A continuación algunos enlaces y textos del libro:
Note 15. Masai Ostrich Mating.
Note 22. Earth's history.
Note 26. Ants...when they fall off a branch, they can maneuver in the air so that, instead of landing on the hostile forest floor, they swoop back to the safety of the tree trunk:
Página 180-181: There is, however, a third explanation for sexual dimorphisms, and it's the simplest of all. It is based on what are called sensory-bias models. These models assume that the evolution of sexual dimorphisms is driven simply by preexisting biases in a female's nervous system. And those biases could be a by-product of natural selection for some function other than finding mates, like finding food. Suppose, for example, that members of a species had evolved a visual preference for red color because that preference helped them locate ripe fruits and berries. If a mutant male appeared with a patch of red on his breast, he might be preferred by females simply because of this preexisting preference. Red males would then have an advantage, and a color dimorphism could evolve. (We assume that red color is disadvantageous in females because it attracts predators.) Alternatively, females may also simply like novel features that somehow stimulate their nervous systems. They may, for example prefer bigger males, males who hold their interest by doing more complex displays, or males who are shaped oddly because they have longer tails. Unlike the models I described earlier, in the sensory-bias model females derive neither direct nor indirect benefits from choosing a particular male.
Página 187-188: Furthermore, because humans are visual animals, we tend to overlook traits that can't easily be se en, like differences in pheromones that often distinguish species of similar-Iooking insects.
And when we think of why we feel that brown-eyed and blue-eyed humans, or Inuit and !Kung, are members of the same species, we realize that it's because they can mate with each other and produce offspring that contain combinations of their genes. In other words, they belong to the same gene pool. When you ponder cryptic species, and variation within humans, you arrive at the notion that species are distinct not merely because they look different, but because there are barriers between them that prevent interbreeding.
Ernst Mayr and the Russian geneticist Theodosius Dobzhansky were the first to realize this, and in 1942 Mayr proposed a definition of species that has become the gold standard for evolutionary biology. Using the reproductive criterion for species status, Mayr defined a species as a group of interbreeding natural populations that are reproductively isolated from other such groups. This definition is known as the biological species concept, or BSC. “Reproductively isolated” simply species have traits-differences in appearance, behavior, or physiology-that prevent them from successfully interbreeding, while members of the same species can interbreed readily.
Página 190: But the BSC isn't a foolproof concept. What about organisms that are extinct? They can hardly be tested for reproductive compatibility. So museum curators and paleontologists must resort to traditional appearance-based species concepts, and classify fossils and specimens by their overall similarity. And organisms that don't reproduce sexually, such as bacteria and some fungi, don't fit the criteria of the BSC either. The question of what constitutes a species in such groups is complicated, and we're not even sure that asexual organisms form discrete clusters in the way that sexual ones do.
But despite these problems, the biological species concept is still the one that evolutionists prefer when studying speciation, because it gets to e heart of the evolutionary question. Under the BSC, if you can explain how reproductive barriers evolve, you've explained the origin of species.
Página 192-193: In many ways biological speciation resembles the "speciation" of two closely related languages from a common ancestor (an example is German and English, two "sister tongues")…During biological speciation, populations change genetically to the extent that their members no longer recognize each other as mates, or their genes can't cooperate to produce a fertile individual Likewise, languages can diverge to the extent that they become mutually unintelligible: English speakers don't automatically ¡;understand German and vice versa. Languages are like biological species in that they occur in discrete groups rather than as a continuum: the speech of any given person can usually be placed unambiguously in one of the several thousand human languages.
The parallel goes even further. The evolution of languages can be traced back to the distant past, and a family tree drawn up, by cataloging _the similarities of words and grammar. This is very like reconstructing an evolutionary tree of organisms from reading the DNA code of their I genes. We can also reconstruct proto-Ianguages, or ancestral tongues, by looking at the features that descendant languages have in common. This is precisely the way biologists predict what missing links or ancestral genes should look like. And the origin of languages is accidental: people don't start to speak in different tongues just to be different. New languages, like new species, form as a by-product of other processes, as in the transformation of Latin to Italian in Italy. The analogies between speciation and languages was first drawn by-who else?-Darwin, in The Origin.
But we shouldn't push this analogy too faro unlike species, languages can "cross-fertilize," adopting phrases from each other, like the English use of the German angst and kindergarten. Steven Pinker describes other striking similarities and differences between the diversification of languages and species in his engrossing book The Language Instinct.
Página 198: The way we discovered how species arise resembles the way astronomers discovered how stars "evolve" over time. Both processes occur too slowly for us to see them happening over our Lifetime. But we can still understand how they work by finding snapshots of the process at different evolutionary stages and putting these snapshots together into a conceptual movie. For stars, astronomers saw dispersed clouds of matter ("star nurseries") in galaxies.
Página 209: Since Dart's time, paleoanthropologists, geneticists, and molecule biologists have used fossils and DNA sequences to establish our place in the tree of evolution. We are apes descended from other apes, an< our closest cousin is the chimpanzee, whose ancestors diverged from our own several million years ago in Africa. These are indisputable facts. And rather than diminishing our humanity, they should produce satisfaction and wonder, for they connect us to all organisms, the living and the dead.
Página 215: Our main question is, of course, to determine the pattern of human evolution. When do we see the earliest fossils that might represent our ancestors who had already diverged from other apes? Which of our hominin relatives went extil1ct, and which were our direct ancestors? How did the features of the ancestral ape become those of modern humans? Did our big brain evolve first, or our upright posture? We know that humans began evolving in Africa, but what part of our evolution happened elsewhere?
Página 225: What happened? There are two theories. The first, called the "multiregional" theory, proposes an evolutionary replacement: H. erectus (and perhaps H. neanderthalensis) simply evolved into H. sapiens independently in several areas, perhaps because natural selection was acting in the same way all over Asia, Europe, and Africa.
The second idea, dubbed the "out of Africa" theory proposes that modern H. sapiens originated in Africa and spread, physically replacing H. erectus and the Neanderthals, perhaps by outcompeting them for food or killing them.
Página 226-227: Looking at the whole array of bones, then, what do we have? Clear and indisputable evidence for human evolution from ape-like ancestors. Granted, we can't yet trace out a continuous lineage from an ape-like early hominin to modern Horno sapiens. The fossils are scattered in time and space, a series of dots yet to be genealogically connected. And we may never have enough fossils to join them. But if you put those dots in chronological order, as in figure 24, you see exactly what Darwin predicted: fossils that start off ape-like and become more and more like modern humans as time passes. It's a fact that our divergence from the ancestor of chimps occurred in East or Central Africa about seven million years ago, and that bipedal walking evolved well before the evolution of large brains. We know that during much of hominin evolution, several species existed at the same time, and sometimes at the same place. Given the small population size of humans and the improbability of their fossilization (remember, this usually requires that a body find its way into water and be quickly covered with sediment), it's amazing that we have as good a record as we do. It seems impossible to survey the fossils we have, or look at figure 25, and deny that humans have evolved.
Página 230-231: But recent work shows that our genetic resemblance to our evolutionary cousins is not quite as close as we thought. Consider this. A 1.5 percent difference in protein sequence means that when we line up the same protein (say, hemoglobin) of humans and chimps, on average we'll see a difference at just one out of every 100 amino acids. But proteins are typically composed of several hundred amino acids. So a 1.5 percent difference in a protein 300 amino acids long translates into about four differences in the total protein sequence. (To use an analogy, if you change only 1 percent of the letters on this page, you will alter far more than 1 percent of the sentences.) That oft-quoted 1.5 percent difference between ourselves and chimps, then, is really larger than it looks: a lot more than 1.5 percent of our proteins will differ by at least one amino acid from the sequence in chimps. And since proteins are essential for building and maintaining our bodies, a single difference can have substantial effects.
Now that we've finally sequenced the genomes of both chimp and human, we can see directly that more than 80 percent of all the proteins shared by the two species differ in at least one amino acid. Since our genomes have about 25,000 protein-making genes, that translates to a difference in the sequence of more than 20,000 of them. That's not a trivial divergence. Obviously, more than a few genes distinguish us. And molecular evolutionists have recently found that humans and chimps differ not only in the sequence of genes, but also in the presence of genes. More than 6 percent of gene s found in humans simply aren't found in any form in chimpanzees. There are over 1,400 novel genes expressed in humans but not in chimps. We also differ from chimps in the number of copies of many gene s that we do share. The salivary enzyme amylase, for example, acts in the mouth to break down starch into digestible sugar. Chimps have but a single copy of the gene, while individual humans have between two and sixteen, with an average of six copies. This difference probably resulted from natural selection to help us digest our food, as the ancestral human diet was probably much richer in starch than that of fruit-eating apes.
Página 232: Races (also called "subspecies" or "ecotypes") are simply populations of a species that are both ¡ geographically separated and differ genetically in one or more traits. There are plenty of animal and plant races, inc1uding those mouse populations that differ only in coat color, sparrow populations that differ in size and song, and plant races that differ in the shape of their leaves. Following this definition, Homo sapiens c1early does have races. And the fact that we do is just another indication that humans don't differ from other evolved species.
Página 234-235: Some racial differences give us clear evidence of evolutionary pressures that acted in different areas, and can be useful in medicine. Sickle-cell anemia, for example, is most common in blacks whose ancestors came from equatorial Africa. Because carriers of the sickle-cell mutation have some resistance to falciparium malaria (the deadliest form of the disease), it's likely that the high frequency of this mutation in African and African-derived populations resulted from natural selection in response to malaria. Tay-Sachs disease is a fatal genetic disorder that is common among both Ashkenazi Jews and the Cajuns of Louisiana, probably reaching high frequencies via genetic drift in small ancestral populations. Knowing one's ethnicity is a tremendous help in diagnosing these and other genetically based diseases. Moreover, the differences in allele frequencies between racial groups mea n that finding appropriate organ donors, which requires a match between several "compatibility genes," should take race into account.
Some of these differences make sense as adaptations to the different environments in which early humans found themselves. The darker skin of tropical groups probably provides protection from intense ultraviolet light that produces lethal melanomas, while the pale skin of higher latitude groups allows penetration of light necessary for the synthesis of essential vitamin D, which helps prevent rickets and tuberculosis.52 But what about the eye folds of Asians, or the longer noses of Caucasians? . These don't have any obvious connection to the environment. For some biologists, the existence of greater variation between races in genes that affect physical appearance, something easily assessed by potential mates, points to one thing: sexual selection.
Apart from the characteristic pattern of genetic variation, there are other grounds for considering sexual selection as a strong driving force for the evolution of races. We are unique among species for having .developed complex cultures. Language has given us a remarkable ability to disseminate ideas and opinions. A group of humans can change their culture much faster than they can evolve genetically. But the cultural change can also produce genetic change. Imagine that a spreading idea or fad involves the preferred appearance of one's mate. An empress in Asia, for example, might have a penchant for men with straight black hair and almond-shaped eyes. By creating a fashion, her preference spreads culturally to all her female subjects, and, lo and behold, over time the curly-haired and round-eyed individuals will be largely replaced by individuals with straight black hair and almond-shaped eyes. It is this "gene-culture coevolution" -the idea that a change in cultural environment leads to new types of selection on genes-that makes the idea of sexual selection for physical differences especially appealing.
Moreover, sexual selection can often act incredibly fast, making it an ideal candidate for driving the rapid evolutionary differentiation of physical traits that occurred since the most recent migration of our ancestors from Africa. Of course, all this is just speculation, and nearly impossible to test, but it potentially explains certain puzzling differences between groups.
Página 236: As the psychologist Steven Pinker noted, "If you adopt children from a technologically undeveloped part of the world, they will fit in to modern society just fine." That suggests, at least, that races don't show big innate differences in behavior.
My guess-and this is just informed speculation-is that human races are too young to have evolved important differences in intellect and behavior. Nor is there any reason to think that natural or sexual selection has favored this sort of difference. In the next chapter we'll learn about I the many "universal" behaviors seen in all human societies-behaviors like symbolic language, childhood fear of strangers, envy, gossip, and gift-giving. If these universal s have any genetic basis, their presence in I every society adds additional weight to the view that evolution hasn't l produced substantial psychological divergence among human groups.
Although certain traits like skin color and hair type have diverged among populations, then, these appear to be special cases, driven by I environmental differences between localities or by sexual selection for I external appearance. The DNA data shows that, overall, gene tic differences among human populations are minor. It's more than a soothing platitude to say that we're all brothers and sisters under the skin. And, that's just what we'd expect given the brief evolutionary span since our most recent origin in Africa.
Página 238-239: The evolution of lactose tolerance is another splendid example of gene-culture coevolution. A purely cultural change (the raising of cows, perhaps for meat) produced a new evolutionary opportunity: the ability ~ to use those cows for milk. Given the sudden availability of a rich new source of food, ancestors possessing the tolerance gene must have had I a substantial reproductive advantage over those carrying the intolerant gene. In fact, we can calculate this advantage by observing how fast the 1: tolerance gene increased to the frequencies seen in modern populations. , It turns out that tolerant individuals must have produced, on average, I 4 to 10 percent more offspring than those who were intolerant. That is pretty strong selection.
Anybody who teaches human evolution is inevitably asked: Are we still evolving? The examples of lactose tolerance and duplication of the amylase gene show that selection has certainly acted within the last few thousand years. But what about right now? It's hard to give a good answer. Certainly many types of selection that challenged our ancestors no longer apply: improvements in nutrition, sanitation, and medical care have done away with many diseases and conditions that killed our ancestors, removing potent sources of natural selection. As the British geneticist Steve Jones notes, 500 years ago a British infant had only 50 percent chance of surviving to reproductive age, a figure that has now risen to 99 percent. And for those who do survive, medical intervention has allowed many to lead normal lives who would have been ruthlessly culled by selection over most of our evolutionary history. How many people with bad eyes, or bad teeth, unable to hunt or chew, would have perished on the African savanna? (I would certainly have been among the unfit.) How many of us have had infections that, without antibiotics, would have killed us? It's likely that, due to cultural change, we are going downhill genetically in many ways. That is, genes that once were detrimental are no longer so bad (we can compensate for "bad" genes with a simple pair of eyeglasses or a good dentist), and these genes can persist in populations.
Conversely, genes that were once useful may, due to cultural change, now have destructive effects. Our love of sweets and fats, for example, may well have been adaptive in our ancestors, for whom such treats were a valuable but rare source of energy.54 But these once rare foods are now readily available, and so our gene tic heritage brings us tooth decay, obesity, and heart problems. Too, our tendency to lay on fat from rich food may also have been adaptive during times when variation in local food abundance produced a feast-or-famine situation, giving a selective advantage to those who were able to store up calories for lean times.
Does this mean that we're really de-evolving? To some degree, yes, but we're probably also becoming more adapted to modern environments that create new types of selection.
http://www.archaeologyinfo.com/evolution.htm A good (albeit slightly outdated) depiction and description of the various stages of human evolution.
http://www.darwin-online.org.uk/ The complete work of Charles Darwin online.
IncIudes not only all of his books (incIuding all six editions of The Origin), but also his scientific papers. You can find many of Darwin's personal letters at the Darwin Correspondence Project: http://www.darwinproject.ac.uk/home
http://www.gate.net/~rwms/EvoEvidence.html large website collecting various lines of evidence for evolution.
http://www.gate.net/~rwms/crebuttals.html website that examines and thoroughly debunks many creationist claims.
http://ncse.com/ An online set of resources assembled by the National Center for Science Education, an organization devoted to defending the teaching of evolution in America's public schools. It gives updates on ongoing battles with creationism, and includes links to many other sites.
http://www.pbs.org/wgbh/evolution/ A large Web site inspired by the PBS series Evolution, this contains a large selection of resources for both students and teachers, including the history of evolutionary thought, the evidence for evolution, and theological and philosophical issues. The sections on human evolution are particularly good.
http://pandasthumb.org/ The Panda's Thumb Web site (named after a famous essay by Stephen Jay Gould) deals with recent discoveries in evolutionary biology as well as ongoing opposition to evolution in America.
http://www.talkorigins.org/ A comprehensive online guide to all aspects of evolution. Included within it is the best online guide to the evidence for evolution, at http:/www.talkorigins.org/faqs/comdesc/.
Among many good blogs on evolutionary biology, two stand out. One is "Laelaps" (http://scienceblogs.com/laelaps/ & http://brianswitek.com/), the blog of Britan Switek, a graduate student in paleontology at Rutgers, which covers not only paleontology but also broader issues in evolutionary biology and the philosophy of science. The other is "This Week in Evolution:' the blog of Cornell professor R. Ford Denison, at http://blog.lib.umn.edu/denis036/thisweekinevolution/ It presents new discoveries in evolutionary biology and is accessible to anyone who has had a college-Ievel course in biology.