Archive for the 'Evolution' category

Know Your Biomes IX: Chaparral

Fynbos in the Western Cape, South Africa*

As much as any biome or global ecoregion is a challenge to group, differentiate or otherwise generalize, the chaparral or Mediterranean woodlands (scrubland/heathland/grassland) biome may be the best example such classification difficulties. There’s perhaps more general agreement regarding the features of this biome, even if the name tends to change from author to author. Many texts will not even include this biome in their list of major regions, instead making a small reference to it in the section regarding deserts. However, these areas, considering their combined territory, contain about 20 percent of the world’s species of plants, many of them endemic gems found nowhere else. On the flipside, due to the often environmentally heterogeneous nature of this biome, organisms that are prominent, integral members of other biome classifications are found in the chaparral as well. For the sake of consistency in this post, I’ll continue to refer to this biome as chaparral, as incomplete a descriptive designation as that may be.

Specifically, chaparral biomes exist in five major regions: South Africa, South/Southwest Australia, Southwestern California/Mexico, Central Chile and in patches wrapped around the Mediterranean Sea, including Southern Europe and Northern Africa. These regions are unified by their hot, dry summers and mild winters, referred to as an archetypal Mediterranean climate at 40 degrees north and south approximately.

The vast majority of rainfall usually comes with the cold fronts of winter. Annually, chaparral can experience anywhere from 250 mm of rain all the way up to 3000 mm in isolated subregions like the west portion of Fynbos in South Africa.

Plants in chaparral areas tend to be sclerophyllous (Greek: “hard-leaved”), meaning the leaves are evergreen, tough and waxy. This adaptation allows plants to conserve water in an area where rainfall is discontinuous, but probably evolved to compensate for the low levels of phosphorous in ancient weathered soils, particularly in Australia where there have been relatively few volcanic events to reestablish nutrients over millions of years. Obviously, these plants also happen to do very well during the xeric summers of the chaparral where drought is always a threat.

Because of the aridity and heat, the chaparral plant communities are adapted to and often strategically dependent on fire. Evolutionary succession scenarios constructed by scientists typically point to fire as one of the major factors that created much of chaparral areas in Australia and South Africa from Gondwanaland rainforest. (Fire ecology really deserves at least a post of its own, which I’d like to discuss given the time in the future.)

Some of the regions in the chaparral are exceptional. In South Africa, the area known as the Fynbos constitutes its own floristic region (phytochorion) among phytogeographers, the Cape Floristic Region. While it is the smallest of these floral kingdoms, it contains some 8500 species of vascular plants, 70 percent of which are endemic. The March rose (Oromthamnus zeyheri) is one of the standout specimens of the group as well as the national flower of South Africa, the King protea (Protea cynaroides). P. cynaroides is a “resprouter” in its fire-prone habitat, growing from embedded buds in a subterranean, burl-like structure. Another endemic species, the Cape sugarbird, is shown feeding on a King protea below**.

There is one unique threat to the chaparral: anthropogenic fire. In the past, if nature had not provided a fire to burn back the accumulated brush in these areas, often the native peoples would do so, and generally speaking, the fires seemed to be controlled and effective. But increased frequency of fires due to negligence or downed power lines can potentially cause catastrophic, unrecoverable fire. Only so much tolerance to such a destructive force can be built by evolutionary processes.

*Image by Chris Eason
**Image by Derek Keats

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Climate change, invasives and extinction in Thoreau's Woods

...I walk encouraged between the tufts of Purple Wood-Grass, over the sandy fields, and along the edge of the Shrub-Oaks, glad to recognize these simple contemporaries. With thoughts cutting a broad swathe I “get” them, with horse-raking thoughts I gather them into windrows. The fine-eared poet may hear the whetting of my scythe. These two were almost the first grasses that I learned to distinguish, for I had not known by how many friends I was surrounded — I had seen them simply as grasses standing. 

From "Autumnal Tints" by Henry David Thoreau The Atlantic Monthly October 1862. In this photo from 1908, the rocks mark the location of his cabin in relation to Walden Pond.

ResearchBlogging.orgAround 1851, after completing the retreat that inspired Walden, Thoreau had taken his interest in nature and made it a more scientific part of his work routine, walking the woods and fields around Concord, Massachusetts recording his observations of plants and animals through the seasons in the area. He paid particularly close attention to the flowering days of local plants, which has been of interest to the scientific community of late.

The data that Thoreau collected is meticulous enough to be considered a viable, useful data source by modern researchers. Thoreau's records of the area's wildlife have been carried on by others, providing us with over 150 years of data regarding the phenology of Northeast American flora; that is, life cycle events like fruiting or flowering days or migration and how these events are influenced by the seasons and the climate. Simply put, after 150 years of suffering the effects of disturbance and climate change, the natural communities of Concord are not quite the forests and fields of yore.

In the past two years or so there have been a handful of studies based on the data set that Thoreau started. In February 2008, Rushing and Primack published a study in Ecology discussing how global warming had affected flowering times in Concord. The average temperature has increased in the area by approximately 2.4° C since 1852, which has, on average, pushed flowering times up by 7 days since Thoreau's time. It was also observed that two non-native plants common in the Northeast, St. John’s wort (Hypericum perforatum) and highbush blueberry (Vaccinium corymbosum), could be useful as bioindicators of the future effects of climate change due to how quickly they responded to the changing temperatures; their mean first flowering days shifted forward approximately three days per 1° C increase in temperature.*

Later that year, Willis et al. published a study in PNAS using the data set started by Thoreau, this time looking at the data from a phylogenetic perspective. It was shown that flowering time was strongly correlated with abundance and that the species seemingly incapable of a relatively quick response to the change in climate were declining. The pattern is phylogenetically selective, strong evidence of climate change as an extinction risk.

In the near term, this pattern of phylogenetic selectivity is likely to have an accelerated impact on the loss of species diversity: groups of closely related species are being selectively trimmed from the Tree of Life, rather than individual species being randomly pruned from its tips.

A more recent study from Willis and his colleagues published in PLoSONE takes a look at how these flowering times differ between native and non-native species, determining how each has been able to respond over the past 150 years. It was previously demonstrated that the non-natives St. John's wort and highbush blueberry have been apt conformers to the changing climate, but neither are considered invasive.

The researchers placed the Concord flora in four comparative categories for analysis - Native vs. non-native, Native vs. non-native, non-invasive, Native vs. invasive, Non-native, non-invasive vs. invasive - and examined phenologically and ecologically important traits such as plant weight at maturity and flower diameter.

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The results are remarkable, and reveal another layer of danger to native plants in the area. In general, non-natives were shown to adapt to changing temperatures better than the natives. Invasive species are particularly apt; they flower 11 days earlier than natives and 9 days earlier than the non-native, non-invasives. The results of the study also backed up earlier evidence that abundance was tied to earlier flowering days; invasives displayed greater relative abundance than the natives and non-native, non-invasives. But in general, non-natives in the area are equipped with certain traits that better prepare them for changes in climate.

Already the Concord area has lost about 27 percent of the species that once inhabited Thoreau's woods and another 36 percent have become incredibly rare. If the projections of 1.1° - 6.4° C increases in average temperature over the next century are correct, this trend will continue, progressively selecting traits that promote invasive growth and pushing natives that much closer to extinction.

*It's not always a boon for the flowering days of plants to be pushed forward in the season. If flowering too early, they may miss their pollinators or succumb to a late frost.

Willis, C., Ruhfel, B., Primack, R., Miller-Rushing, A., Losos, J., & Davis, C. (2010). Favorable Climate Change Response Explains Non-Native Species' Success in Thoreau's Woods PLoS ONE, 5 (1) DOI: 10.1371/journal.pone.0008878

Willis CG, Ruhfel B, Primack RB, Miller-Rushing AJ, & Davis CC (2008). Phylogenetic patterns of species loss in Thoreau's woods are driven by climate change. Proceedings of the National Academy of Sciences of the United States of America, 105 (44), 17029-33 PMID: 18955707

Miller-Rushing AJ, & Primack RB (2008). Global warming and flowering times in Thoreau's Concord: a community perspective. Ecology, 89 (2), 332-41 PMID: 18409423

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Know Your Biomes VIII: Boreal Forest

Sep 24 2007 Published by under Basic Concepts, Ecology, Environment, Evolution

Larix_gmelinii0.jpg

A stand of Dahurian Larch beginning to change color in Northeast Siberia.

Between 50 and 65 degrees N latitude lies a globe encircling band of forest dominated by conifers and chilly winds called the boreal forest (boreal is from the Greek word for north) or the taiga (Russian for "marshy pine forest"). From Iceland's wiry birch forests to the larch covered northern areas of Siberia, the boreal forest grows in a climate where extremes are commonplace, and where much of the wilderness remains preserved.

The boreal forest is generally a cold place; it is winter in these areas for over half the year, and most of the precipitation is locked up in snow and ice, about 20 to 60 cm per year. However, in places like Central Siberia for example, the range of temperatures throughout the year can be over 100 degrees C, reaching 30 degrees C in the summer and dropping to a breath taking -70 degrees C in the winter.

Naturally, the plants and animals are well adapted to the climate. Most of the boreal forest is dominated by evergreen conifers - spruce, fir, pine, etc. - but hardy deciduous trees crop up here and there as well, like birch and aspen. Willows grow near the clear, mountain lakes of the northern forest. The needles of the larch, a deciduous conifer, turn bright yellow in autumn before falling in and around the sparse underbrush, in some areas composed of rose, juniper and blueberry, which provide food for small animals like the red squirrel and the porcupine, as well as nesting crossbills and spruce grouse. Moose and bison are wary of the elusive wolf. Lynx and coyote scour the forest floor for a giveaway twitch from the snowshoe hare. Another of the smaller predators, the tenacious wolverine, has been observed in fierce disputes over prey with the huge brown bears of the North.

Such a high concentration of conifers leads to acidic soils in most areas of the boreal forests. The cold and low pH limits the decay of organic material, so unlike most biomes, the most fertile soil horizon is the leaf litter, where mycorrhizal fungi (fungi that lives within the roots of plants, sharing nutrients fixed from the soil) break down dead plant and animal material. Naturally, the roots that tap into this store of nutrients are shallow ones. In the coldest of climes, permafrost turns the subsoil into a near impenetrable mass.

The boreal forest has been the focus of much ecological research due to the general lack of human influence in the past.* Perhaps one of the best studied cases of predator-prey dynamics was done on two inhabitants of the boreal forest, the lynx and the snowshoe hare, which is famously the food of choice for a hungry lynx. Ecologists were able to take 200 years of pelt purchasing records from the Hudson Bay Company and reconstruct the relative abundances of the hare and the lynx. When graphed and superimposed, they found that population spikes in the hare population were followed by lagging spikes in the lynx population. Additionally, plummeting numbers of hare were followed by a proportional decrease in the lynxes.

A long chain of studies have been done since, which has given us a window in to how predation and food abundance affects animal and plant populations. In a nutshell, as more food (underbrush like rose and willow) became available to the hares, they increased in population up to a certain maximum feeding level. At this level, the population is no longer sustainable. An increased population of hares means more food for the lynxes, which undergo a similar transformation, explaining the lagging population spikes followed by proportional drops in population. (Eventually I will find the time to talk about functional and numerical responses in predator-prey/food availability situations - this post could go on forever just about that.)

Next in the biomes series: Tundra.

*The major threats to these forests in recent times is logging, the power industry and a lack of strong legislation preventing abuse.

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Know Your Biomes VII: Temperate Forest

May 24 2007 Published by under Basic Concepts, Conservation, Ecology, Environment, Evolution

Walking through a streamside copse of eastern hemlock in the ancient Appalachians is revealing for several reasons. First, the sheer size and age of these virgin stands can be humbling - at 45+ meters high, one tree may have been alive for more than 600 years. Second, a closer look at the forest's composition can tell ecologists two things: By assessing the pollen contained within pond sediment, you learn that these hemlocks started repopulating the eastern US about 12,000 years ago, following in the "footsteps" of the maple genus (Acer spp.) after the retreat of the massive glaciers covering most of the United States. We also learn that eastern hemlocks tend to hug water sources, giving way to deciduous trees as the incline of the valley steepens. Mixed forests like these are principle in most of the Appalachian mountains.

But the Appalachian mixed forest is only one small ecoregion in a much larger biome, the temperate forests. Named for relatively mild temperatures and moderate annual precipitation, they stretch across the globe between 30 and 40 50 degrees latitude, from the Gondwanaland throwback Valdivian forests of Argentina and Chile (they resemble forests in New Zealand and Australia), to the home of the pandas.

Temperate forests vary greatly in the amount of rain they receive, anywhere from 650 mm 3,000 mm. On the high end of the scale are regions like the Pacific northwest, where redwoods and sequoias live in what is sometimes classified as a temperate rainforest due to the high levels of precipitation, mid range for a tropical rainforest. They're seasonal. Deciduous (and one or two conifers like the larch) drop their leaves during the winter to conserve energy.

The soils of temperate forests are typically fertile, but their specific properties depend on the composition of the forest. In deciduous dominated forests (oak-hickory, beech-maple, etc.), nutrients cycle quickly, creating a substrate rich in organics. Soils in coniferous dominated forests are much more acidic and nutrient cycling tends to be more conservative.

Fire is important to nutrient cycling and population regulation. Many conifers have specially adapted thick bark to ward off the effects of fire and the cones of some species, the "fire-climax" pines like the pond pine or the Monterey pine, often depend on the touch of flame to open.

Like the tropical rainforest, temperates are vertically stratified, with organisms living and growing in the canopy, a shorter layer of mature trees below, the shrub layer and, of course, the understory, where nematodes, fungi and bacteria break down the thick mat of leaf litter into organically rich soil. Light is relatively abundant in the forest understory, allowing ferns and herbaceous plants to thrive. Mosses and lichens cover tree trunks and rock in the more moist portions of the forest.

Vertebrate life is equally diverse. In China, the red panda and the giant panda live in the same general area and subsist on the same food - bamboo - without being in direct competition. They fill very specific niches, however, predominantly eating different parts of the plant and browse slightly different regions. White-tailed deer, grouse, bobcats and black bear dominate the Appalachian forest. In eastern Russia the and leopard, both highly endangered, found refuge in Manchuria, which the last ice age left untouched by glaciers.

Humans have affected temperate forests more than any other biome due to the habitability, fertility and resource richness of these areas. Forest covered most of the eastern US and western Europe until civilizations moved in to urbanize.

Next time: Taiga

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Know Your Biomes V: Deserts

Mar 24 2007 Published by under Basic Concepts, Conservation, Ecology, Environment, Evolution

From a human perspective, deserts, like tundras, seem barren and desolate, inhabited by organismal oddities, pressed into their respective niches by patch of bad luck, or a salt flat, as it were. But thinking beyond our prejudice, seeing through the eyes of a camel or transpiring through the stomata of a saguaro cactus, some conception of deserts as biologically viable and diverse regions of the planet can be gained. Life may not be particularly abundant in most of these areas, but it is varied, unique and beautiful.

For the most part, deserts occur in a consistent band at 30 degrees north and south, with some exceptions on the coasts of North and South America. Dry, subtropical air robs these areas of moisture as it descends, circulating it to more temperate zones. Not all deserts are as parched as the Sahara in Africa or the Atacama-Sechura in Chile and Peru, which receive less than an inch of rain per year - essentially nil. The Sonoran Desert, for example, receives as much rain as the lower threshold of a temperate grassland, about 300 mm per year. The Sonoran remains a desert because of this cardinal rule of being: evaporation exceeds precipitation.

Temperatures are typically hot during the day and freezing at night because of the lack of cloud cover, though in areas of the Gobi Desert in Mongolia, the average temperature is only about 3.6 degrees C annually, with temperatures dropping well below zero C.

Soil has a low concentration of organic matter, so much so that it is often classified as lithosols, or strictly inorganic soils. This is especially evident in aged, undisturbed soils, where a special limestone horizon called a caliche is formed (because of its inherent low level of disturbance, ecologists can use this layer to accurately age a desert). Great salt flats are common in desert areas, where pools of accumulated water from heavy rains evaporate, leaving crusts of salt crystals spanning large areas, making it more difficult for organisms to extract water from their environment.

But extract they do, in various specialized ways. Desert perennials like the prickly pear have evolved a thick, waxy cuticle capable of retaining water more effectively year round. The stems have low surface area exposure, its "thin" parts facing into the sun. Roots extend horizontally below the surface, increasing their area of absorption. Their leaves, which would have been a liability in the extreme sun and moisture sapping aridity, have become reduced to non-photosynthesizing, defensive spines. Even their cycle of photosynthesis is different than most plants, closing their carbon dioxide absorbing stomata during the day, when the potential for water loss is greater (CAM).

Annuals are a different story. Given the extreme rarity of significant rainfall, these plants grow rapidly when water is available, producing seeds that can lie dormant for years, until the rains come again. Some of these plants keep a death grip on their seeds until the touch of water hydrates the cellulose of their seed pods, opening the pod releasing the seed.

Some plants, called halophytes, have even adapted to thrive in the salt flats. Atriplex is an extraordinary example. It can maintain higher levels of salt within its cells in order to extract water from the salt flats. Some of these cells burst, coating Atriplex with a defensive layer of salt, making the plant a dangerous meal for any water conserving herbivore.

One species, however, has found a way around this problem: The red vizcacha rat (in the same family as the chinchilla) has evolved a series of teeth that can remove the outer salty layer from Atriplex, making it just edible for the resourceful animal.

Other animals have similarly adapted to the heat and aridity; in fact, some, like the camel, have independently evolved the same measures as plants for keeping cool and hydrated. The camel faces into sun, keeping a slim profile, reducing its surface area exposure. It maintains a store of fat in its hump in order to produce metabolic water. A thick coat of hair, much like cactus spines, covers its body, reducing heat absorption. As the prickly pear keeps its stomata closed during the day, the camel does not sweat, reducing its own loss of water.

Most desert animals, however, are nowhere near the size of the camel, preferring to hide in burrows during the day, emerging to hunt and/or forage at dusk.

I could go on indefinitely about this biome. The extremity of the climate has produced fascinating characteristics in desert wildlife, and they are so different from region to region that they deserve descriptions of their own (especially areas like the Madagascar succulent woodlands, home of the bizarre Didiereans, pictured at the beginning of this post). Perhaps I will return to these areas of interest at a later date.

Next we'll look at the bread baskets of the world - the temperate grasslands.

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>Dinosaurs and the Mystery of Body Temperature III: Intertial Homeothermy

Jan 28 2007 Published by under Animals, Evolution, Links, Paleontology, Physiology

>Body size is an important factor in the debate over whether dinosaurs were cold or warm blooded (or something in between). When you have a land animal 42 feet long weighing nearly as much as a blue whale, temperature models tend to break down. If the dinosaurs were ectotherms, relying on the environment for heat, they may lack the surface area to sufficiently heat the blood pumping directly beneath the skin. If dinosaurs are endotherms, and internally heated by its own metabolism, it may not have enough surface area to expel excess heat from the depths of its massive body.

The following chart shows this principle a little more clearly.


As you can see, the second two cubes have the exact same volume (body size), but the surface areas are vastly different. Large animals like dinosaurs and blue whales are like the middle cube with the smaller ratio; it becomes difficult to use surface area to heat/cool its insides. Also, the more massive an animal is, the more heat it produces/requires, generally speaking.

The reason blue whales get away with being the most massive animal to ever live (so far) is that temperature exchange with their environment is rapid. The ambient temperature of the ocean is on average much lower than ambient temperatures on land, allowing the whale to circulate heat through the thinner parts of its body and allowing the cold water to carry away the excess. Plus, the whale's 100 tons is spread out along 100 feet of body as well.

You can see how a creature on land weighing as much as the blue whale, compacted into 40 or 50 feet and lacking the might present a particular problem for scientists to figure out, especially in the absence of direct evidence.

But, the creature did exist. We're just now picking up the pieces, so to speak.

And recently, scientists put those pieces to good use. By simulating the ontogenetic development of eight different dinos using data from recent bone analyses, they were able to determine that the internal temperature of dinos depended on size. Smaller dinosaurs maintained a lower body temperature and probably grew at a rate consistent with extant reptiles, while the larger dinos maintained a higher body temperature, like today's birds and mammals.

The largest animal studied, Sauroposeidon proteles, was estimated to have an internal temperature of 48 degrees Celsius (120 degrees Fahrenheit), a few degrees higher than what was thought to be the upper limit of temperature tolerance for animals. Because of this extremity, the authors believe that temperature may have been the ultimate cap on body size.

Ultimately, this study was transposing a state called "inertial homeothermy," which is observed in ectotherms like crocodiles and the Galapagos tortoise that can maintain their internal temperatures by adjusting their internal physiological conditions, much like endotherms. The researchers performed the same tests on crocodiles of similar size (when they could; there are no crocs alive today to compare with the larger dinos):


Perhaps, if time allows in the near future, I'll detail a bit more about all the thermies: poikilo, homeo, hetero, ecto and endo.

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>Dinosaurs and the Mystery of Body Temperature II: The Evolution of Endothermy

Jan 19 2007 Published by under Animals, Birds, Evolution, Links, Paleontology, Physiology

>There's a fairly significant problem with the evolution of endothermy from ectothermy, a paradox that has no satisfactory conclusion as of yet: How could well-insulated animals with high metabolic rates producing lots of heat from within their bodies evolve from animals with low metabolic rates and poor insulation, expertly absorbing heat from the surrounding environment?

If these characteristics evolved independently what purpose would they serve? An ectotherm has no use for insulation like feathers or hair since heat exchange needs to be rapid with its surroundings, just as it has no use for a heat producing high metabolism without the necessary insulation.

Raymond Cowles' experiment showed that by putting little fur coats on lizards wasn't keeping heat in, it was keeping heat out. The lizards couldn't warm up.

That's the paradox, the catch-22. The ticket out, however, is the exaptation: An adaptation of a structure that becomes useful for one biological purpose that originally evolved for another.

Feathers are are thought to be derived from the long scales of ancient archosaurs. These reptiles could lift their scales and expose their skin directly to the source of heat, or orient them so that they could block heat absorption. Just as the marine iguanas of the Galapagos Islands are able to trap a layer of air within their scales, these reptiles are thought to have done the same, retaining more metabolic heat, leading to a more active life.

Another theory concentrates on our reptilian ancestors from the synapsid lineage. The synapsids were steadily becoming more active (illustrated by changes in bone structure), and those morphological changes could have been accompanied by higher metabolic rates, leading to more heat in the body. The more hair on the body, the better heat retention.

So which came first, the dino or the egg?

It's apparent that endothermy evolved at least twice; once beginning with an exaptational insulator in the case of birds, and once beginning with exaptational skeletal changes and a needed increase in activity for foraging, in the case of mammals.

Largely, however, the jury is still out. (I heard that there is also evidence that pterosaurs might have been endothermic, leading to a third origin of endothermy; please link research if you know of any.)

Its important to realize that there are in-between states of thermoregulation, and the progression from ectothermy to endothermy (and back again, in some cases) took place in baby steps across millennia. Time and again we're shown that organisms tend not to fit our definitions and molds. It's not like flipping a switch.

More on dinos and thermoregulation for part III.

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>Dinosaurs and the Mystery of Body Temperature I: Endothermy vs. Ectothermy

Jan 19 2007 Published by under Evolution, Links, Paleontology, Physiology

>This is repost from September of '06. I never had the chance to finish the series because of school, so I will be finishing in three parts this week.

Why have the dinosaurs been relegated to little kid stuff? Why can't I find a purple triceratops t-shirt in XL?

Were the dinosaurs warm-blooded or cold?

I want to talk about dinosaurs and body temp for a couple of posts, but I think the best place to start is with a little discussion of why this issue is important and the evolutionary implications of endothermal/ectothermal states.

Ectothermy is the state of commonly referred to as "cold-bloodedness" (an inaccurate term; it's more about precise regulation) which is observed in most reptiles (though not all), most fish (not all) and basically every animal that is not a bird or a mammal (though not all). Ectotherms rely on heat from outside sources to maintain their body temperature. The sun is the main source of this heat energy (infrared), which can be transferred directly through absorbing the sun's rays, or indirectly through convection - movement of heat between objects (including organisms) and the air.


When a lizard suns itself, its blood vessels just below the skin open up, allowing more blood to flow next to the skin and the source of heat (the sun's radiation). The lizard's blood warms and transmits heat to the rest of the body.

Endotherms, on the other hands, are animals that regulate their own heat, like us. We don't need external sources because heat production is built in to our metabolic machinery. In fact, most endotherms spend more time trying to dissipate heat than acquire it. As you might expect, there is a formal range of classifications (lower lethal, lower critical, thermoneutral, upper critical, etc.) that biologists can use to determine the tolerances of certain endotherms.

The most important thing to remember is the necessity of temperature regulation in animals, the "why." Metabolism is all about burning calories, and the processes that burn those calories are driven by chemical reactions. Chemical reactions within the body are finicky; they depend on enzymes to catalyze which only work within a relatively narrow band of temperatures.

Endotherms have an advantage in this respect. A constant high body temperature increases the activity of the central nervous system, and subsequently neurotransmitter and enzymatic activity. Ectotherms do not have this advantage; on cool days/nights, they lose the ability to be as active. Keep in mind that this does not mean that endotherms are better, just different.

So what does this have to do with dinosaurs?

The main problem in assessing the body temperature of dinosaurs is that we have no direct evidence. There are no extant dinos, so scientists have to look to their descendents, birds and reptiles. But, as we covered earlier, temperature regulation is far from uniform in these animals.

There is one other problem: temp regulation is a special problem in the case of such huge animals.

Next time we'll look into the evolution of endothermy and how it might have arisen from the dinosaurs.

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Why Spiders Aren't Insects IV: Molecular Homology

Photo by: Gerald Yuvailos


This is the fourth part in an ongoing series discussing the distinction and evolution of spiders and other arthropods.
Part I, part II and part III have led up to this point:

In the last post of this series, we established that spiders descended from marine arthropods called the eurypterids, distinct and separate from insects, appearing in the fossil record in the late Silurian/early Devonian, about 425 million years ago.

The cladogram we used to analyze the spider's history was based on the organism's morphological characteristics, that is, visible structures like chelicerae and book lungs that can be tied to other organisms that possess the same structures. Limulus (the extant horseshoe crab) has both of these structures and predates the spiders, placing them further back in the chelicerates' evolutionary history.

Homologous bones from human (I), dog (II), pig (III), cow (IV), tapir (V) and horse (VI):
r — Radius, u — Ulna, a — Scaphoid, b — Lunare, c — Triquetrum, d — Trapezium,
e — Trapezoid, f — Capitatum, g — Hamatum, p — Pisiforme

Paleontologists call this comparison of physical characteristics homology (coined by Richard Owen an anatomist and, ironically an opponent of Darwin). The mouth parts of a spider are homologous to the mouth parts of Limulus because of the cherlicae's exact form and function. This is a different designation than analogy; analogous structures may function in the same way, but they are different in form because of their different lineage. For this reason, scientists call analogy an artificial classification system.

A good example of analogous structures are wings from bat and bird. They perform the same function in varying degrees, but they have evolved very different forms. A bat's wing is basically a modified mammal hand, while the bird wing is a modified tetrapod arm.

Homology is essential in building an organism's phylogeny (evolutionary history). More recently, geneticists have employed this classification technique to analyze and find similarities among the less visible traits in life, RNA (ribonucleic acid) and the building blocks of proteins, amino acids.

Think of these cellular chemicals this way: If DNA is the blueprint of life, RNA is the builder and its materials are amino acids. When these amino acids are placed in the correct sequence by RNA, they become proteins, the framework of our body. And, since the genetic code for protein constructions is nearly universal*, geneticists can compare entire swaths of RNA from one organism to those of another and find homology at the molecular level.

Here's an example (sequences are greatly abbreviated for the sake of our sanity):

Organism 1: ACGC-CCCCC
Organism 2: ACGC-CCCUC
Organism 3: ACGU-CUCUC

Basically, from noting the differences in each RNA sequence, and determining the homologous sequences (such as the ACGU sequence above), a cladogram can be constructed that shows common ancestry without the murky distinctions that sometimes cloud the comparison of bones to bones, or mouth parts to mouth parts.

The problem with this molecular system of analysis is that it often provides vastly different cladograms than the ones crafted through morphological analysis. This is not necessarily the case between the spiders and and Limulus, the molecular evidence supports the fossil record's interpretation of ancestry, but it calls into question the descent of insects from chelicerates like spiders.

In short, the molecular evidence agrees with the morphological evidence: spiders are more closely related to horseshoe crabs than insects. But where and when did the insects arise?

Next time we'll tackle the more recent movements to elucidate the phylogeny of arthropods, including a discussion on the significance Hox genes and evolutionary-developmental biology (evo-devo).

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