|Publisher:||University of Chicago Press|
|Sold by:||Barnes & Noble|
|File size:||19 MB|
|Note:||This product may take a few minutes to download.|
About the Author
Read an Excerpt
By KARL J. NIKLAS HANNS-CHRISTOF SPATZ
THE UNIVERSITY OF CHICAGO PRESSCopyright © 2012 The University of Chicago
All right reserved.
Chapter OneAn Introduction to Some Basic Concepts
When you have eliminated the impossible, whatever remains, however improbable, must be the truth.—Sir Arthur Conan Doyle, The Sign of Four
Our goal in this chapter is to introduce some basic concepts in the study of plant life and biophysics, concepts that might be unfamiliar to physicists, engineers, or mathematicians interested in learning about plants or to biologists who want to learn more about physics.
The topics discussed in this chapter can be thought of as a philosophical prolegomena to the rest of the book. They include the limits of natural selection, the role of endosymbiosis in the evolution of plants, and some practical and philosophical issues, among which the value and pitfalls of reductionism and modeling are important launching pads for interpreting the concepts introduced in other chapters.
To begin with, it is important to recognize that the biological and physical sciences have much in common. Both can be used to explore the relationships that exist between form and function. Both help us to recognize that these relationships are contingent on local environmental conditions (i.e., the "working place" of the engineered artifact and the "habitat" of the organism). Both are experimental sciences that can be used to achieve great quantitative rigor. Both have rich theoretical frameworks on which to draw. And both are very practical sciences in the sense that physicists, chemists, engineers, and biologists recognize that the phenomena they study often resist the tidy, elegant analytical solutions that appear in the pages of many textbooks.
Despite these parallels in perspective, the physical and biological sciences are not entirely compatible. Typically, the physical scientist does not encounter systems that can alter form and substance in response to environmental changes. Nor does he or she deal with systems that can reproduce, mutate, or evolve. In the physical sciences, form-function relationships are known in advance when a machine or structure or synthetic enzyme or material is constructed. In addition, the building materials are specified in advance. In contrast, the biologist must deduce form-function relationships, an activity that, with very few exceptions, tends to be a risky adventure in speculation. The biologist must also deal with organic shapes, geometries, structures, and materials that have few, if any, counterparts in the physical sciences. Indeed, the distinction between a "structure" and a "material" as traditionally defined by engineers often becomes blurred when we examine the ultrastructure of a plant, animal, or some other life-form.
1.1 What is plant physics?
The title of this book reflects the juxtaposition of plant biology and classical physics to better understand the physical factors that have helped to shape plant form-function relationships, ecology, and the broad evolutionary patterns we see in the fossil record. This approach uses physical laws and processes, engineering principles, and mathematical tools to discover how an organism functions, grows, and reproduces. It also uses these tools to explore adaptive evolution by means of natural selection. The fundamental premise of any biophysical enquiry is that organisms cannot obviate physical laws and processes, which must therefore influence the course of organic evolution. That is not to say that all of biology can be reduced to mathematics and physical phenomena and processes, nor does it mean that evolutionary history is prefigured in the same way that Newton believed the universe was deterministic. Rather, the approach presented here merely assumes that much of biology can be understood by taking a reductionist approach and that whatever remains must be approached from a strictly biological perspective, one that fully acknowledges the important roles played by random events and historical contingency during the course of life's long and complicated history on earth.
For this reason, a biophysical approach can explain why certain hypothetical organisms are physical impossibilities, but it cannot explain why certain kinds of organisms exist. The course of organic evolution is influenced by random processes such as mutation, genetic recombination, and extinction events just as it involves the participation of nonrandom processes, among which natural selection is extremely important (Futuyma 1998). The concept of natural selection is complex and often misunderstood, so much so that it is sometimes accused of having circular logic; that is, "that which is fit survives, that which survives is fit." Indeed, many essays and symposia have been dedicated to the topic of natural selection, which can be defined in a variety of ways (e.g., Sober 1984). For example, natural selection can be defined as the process by which genetic variants in populations are winnowed to eliminate those variants that are less suited to the environment. This simply means that the traits exhibited by successfully reproducing individuals in any population are not identical to those of the entire population of potentially reproducing individuals. The disparity between reproductively successful individuals as a group and the rest of the population is natural selection. Accordingly, when natural selection is said to result in the adaptation of individuals to their environment, what is really being said is that a continual contrast exists between parent and offspring and that this contrast is generally advantageous to survival and reproduction for at least some individuals under particular environmental conditions.
Evolution by means of natural selection has no "purpose," and so it can have neither foresight nor intent. The traits that allow individuals in one generation to reproduce successfully may not be those that allow future generations to reproduce successfully, especially if the environment changes abruptly and unpredictably. Nevertheless, evolutionary trends that appear "directional" are not uncommon. Trends in the fossil record are discernible for both plants and animals. For example, the geological record shows that during periods of stable environmental conditions, the size range in many fossil lineages increased. Tree-sized lycopods and horsetails flourished and adapted to the comparatively stable and luxuriant coal swamps of the Carboniferous period (Taylor et al. 2009). Today, the descendants of these plants are herbaceous and small. In contrast, the size range of many lineages was reduced during periods of global attrition or environmental instability. The paleoecologist might suggest that stable environments permit plants to evolve longer maturation times and thus achieve greater size whereas, in contrast, environmental instability or attrition selects against organisms that require long periods to achieve reproductive maturity. Certainly, these are reasonable hypotheses that echo the classical distinction between the environmental regimes that favor K- or r-selection. The concept of K- and r-selection regimes deals with the trade-offs between the degree to which environmental conditions are stable or unpredictable and the fecundity and precocity of the species that cope with them. Habitats with environmental conditions that are predictable permit species to exist that require longer times to reach reproductive maturity (i.e., K-selection). Such species typically cannot maintain viable populations in habitats characterized by conditions that are unpredictable because individuals die before they reach reproductive maturity. Thus, natural selection in unpredictable environmental conditions favors the existence of species that grow rapidly, reach reproductive maturity quickly, and produce numerous progeny (i.e., r-selection). To a certain extent, K-selection is really not "selection" at all because it does not exclude a priori species that are very fecund and reproductively precocious.
Regardless of whether species experience K- or r-selection, throughout the vicissitudes of Earth's long history, they experience the results of physical laws and processes that are invariant and ubiquitous. For this reason, evolutionary history has other equally strong directional components or, at least, "historical signals" that reflect the inextricable interconnectedness of organic form, function, and environment. The first land plants were small by present-day standards and lacked specialized tissues for the conduction of water and sap (Taylor et al. 2009). They also possessed none of the organographic distinctions among leaves, stems, and roots that characterize the vast majority of land plants today. Yet within a comparatively short time (by geological standards), the surface of the earth was colonized and made green by organisms that manufactured wood with which to elevate specialized leaves and reproductive structures to heights that rival those of modern trees. Importantly, the fossil record shows that many of the form-function relationships observed in one ancient plant lineage are mirrored in other plant lineages that evolved independently but along the same morphological or anatomical pathways. Thus, the capacity to form wood evolved in the lycopods, the horsetails, and the progymnosperms independently. Within each of these three lineages, roots and large leaves also evolved, albeit in very different ways. And within each of these lineages, plants evolved the capacity to produce spores that produce unisexual gametophytes (heterospory). These and other examples of convergent evolution attest to the strong bond between form, function, and environmental context.
Plant physics provides the tools to explore this triumvirate quantitatively and to learn how organic form functions in the environmental setting of organisms that exist today and—by inferences drawn from the fossil record and what we know about biology in general—organisms that are now long extinct. By doing so, it sheds light on present-day ecology and the evolutionary history of every form of life, past and present and—yes, in theory—life-forms that have yet to evolve.
1.2 The importance of plants
Over 90% of all visible living matter is plant life—the substance that cleans the air and provides food, wood, fibers for clothing, important pharmaceuticals, the coal that fueled the Industrial Revolution, and many model organisms with which to explore genetics and development. In addition, plants have evolved into the largest life-forms on earth. Consider the largest extant animal, the magnificent blue whale (Balaenoptera musculus), which can weigh as much as 1 6 metric tons and measure 4 m in length, thus surpassing what may have been the largest dinosaur, Argentinosaurus, which is estimated to have weighed 60 metric tons with a body length of m. Yet, however impressive these body sizes may appear, they pale in comparison to modern plants. For example, the brown alga Macrocystis pyrifera can grow 60 m in length annually, whereas the General Sherman tree (a specimen of Sequoia sempervirens) is estimated to weigh 1,814 metric tons and measures over 84 m in height.
It is surprising, therefore, that plants receive comparatively little attention in many textbooks devoted to biology or evolution. Apparently, attempts to drive home their importance can fail miserably. But consider food production, for example—or, more precisely, the annual production of organic carbon by plants—and its ecological consequences in terms of oxygen and carbon dioxide processing.
Aside from a small amount of organic carbon produced by chemoautotrophic organisms, plants provide virtually all of the organic carbon used by heterotrophs as food. The magnitude of organic carbon produced annually by land plants is on the order of billion tons, as can be shown by a simple calculation (box 1.1). Regardless of how large this number seems, it pales in comparison to the estimated amount of organic matter produced annually by algae! Naturally, not all of this organic matter is available as food for humans. Roughly 70% of the organic carbon produced annually is in the form of cellulose and lignin, both of which contain carbon and neither of which is digestible by humans, although both are consumed in the form of paper and wood products.
1.3 A brief history of plant life
We have used the word plant rather glibly so far. Yet the meaning of this word is sometimes ambiguous because it can be used in at least two ways: the traditional way, which groups organisms based on their shared characteristics (the grade level of organismal construction), and the phylogenetic way, which groups organisms based on their evolutionary histories (the clade level of evolutionary ancestor-descendant relationships).
The traditional way defines any eukaryotic organism as a plant if it carries out photosynthesis and possesses cell walls. This definition groups all of the unicellular and multicellular algae together with the more familiar nonvascular and vascular land plants, collectively called the embryophytes. This definition has some advantages. For example, it draws attention to a shared metabolism that requires the acquisition of photosynthetically active radiation, carbon dioxide, water, and minerals to support growth and reproduction. It also highlights the physical presence of an external layer of materials that provides the protoplasm of cells with mechanical rigidity and protection. By doing so, the traditional definition focuses our attention on convergent evolution among otherwise dissimilar evolutionary lineages. In turn, this convergence invites us to explore whether these and other shared features confer adaptive advantages.
Nevertheless, the traditional definition has two drawbacks. Unless qualified in some way, it excludes nonphotosynthetic organisms that have evolved from photosynthetic ancestors, such as the "fungus" Saprolegnia and the parasitic flowering plant Monotropa. More important, it neglects a complex evolutionary history that shows us that photosynthetic eukaryotes possessing cell walls have evolved independently many times during earth's history—which is the basis for affirming that convergent evolution has occurred in the first place!
Evidence for convergent evolution comes from the many detailed comparative studies using DNA sequence, biochemical, ultrastructural, molecular, and morphological data that reveal the polyphyletic nature of the organisms we call algae (Palmer et al. 2004). These studies indicate that there are at least five separate algal lineages (table 1.1). In contrast, similar phylogenetic analyses reveal that the embryophytes are a monophyletic group of organisms that includes the mosses, liverworts, hornworts, lycopods, ferns, horsetails, gymnosperms, and angiosperms. Collectively, all of these land plant groups trace their evolutionary history back to a common ancestor that was shared with the closest living relatives of the land plants, the modern charophycean algae.
Phylogenetic analyses also show that the multiple evolutionary origins of the algal lineages (including the green algal lineage that ultimately gave rise to the embryophytes) were the consequence of primary, secondary, and even tertiary endosymbiotic events (see table 1.1). In a very real sense, therefore, the history of plants, as traditionally defined, is reticulate by means of extensive lateral gene transfer (Kutschera and Niklas 2004, 2005).
As the word implies, endosymbiosis refers to the evolution of symbiotic relationships among different kinds of organisms in which one or more attained the physiological status of being an organelle in a host cell. For example, it is now widely accepted that plant plastids (of which the chloroplast is one) evolved when an ancient heterotrophic or chemoautotrophic prokaryote engulfed a photosynthetic prokaryote and evolved a mutually beneficial endosymbiotic relationship with it. This primary endosymbiotic event led to the evolution of the red algae (Rhodophyta), the green algae (Chlorophyta), the charophycean algae (Charophyta), and ultimately, the embryophytes. Phyletic molecular analyses indicate that the ancestral proto-plastid was very much like modern cyanobacteria, which liberate oxygen as a by-product of photosynthesis. Similar studies based on DNA sequences indicate that the first mitochondria probably evolved from prokaryotes very much like extant free-living a-proteobacteria (for a review, see Kutschera and Niklas 2004, 2005).
Excerpted from Plant Physics by KARL J. NIKLAS HANNS-CHRISTOF SPATZ Copyright © 2012 by The University of Chicago. Excerpted by permission of THE UNIVERSITY OF CHICAGO PRESS. All rights reserved. No part of this excerpt may be reproduced or reprinted without permission in writing from the publisher.
Excerpts are provided by Dial-A-Book Inc. solely for the personal use of visitors to this web site.
Table of Contents
Introduction: Ladies and Gentlemen, I Bring You . . .
Part One: Street Spectacles
Chapter One: Glimpsing Urban Savages
Chapter Two: Artful Promotion
Part Two. Metropolitan Encounters
Chapter Three: Managing Performance
Chapter Four: Recruiting Entertainers
Chapter Five: Interpreting Exhibitions
Part Three: The Natural History of Race
Chapter Six: Transforming "Unfruitful Wonder"
Chapter Seven: The End of an Affair