ZOOL 304

Class Notes

Prologue and Chapter 1, Jan. 13 - Jan. 17.  

Text reading: Prologue and Chapter 1.

• ASSIGNMENT for written preparation.
• Introductory comments (on this course, on evolution's place in biology, and on sources of scientific information.
• Discussion (elaboration upon ideas presented in class or textbook).
  • Prelude to evolution (some preliminary background on Darwin's life).
  • Comment on text prologue.
  • Comment on text Chapter 1.
  • Consilience of induction (evidence for evolution)
  • Side discussion on the question, Is evolution progressive?
  • What is "evolution"? (definitions).
• Study questions for text reading.

Notes for chapter 1 / 2 / 3 / 4 / 5 / 6 / 7 / 8 / 9 / 10 / 11 / 12 / 13 / 14 / 15 / 16 / 17

Prelude to evolution

 Like many an "idle sporting man", a young Charles Darwin had no clear plans for his future.  Mostly he enjoyed collecting beetles.  His father was afraid he would have no higher aspirations than shooting rabbits.  Nonetheless, he fared well at university.  Since only a few vocations were suitable for an English gentleman, when young Darwin found that he had no stomach for medical studies he began to prepare for the clergy.  Then, through the grapevine, he learned of an opportunity to travel and see the world.   The modern science of evolutionary biology begins with a sea voyage.  (For more, see Darwin's Autobiography, or additional links.)

Through friends, Darwin learned that a surveying ship, the H.M.S. Beagle, would soon set sail to chart the coasts of South America and nearby islands for the British admiralty.  The expedition would last for five years, from 1831 to 1836.  Darwin joined the voyage, at his own expense (or rather, at his family's expense), as a gentleman companion for the ship's captain, Robert FitzRoy.  (Class expectations prohibited a ship's captain from socializing with his crew.)  A few weeks into the voyage, the ship's surgeon and naturalist gave up and returned to England.  Darwin, while retaining his primary role as captain's companion, took on the naturalist's duties of recording the natural history and geology of the various countries visited by the Beagle.  While the Beagle sailed up and down a stretch of coast for weeks at time, Darwin would take long journeys inland -- observing, describing, and collecting.  

This voyage was to change Darwin's life and the course of science.  The extent of his collections and the quality of his letters (later published as a Journal of Researches, with a popular edition known as The Voyage of the Beagle) earned him high regard as an eminent naturalist even before his return.  His reputation grew with subsequent publications, include a massive monograph on barnacles.  (0nline text of The Voyage of the Beagle.)

But for a quarter century after his return, Darwin's considerations of "the species problem" would remain private.  Gradually, he mustered evidence for the conclusion that had grown ever more compelling to him.  Living species were related by "descent with modification", diverging and adapting to various habits by the process which he called "natural selection".  (Darwin avoided the term "evolution", which in his century carried implications of unfolding or emergence of a predetermined pattern, a meaning closer to our modern use of "development".)  Darwin's finally published The Origin of Species in haste, in 1859, after receiving notice that the same basic thesis -- descent with modification powered by natural selection -- had also been realized by Alfred Russel Wallace.  (Online text of The Origin.)

Darwin's life has been exceptionally well documented by his own extensive notes, diaries, and letters.  Of many biographies, my favorite is Janet Browne's Charles Darwin: Voyaging.  There is also abundant information on the World Wide Web.  Here is an initial link:  More on Darwin.  Or go straight to his autobiography.

Upon publication of the Origin, the idea of evolution was greeted with enthusiastic acceptance by some scientists, with skepticism by others, and with ridicule and scorn by yet others.  We shall explore some of these reactions through the remainder of the course.

The textbook Prologue introduces a few selected examples to illustrate what evolutionary biology is about.  Be sure you appreciate the significance of the examples.  That is, these are not factoids to memorize, they are particular representations of general concepts.

Sexual cannibalism by Australian red-backed spiders:  Seemingly bizarre, self-destructive behaviosr -- which appear counter-intuitive as a product of natural selection based on individual fitness -- can make sense as strategies to maximize reproductive success when properly interpreted in terms of the ecological circumstances in which it occurs.  (More in Chapters 2, 8 and 9.)

Rapid maturation and small adult size in guppies:  Under appropriate circumstances (strong selection pressure and abundant variation), natural selection can operate remarkably quickly, producing significant adaptive effects in just a few generations.  (More in Chapters 2, 4 and 5.)

Relationships of pentastomid parasites:  An evolutionary history of ancestor-descendent relationships ("phylogeny") results in nested sets of similarities (i.e., species with a more recent common ancestor generally share more similarties than more distant relatives).  Relationships which are particularly obscure in terms of morphological similarities can be illuminated by comparison of DNA sequences.   (More in Chapters 12 and 15.)

Morphology of plethodontid salamanders and vertebrate ears:  Many evolutionary transformations involve complex rearrangements which would make any reversal of the evolutionary process impractical.  Thus there are "constraints" upon the power of natural selection.  (This topic is not well-developed elsewhere in the text.)

Chapter 1 is the longest chapter in the text.  By previewing the entire subject, this chapter introduces a lot of extremely important concepts.  Here in summary are several of the major themes.

Evolution encompasses a broad range of processes and effects, from microevolution (allele frequency changes in populations) to adaptation to macroevolution (major transformations over time) to phylogeny (the evolutionary "tree").

Evolutionary change may be adaptive or neutral (i.e., not all changes and not all traits result from the direct action of natural selection).

Any evolutionary change requires variation among individuals.  This variation must be heritable.  To be influenced by natural selection, heritable variation must be correlated with variation in reproductive success.

Pages 9 - 12 may be the most important three pages in the entire book.  These pages describe, in brief, the essential mechanism of evolution.  UNDERSTAND these words.  Come back to these pages as often as you need to.

Here is how Charles Darwin summarized these ideas in the concluding paragraph of The Origin [full text of the source chapter]:

"It is interesting to contemplate an entangled bank, clothed with many plants of many kinds, with birds singing on the bushes, with various insects flitting about, and with worms crawling through the damp earth, and to reflect that these elaborately constructed forms, so different from each other, and dependent on each other in so complex a manner, have all been produced by laws acting around us. These laws, taken in the largest sense, being Growth with Reproduction; Inheritance which is almost implied by reproduction; Variability from the indirect and direct action of the external conditions of life, and from use and disuse; a Ratio of Increase so high as to lead to a Struggle for Life, and as a consequence to Natural Selection, entailing Divergence of Character and the Extinction of less-improved forms."

Natural selection can rapidly produce highly improbable states.  In the words of R.A. Fisher, "Natural selection is a mechanism for generating an exceedingly high degree of improbability."  

Many biologists, including the authors of your textbook, celebrate the power of natural selection to produce adaptations of extreme precision and complexity.  Nevertheless, the precision of adaption is NOT, by itself, evidence for the power of natural selection.  Rather, the power of natural selection is a deduction from the presumption (justified on more subtle grounds, see consilience) that all adaptation is indeed shaped by natural selection.

Adaptation may be limited by constraints on the power of natural selection.

Natural selection does not automatically produce whatever adaptations would be useful.  In spite of abundant evidence that evolution has produced some wonderful adaptations, the process of adaptation is nonetheless limited or constrained by the conditions and processes under which it occurs.  These constraints include prior history, availability of variation, opportunity for gene flow, functional tradeoffs, genomic organization, relationship between DNA and fitness, and time.

Furthermore, "Traits do not evolve for the good of the species... [they] evolve because they improve the reproductive success of individuals and their kin."  

Traits do not evolve because they WILL improve reproductive success, but because in the past each step leading up to the trait has already improved reproductive success for the individual bearers of the trait. Adaptation is the RESULT of past success, NOT a purposeful preparation for the future. Nevertheless, since future conditions are often similar to past conditions, adaptation to past conditions is often advantageous for future circumstances.

The mechanisms of sexual reproduction (meiosis and fertilization, and associated genetic processes like recombination) have important consequences for evolution.  These will become evident in later discussions.

Fitness is rather simply defined as relative reproductive success.  But fitness is NOT a simple concept.  It embraces many subtle (and sometimes counter-intuitive) aspects.  Understanding how fitness emerges involves understanding ecology, morphology, physiology, genetics, behavior, and any other aspect of biology you care to consider.

Speciation is also rather simply defined but involves complex processes of genetics, population structure, geography, and adaptation.  

Macroevolution (large-scale, long-term patterns of change) emerges from microevolution (mutation and changes in allele frequency), but many aspects of the relationship remain obscure.

Clear thinking about evolution requires some novel styles of thought.  Most notably, these are: 

• Emphasis on individual variation and probability ("population thinking").
• Constant awareness that everything in biology is caused by past history, not by future purpose (i.e., rejection of teleology, of Aristotle's final cause).
• Appreciation for the flaws and limits on adaptation, more than concentration on the perfection of adaptation.
• Simultaneous consideration of multiple levels of causation, from immediate short-term (proximate, "mechanical") effects to consequences which extend from the farthest reaches of time.  [ Example:  Most of us have five fingers on each hand because that's what happens when during embryonic development our molecules (DNA, proteins, etc.) interact within our cells and with our environment.  We also have five fingers because an ancient vertebrate ancestor happened to have five appendages on each limb.  Note that in this example there is no reason to invoke natural selection.  That is, there is no evidence that five fingers has ever been "better" (i.e., contributing a higher degree of fitness) than four or six .]

Do read the Landmarks in evolutionary biology on pages 31-34.  Recite to yourself the name of each person listed.  Many of these names are associated with concepts of great significance.  (For more on famous names in evolution, click here.)

Consilience of induction

• consilience defined
• consilience as evidence of descent with modification
• consilience as evidence of natural selection as a mechanism for evolution
• Occam's Razor

The strongest argument for biological evolution is what William Whewell called consilience of induction -- the concurrence of many separate lines of evidence and argument into a single, integrated explanatory framework.  In his Origin, Darwin explained how a host of diverse observations, including hierarchical classification, comparative anatomy (homology), biogeography, the fossil record, and adaptation, could each lead to the same coherent theory of descent with modification.  This demonstration of consilience of induction is what made Darwin famous.  

Consilience defined

• Consilience -- "The fact of 'jumping together' or agreeing; coincidence, concurrence; said of the accordance of two or more inductions drawn from different groups of phenomena" [from the Oxford English Dictionary].

• 1840 Whewell The philosophy of the inductive sciences, founded upon their history, vol. II, p. 230.  "Accordingly the cases in which induction from classes of facts altogether different have thus jumped together, belong only to the best established theories which the history of science contains... I will take the liberty of describing it by particular phrase; and will term it the consilience of inductions" [Oxford English Dictionary].

• 1847 Whewell History of the inductive sciences, from the earliest to the present times, vol. II, p. 583.  "Such coincidences, or consiliences ... are the test of truth" [Oxford English Dictionary].

• "The Consilience of Inductions takes place when an Induction, obtained from one class of facts, coincides with an Induction, obtained from another different class.  This Consilience is a test of the truth of the theory in which it occurs" [Audi, R. (1995) The Cambridge Dictionary of Philosophy, Cambridge University Press, Cambridge].

• Criticism based on an appreciation of consilience:
  1859  Darwin wrote to Lyell concerning an objection by Richard Owen that Darwin's Origin of Species "explained everything & that it was improbable in the highest degree that I sh^d. succeed in this.  I quite agree with this rather queer objection, & it comes to this that my book must be very bad or very good."  (Charles Darwin's Letters: A Selection 1825-1859, edited by F. Burkhardt, Cambridge Univ. Press, 1996.)  Of course, it has turned out that Darwin's book was very good indeed.

Consilience of induction for descent with modification.

Before Darwin, many independent sets of observations were all sources of puzzlement with many independent explanations.  After Darwin, one simple explanation was found to work for all of these, that present kinds of organisms are descended with modification from organisms that lived in the distant past.  These lines of evidence include:

• Biogeographical distribution of similar organisms.
• Biogeographical distribution of recent fossils.
• Linnean hierarchical classification.
• Anatomical homology.
• Embryological homology.
• Geological sequence of fossils of major groups.
• Fossil record of gradual transformation within a lineage (limited data in Darwin's time).
• Molecular homology (data not available to Darwin).

None of these would, by itself, constitute overwhelming evidence for the divergence and transmutation of species.  But, taken all together, the fact that every one of these various phenomena can be accounted for by the same explanation is quite powerful.  The coincidence of these independent lines of evidence constitutes consilience of induction for descent with modification.

Consilience of induction for natural selection as the mechanism of evolution.

Similarly, several independent lines of reasoning or empirical observation all suggest that natural selection is the mechanism for descent with modification (and hence for adaptation).  These lines of evidence include:

• Artificial selection can indeed be a powerful agent of change, as evidenced by diversity in domesticated plants and animals.
• Natural selection can explain adaptation [but see limitations below].
• Natural selection is logically necessary if certain conditions are met.
• The conditions necessary for selection are indeed empirical facts.
• No empirical evidence has been found for alternative mechanisms for adaptation (e.g., inheritance of acquired characters, "intelligent design").

In other words, a causal mechanism underlying descent with modification could be induced without calling upon any new or mysterious principles, just from the empirical facts of reproductive excess (noted by Malthus), from heredity with variation, and from the correlation of variation in heritable traits with variation in reproductive success.  The strongest evidence for Darwin's mechanism is exactly these empirical observations.  

If mutation did not provide an endless supply of novel variants, if such variation were not heritable, if all offspring survived, if hereditary variation in traits made no difference for survival, in any of these cases evolution by natural selection would be disproven.  But all of these are facts which have been abundantly confirmed.  With this causal mechanism, the consilience grew ever so much stronger.  Now not only could so much be explained through descent with modification, but descent with modification could itself be explained by processes which were reliably observable and readily appreciated already in Darwin's time.

Because of abundant evidence that the mechanism (natural selection) operates and also abundant evidence that the process (evolution) has happened, there is a STRONG PRESUMPTION that the one accounts for the other.  However, note that (in our current state of knowledge) it must still be presumed that there is a sequence of spontaneous, undirected mutations that has led along a naturally selectable route from one ancestral set of adaptations (e.g., those of a paleozoic fish) to another, transmuted descendent set (e.g., those of any modern bird or mammal).  

This presumption is a strong one, based on the principle known as Occam's Razor (named after English philosopher William of Occam, ca. 1350).  Occam's Razor states that "Essentia non sunt multiplicanda praeter necessitatem" or "Essentials shall not be multiplied beyond necessity".  This principle asserts that one should not complicate an explanation unnecessarily.  Applied to evolution, it is generally taken that we should not deny the appropriateness of extrapolation without compelling reason.  No sufficiently good reason has yet been found.  Nevertheless, like any other sharp tool, Occam's Razor can be dangerously misused; its application must always be treated with some degree of skepticism.

Concerns are often raised about the adequacy of microevolutionary processes to explain macroevolutionary results.  Such concerns are at the heart of Mivart's old "incipient stages of complex adaptations" argument, and also lie behind the outdated "acquired characters", "orthogenesis", and "hopeful monsters" hypotheses.  But because there is no substantial evidence for any of these alternatives, the initial presumption (and the consilience upon which it is based) stands as the most promising candidate for a necessary and sufficient explanation of evolution.

Plausible "just-so" stories can be told to make the presumption of extrapolation seem reasonable in particular cases (such as the famous case of the vertebrate eye), but we do not yet have a theory relating genetics, mutations, adaptations, and fitness that is adequate to explain the transformation in any detail.

Notes for chapter 1 / 2 / 3 / 4 / 5 / 6 / 7 / 8 / 9 / 10 / 11 / 12 / 13 / 14 / 15 / 16 / 17

Various definitions of evolution are discussed on a separate page.

The question, Is evolution progressive? is discussed on a separate page.

304 index page

Comments and questions: dgking@siu.edu
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SIUC / College of Science / Zoology / Faculty / David King / ZOOL 304
URL: http://www.science.siu.edu/zoology/king/304/ch01.htm
Last updated:  15 December 2005 / dgk

From Charles Darwin's Autobiography:

"But no pursuit at Cambridge was followed with nearly so much eagerness or gave me so much pleasure as collecting beetles. It was the mere passion for collecting, for I did not dissect them, and rarely compared their external characters with published descriptions, but got them named anyhow. I will give a proof of my zeal: one day, on tearing off some old bark, I saw two rare beetles, and seized one in each hand; then I saw a third and new kind, which I could not bear to lose, so that I popped the one which I held in my right hand into my mouth. Alas! it ejected some intensely acrid fluid, which burnt my tongue so that I was forced to spit the beetle out, which was lost, as was the third one." 

The cartoon at right was drawn by one of Darwin's chums at Cambridge.  The caption reads "Go it Charlie!"