evolutionary history

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Phylogenetic Arts and Crafts

This is a guest post by Rachel Rodman.

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The foods we eat – namely fruits, vegetables, and grains – are all products of their own evolutionary stories. Some of the most well-known chapters in these stories are the most recent ones – dramatic changes in size and shape mediated by human selection.

One especially striking example is that of Brassica oleracea –the source of broccoli, cauliflower, kale, Brussels sprouts, kohlrabi, and cabbage. Each of these diverse vegetables belongs to the same species, and each is the product of a different kind of selection, exerted on different descendants of a common ancestor.

Corn is another famous chapter. The derivation of corn – with its thick cobs and juicy kernels developed from the ancestral grain teosinte, which it barely resembles – has been described as “arguably man’s first, and perhaps his greatest, feat of genetic engineering.”

But these, again, are recent chapters. Relatively. They unfolded over the course of consecutive human lifetimes –hundreds of years or thousands at the outset (sometimes much less). They are the final flourishes (for the moment) on a much older story — a story that significantly precedes agriculture as well as humans.

It is this older story that lies at the heart of truly deep differences, like those at play in the idiom “apples and oranges.” The contrast between these two fruits can be mapped according to many measures: taste, smell, texture, visual appearance, and so on. When used colloquially, the phrase serves as a proxy for unmanageable difference — to describe categories that differ along so many axes that they can no longer be meaningfully compared.

However, in evolutionary terms, the difference between apples and oranges is not ineffable. It is not a folksy aphorism or a Zen puzzle at which to throw up one’s hands. To the contrary, it can be temporalized and quantified; or at least estimated. In fact, in evolutionary terms, that difference comes down to about 100 million years. That is, at least, the date (give or take) when the last common ancestor of apples and oranges lived — a flowering plant from the mid-Cretaceous.

The best way to represent these deep stories is with a diagram called a phylogenetic tree. In a phylogenetic tree, each species is assigned its own line, and each of these lines is called a branch. Points at which two branches intersect represent the common ancestor of the species assigned to these branches.

Phylogenetic trees can serve many purposes. Their classical function is to communicate a hypothesis – a pattern of familial relationships supported by a particular set of data based on DNA sequence, fossils, or the physical characteristics of living organisms.

But here are two alternate reasons to build trees:

  • To inspire wonder
  • Or (my favorite) just because

To reflect these additional motivations – this conviction that trees are for everyone and for all occasions and that an evolutionary tree belongs on every street corner – when I build trees, I often avail myself of a range of non-traditional materials. I’ve written previously about creating edible trees using cake frosting and fruit, as well as building trees out of state symbols and popular songs. Now here are two additional building materials, which are arguably even more fun.

First: Stickers. This one is titled: “Like Apples and Oranges…and Bananas.”

Bananas split ways with the common ancestor of apples and oranges about 150 million years ago, 50 million years before the split between apples and oranges. On this tree, these relationships are represented like so: the banana branch diverges from the apple branch at a deeper position on the trunk, and the orange branch diverges from the apple branch at a shallower position. 

All of the data required to build this tree  (and essentially any tree) is available at TimeTree.org. On TimeTree, select “Get Divergence Time For a Pair of Taxa” at the top of the page. This is where one can obtain a divergence time estimate for most pairs of species. The divergence time is an approximate date, millions of years ago, at which the organisms’ last common ancestors may have lived. For more heavy duty assistance, there is the “Load a List of Species” option at the bottom of the page. Here, one can upload a list of species names (.txt), and TimeTree will generate a complete tree – a schematic that can serve as a guide in patterning one’s own phylogenetic artwork.

Here, by way of additional illustration, are three more sticker trees, equally charming and equally mouthwatering:

Carrot, watermelon, broccoli, strawberry, and pear.

Onion, asparagus, tomato, cucumber, and cherry.

Raspberry, apricot, pea, grape, and green pepper.

Sticker trees are festive takes on traditional trees. They are brighter, livelier, and more lovely. But, like traditional trees, they are also 2D, restricted to a flat sheet of paper. To extend one’s phylogenetic art projects into three dimensions, one must modify the choice of materials. There are many options. The following 3D tree, for example, employs 13 pieces of plastic toy food, the accouterments of a typical play kitchen. Segments of yarn serve as branches.

Trees like these, made of stickers or toys, constitute playful takes on deep questions. In pencil and yarn, they sketch a network of primeval relationships. They tell the history of our foods, a narrative whose origins profoundly precede us, as well as our intention to selectively breed them. To the Way-Before, to the Way-Way-Way-Before, these projects give shape and color. If and where they succeed, it is because they manage to do two things at once: To communicate a vast biological saga extending across many millions of years, and to be completely cute. Perhaps best of all – and let it not go unmentioned – anyone can make them.

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Bio: Rachel Rodman has a Ph.D. in Arabidopsis genetics and presently aspires to recast all of art, literature, and popular culture in the form of a phylogenetic tree.

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The Agents That Shape the Floral Traits of Sunflowers

Flowers come in a wide array of shapes, sizes, colors, and scents. Their diversity is downright astounding. Each individual species of flowering plant has its own lengthy story to tell detailing how it came to look and act the way it does. This is its evolutionary history. Unraveling this history is a nearly insurmountable task, but one that scientists continue to chip away at piece by piece.

In the case of floral traits – particularly for flowers that rely on pollinators to produce seeds – it is safe to say that millennia of interactions with floral visitors have helped shape not only the way the flower looks, but also the nature of its nectar and pollen. However, flowers are “expensive” to make and maintain, so even though they are necessary for reproduction, plants must find a balance between that and allocating resources for defense – against both herbivory and disease – and growth. This balance can differ depending on a plant’s life history – whether it is annual or perennial. An annual plant has one shot at reproduction, so it can afford to funnel much of its energy there. If a perennial is unsuccessful at reproduction one year, there is always next year, as long as it has allocated sufficient resources towards staying alive.

Where a plant exists in the world also influences how it looks. Abiotic factors like temperature, soil type, nutrient availability, sun exposure, and precipitation patterns help shape, through natural selection, many aspects of a plant’s anatomy and physiology, including the structure and composition of its flowers. Additional biotic agents like nectar robbers, florivores, and pathogens can also influence certain floral traits.

This is the background that researchers from the University of Central Florida and University of Georgia drew from when they set out to investigate the reasons for the diverse floral morphologies in the genus Helianthus. Commonly known as sunflowers, Helianthus is a familiar genus consisting of more than 50 species, most of which are found across North America. The genus includes both annuals and perennials, and all but one species rely on cross-pollination to produce viable seeds. Pollination is mainly carried out by generalist bees.

Maximilian sunflower (Helianthus maximiliani)

Helianthus species are found in diverse habitats, including deserts, wetlands, prairies, rock outcrops, and sand dunes. Their inflorescences – characteristic of plants in the family Asteraceae – consist of a collection of small disc florets surrounded by a series of ray florets, which as a unit are casually referred to as a single flower. In Helianthus, ray florets are completely sterile and serve only to attract pollinators. Producing large and numerous ray florets takes resources away from the production of fertile disc florets, and sunflower species vary in the amount of resources they allocate for each floret form.

In a paper published in the July 2017 issue of Plant Ecology and Evolution, researchers selected 27 Helianthus species and one Phoebanthus species (a closely related genus) to investigate “the evolution of floral trait variation” by examining “the role of environmental variation, plant life history, and flowering phenology.” Seeds from multiple populations of each species were obtained, with populations being carefully selected so that there would be representations of each species from across their geographic ranges. The seeds were then grown out in a controlled environment, and a series of morphological and physiological data were recorded for the flowers of each plant. Climate data and soil characteristics were obtained for each of the population sites, and flowering period for each species was collected from various sources.

The researchers found “all floral traits” of the sunflower species to be “highly evolutionarily labile.” Flower size was found to be larger in regions with greater soil fertility, consistent with the resource-cost hypothesis which “predicts that larger and more conspicuous flowers should be selected against in resource-poor environments.” However, larger flower size had also repeatedly evolved in drier environments, which goes against this prediction. Apart from producing smaller flowers in dry habitats, flowering plants have other strategies to conserve water such as opening their flowers at night or flowering for a short period of time. Sunflowers do neither of these things. As the researchers state, “this inconsistency warrants consideration.”

The researchers speculate that “the evolution of larger flowers in drier environments” may be a result of fewer pollinators in these habitats “strongly favoring larger display sizes in self-incompatible species.” The flowers are big because they have to attract a limited number of pollinating insects. Conversely, flowers may be smaller in wetter environments because there is greater risk of pests and diseases. This is supported by the enemy-escape hypothesis – smaller flowers are predicted in places where there is increased potential for florivory and pathogens. Researchers found that lower disc water content had also evolved in wetter environments, which supports the idea that the plants may be defending themselves against flower-eating pests.

Seed heads of Maximilian sunflower (Helianthus maximiliani)

Another interesting finding is that, unlike other genera, annual and perennial sunflower species allocate a similar amount of resources towards reproduction. On average, flower size was not found to be different between annual and perennial species. Perhaps annuals instead produce more flowers compared to perennials, or maybe they flower for longer periods. This is something the researchers did not investigate.

Finally, abiotic factors were not found to have any influence on the relative investment of ray to disc florets or the color of disc florets. Variations in these traits may be influenced instead by pollinators, the “biotic factor” that is considered “the classic driver of floral evolution.” This is something that will require further investigation. As the researchers conclude, “determining the exact drivers of floral trait evolution is a complex endeavor;” however, their study found “reasonable support for the role of aridity and soil fertility in the evolution of floral size and water content.” Yet another important piece to the puzzle as we learn to tell the evolutionary history of sunflowers.

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Thoughts on Equisetum Phylogenesis

This a guest post. Words and photos by Jeremiah Sandler.

These notes do not discuss either anatomy or medicinal uses of Equisetum. Both topics are worthy of their own discourse.

Plants in the genus Equisetum can be found on each continent of our planet, except for Antarctica. The plants are collectively referred to as scouring rush or horsetail.  Equisetum is in the division of plants called Pteridophytes, which contains all of the ferns and fern-allies (lycopods, whisk ferns, etc.) Pteridophytes are characterized by having a vascular system and by reproducing with spores, rather than seeds. Equisetum is the only living genus within the entire class Equisetopsida.  Within this single genus, there are a mere 20 species.

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Equisetums can live pretty much anywhere. They can tolerate lots of shade, lots of sun, and virtually any soil condition (including submerged soil). Rhizomatous stems make it difficult for either disease or insects to kill an entire population. They do not require pollinators because they reproduce with spores.  Sounds like a recipe for reproductive and evolutionary success. Yet with all of these traits working in their favor, there is only a single genus left.  

Where’d they all go?

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Let’s briefly consider the origin of these plants first. In the late Paleozoic Era, during the end of the Cambrian Period, these plants began their takeover. Shortly thereafter (about 70 million years later), in the Devonian Period, land plants began to develop a tree-like habit, also called “arborescence.” Tree-sized ferns and fern-allies ruled the planet. They formed the ancient forests.

The elements required for photosynthesis were plentiful. The planet was warm. Competition from the Cambrian Explosion of flora and fauna drove plants upwards towards the sky. Larger plants can both shade their competition and remain out of reach of herbivores. None of the Equisetum species alive today are near their ancestors’ height.  

picture 3

It is rather obvious why we don’t see as many Equisetum species, and why they are not as large: The planet now is not the same planet it once was. Oxygen levels back in those times were about 15% higher than today’s levels. Seed plants can diversify much faster than non-seed-bearing plants; Equisetum cannot compete with the rate of diversification of seed-bearing plants.

The most interesting predicament comes when Equisetum is compared with other Pteridophytes. Some ancient Pteridophytes still do have diversity of genera. True Ferns, as they’re called, are broad-leaved ferns. In the class Filicopsida, there are 4 orders of True Ferns containing about 100 genera combined. Equisetum has 1 order and 1 genera.

What’s the primary difference between these two classes of Pteridophytes?  Broad leaves.

Most pteridophytes tolerate some shade; most other plants can’t tolerate as deep of shade as ferns. More specifically, the amount of shade the plants create could be a deciding factor in this question. True ferns have all of the traits equisetums have, with one additional physical trait that has pulled them ahead: Broad leaves allow true ferns to actively shade out local competition while creating more habitat for themselves. Equisetums don’t have this aggressive capacity.

Of course there are other biological and evolutionary pressures affecting equisetums beside their lack of broad leaves. The structure they do possess has benefited them at a time when it was advantageous to have it.  Otherwise why would it exist? Equisetums remind me of the dynamic nature of a planet. I don’t anticipate equisetums coming back. 

Although, I find it entertaining to humor the idea that they might return to their former glory. The planet’s climate could change toward any direction (I’m not a climatologist, though). Maybe equisetums are adequately prepared to adapt to whatever changes come – or maybe we are observing the gradual decline of an old branch on the tree of life.  

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