The difference between blackberries and mulberries and why it matters

Blackberries grow on prickly vines or brambles, 
and are members of the Rose Family (Rosaceae).

 As I was picking mulberries from a tree in my back yard the other day, I was reminded of the similarity between blackberries and mulberries. They are strikingly similar in appearance. 

Like most dark fruits, they are both rich in nutrients and protective phytochemicals. For the consumer, the primary differences are the somewhat milder, less sweet flavor, and the annoying little green stems of of mulberries. Depending on the climate, one or the other may be easier to grow, and fresh blackberries are generally more widely available in stores. Dried mulberries, however, are becoming increasingly popular and more available. Beyond all that, it's a matter of taste.

Mulberries grow on trees, and are 
members of the Mulberry Family
(Moraceae).

In terms of the teaching of botany and evolution, however, the similarities and differences between the two berries tell a powerful story. Though they function the same way in natural fruit dispersal, they are not related at all. Blackberries are members of the the very fruitful family, Rosaceae, which includes raspberries, strawberries, plums, cherries, peaches, apricots, apples, pears, rose hips and many more.  Mulberries, on the other hand are members of the Moraceae, which includes figs, breadfruit, and rubber trees. 

The structures of the two fruits are quite different. Blackberries are aggregate fruits, which means that the cluster of drupelets derive from a cone of separate carpels belonging to a single flower. Mulberries on the other hand are technically infructescences, or compound fruits, similar to pineapples. Each drupelet forms from its own tiny flower. So there are fundamental developmental differences that lead to the similar looking fruits.

This might seem like a geeky bit of botanical trivia that would quickly make dinner guests fall asleep, but in the classroom, however, it illustrates some of the most central phenomena of evolution: adaptation, adaptive radiation, and convergent evolution.

These fruits, first of all, are adapted for dispersal by animals, primarily birds, though I have had to keep an eye out for hungry black bears as well while picking berries along roadsides in Washington State. Both go through green and red phases before turning black at ripening. This primes the animals for the coming feast. The berries are sweet, juicy, and flavorful. The animals gobble down the fruits, and the digestive process strips away the juicy tissues, leaving the tiny seeds to pass through the alimentary canal. The animals tend to move about after feeding, leaving seeds in their feces. (See also "What is an Adaptation?)

The different kinds of fruit to be found in the Rose Family are an example of adaptive radiation - the evolution  of a variety of descendant species from a common ancestor. As the descendants of the common ancestor  began spreading into new habitats and new geographic areas, they adapted to local conditions, including local fruit dispersers. 

As other families went through their own adaptive radiations, some descendants encountered the same opportunity for dispersal, and developed similar physical characteristics, but with tell-tale differences in underlying structure. This is convergent evolution - the development of very similar adaptations from unrelated ancestors. The ancestors of the Rose Family happened to have flowers with multiple separate carpels, and so easily evolved into aggregate fruits, while the ancestors of the Mulberry Family had tiny flowers with just one carpel in each, so a similar fruit was most easily developed by grouping the fruits of many flowers together.  I have posted earlier about evolution of cactus-like members in  unrelated families, along with numerous examples of convergent evolution in animals. (See "Of cacti and humans – are certain life forms inevitable?"

Theme and Variation - the Amaryllidaceae

The primary types of cultivated amaryllis are in the genus Hippeastrum.  

Their flowers are mostly shades and mixes of red, pink, and white. 

This is  one of my favorite cultivars, "Eye-catcher."

This spring, while I was waiting eagerly for the amaryllis plants in my yard to bloom, I started reflecting on
the marvelous family to which they belong, and how nicely they represent a fascinating aspect of plant evolution.


The Amaryllis family is known and beloved worldwide, even by people unfamiliar with its technical name or taxonomy, for it provides us with a variety of unique spring-flowering bulbs and perennials, from daffodils to subtropical amaryllis and tropical Crinums.

As presently defined, Amaryllis (technically the genus Hippeastrum), daffodils (Narcissus) and Crinum all belong to the subfamily Amarylliodeae. Onion, garlic, etc.are also members of the family, constituting the subfamily Allioideae.   Finally, the blue-flowered "Lily-of-the-Nile" (Agapanthus), from southern Africa, is technically in it's own subfamily, Agapanthoideae.  Altogether, there are some 1600 species in 75 genera, found naturally on every unfrozen continent.

Daffodils are specialized members of  the genus Narcissus, in which the umbel has been reduced to a single flower.

The subfamily Agapanthoideae consists of the single genus Agapanthus from
southern Africa. Flowers are blue to white.

The onion subfamily,  Allioideae, contains numerous aromatic and edible species.
The characteristic pungent fragrances are based on allyl sulfides,
which in nature  act as deterrents to insect pests.

The true bulbs of onions and amaryllis are made up of
the swollen bases of recent leaves that encircle the
central stem.  The outermost layers, representing
 older leaf bases, become dried and paper-like,
 which protects the fresh inner layers from drying out.
 This gives rise to the designation "tunicate bulbs,"
differing from the scale-bulbs of the true lilies.
Photo by Amada44, CC BY-SA 3.0.

So what defines this family? What is the common theme upon which the 1600 species are variants? The vast majority of the species in this family are geophytes, plants that survive adverse seasons underground. Most species form bulbs, but some, like Agapanthus and certain members of the Allioideae, employ underground rhizomes instead.  The leaves are strap-shaped (sometimes tubular and hollow in the onions) and extend themselves upward from the bulb by basal intercalary meristems (see "How the grass leaf got its stripes").  This is the most common form of leaf in the monocots, and it varies little in this family.

True, or tunicate bulbs (see illustration to the left), differing from the scale-bulbs of the true lilies, do seem to be a unique invention of this family,  though some members of the Lily family, such as tulips, have evolved a similar type of bulb independently.

Flowers in the Amaryllidaceae  undergo preliminary 
development below ground, between leaves or within the 
bulb and are protected by a closed sheath.  The enclosed 
bud is then pushed upward by the intercalary growth of  
the stalk.

But it is how the flowers emerge from the bulbs that is the most iconic, revolutionary, and consistent theme of the family.  Flowers form below ground, tightly enveloped in a protective sheath.   Below each inflorescence bud, a stalk (the peduncle) develops and lengthens through basal intercalary growth (i.e. new tissues are produced at the base of the stalk, pushing older tissues and the inflorescence bud upwards).  After rising to optimum height for pollination and eventual seed dispersal, the sheath splits open to reveal a simple umbel, i.e. one to many flowers arising from a single point at the tip of the stalk, roughly forming the shape of an umbrella or sometimes an entire sphere.

This proved to be a remarkably effective way to protect and elevate the flowers, for after it evolved in the common ancestors of the family, descendant species spread worldwide, adapting to different climates, soils, and pollinators. Such a spreading diversification is called an adaptive radiation. Note that the special structure and growth form of the inflorescence remained essentially unchanged throughout the family, while details of flower structure and color, fruit type, and physiological adaptations diversified.

Yellow flowers are uncommon in the Amaryllidaceae, but found here in
Lycoris aureus.  Photo by Tomago Moffle, CC BY-SA 3.0.

 The importance of this discussion is not simply to say how wonderful and unique the Amaryllidaceae is, but to stimulate us, particularly those of us who are teachers, to look for similar patterns of breakthrough adaptations followed by adaptive radiation throughout the plant kingdom.

Almost any genus, and sometimes a whole family can be seen to be based on some "great idea," i.e. some new structure, growth pattern, flower type, etc., that gave the ancestral species an advantage and allowed its descendants to diversify into great numbers.  Two simple examples are the genus Aquilegia (columbines) with its nectar spurs arising from each of the five petals, and the genus Euphorbia, with its highly compact flowering units called cyathia.

How many examples can you find? Can you explain the adaptive value of the distinctive features?

Each yellowish, red-tipped structure in this Poinsettia
(genus Euphorbia) is a cyathium, a cupule containing several
tiny flowers.

The highly distinctive flowers of Aquilegia feature a nectar
spur projecting backwards from each petal.

The giant crinum, C. asiaticum, from southern China, is a tropical evergreen
plant that develops a pseudostem, similar to that of the banana, made of the
tubular bases of the leaves.

What is an adaptation?

What do we mean by adaptation?  We can use that word  both as a process and as the observable result of that process.   Adaptation is the process of evolutionary change under the guidance of natural selection.  It is the process in which populations become genetically modified to function more efficiently in their specific environment, to respond to changes in the environment, or to move into new environments.  The result of that process is new or altered characteristics that we refer to as adaptations.

An important working assumption, or hypothesis, in biology is that every observable characteristic or trait of an organism has some adaptive significance, or at least had adaptive significance sometime during the ancestry of the organism.  A related assumption is that the total set of adaptations (and hence the total set of observable characteristics) is unique for each species, and defines a unique ecological niche.  That in turn means that each species "fits" into the biosphere in a different way from every other species. Discovering the adaptive meaning of everything from leaf shape to flower color is to me the most exciting part of botany, or biology in general.

Let's just take one example: the shape of cactus stems.  First, of course, cactus stems are succulent, i.e. filled with water-storage tissue.  They gather water during the brief and infrequent rain storms, store it, and utilize it sparingly during the long dry spells.  It allows cacti to continue to function, even to bloom at predictable times, rather than become dormant during those dry periods. That is the signature adaptation made by early members of the cactus family.

Cactus stems are also, in the absence of leaves, photosynthetic.  The two major functions of cactus stems requires some interesting compromises. They need to gather light, but exposure to the intense sunlight and heat of the desert environment can potentially result in overheating and tissue damage.   Imagine leaving a plastic jug of water out in the full sun, with surrounding air temperatures over 100 degrees F.

The approximately 1500 species of the cactus family have evolved a variety of mechanisms to cope with this heating problem.  The evolution of many species from a single common ancestor is called adaptive radiation.

Many cacti are round but narrow,optimizing water storage while reducing
exposure along the sides at mid-day sun and optimizing exposure in
the early morning or late afternoon, Photo by RC Designer t-w-m-c _stockarch.com.

Most cacti have adaptations that minimize exposure during the hottest hours of the mid-day.  One strategy is to take on an erect and narrow shape.  This allows full exposure to early morning and late afternoon sun, when temperatures are somewhat cooler.  In the middle of the day, however, only the small tip of the stem faces directly into the sun, and the sides receive light obliquely.

Beavertail cacti (genus Opuntia) take that strategy a step further. Their stems develop as flattened segments, which expose even less surface to the noon-time sun, and even more direct exposure  early and late in the day.

The flattened segments of a beavertail cactus (Opuntia) gather
light optimally when the sun is low in the sky, and provide
minimal exposure in mid-day.  Photo by Stan Sherm, Wikipedia.

A spherical or barrel-shaped stem would seem to be all wrong - exposed maximally at high noon.   It is however the most efficient way to store water.  The round shape provides the minimum ratio of evaporative surface area to water storage volume, but it does potentially provide the greatest proportion of its surface facing directly to the noonday sun.  To compensate, barrel cacti often have large curved spines, or numerous long hair-like spines that provide protection from the intense sun.

Cactus spines, which are modified leaves, are particularly well-developed in
broadly rounded cacti, and serve both for protection against herbivores and for
shading from mid-day sun.  Photo by t-w-m-c _stockarch.com.

Most barrel cacti are ribbed, allowing expansion of the water-storage tissues, and
 also decreasing exposure of the surface tissues to direct sunlight.
Photo by F. B. Essig

Barrel cacti are often fluted, or corrugated, as well - their surfaces consist of accordion-like ridges and valleys.  This further reduces the amount of surface exposed to intense sunlight.   This fluting has a second function as well, allowing the stems to shrink or expand neatly as their internal water stores fluctuate.  The vascular tissues in these stems are concentrated into a series of parallel ribs, so as to allow the expansion of tissues between them.

Another aspect of adaptation is how they are chained together over time, one leading to another to arrive at the characteristic features of a current organism.  We can say that adaptive change is canalized, (see G. L.Stebbins and the process of adaptive modification),  and develops momentum in a particular direction.  Certain kinds of change come naturally based on what has come before; others are extremely unlikely.  I have referred to this as "adaptive parsimony" in some of my other essays (see Were the first monocots syncarpous?) A flying elephant is unlikely, but the evolution of flight is quite possible in lightweight animals that already leap around in trees (e.g. the ancestors of bats and flying squirrels). 

Another thing lightweight arboreal mammals can become is human.  When I taught introductory biology, I had the students do a thought experiment dealing with human evolution: could humans (or equally sentient beings) have evolved from some other starting point than primates adapted to life in the trees?  Could they have evolved from grazing ungulates or dog-like carnivores?  Could they have evolved from octopi or cuttlefish? Or from insects? The evolution of stereoscopic color vision, grasping hands with opposable thumbs, and rotatable arms in arboreal primates pre-adapted some of their descendants to walk upright and use their hands to craft and utilize tools and weapons - an essential ability for developing technology.  What other path to humanity could have occurred?  We also applied this logic to fictional aliens: how might Wookies or Huts have evolved (especially the huts!)?  Well, that's another story altogether.

Returning to plants, a number of my previous postings (including the ones mentioned above) have centered around logical chains of adaptations.  In the case of cacti, the original adaptations for storing water within the stems led to modifications of the stem to avoid overheating.  In other succulent plants, leaves were modified for water storage instead of the stems (aloes, sedums, etc.).  An aloe is not likely to abandon its water-filled leaves and transfer that function to its stem, just as a cactus is highly unlikely to sprout leaves and transfer water storage to them.  The modification of stems or leaves for water storage is an either/or situation, constrained by their separate canalized adaptive trends.

Stem segments of some epiphytic cacti, such as this Schlumbergera, have
become thin and leaf-like. Photo by Peter Coxhead, Wikipedia.

When some species of cactus adapted to life as epiphytes in tropical rain forests, overheating was not as big a problem, but they needed to absorb more of the light that came to them, They did not sprout leaves again, but instead developed flattened, leaf-like stem segments.  It was a simpler adaptive path.  They also ditched the stem fluting and heavy spines so as to expose more of their light-gathering surface.

Focusing on adaptation can be a highly useful way to teach botany.  It allows one to tell engaging stories that combine systematics (the differences among plants), ecology, anatomy, and physiology.