Tag Archives: adaptation

Zebra Stripes

Zebra_Botswana

Why do zebras have stripes? Well, we’ve all learned from an early age that their peculiar and unique black and white stripes are an adaptation to combat predators. One theory suggests that the stripes are camouflage. Another theory suggests that the stripes are there to confuse predators. Yet another proposes that the stripes are a vivid warning signal.

But Tim Caro, professor of wildlife biology at the University of California, has a thoroughly different idea, conveyed in his new book, Zebra Stripes. After twenty years of study he’s convinced that the zebra’s stripes have a more mundane purpose — a deterrent to pesky biting flies.

From Wired:

At four in the morning, Tim Caro roused his colleagues. Bleary-eyed and grumbling, they followed him to the edge of the village, where the beasts were hiding. He sat them down in chairs, and after letting their eyes adjust for a minute, he asked them if they saw anything. And if so, would they please point where?

Not real beasts. Despite being camped in Tanzania’s Katavi National Park, Caro was asking his colleagues to identify pelts—from a wildebeest, an impala, and a zebra—that he had draped over chairs or clotheslines. Caro wanted to know if the zebra’s stripes gave it any sort of camouflage in the pre-dawn, when many predators hunt, and he needed the sort of replicability he could not count on from the animals roaming the savannah. “I lost a lot of social capital on that experiment,” says Caro. “If you’re going to be woken up at all, it’s important to be woken up for something exciting or unpredictable, and this was neither.”

The experiment was one of hundreds Caro performed over a twenty year scientific odyssey to discover why zebras have stripes—a question that nearly every major biologist since Alfred Russel Wallace has tried to answer. “It became sort of a challenge to me to try and investigate all the existing hypotheses so I could not only identify the right one,” he says, “but just as importantly kill all those remaining.” His new book, Zebra Stripes, chronicles every detail.

Read the entire story here.

Image: Zebras, Botswana. Courtesy: Paul Maritz, 2002. Creative Commons Attribution-Share Alike 3.0.

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How and Why Did Metamorphosis Evolve?

papilio_machaon

Evolution is a truly wondrous thing. It has given us eyes and lots of grey matter [which we still don’t use very well]. It has given us the beautiful tiger and shimmering hues and soaring songs of our birds. It has given us the towering Sequoias, creepy insects, gorgeous ocean-bound creatures and invisible bacteria and viruses. Yet for all its wondrous adaptations one evolutionary invention still seems mysteriously supernatural — metamorphosis.

So, how and why did it evolve? A compelling new theory on the origins of insect metamorphosis by James W. Truman and Lynn M. Riddiford is excerpted below (from a detailed article in Scientific American).

The theory posits that a beneficial mutation around 300 million years ago led to the emergence of metamorphosis in insects:

By combining evidence from the fossil record with studies on insect anatomy and development, biologists have established a plausible narrative about the origin of insect metamorphosis, which they continue to revise as new information surfaces. The earliest insects in Earth’s history did not metamorphose; they hatched from eggs, essentially as miniature adults. Between 280 million and 300 million years ago, however, some insects began to mature a little differently—they hatched in forms that neither looked nor behaved like their adult versions. This shift proved remarkably beneficial: young and old insects were no longer competing for the same resources. Metamorphosis was so successful that, today, as many as 65 percent of all animal species on the planet are metamorphosing insects.

And, there are essentially three types of metamorphosis:

Wingless ametabolous insects, such as silverfish and bristletails, undergo little or no metamorphosis. When they hatch from eggs, they already look like adults, albeit tiny ones, and simply grow larger over time through a series of molts in which they shed their exoskeletons. Hemimetaboly, or incomplete metamorphosis, describes insects such as cockroaches, grasshoppers and dragonflies that hatch as nymphs—miniature versions of their adult forms that gradually develop wings and functional genitals as they molt and grow. Holometaboly, or complete metamorphosis, refers to insects such as beetles, flies, butterflies, moths and bees, which hatch as wormlike larvae that eventually enter a quiescent pupal stage before emerging as adults that look nothing like the larvae.

And, it’s backed by a concrete survival and reproductive advantage:

[T]he enormous numbers of metamorphosing insects on the planet speak for its success as a reproductive strategy. The primary advantage of complete metamorphosis is eliminating competition between the young and old. Larval insects and adult insects occupy very different ecological niches. Whereas caterpillars are busy gorging themselves on leaves, completely disinterested in reproduction, butterflies are flitting from flower to flower in search of nectar and mates. Because larvas and adults do not compete with one another for space or resources, more of each can coexist relative to species in which the young and old live in the same places and eat the same things.

Read the entire article here.

Image: Old World Swallowtail (Papilio machaon). Courtesy: fesoj – Otakárek fenyklový [Papilio machaon]. CC BY 2.0, https://commons.wikimedia.org/w/index.php?curid=7263187

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Non-Adaptive Evolution of the Very Small

Is every feature that arises from evolution an adaptation?  Some evolutionary biologists think not. That is, some traits arising from the process of natural section may be due to random occurrences that natural selection failed to discard. And, it seems that smaller organisms show this quite well. To many adaptationists this is heretical — but too some researchers it opens a new, fruitful avenue of inquiry, and may lead to a fine tuning in our understanding of the evolutionary process.

From New Scientist:

I have spent my life working on slime moulds and they sent me a message that started me thinking. What puzzled me was that two different forms are found side-by-side in the soil everywhere from the tundra to the tropics. The obvious difference lies in the tiny stalks that disperse their spores. In one species this fruiting body is branched, in the other it is not.

I had assumed that the branched and the unbranched forms occupied separate ecological niches but I could not imagine what those niches might be. Perhaps there were none and neither shape had an advantage over the other, as far as natural selection was concerned.

I wrote this up and sent it to a wise and respected friend who responded with a furious letter saying that my conclusion was absurd: it was easy to imagine ways in which the two kinds of stalks might be separate adaptations and co-exist everywhere in the soil. This set me thinking again and I soon realised that both my position and his were guesses. They were hypotheses and neither could be proved.

There is no concept that is more central to evolution than natural selection, so adding this extra dimension of randomness was heresy. Because of the overwhelming success of Darwin’s natural selection, biologists – certainly all evolutionary biologists – find it hard to believe that a feature of any organism can have arisen (with minor exceptions) in any other way. Natural selection favours random genetic mutations that offer an advantage, therefore many people believe that all properties of an organism are an adaptation. If one cannot find the adaptive reason for a feature of an organism, one should just assume that there was once one, or that there is one that will be revealed in the future.

This matter has created some heated arguments. For example, the renowned biologists Stephen Jay Gould and Richard Lewontin wrote an inflammatory paper in 1979 attacking adaptionists for being like Dr Pangloss, the incurable optimist in Voltaire’s 1759 satire Candide. While their point was well taken, its aggressive tone produced counterattacks. Adaptionists assume that every feature of an organism arises as an adaption, but I assume that some features are the results of random mutations that escape being culled by natural selection. This is what I was suggesting for the branched and unbranched fruiting bodies of the slime moulds.

How can these organisms escape the stranglehold of selection? One explanation grabbed me and I have clung to it ever since; in fact it is the backbone of my new book. The reason that these organisms might have shapes that are not governed by natural selection is because they are so small. It turns out there are good reasons why this might be the case.

Development is a long, slow process for large organisms. Humans spend nine months in utero and keep growing in different ways for a long time after birth. An elephant’s gestation is even longer (about two years) and a mouse’s much shorter, but they are all are vastly longer than a single-cell microorganism. Such small forms may divide every few hours; at most their development may span days, but whatever it is it will be a small fraction of that of a larger, more complex organism.

Large organisms develop in a series of steps usually beginning with the fertilisation of an egg that then goes through many cell divisions and an increase in size of the embryo, with many twists and turns as it progresses towards adulthood. These multitudinous steps involve the laying down of complex organs such as a heart or an eye.

Building a complex organism is an immense enterprise, and the steps are often interlocked in a sequence so that if an earlier step fails through a deleterious mutation, the result is very simple: the death of the embryo. I first came across this idea in a 1965 book by Lancelot Law Whyte called Internal Factors in Evolution and have been mystified ever since why the idea has been swallowed by oblivion. His thesis was straightforward. Not only is there selection of organisms in the environment – Darwinian natural selection, which is external – but there is also continuous internal selection during development. Maybe the idea was too simple and straightforward to have taken root.

This fits in neatly with my contention that the shape of microorganisms is more affected by randomness than for large, complex organisms. Being small means very few development steps, with little or no internal selection. The effect of a mutation is likely to be immediately evident in the external morphology, so adult variants are produced with large numbers of different shapes and there is an increased chance that some of these will be untouched by natural selection.

Compare this with what happens in a big, complex organism – a mammal, say. Only those mutations that occur at a late stage of development are likely to be viable – eye or hair colour in humans are obvious examples. Any unfavourable mutation that occurs earlier in development will likely be eliminated by internal selection.

Let us now examine the situation for microorganisms. What is the evidence that their shapes are less likely to be culled by natural selection? The best examples come from organisms that make mineral shells: Radiolaria (pictured) and diatoms with their silica skeletons and Foraminifera with their calciferous shells. About 50,000 species of radiolarians have been described, 100,000 species of diatoms and some 270,000 species among the Foraminifera – all with vastly different shapes. For example, radiolarian skeletons can be shaped like spiny balls, bells, crosses and octagonal pyramids, to name but a few.

If you are a strict adaptionist, you have to find a separate explanation for each shape. If you favour my suggestion that their shapes arose through random mutation and there is little or no selection, the problem vanishes. It turns out that this very problem concerned Darwin. In the third (and subsequent) editions of On the Origin of Species he has a passage that almost takes the wind out of my sails:

“If it were no advantage, these forms would be left by natural selection unimproved or but little improved; and might remain for indefinite ages in their present little advanced condition. And geology tells us that some of the lowest forms, as the infusoria and rhizopods, have remained for an enormous period in nearly their present state.”

Read the entire article here.

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DarwinTunes

Charles_DarwinResearchers at Imperial College, London recently posed an intriguing question and have since developed a cool experiment to test it. Does artistic endeavor, such as music, follow the same principles of evolutionary selection in biology, as described by Darwin? That is, does the funkiest survive? Though, one has to wonder what the eminent scientist would have thought about some recent fusion of rap / dubstep / classical.

From the Guardian:

There were some funky beats at Imperial College London on Saturday at its annual science festival. As well as opportunities to create bogeys, see robots dance and try to get physics PhD students to explain their wacky world, this fascinating event included the chance to participate in a public game-like experiment called DarwinTunes.

Participants select tunes and “mate” them with other tunes to create musical offspring: if the offspring are in turn selected by other players, they “survive” and get the chance to reproduce their musical DNA. The experiment is online – you too can try to immortalise your selfish musical genes.

It is a model of evolution in practice that raises fascinating questions about culture and nature. These questions apply to all the arts, not just to dance beats. How does “cultural evolution” work? How close is the analogy between Darwin’s well-proven theory of evolution in nature and the evolution of art, literature and music?

The idea of cultural evolution was boldly defined by Jacob Bronowski as our fundamental human ability “not to accept the environment but to change it”. The moment the first stone tools appeared in Africa, about 2.5m years ago, a new, faster evolution, that of human culture, became visible on Earth: from cave paintings to the Renaissance, from Galileo to the 3D printer, this cultural evolution has advanced at breathtaking speed compared with the massive periods of time it takes nature to evolve new forms.

In DarwinTunes, cultural evolution is modelled as what the experimenters call “the survival of the funkiest”. Pulsing dance beats evolve through selections made by participants, and the music (it is claimed) becomes richer through this process of selection. Yet how does the model really correspond to the story of culture?

One way Darwin’s laws of nature apply to visual art is in the need for every successful form to adapt to its environment. In the forests of west and central Africa, wood carving was until recent times a flourishing art form. In the islands of Greece, where marble could be quarried easily, stone sculpture was more popular. In the modern technological world, the things that easily come to hand are not wood or stone but manufactured products and media images – so artists are inclined to work with the readymade.

At first sight, the thesis of DarwinTunes is a bit crude. Surely it is obvious that artists don’t just obey the selections made by their audience – that is, their consumers. To think they do is to apply the economic laws of our own consumer society across all history. Culture is a lot funkier than that.

Yet just because the laws of evolution need some adjustment to encompass art, that does not mean art is a mysterious spiritual realm impervious to scientific study. In fact, the evolution of evolution – the adjustments made by researchers to Darwin’s theory since it was unveiled in the Victorian age – offers interesting ways to understand culture.

One useful analogy between art and nature is the idea of punctuated equilibrium, introduced by some evolutionary scientists in the 1970s. Just as species may evolve not through a constant smooth process but by spectacular occasional leaps, so the history of art is punctuated by massively innovative eras followed by slower, more conventional periods.

Read the entire story here.

Image: Charles Darwin, 1868, photographed by Julia Margaret Cameron. Courtesy of Wikipedia.

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