Speciation, Extinction, and the Diversity of Life

Posted by on March 2, 2016
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Even Mosquitoes Have Value

The other day, while being interviewed about the Zika virus, Dr. Anthony Fauci of NIH was asked if he thought mosquitoes had any value at all. He said it would be wonderful if we could eliminate them totally, once and for all. I understand his perspective, given the range of diseases that hop from person to person via a mosquito, but I look at it rather differently. Mosquitoes are amazing. They are equipped with a wonderful example of a finely tuned, piercing/sucking device, way more complex than a hypodermic, built by the evolutionary modification of a much simpler chewing mouth. Like all insects, female mosquitoes possess a bewilderingly large number of different mouth appendages with names like labium, labrum, and mandible. These have evolved into a tubular sucking organ, with knives at the tip for piercing. The males possess the same feeding structure minus the knives and feed only on nectar.

Mosquito mouthparts Krenn Elsevier

Scanning electron micrographs of the head of a female Anopheles mosquito. In A) the mouthparts are all contained within the proboscis, a scaled sheath modified from the labium (La). The labella (Lb) is seen at its tip. C) and D) provide more detailed images of the tip of the proboscis – visible are the labrum (Lr) which forms the feeding tube (Fc) within the labium, and the laciniae (Lc) and mandibles (Ma), together called the stylets, the bits that do the piecing/slicing. The males lack these serated blades and can only suck nectar. Figure © H. Krenn and Elsevier.

Insects that feed on blood evolved several times during the Jurassic and Cretaceous. Recognizably modern mosquitoes first appeared about 75 to 100 million years ago during the Cretaceous, and their females have been having blood meals ever since. About 3500 species of mosquito now exist, and if such creatures had never evolved, our world would be just a bit more common-place (and our summer evenings would be more comfortable!).

I’m not discussing insect evolution today; I want to reflect on biodiversity, the complex spatio-temporal variation in this thing we call life, and the roles of speciation and extinction in determining how much biodiversity is around us. And to reflect on what it means to lose diversity, because we are currently losing diversity very rapidly.

Biodiversity – The Richness of Life on This Planet

Biodiversity, the variety of life, is a simple product of mutation and extinction. For typically sexually reproducing creatures like mosquitoes, speciation is a consequence of the accumulation of mutations over time until the point is reached where one population of creatures is now sufficiently different from its nearest relatives that they can no longer interbreed successfully. Mutations are mistakes in replication of genetic information, mistakes that happen randomly as long strands of DNA duplicate themselves, base pair by base pair, each time a cell divides. Think of them as typos that occur during the copying of long strings of the letters a, c, g, and t – the first letters of the names of the bases that make up the genetic code. Many mistakes are deleterious, and the organism unlucky enough to carry them dies early, or is less successful at reproducing. In this way those deleterious mistakes are constantly being removed. Many other mistakes are harmless neither benefiting nor harming the organism that carries them; these accumulate to provide the rich genetic diversity within any species. Some of the mistakes equip the organism to be more successful than it otherwise was in the environment it inhabits. Evolution is the inevitable process whereby such a favorable mistake gradually becomes common (or even universally present) in the population in which it occurred (because the more successful individuals have more offspring than their neighbors), and that population becomes genetically distinct from, and no longer able to breed with, others to which it is related.

Technically, biodiversity is not simply measured as numbers of species. Every species contains an enormous amount of genetic variation within in. If it didn’t, creatures belonging to one species would be genetically identical, and life would be a whole lot duller. It is possible to think of biodiversity at different levels – the genetic variability within populations or species, the numbers of different species at a location, the differences among the species present at different locations – however, thinking about numbers of species is a convenient way of thinking about variation in biodiversity through space a time (plus it’s a lot easier to deal with numbers of species than to deal with genetic sequences, especially if you are out in the woods being bitten by mosquitoes which is where I started out).

Biodiversity, at the level of the species, is basically set by the balance between speciation and extinction. If the rate of speciation is higher than the rate of extinction, the number of species present will grow, and conversely, if extinction rate is higher, the number of species present will fall. There are lots of reasons to anticipate that the rate of species formation is likely to be more or less constant through time. The transcriptional mistakes that create the genetic differences that can allow a population to become different to its neighbors are simple chemical or physical errors made while molecules are being assembled into long strands of DNA. Indeed, a recent paper in Molecular Biology and Evolution, based on an enormous set of molecular genetic data, has just declared that across all the different types of eukaryote life there has been a continuous, or perhaps a slowly increasing, rate of diversification through the 2 billion years of geological time since cellular creatures first evolved, and that the rate of species production is also rather uniform across vertebrates, arthropods and plants taking on average about 2 million years for one species to split into two in each of these broad groups. In saying this Blair Hedges and five colleagues at Temple University also produced a lovely spiral diagram to represent all of life from bacteria and protists to humans (and mosquitoes).

Tree of Life Hedges MolBiolEvol 2015 F1.large

The molecular tree of life is presented as a spiral, in which time, and therefore lines of descent of each group from its shared ancestors, runs across the arm, from inner to outer edge (scale at innermost terminus) while all the many different types of organism are arrayed in groups, adjacent to their nearest living relatives, along the length of the spiral. Humans fall, somewhere between 1 and 2 o’clock on the second ring in, along with the rest of the mammals. Image © B. Hedges.

It’s quite a beautiful spiral, and it does convey a sense of just how many species exist on this planet, but it is also quite useless because the resolution at which it appears on page or screen prevents you seeing the identities of the species or the patterns of linkage among them. Speciation from the beginning of the first ancestral type to the present (a span of perhaps 2 billion years) is compressed into the WIDTH of the spiral arm, not its length. Just like other people, scientists can be overly impressed by their growing abilities to manage vast quantities of data and display them in two or more dimensions. Undoubtedly they have this spiral stored somewhere so it can be expanded multiple times to home in on the details surely encoded in it – but that is of scant help to the reader confronted with the spiral in the pages of their article. In some ways, old-fashioned trees with limbs and twigs were more helpful ways of explaining how members of the web of life are interrelated – even if the makers of such diagrams had the hubris to always put humans at the top of the tree. The geneticists have put us close to the outer end of the spiral, but one rotation in.

Change in Biodiversity

The idea that genetic variation increases continuously and at a relatively uniform rate is not new, but Hedges and colleagues have now backed up this conclusion with a very rich set of data. That does not mean that there will cease to be discussion. Other biologists are divided on whether evolution has proceeded more or less continuously versus whether there have been dramatic radiations of species at particular times and places. Last year, two papers were published back to back in American Naturalist. They were the result of a debate organized at a conference the year before, and the authors took opposite sides on the question of whether the number of species that can be present at one time and place is set (at a continental scale) by the ecological interactions that occur among species. This is a question that has long concerned ecologists and evolutionary biologists. One widespread view (I’ll call it the ecological limits view) is that the number of organisms, and the number of different kinds of organisms in an environment is set by the food and other resources available and by processes of competition and predation among the species to use those resources. This is the world of populations at carrying capacity and communities at equilibrium. In such a world, we’d expect the number of species over a large region, such as a continent, to be constant through time, or nearly so, even though the species present would change through time as foreign species invade from elsewhere displacing residents, and as new species evolve. I remember well a time when this was overwhelmingly the favored view.

The alternative viewpoint (which I’ll call the unbounded dynamic view), is that while competition and predation, and available supplies of food and other resources undoubtedly play a role in determining the success or otherwise of a species, there are enough other factors that determine how well a species is doing, and hence, how numerous it is becoming, that most of the time, on a continent-wide scale, species are not in constant competition for resources, and the numbers of individuals and numbers of species present vary through time as conditions favoring each species wax and wane. This, for me, is the more realistic world, especially now, when the world is changing in so many ways that achieving an equilibrium state seems a difficult task indeed. The two articles, by Luke Harmon and Susan Harrison, of U. Idaho and UC Davis respectively, for the unbounded dynamic viewpoint, and by Daniel Rabosky and Allen Hurlbert, of U. Michigan and U. North Carolina respectively, for the ecological limits viewpoint, provide excellent summaries of the arguments and data on the two sides. This almost philosophical argument within the ecological/evolutionary community is likely to continue for some time, because the processes involved take evolutionary time, and the spatial scales involved mean that data can seldom be really precise: it’s also a question that is not readily subjected to powerful experimental testing. The article by Hedges and colleagues provides molecular evidence favoring the unbounded dynamic viewpoint, and perhaps offers new ways of looking at the problem.

I tell you all this not only to give a tiny glimpse into the esoterica that keep scientists happy being scientists, but also to have you appreciate that biologists are unsure whether they live in a more or less orderly, predictable world in which such things as rates of species formation are closely controlled, or in a less orderly world in which most ecological patterns are produced by a multitude of simultaneously acting causal processes. As someone who every now and then remembers we are all clinging to a rocky mass hurtling through the cosmos on a path and at a speed that is totally beyond our ability to control, while being warmed by the light of a star that will one day die, I personally find the less constrained unbounded dynamic viewpoint the more realistic, and I am quite comfortable with speciation as something that happens inevitably because long chains of base pairs don’t always get precisely duplicated every time they are copied. Nature is not perfect, or, as some people say, “shit happens”.

Speciation may have gone on creating species at a constant rate through time (actually at an exponential rate of species production, because it is a constant rate of divergence – all populations growing further and further apart, while simultaneously becoming genetically more heterogeneous themselves, creating subpopulations which then diverge from each other, on and on, ad infinitum). But the rate of extinction has not been similarly constant.

Mass Extinctions – Is a Sixth One Coming?

Almost since the dawn of paleontology, it has been apparent that there have been times in the planet’s past when a rapid die-off of species occurred. Rocks of one age would contain numerous fossils of various species, and then, a little higher up the cliff and therefore slightly younger, the rocks would contain none of those fossils, and perhaps contain fossils that were new, never present in the older rocks. At first, the assumption was that the younger rocks were a lot younger than those beneath them, and that some long period of time must have elapsed for which no rocks remained at that site. However, with more data, from more sites, it became apparent that there were indeed short periods in the planet’s history when dramatic changes in the species present took place. Many of the time periods into which geologists have organized the Earth’s history have been defined by such discontinuities in the fossil record, and we now recognize about five times at which the extent of the die-off of species was so great that we call them mass extinction events.

640px-Extinction_intensity.svg

This graph shows the frequency of extinction as percent of extant genera lost in each time interval from the start of the Cambrian period, 542 million years ago to the present. The figure is based on Sepkoski’s 2002 data set for marine genera in groups that are reliably present in the fossil record (i.e. it is not based on all genera living at any particular time, but on a sub-set of 36 thousand reliably fossilized forms). Figure in Wikimedia Commons originally from paper in Nature in 2005 by Robert Rohde and Richard Muller of UC Berkeley.

The greatest mass extinction event occurred roughly 252 million years ago, and this event not only marks the boundary between the Permian and the Triassic periods, but the boundary between the Paleozoic and the Mesozoic eras in the history of Earth. During the end-Permian extinction event – which took place over about 60 thousand years – some 96% of all living marine species and 49% of marine families became extinct. On land, extinctions during this event were also very high; about 83% of all insect genera and 70% of vertebrate species disappeared.

The other best-studied mass extinction is the end-Cretaceous event, 66 million years ago, marking the boundary between the Cretaceous and the Paleogene periods, and between the Mesozoic and the Cenozoic eras. This event, which may have occurred over an even shorter timespan given that one of its primary causes was the impact of a massive asteroid with Mexico’s Yucatan Peninsula, resulted in the extinction of about 75% of all species on the planet, including the last of the dinosaurs, other than the feathered ones we call birds.

I wrote about mass extinction events last February, and, very briefly, in June of 2014. They are fascinating because the rapidity and extent of species loss demands explanation – what happened to cause them? And could such things happen today? There is a widespread belief among scientists that we are at the beginning of the 6th mass extinction, with rates of extinction estimated to be somewhere between 10 and 1000 times higher than normal. I wrote about this Anthropocene defaunation in July 2014, but there is more to say.

The most obvious reason for concern about loss of biodiversity is that if it continues there has to come a time when important ecological functions – such as the production of species we depend on for food — no longer are done. Biologists are unsure how much of present biodiversity can be lost before that happens, but if we are at the start of another mass extinction, perhaps we’d be wise to proceed cautiously.

An article appeared in Science last April which sheds a little light. Authored by Yann Hautier, Oxford University, David Tilman, U. Minnesota, and four colleagues, the paper evaluates results of 12 long-term experiments on growth of grassland plants that each ran for from 4 to 28 years and involved manipulation of factors such as nitrogen, water, CO2, fire, and grazing. All experiments had been done at the same experiment station in Minnesota. Factors like number of plant species present, biomass and productivity had been recorded. The results showed that, in many cases the changes in nitrogen, water, and so on had direct effects on biodiversity, and the grassland plots became simpler (fewer species), but these changes in biodiversity then had effects on productivity and the stability of production through time. Effectively the authors have provided evidence that biodiversity decline does impact plant production in important ways, and that other factors have their effects on productivity because they alter biodiversity. It only applies to Minnesota grasslands, but it is another brick being added to the solid structure of evidence of biodiversity effects that should cause us concern as biodiversity declines.

Sixth-Mass-Extinction

Our damage to the planet is profound, and we could be entering a 6th mass extinction because of it. Image © Guardian Liberty Voice.

To complicate the issue, however, Brian McGill of University of Maine, and three colleagues had published an article in Trends in Ecology and Evolution (known to all its readers as TREE) in February 2015. This article looks critically at the issue of present-day biodiversity decline and points out just how sparse the data are to support claims of a sixth mass extinction. The fact that estimates of current rates of extinction are from 10 to 1000 times higher than the average through geological time should tell us immediately that data on such topics are imprecise. McGill and colleagues point out that the expectation among biologists that global biodiversity is trending rapidly downwards is based largely on a logical deduction from observations of the many ways in which humanity is impacting the planet.

Humans have modified 50% of the Earth’s land surface, consumed about 40% of all plant production each year, doubled the rate at which nitrogen is converted from gaseous into biologically reactive forms, created vast dead zones in the oceans through their pollution, released sufficient CO2 that the climate is changing in dangerous ways, and largely eliminated top predators on land and in the ocean through over-exploitation. We’ve also greatly reduced the abundances of many species, bringing their populations closer to the extinction limit of zero. Logically, it is hard to see how biodiversity could not be falling fast with all these things going on, and the declining numbers of particularly charismatic species provide heart-rending evidence of what is happening. But logical deductions are not measurements, and as McGill and colleagues point out, measuring global biodiversity would be a hugely difficult undertaking, and monitoring it through time to see if it is falling would be more difficult still. So science uses deduction, inference and models.

McGill and colleagues go on to suggest there are a number of different trends in biodiversity that need to be considered. They point out that one can consider biodiversity at a variety of spatial scales from the local habitat to the globe. At each of these scales, α diversity, the diversity of species occurring at the same place, can be measured, and a trend in time can be determined. But there is also β diversity, the differences in species present in nearby places, and another β diversity, the differences in species present at different times in the same place, and both of these can be measured at a variety of scales. They suggest there are about 15 different trends that could be measured through time, depending on which type of diversity is measured and at what spatial scale.
While this could be seen as yet another speculation on the number of complicated concepts that could be balanced on the head of a pin (and biologists do sometimes fall into this trap), McGill and colleagues point out that there is solid empirical evidence for only three of these 15 trends, and for only one of these three (global α diversity) is it negative.

Recent detailed studies at local scales have surprised ecologists by showing that local α diversity is remarkably constant through time in recent years, although that constancy masks a rapid turnover as new species enter and earlier species disappear from local sites. In fact, what appears to be happening over recent years is that our negative impacts on natural systems have homogenized the planet – widespread species have become more widespread while species with restricted ranges have been lost. In other words, our impact on biodiversity, until now, seems to have been to reduce β diversity. Of course, if we continue the way we are going, our impacts will start to cut into α diversity, and then we need to really start to worry.

Fishing down the food web Wikipedia small

Over time, our fishing has grossly simplified marine food webs, primarily by removing larger, higher trophic level species and by reducing overall biomass by about 90%. This is just one of many ways in which we are reducing global biodiversity.  Image courtesy Wikimedia Commons.

I said at the beginning (well, after going on about mosquito mouthparts) that biodiversity was being lost rapidly. I accept the caution by McGill and colleagues about data on extinction rates, but we are also losing biodiversity simply by reducing the sizes of populations of other species. Smaller populations are inevitably less diverse than larger ones because they have fewer copies of all those genes, and fewer opportunities for different versions to be present. (When people gossip about the residents of tiny, remote towns all looking alike, because they are inbred, they are actually commenting on sound genetic principles!) Not only has our hunting and fishing largely removed top predators from the planet, we have reduced the abundance of fishes in the oceans by 90%, and we have had similar effects on many terrestrial creatures as well. We have taken vast areas of savannah, rangelands and pastures and turned them into monocultures of single domestic strains of food crops. And in destroying natural habitat as we take over that 50% of the world’s land surface, we have destroyed habitats vital to the lives of many species. For organisms other than the protists and bacteria, we have pushed numerous species closer to those low numbers where extinction next year becomes a real possibility. And, yes, I think we will discover that losing biodiversity brings big problems for us (and by then, it will be too late).

Problems Coming for Pollinators – Does This Matter?

The United Nations certainly sees biodiversity as a topic for concern. The Intergovernmental Science-Policy Platform on Biodiversity and Ecosystem Services (IPBES), a technical panel modeled after the IPCC, just held its fourth annual plenary in Malaysia. One of the major news items from that conference was a new report on losses of pollinators. Not yet on line, the written report should be available shortly. However, the press release and reports in the New York Times and on the CBC News highlighted that there are some 20,000 species of wild bee and numerous other insect and avian species that pollinate plants including 75% of our crops, and that around 40% of these species currently face extinction due to our use of agrichemicals (including neonicotinoids), our elimination of native vegetation that can sustain these species during the bulk of the growing season when our crops are not flowering, our more intensive patterns of land use, the introduction of invasive species including pathogens, and of course climate change. In short, pollinators are being hit by a multiplicity of changes, but with a common result – loss of pollinators means loss of an ecosystem service that is essential to some 90% of native plants, and enormously important to our agricultural production. Yes, biodiversity decline can cost us money big time, as well as food production. Not a problem?

pollinator via naturalcapital-us lilium_superbum_lg

It’s not just bees that pollinate, but all pollinators are impacted by our impacts on environment. Some 40% of them may now be at risk of extinction. Image © Center for Natural Capital.

What We Lose as Biodiversity Falls

I began this post marveling at the mouthparts of a mosquito, and I want to end with some additional comments on what it really means to humanity to lose biodiversity. Put the possible loss of global agricultural production aside – that’s just economics and food. Think instead about what biodiversity loss does to this planet. There may be many Earth-like planets in this universe, and many of them may support life, but this is the only planet we know of that supports life. It has supported such an exuberance of life over millions of years that evolution, making little mistakes in copying genetic sequences, has happened to generate richly different species of many different types. Some of them are long since extinct, but must have been wondrous creatures in their day. Others are with us now, and offer our senses an array of different stimuli that is so vast it is impossible to fully comprehend, while also offering our analytical brains examples to draw on as we invent and create. Our art, our engineering, our computer code, our poetry, our social linkages, all of what we create as humans is but a pale shadow of the wealth of form, function, color, sound, smell, taste and feel offered to us, for free, by the natural world. And, of course, we too are part of this biodiversity – beginning with mistakes in DNA replication, evolution has built a rich biodiversity that includes at least one species capable of reflecting on and appreciating that richness.

From the hypodermic feeding structure of the mosquito, to the geometric precision of the mollusk shell, to the fluorescent geometry of the scales of a butterfly wing, to the wise old eyes of a mountain gorilla and the exquisite courtship dance of a bird of paradise – shapes, structures, movements we could never imagine if they were not already here. From the intricate relationships between dinoflagellates and their coral polyp host, to the interspecific social interactions at a coral reef fish cleaning station, and the exquisite endocrine sabotage whereby some parasites control the behavior of their hosts – relationships of complexity that can tax our ability to understand. From the aural landscape of a pond in the springtime, to the navigation and prey interception of a colony of bats, and to the haunting sounds of the song of the humpback whale – diverse uses of sound that reveal to us what might be possible. In every one of these ways, and many more, the biodiversity currently alive on this planet provides us with an environment of a richness that could never have been imagined if it did not already exist. Losing biodiversity is a process of progressively losing those attributes of our world that makes our own lives worth living.

In his new book, Half-Earth: Our Planet’s Fight for Life, Harvard University Professor E.O. Wilson has just suggested that we need to set aside fully half the planet’s surface for the support of natural populations. People would live within these natural areas, but in ways that would explicitly favor the continuance of natural systems. He says that unless we take the kind of bold action required to achieve this half-earth goal, our destruction of biodiversity will continue unchecked. His ideas are not as far-fetched as they might at first appear. We’d still have our modern, technically advanced civilization, but we’d just be a whole lot more careful about how and where we did things than we currently are. In short, we’d begin to show the natural world the respect it deserves – and we’d be the beneficiaries in the long run. I, for one, would like to see his ideas take hold. We will not benefit by turning our planet into a concrete encased wasteland.

moorish_idol villabossibali-com

If the Moorish Idol, Zanclus cornutus, had never evolved, would humans have imagined it? If it disappears forever, will humans miss it? Photo © Villa Bossi Bali.

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