Geography 140
Introduction to Physical Geography

Lecture: Basics of Evolution

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  I. Evolutionary theory is the core of biogeography, and the ultimate source 
     of the theories tested in it. Though it is the reigning paradigm in 
     biogeography, ecology, and other biosciences, it is not widely accepted 
     among the general public (e.g., the recent Kansas debacle) and, as such, 
     is not well-treated in the K-12 curriculum.
     A. Contrary to popular opinion, neither the term nor the idea of 
        biological evolution began with Charles Darwin and his foremost work, 
        On the Origin of Species by Means of Natural Selection (1859). 
     B. Many scholars from the ancient Greek philosophers on had inferred that 
        similar species were descended from a common ancestor, most famously 
        Jean-Baptiste Lamarck (1744-1829).
     C. In 1799 an engineer named William Smith reported that, in undisrupted 
        layers of rock, fossils occurred in a definite sequential order, with 
        more modern-appearing ones closer to the top. 
        1. Because bottom layers of rock logically were laid down earlier and 
           thus are older than top layers, the sequence of fossils also could 
           be given a chronology from oldest to youngest. 
        2. His findings were confirmed and extended in the 1830s by the 
           paleontologist, William Lonsdale, who recognized that fossil 
           remains of organisms from lower strata were more primitive or 
           general looking than the ones above. Today, many thousands of 
           ancient rock deposits have been identified that show corresponding 
           successions of fossil organisms. 
     D. Thus, the general sequence of fossils had already been recognized 
        before Darwin conceived of descent with modification. The 
        paleontologists and geologists before Darwin used the sequence of 
        fossils in rocks not as proof of biological evolution, but as a basis 
        for working out the original sequence of rock strata that had been 
        structurally disturbed by earthquakes and other forces. 
     E. Darwin's great contribution, then, was NOT the idea of evolution, for 
        that had long been established as fact; rather, it was a coherent and 
        testable mechanism for evolution, or what he called "descent 
        with modification." 
        1. Darwin proposed that evolution could be explained by the 
           differential survival of organisms following their naturally 
           occurring variation--a process he termed "natural selection." 
        2. According to this view, the offspring of organisms differ from one 
           another and from their parents.  
        3. Some of these differences are heritable--that is, organisms can 
           pass on the differences genetically to their own offspring. 
        4. Furthermore, organisms in nature typically produce more offspring 
           than can survive and reproduce given the constraints of food, 
           space, and other environmental resources.  This is an idea he got 
           from Robert Malthus (An Essay on the Principle of Population, 
           1798), who had said that food supply can only grow arithmetically 
           (for humans, who engage in farming, and not really at all for other 
           species), but population can grow exponentially (producing more and 
           more offspring at faster and faster).  See the graph below for the 
           growing contrast in population levels produced by arithmetic and 
           exponential growth functions:
           [ arithmetic vs exponential growth ]

        5. If a particular offspring has traits that give it even a slight 
           advantage in a particular environment, that organism will be 
           more likely to survive and pass on those traits in that particular 
           environment. 
        6. As differences accumulate over generations, populations of 
           organisms diverge more and more widely from their ancestors. Over 
           millions of years, this can produce tremendous numbers of wildly 
           different species.
     F. Darwin's original hypothesis has undergone extensive modification and 
        expansion, but the central concepts stand firm.  
        1. For example, he had no clear idea how heritable traits were 
           inherited:  He thought it had to do with some kind of blending of 
           substances from parents, which he understood made systematic 
           directional change difficult to explain (you'd lose the 
           distinctiveness of new traits as they were blended in with other 
           traits in the offspring).  Darwin didn't live long enough to see 
           the solution to what was for him a big problem.
        2. The mechanism awaited Gregor Mendel's experiments with peas. 
           a. Mendel was the Augustinian monk in Austria who followed seven 
              clearly different pairs of genetic traits through time (he 
              experimented on 28,000 plants!).
           b. He was the first to approach a biological question with a 
              statistical and mathematical methodology.
           c. He presented and then published his work in a local scientific 
              society's conference and journal series back in 1865, where no-
              one seemed to know what he had accomplished.  A few people did 
              cite his work now and again, but, again, didn't seem fully to 
              comprehend what he had achieved.
           d. His work was independently rediscovered by Carl Correns in 
              Germany, Hugo de Vries in the Netherlands, and Erich von 
              Tschermak-Seysenegg in Austria, who all three separately 
              realized what a gold mine they'd found in Mendel's work and 
              began to promote it.
           e. The mechanism is of discrete gene units, called alleles, of 
              which more than one type can be found at a gene locus.  
                i. The alleles are inherited wholly, one from each parent.  
               ii. The offspring can have two copies of the same allele at a 
                   given gene locus, which makes them "homozygous" for that 
                   gene, or they can have one copy of two different alleles, 
                   which makes them "heterozygous" for that gene.
              iii. This genetic inheritance is called the offspring's 
                   genotype.  
           f. The alleles then enter into relationships with one another that 
              determine much of the offspring's phenotype, which is how the 
              genotype expresses itself.  These relationships include:
                i. Dominant-recessive interactions among homozygous individual 
                   (e.g., in humans, the interaction between brown eyes and 
                   light blue eyes, where two homozygous people, one with 
                   brown eyes and the other with light blue eyes, can only 
                   produce brown-eyed children, because brown is dominant and 
                   light blue is recessive).
               ii. Dominant-recessive interactions among heterozygous 
                   individuals (e.g., two brown-eyed people who each had a 
                   light blue-eyed parent getting together will typically 
                   produce one blue-eyed homozygous child who resembles two of 
                   the grandparents, two heterozygous brown-eyed children like 
                   themselves, and one homozygous brown-eyed child like the 
                   other two of the grandparents).
              iii. Blending forms among heterozygous individuals (e.g., 
                   crossing a white or cream colored horse with pink skin with 
                   a chestnut or solid red horse will produce palominos, gold-
                   colored horses with white or cream-colored manes and tails, 
                   but, if you breed two palominos, you'll only get half of 
                   the offspring being palomino, with one quarter being white, 
                   and one quarter being chestnut).
        3. The incorporation of Mendellian genetics into Darwin's idea is 
           termed the "Modern Synthesis." 
     G. The Modern Synthesis has received still further support through 
        studies in molecular biology -- a field unimaginable either to Darwin 
        or Mendel.  
        1. These have explained the occurrence of the hereditary variations 
           that are essential to natural selection. 
        2. Genetic variations result from changes, or mutations, in the 
           nucleotide sequence of DNA, the molecule that genes are made from. 

           [ DNA molecule animation showing helix structure and double
           binary links among adenine-thymine and guanine-cytosine ]
        3. Such changes in DNA now can be detected and described with great 
           precision, and, indeed, the complete genetic map for 
           humans was finished in 2003.  You can learn more about the Human 
           Genome Project at http://www.ornl.gov/sci/techresources/Human_Genome/home.shtml.
     H. Genetic mutations arise by chance. 
        1. They may or may not equip the organism with better means for 
           surviving in its environment.
        2. Indeed, the vast majority are harmful. 
        3. But if a gene variant does improve adaptation to the environment 
           (for example, by allowing an organism to make better use of an 
           available nutrient, or to escape predators more effectively -- such 
           as through stronger legs or disguising coloration), the organisms 
           carrying that gene are statistically more likely to survive and 
           reproduce than those without it. 
        4. Over time, their descendants will tend to increase, changing the 
           average range of characteristics of the population. 
        5. Although the genetic variation on which natural selection works is 
           based on random or chance elements at the molecular level, natural 
           selection itself produces "adaptive" change -- which is the very 
           opposite of chance. 
           a.  To give you a sense of this distinction between random, chance
               generation of change and the very non-random process of natural
               selection, consider the famous million monkeys and Shakespeare
               analogy for the probability of evolution.
           b.  One million monkeys, given typewriters and pecking away at them
               randomly, would need 78,800 years to produce the single sentence,
               "To be or not to be" from Hamlet.  
           c.  But if you incorporate selection acting on the random chaos,
               you get startlingly higher probabilities and shorter timeframes.
           d.  Richard Hardison up at Glendale College wrote a computer program
               that mimics the million monkeys but every time one of the virtual
               monkeys happens to type a letter in the right place, it is 
               allowed to keep it there:  "natural" selection.
           e.  His program can produce "To be or not to be" in 366 iterations
               in under 90 seconds!
           f.  In fact, his program can reproduce all of Hamlet in just
               4.5 days!
           g.  The very non-random selection pressure acting on randomly given
               variations can produce directional change very fast, which might
               be why life seems to have arisen so soon after Earth solidified
               (the planet goes back about 4.5 billion years and the first 
               traces of simple life forms about 3.5 billion years ago.             

 II. We have also gained an understanding of the processes by which new 
     species originate, and this is where geography comes in big time. 
     A. A new species is one in which the individuals cannot mate and produce 
        viable descendants with individuals of a pre-existing species.  
     B. Now, viability doesn't just mean the creation of living offspring, 
        because you can get mule hybrids between distinct species (e.g., when 
        you cross a female horse with a jackass, you get a mule, and such 
        hybrids have been produced between other species, such as between 
        street pigeons and ring-neck doves).  
        1. Most such mules cannot themselves reproduce:  You want a baby mule, 
           you have to go back to a mare and a jackass.  
        2. Some hybrids can produce babies, but the resulting creature does 
           not have an ecological niche in which it competes equally with or 
           better than either parent species, so it's a waste of the parents' 
           reproductive labor to make such a mating, and this generates 
           pressure to make darned sure you're messing just with your own 
           species (reproductive isolation).  
     C. The split of one species into two often starts because a group of 
        individuals becomes geographically separated from the rest. 
        1. This is particularly apparent in distant remote islands, such as 
           the Galápagos and the Hawaiian archipelago, whose great 
           distance from the Americas and Asia means that arriving colonizers 
           will have little or no opportunity ever to mate with individuals 
           remaining on those continents. 
        2. Mountains, rivers, lakes, and other natural barriers also account 
           for geographic separation between populations that once belonged to 
           the same species. 
        3. There is some evidence accumulating that even something like an 
           interstate highway is creating reproductive isolation among 
           populations of small species, causing them to diverge in traits!
     D. Once isolated, geographically separated groups of individuals become 
        genetically differentiated as a consequence of mutation and other 
        processes, such as the founder effect, gene drift, and natural 
        selection. 
        1. Founder effect means that a small, isolated population does not 
           have the full range of alleles in its gene pool, so natural 
           selection starts working with different material, which biases the 
           population's genetic evolution.
        2. Gene drift means that small populations tend to lose even more 
           alleles, just due to random events wiping out the rare individuals 
           with uncommon alleles.  If you have two alleles present in a small 
           population at one gene locus, and one is slightly less common than 
           the other and if the population commonly experiences random 
           episodes of high mortality (e.g., a hurricane coming through), the 
           less common one typically disappears entirely within a few 
           generations.  The impact of such disasters is disproportionate on 
           the less common allele from generation to generation.
        3. The environment in which the small population is isolated will 
           almost certain differ from the range of environments that the large 
           bulk of the species experiences.  This means that natural 
           selection, working through different environmental conditions, will 
           also cause the isolated population to evolve in a different 
           direction than the big parent population.
     E. The origin of a species is often a gradual process, so that at first 
        the reproductive isolation between separated groups of organisms is 
        only partial, but it eventually becomes complete. Biogeographers and 
        other life scientists pay special attention to these intermediate 
        situations, because they help to reconstruct the details of the 
        process and to identify particular genes or sets of genes that account 
        for the reproductive isolation between species.  There's an 
        interesting case in Southern California:  
        1. Penstemon spectabilis is a five-petalled flower with a showy 
           blue-purple flower that is short and plump in profile, because it 
           is pollinated by bees 
        2. P. centranthifolia is another five-petalled flower, but it's 
           bright red with a long slender profile, being pollinated by 
           hummingbirds.
        3. These two species have moved into one another's ranges.
        4. Bees can't pollinate P. centranthifolia, but hummingbirds 
           can pollinate P. spectabilis.
        5. The result is a beautiful, perfectly intermediate hybrid:  It's a 
           bright violet color and intermediate in profile between its two 
           parents.
        6. Unfortunately, the hybrid doesn't compete as well as either parent, 
           particularly for pollination, so it's a waste of reproductive 
           effort for P. spectabilis to let those pesky hummingbirds in 
           to mess things up.
        7. Over the millenia, the pressure will be on P. spectabilis to 
           find some way to gross out hummingbirds.  
        8. Had the hybrid been able to compete with its parents, then the two 
           species would have merged, because that would have been the smart 
           way to invest their reproductive charms.
     F. Evolution can be sped up by aggressive selection processes, such as 
        the pressures we humans have exerted on the other species with which 
        we interact:
        1. We have sped up evolution in the last few thousand years in the 
           case of animal and plant domestications.
           a. Dogs have been genetically determined to be wolves, all of 
              them, including chihuahuas and poodles (oh, how the mighty wolf 
              has fallen <G>!).  All dogs are the same species, but we 
              have pushed the definition of a species when its end points 
              (say, trying to mate a chihuahua female with an Irish wolfhound 
              male) are not viable, though the end points can eventually share 
              genes through a long series of mutts.
           b. We have also accidentally created new strains of bacteria, 
              through careless use of antibiotics, which create severe 
              selection pressure on pestilential species, which only 
              eventually creates "superbugs." 
                i. The worst abuses of antibiotics as superbug breeders 
                   involves the meat industry, where it's cheaper to medicate 
                   animals than to keep meat animals clean enough and in 
                   uncrowded conditions to reduce the transmission of 
                   bacteria.
               ii. People help this process along in their everyday lives, too 
                   -- demanding antibiotics of their doctors for every cold 
                   they or their kids get -- antibiotics are utterly useless 
                   in fighting colds and flus, since those diseases 
                   are caused by viruses, which do not respond to antibiotics
     G. We used to think that all speciation involved geographic (allopatric) 
        isolation, but we now have evidence that this can happen in a 
        sympatric (non-geographically separated) situation.
        1. In plants, it can happen instantaneously through polyploidy. That 
           is, some organisms occasionally make a mistake in meiotic cell 
           division (the kind that is supposed to divide chromosome pairs into 
           single chromosomes in sex cells, so that fertilization  of the 
           female sex cell will restore the normal number of pairs of 
           chromosomes, this time having one set of each pair from the mother 
           and the other set from the father).  As a result, plants can create 
           daughter cells with twice the number of chromosomes normal to the 
           species.  The offspring grows up, functioning normally, but cannot 
           reproduce with anyone of its parents' species, because half of its 
           chromosomes can't link up with its mate's chromosomes.  In complex 
           animals, this kind of accident would be the end of the new species.  
           In many plants (and even a few species of animal, such as 
           earthworms), however, the offspring can fertilize itself.  Thus 
           a new species can be born instantly.  
        2. In sexually reproducing species, mate selection by females can lead 
           to sympatric reproductive isolation, and it can take place quite 
           quickly.  Probably the most famous case of this is the brightly 
           colored cichlid fishes of Lake Victoria in East Africa.  For a map, 
           click here.
           This lake is known to have dried up completely about 12,400 years 
           ago.  The ancestor of the cichlids got in there after the lake 
           water level began to rise, and, though the fish environment shows 
           relatively little variation, there are now more than 300 
           reproductively isolated cichlid species in there!  It turns out 
           that, while the females often resemble those of other species, the 
           males are wildly different, and the females of each species are 
           really, really picky about exactly which color and pattern they 
           will accept in a boyfriend.  The result is the popular types of 
           guys got to breed and each generation picked mutant guys even more 
           extreme along the favored line of appearance and the process 
           snowballed into 300+ species that cannot swap genes because the 
           females are so darned fussy.  There's a troublesome footnote to 
           this example:  The lake is being polluted so bady that the 
           underwater visibility is declining, and that means the gals can't 
           see clearly enough to pick the pretty guys and there's some 
           evidence that some interbreeding is starting to happen, which will 
           eventually wipe out the unbelievable diversity of this unique bunch 
           of fish.
        3. There's also evidence of sympatric evolution when a new, acceptable 
           food source shows up in a region.  Some of the members of a species 
           will start exploiting that resource and will start to develop 
           quirks that help them exploit it better.  This means that 
           interbreeding back with everyone still using the traditional 
           resource will disrupt their new adaptation, and so pretty soon they 
           get snotty towards the other members of their species still 
           concentrating on the traditional resource and that leads to 
           reproductive isolation.  This happened very famously in the case of 
           the hawthorne/apple maggot fly, Rhagoletis pomonella, which 
           lives in the north central USA.  Its traditional resource was 
           hawthorne fruit, in which it would lay its eggs.  The maggots would 
           have a grand old time in the fruit, which would eventually fall on 
           the ground and the maggots would then burrow into the ground to 
           pupate and eventually emerge as flies.  Well, apple trees were 
           introduced to the area and, sure enough, some of the flies 
           discovered it and accidentally lay eggs in the apples.  The maggots 
           hatched to a surprising new environment and some of them managed to 
           thrive and that led to more of the indiscriminant flies dumping 
           eggs on apples.  They began to adapt to this strange and yummy new 
           resource, and now there are two races of the fly:  the hawthorne 
           maggot fly and the apple maggot fly.  They are now reproductively 
           isolated by the different timings of the peak fruiting seasons of 
           their host species.

III. Evidence 
     A. In Darwin's time, paleontology was still a rudimentary science. Large 
        parts of the geological succession of stratified rocks were unknown or 
        inadequately studied. Darwin, therefore, worried about the rarity of 
        intermediate forms between some major groups of organisms.  This is 
        the origin of that misleading term, "missing links."
     B. Today, many of the gaps in the paleontological record have, in fact, 
        been filled by the research of paleontologists. Hundreds of thousands 
        of fossil organisms, found in well-dated rock sequences, represent 
        successions of forms through time and manifest many evolutionary 
        transitions. 
        1. Microbial life of the simplest type was already in existence 3.5 
           billion years ago. 
        2. The oldest evidence of more complex organisms (that is, eucaryotic 
           cells, which are more complex than bacteria) has been discovered in 
           fossils sealed in rocks approximately 2 billion years old. 
        3. Multicellular organisms, which are the familiar fungi, plants, and 
           animals, have been found only in younger geological strata.




           Life Form                           Millions of Years Since 
                                               First Known Appearance 
                                               (Approximate)
           -----------------------------------------------------------
           
           Microbial (procaryotic cells)                3,500    
                                                                                      
           Complex (eucaryotic cells)                   2,000    
                                                                                      
           First multicellular animals                    670    
                                                                                      
           Shell-bearing animals                          540    
                                                                                      
           Vertebrates (simple fishes)                    490    
                                                                                      
           Amphibians                                     350    
                                                                                       
           Reptiles                                       310    
                                                                                      
           Mammals                                        200    
                                                                                      
           Nonhuman primates                               60    
                                                                                      
           Earliest apes                                   25    
                                                                                      
           Australopithecine ancestors of humans           4    
                                                                                      
           Modern humans                                    0.15 (150,000 years)
           
           ---------------------------------------------------------------------

        4. So many intermediate forms have been discovered between fish and 
           amphibians, between amphibians and reptiles, between reptiles and 
           mammals, and along the primate lines of descent that it often is 
           difficult to identify categorically when the transition occurs from 
           one to another particular species. Actually, nearly all fossils can 
           be regarded as intermediates in some sense; they are life forms 
           that come between the forms that preceded them and those that 
           followed (we are the intermediates between our parents and our 
           children, so to speak). 
        5. The fossil record thus provides consistent evidence of systematic 
           change through time--of descent with modification. From this huge 
           body of evidence, it can be predicted that no reversals will be 
           found in future paleontological studies. That is, amphibians will 
           not appear before fishes, nor mammals before reptiles, and no 
           complex life will occur in the geological record before the oldest 
           eucaryotic cells. This prediction has been upheld by the evidence 
           that has accumulated until now: no reversals have been found. 
     C. Other evidence includes:
        1. Common structures (homologies) most economically explained as 
           descent with modification of a common ancestral form.
           a. Limb bones show direct homologies across wildly different 
              species.  
                i. For example, examination of a horse's hoof and leg bones 
                   show that the hoof is homologous to our fingernails and 
                   that horses run on their middle finger bones. Two of the 
                   other "fingers" exist as "splints" or toothpick-sized bones 
                   on either side of their lower legs, which become evident 
                   only when they manage to break them!  Their thumbs are 
                   reduced to soft fingernail-like areas on their upper limbs, 
                   called "chestnuts." Their "knees" are homologous to our 
                   wrists!  The development of the modern equine's limb bones 
                   can be traced very clearly through the fossil record from 
                   the early Hyracotherium or Eohippus ancestor 
                   that had four hoofed toes in front and three in back, 
                   through a number of other genera with gradually reduced 
                   outer toes to the modern Equus. 
               ii. Other examples of homologies are the hand bones that 
                   support whale fins and bird wings.
           b. The mammalian ear and jaw are instances in which paleontology 
              and comparative anatomy combine to show common ancestry through 
              transitional stages. The lower jaws of mammals contain only one 
              bone, whereas those of reptiles have several. The other bones in 
              the reptile jaw are homologous with bones now found in the 
              mammalian ear. Paleontologists have discovered intermediate 
              forms of mammal-like reptiles (Therapsida) with a double jaw 
              joint--one composed of the bones that persist in mammalian jaws, 
              the other consisting of bones that eventually became the hammer 
              and anvil of the mammalian ear. 
           c. A good review of such homologies and transitions can be found here, 
              if you're curious about this subject.
        2. Biogeography
           a. Island groups closely related to distinct species on the 
              mainland, but which diversify like crazy in isolation: Hawai'is 
              diversity of species in certain groups -- adaptive radiation by 
              accidental colonizer species into virtually unoccupied niches 
              (kind of like those fruit maggot flies).
           b. Extinctions caused by new exotic invaders:  
                i. One of the derivatives from the Modern Synthesis is called 
                   the competitive exclusion principle:  Two species cannot 
                   indefinitely co-exist on the same resources in the same 
                   area at the same time.  One species will always 
                   outreproduce the other in that circumstance, unless there 
                   is enough time for the two to hit on some specialization 
                   that will allow them to co-exist.  
               ii. Humans have often introduced species to other areas and 
                   produced extinctions through this effect, such as the 
                   extinction of indigenous island rodent species by the 
                   introduction of ship rats. 
        3. Similarities during embryological development
           a. Embryos of vertebrates look virtually identical early on
           b. Barnacles are crustaceans, like lobsters and shrimps, that they 
              do not resemble, due to their sedentary adult lives, but their 
              larvae are free-swimming and strongly resemble other crustacean 
              larvae. 
        4. New evidence from molecular biology          
           a. The code used to translate nucleotide sequences into amino acid 
              sequences is essentially the same in all organisms. Moreover, 
              proteins in all organisms are invariably composed of the same 
              set of 20 amino acids. This unity of composition and function is 
              a powerful argument in favor of the common descent of the most 
              diverse organisms. 
           b. Family histories have been obtained from the three-dimensional                
              structures and amino acid sequences of all sorts of proteins. 
              The examination of molecular structure offers a new and 
              extremely powerful tool for studying evolutionary relationships. 
                i. These molecular clocks run rapidly for less constrained 
                   proteins and slowly for more constrained proteins, but they 
                   all time the same evolutionary events the same way. 
               ii. Pseudogenes are very interesting.  They are remnants of 
                   genes that no longer function but continue to be carried 
                   along in DNA as excess baggage, usually kept out of action 
                   by the folding of the DNA to keep it from generating 
                   proteins that might be disruptive if produced. Like all 
                   other genes, pseudogenes also mutate and change through 
                   time. 
                   a. This junk DNA offers an especially useful way of 
                      reconstructing evolutionary relationships. 
                   b. With functioning genes, one possible explanation for the 
                      relative similarity between genes from different 
                      organisms is that their ways of life are similar -- for 
                      example, the genes from a horse and a zebra could be 
                      more similar because of their similar habitats and 
                      behaviors than the genes from a horse and a tiger.  
                   c. But this possible explanation does not work for 
                      pseudogenes, since they perform no function. Rather, the 
                      degree of similarity between pseudogenes must simply 
                      reflect nothing more than their evolutionary 
                      relatedness.


Well, that's enough for now.  Come away from this lecture knowing the basics 
of Darwin's and Mendel's achievements.  Understand the source of novelty in 
genetic makeup (chance mutations) and how that random chaos is converted into 
well-ordered life forms (natural selection for survival and reproductive 
success in a particular environment in a particular timeframe).  Know what 
alleles are and how they relate to genes (which are segments of DNA grouped 
along chromosomes).  Be able to differentiate the two big forms of speciation: 
allopatric (geographical) speciation and sympatric speciation.  Know what the 
founder effect and gene drift are and how they affect allopatric speciation. 
Be able to describe two main mechanisms of sympatric speciation (sexual 
selection and ecological selection).  Be able to identify two forms of 
accelerated evolutionary change associated with human activities 
(domestication and the creation of resistant pest species).  Know why the 
term, "missing links," is so misleading now.  

In the next lectures, I'll introduce systems of classifying the astounding 
diversity of life on Earth:  genetically, morphologically, and by vegetation 
typologies.


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Document and © maintained by Dr. Rodrigue
First placed on web: 03/23/01
Last revised: 06/26/07

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