syndromes that takes advantage of a specific dietary affiliation of their target pollinator. Instead of appearing to be an ambiguous food source, they appear to be something altogether different. One example of this are the Dracula orchids of Ecuador, of which there are many species. Dracula flowers tend to be large, drab, and in possession of a modified petal that is pale, upturned and folded to resemble a gilled mushroom. As if that wasn’t enough, the orchid steps it up a notch and also produces an aromatic compound that mimics, perfectly, the rich smell of a rainforest fungus…the same fungus that the fungus gnat uses for sustenance. The gnat follows the scent, comes across a convincing visual mimic in the warped mushroom petal, and attempts to dig in. By the time the gnat figures out it’s not on top of its favorite food, it has already been covered in the orchid’s pollen. Fungus gnats must live a pitiful and frustrating life, constantly being duped by a goddamn plant.
Plant-animal coevolution doesn’t always have to entail trickery, callous self-interest, and feigned kindness. Sometimes what’s known as a symbiotic relationship can evolve, in which both parties legitimately benefit from the interaction, and are completely dependent on each other for survival. Individual fitness is still the ultimate factor, but a car salesman-esque swindle isn’t the primary way of getting there.
This is the bullhorn acacia (Acacia cornigera), native to much of Central America. Taxonomically, it shares a family with such familiar plants as beans, peas, lupine, and vetch; a look at the leaves provides a good hint. Anyways, the bullhorn acacia is so named for its swollen and hollowed out stipular spines, pictured above, which resemble cow horns. Most acacia trees, found in tropics worldwide, possess very bitter alkaloid compounds in their leaves which serve as a deterent to being eaten by insects and large herbivores. This species went on a different evolutionary trajectory, nixed the nasty taste, and formed a symbiotic relationship with the ant Pseudomyrmex ferruginea in order to protect itself. The ants spent a lot of time in the shelter of the hollowed out spines, and when any sort of animal interacts with the tree, be it a frog, deer, or human, they rush out in a rage and swarm and sting the unfortunate beast that brushed up against the coveted acacia. The ants pack a hell of a punch in their sting, and are more like wasps in that regard. Most herbivores learn quite quickly, of course, not to fuck around with bullhorn acacias. It is also thought that some herbivores, after interacting with these plants, learn what the alarm pheromones of the ants smells like, and give the plants quite a bit of room based on that. The ants are also so dutiful to their gracious plant host that they routinely clear away seedlings of other plants growing around the acacia that threaten to grow up and block access to sunlight. The ants do all this because their beloved acacia provides more than just a boss as hell spiny loft in the jungle to crash in; the bullhorn acacia produces protein and lipid-rich nodules on its leaflet tips called Beltian bodies (pictured below), along with a sugary nectar from glands on the leaf stalk. The ants essentially live purely on these products, and will go apeshit on any threat to their food supply. So, while the relationship works well both ways, there is some passive manipulation going on. The acacia gets a private army of hyperaggresive ant slaves by getting them hooked on readily available, energy rich food, and the ants live contently…but have to fight and die in order to get their fix.
Stepping away from plants and bugs, and towards the antagonistic predator-prey interactions, one must look on the west coast of North America, a place very dear to my heart. Anyone who has lived there undoubtably is familiar with the rough-skinned newt (Taricha granulosa), with its ubiquitous presence in rivers and streams, pebbly gray back, bright orange belly, and adorable amphibian face. As children we were wisely warned by our elders to wash our hands after handling the newts, not because they are dirty, slimy, bacteria-ridden critters…but because rough-skinned newts are among the most toxic amphibians in the U.S. They produce tetrodotoxin, which then seeps through their skin; tetrodotoxin is same toxin that pufferfish possess…and that kills a certain number of Japanese and too-ballsy-for-their-own-good tourists from fugu consumption every year. Tetrodotoxin is also used by some lethally toxic poison dart frogs. For humans, the toxin isn’t much of a concern unless it’s ingested or introduced through the mucuous membranes or through a cut. SO ATTENTION FELLOW WEST-SIDE OREGONIANS: If you are “upriver”, and you cut your hand on a blackberry bush struggling to get down to a sandbar, because you didn’t want to spill your Ninkasi or smash your Newman-O’s…you might want to think twice about handling that cute little newt you spotted on your last dive to the river bottom. Part of the reason these little guys are so damn toxic is because of this:
LITTLE BUDDY!!!! D:
Garter snakes (Thamnophis sirtalis) have a penchant for newt flesh. In theory, the tetrodotoxin messes with a sodium channel in the snake’s nerve cells, making the above dining risky, if not deadly. However, some populations have developed genetic dispositions that make them resistant to the newt’s toxin, allowing them to munch on as many squishy, defenseless newts as they want, which gives them a unique advantage over other predators, as they are the only creatures that can exploit this food resource. In areas where toxin-resistant snakes occur, there is a selective pressure on newts for more potent toxins to protect against the super-snakes. This coevolutionary back and forth, with ever-increasing toxicity and reactionary resistance evolution, goes on and on, in what is referred to as an evolutionary “arms race.”
This has occurred for a long time, sporadically, throughout the entire range of the rough-skinned newt. The end result is, in general, a species of newt that produces toxin levels far beyond what would be necessary to kill any other predator. This was made adundantly clear in 1979 when a 29-year-old college student in Oregon (of course) died not too long after swallowing a newt on a dare at a party. So yeah, thanks a lot garter snakes. Now we Oregonians are forced to resort to boring things like goldfish to throw down the gullet on a drunken whim. There’s no telling how far this arms race will go, but given the long history of such things, I’m worrisome.
Extremes exist on the more laid back, symbiotic side of things too.
This is lichen. It looks a little like a hardy plant, or a fungus, but in reality, it’s neither. Lichen is not actual a single “thing”, but a composite organism made of a species of fungus and a separate species of green algae or a cyanobacteria. The fungal component is known as the “mycobiont” and provides the greater structure and framework for the lichen. The cyanobacteria or algae, contained inside the mycobiont itself, is the “photobiont.” The photobiont is, predictably, a photosynthesizer, and produces carbohydrates for both partners from sunlight. The mycobiont provides shelter for the photobiont, and assists in the retention of water and minerals. Lichens have evolved multiple times over the past several hundreds of millions of years, and rightfully so, as lichens tend to be highly successful in extremely dry and cold environments. They represent some of the most specialized, ancient symbiotic marriages in the history of life. And we humans think making it to our 25 year anniversaries are hard.
But even lichens may not corner the market on symbiotic supremecy. Eukaryotes, which are organisms whose cells contain membrane-bound organelles (a nucleus, chloroplasts/mitochondria, Golgi apparatus), include all multicellular life. You, me, all animals, plants, fungi, algae, and single-celled protists are all eukaryotes. They arose roughly 2 billion years ago, and the evolutionary step towards having complex, membrane-bound structures like organelles inside each and every cell is arguably one of the most significant transitions in the history of life on Earth. One theory, which is now heavily supported, is that these organelles were not just manufactured by a bacterial or archaean cell. Endosymbiotic theory posits that eukaryotic cells are derived from a collection of bacterial cells that became symbiotic upon each other deep in evolutionary time. Mitochondria could be derived from a proteobacterium that became internalized inside another bacterium, becoming a dependent “endosymbiont.” Chloroplasts could be descendents of cyanobacteria that underwent the same process. Some evidence for this comes from the fact that mitochondrial and chloroplast contain their own, distinct DNA from the main cell’s nuclear DNA. They also replicate themselves in a similar manner to bacterial cells. If endosymbiotic theory is indeed true, you and I are less a cohesive, singular organism than we ever thought. Not only are we technically a giant collective of cells masquerading as a single, whole unit…but within those cells, we may have another level of collective organization. It may be that, although we conceive of ourselves as individuals (and certainly from an evolutionary and ecological standpoint, we are), at our most basic level, we are a collection of interdependent machines, supporting each other for billions of years, with the only thing “individual” about us being the singular goal of the replication of the “main” cell’s genetic material.
Coevolution adds an additional layer of complexity to evolutionary dynamics on this planet, and is responsible for the much of the diversity of life on this planet. Life has a profound impact on other life, and by driving the biosphere into dizzying heights of diversity, helps ensure the long-term survival of life on Earth.
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