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: 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. 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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First placed on web: 03/23/01 Last revised: 06/26/07