My New Book (May, 2022):
THE PATCHWORK HUMAN: TWO BILLION YEARS OF EVOLUTION
Click on the link for details:
(at the McFarland site (link, above),
see “About the Book” and “Book Reviews & Awards”)
TABLE OF CONTENTS
1. Evidence for evolution / 2. Darwin & God / 3. Dinosaurs and humans / 4. Origin of dogs
5. Hairy males / 6. Flamingos / 7. Survival of the fit / 8. Stronger fingers and weaker legs? /
9.What is the evidence that humans originated in Africa? / 10. Are humans still evolving? /
11. What is natural selection? / 12. Evolution and generation time /
13. Gene change in just one generation? /
14. The 1% human-chimp DNA difference: what does it mean?
Scroll down to a numbered question and click on it to find its answer.
1. How many independent kinds of evidence for evolution are there? – Answer – Many. “Evolution” can be defined as (A) the evolution of one species from another, usually over long geological time scales (macroevolution); (B) genetic changes within a species, usually over relatively short time scales (microevolution); (C) the processes or mechanisms by which these changes occur.
For evolution’s having occurred, and the processes by which it occurs, we have the following kinds of evidence. (This is a list of the kinds of evidence, not the evidence itself.)
(1) Fossils, dated by various physical & chemical means
(2) The relatedness of living species to fossil species
(3) Geographic distributions of living and related fossil species
(4) The existence of a nearly continuous range of degrees of relatedness (from very close to very distant) among living organisms,
(5) The nested pattern of traits in related species; e.g., (((AB)C)D), where A, B, C, D share one trait, only A, B, C share another trait, and only A, B, share a third trait. Such patterns reflect successive add-ons to pre-existing traits, or successive modifications of traits. (This is reflected in biological classifications: races, subspecies, sibling species, full species, subfamilies, families, suborders, orders, subclasses, classes, and so on.) Many different kinds of trait show these patterns: (a) anatomical traits; (b) embryological traits; (c) biochemical and metabolic pathways; (d) chromosome numbers; (e) DNA and protein sequences.
(6) Vestigial features in a species. These make no sense in terms of present adaptation, but make sense if seen as genetic holdovers from earlier stages of evolution. Vestigial features are (a) anatomical structures; (b) embryological stages and structures; (c) inactive genes and pseudogenes.
(7) Experimental evolution and evolutionary principles seen in laboratory and natural settings: (a) evolution of replicating molecules in test tubes; (b) evolution of viruses in the laboratory and in people – e.g., human immunodeficiency virus (HIV), hepatitis-C virus, avian flu virus, influenza virus; (c) evolution of drug-resistance in bacteria; (d) evolution of cancer cells in a cancer patient; (e) evolution of insecticide resistance in insects, herbicide resistance in plants, and many other examples; (f) genetic changes in humans related to diet, climate, pathogen resistance.
(8) Mathematical and computer models that replicate evolutionary patterns in nature (selection and genetic drift).
2. Did Darwin believe in God? – Answer: Originally, yes; finally, no, with a qualification.
Several pages of Charles Darwin’s autobiography were devoted to his religious beliefs. He was born in 1809 into a quietly freethinking family, although nominally Unitarian. He was baptized in an Anglican church, and his family’s social circles were conventionally religious. As a young man he accepted the general view that all of nature’s variety was evidence of God’s handiwork. However, like many other thinkers during their youth, as time went on he began to view the Bible as a highly implausible and even contradictory account of history – beautiful in its moral teachings perhaps, but based on metaphors and allegories. In his late twenties, he began serious work on the question of the origin of the various species of plants and animals in the world, and evaluating evidence became a habit of mind. Christians’ firm belief in their version of God could no more be taken as evidence, because other religious believers had equally firm beliefs in their own religions. He found repugnant the idea that his father, his brother, and many of his best friends would be condemned to everlasting punishment for their independent ways of thinking. “Thus disbelief [in the existence of God] crept over me at a very slow rate, but was at last complete.”
After he had seen how natural selection could account for the variability and adaptations of living beings, he wrote to a friend, “I can see no evidence of beneficent design, or indeed of design of any kind, in the details.”
An argument against any sort of all-loving God was the unjustified suffering of men and women, supposedly one of God’s means of instructing humankind. When this was added to the sufferings of innocent children and, nature having as much cruelty as beauty, of even greater numbers of animals experiencing torments at the hand of violent and implacable nature, without any possibility of “moral uplift through suffering,” the idea of a benevolent God became even more implausible.
The idea of the immortality of the soul, and of the importance of a God in inspiring moral behavior in humans, he came to view as creations of the human mind. He felt he did not have to believe in God to live a moral life. The moral life, with all the social instincts for acting for the good of others, is simply a better way to live than merely to obey one’s sensuous passions rather than the higher impulses. “As for myself I believe that I have acted rightly in steadily following and devoting myself to science. I feel no remorse from having committed any great sin, but have often and often regretted that I have not done more direct good to my fellow creatures. . . . though this would have been a far better line of conduct.”
Considering also how much the incompletely developed minds of children are influenced by constant references to God around them, it would seem that a grown person’s unshakable belief in God could not really count as real evidence.
As for how the Universe might have originated, Darwin thought this might be the best argument for God’s existence. There must have been a First Cause, some Creative Intelligence to bring it all about, with all its immensity and wonder. But even that thought was weakened by the human mind’s probably having had its origins in the lesser minds of ancestral animals; and since even the best philosophical minds are not clear on the meaning of “cause” and “effect,” whatever conclusions the human mind might come to concerning such a huge and difficult question cannot be trusted.
Darwin finished writing his Autobiography when he was sixty-seven. Toward the end of the section on his religious beliefs he wrote, “I cannot pretend to throw the least light on such abstruse problems. The mystery of the beginning of all things is insoluble by us; and I for one must be content to remain an Agnostic.”
3. If dinosaurs hadn’t gone extinct, would humans have evolved? –
Answer: Probably not.
Dinosaurs along with lots of other animals and plants went extinct about 66 million years ago as a result of a major asteroid impact in Mexico, which affected temperature and climate all over the world for decades and maybe centuries. A few small mammals survived, and from them evolved all the diversity of modern mammals – elephants, horses, whales, camels, tigers, hedgehogs, bats, mice, rabbits, anteaters, and others, as well as primates and our special group of primates, the great apes, which eventually, about 300,000 years ago, included Homo sapiens, human beings.
Evolution almost certainly would have followed a different path if dinosaurs hadn’t gone extinct. Dinosaurs would probably have flourished for millions of years more, evolving new species that would have dominated all the Earth’s habitats, evolving adaptations that gave them new advantages and lifestyles, and preventing mammals from ever gaining an evolutionary foothold. The dominant species today might have turned out to be a social, intelligent, creative, technologically sophisticated and language-adept dinosaur. If by some remote chance some clever ape did somehow manage to arise somewhere, it probably would have been exploited immediately, and these superior dinosaurs would now be grilling apeburgers in their backyards on weekends.
(revised 13 Sept. 2019)
4. Where did dogs come from? – Answer: Dogs originally descended from prehistoric wolves at least 15,000 years ago. They were the first animals to be domesticated, thousands of years before cats, horses, cattle, sheep, goats, and pigs. Wolves (Canis lupus) and dogs (Canis familiaris) are usually considered to be separate species because of differences in their teeth and skull and other parts of the skeleton; but they remain genetically very similar and can still interbreed, although they generally don’t.
Because for thousands of years ancient wolves were widespread throughout the northern hemisphere, they were probably domesticated to evolve into dogs at least twice: once in Central Asia and again, independently, in Europe. According to extensive worldwide genetic comparisons among modern dogs, supplemented with archeological studies, the best guess is that the ancient dogs of the Asian branch migrated with humans westward over thousands of years, where they substantially replaced the dogs of the earlier European branch. Although you couldn’t tell it from their outward appearance, among the dogs still showing DNA traces of their ancient European domestication are the Samoyed, the German Shepherd, the Norwegian Elkhound and the street dogs of the Middle East. (Among the dogs with signs of their ancient Asian ancestry in their DNA are the Shar Pei, Greenland sledge dogs, the Siberian Husky, and the street dogs of Vietnam.) Because of this history, the majority of the dogs in the world today have predominantly an old Asian origin.
Prehistoric humans and prehistoric wolves were competitors for prey before their descendants became friends. Human hunters were probably out-competing wolves for deer, antelopes, wild boars, wild goats, and wild sheep. So the wolves, in order to survive, had to resort to scavenging near human camps, eventually coming to rely on direct handouts from humans. The transition was probably made easier by the fact that wolves, like humans, were (and are) highly social animals.
The canid bones that archeologists have found at ancient human campsites are often both wolf-like and dog-like. Therefore early on there was probably a lot of interbreeding between fully wild wolves and partially domesticated ones. Full domestication involves not only anatomical changes but also changes in genes that influence behavior: wolves are innately more aggressive and wary of humans, while dogs are naturally tame and docile, maintaining their playful and puppy-like behavior into adulthood. (Wolves do not generally make good pets; even when raised from a very young age with humans, they retain a large measure of their innate wildness.)
Most of the dogs of the world do not belong to anyone; they are “street dogs” (also called “village dogs” or “community dogs”). They make their living hanging around human habitation, picking up scraps of food where they can find them. As humans migrated from place to place, dogs spread with them into southeast Asia, southern Europe, North America, as well as into the southern hemisphere: Africa, South America, and Australia (where three or four thousand years ago they reverted to the wild, as dingoes).
Most of the 400 or so “pure breeds” recognized by various national and international Kennel Clubs are, by contrast, descended only recently from relatively modern European dogs. In spite of their great variation in size and appearance (compare the poodle and the Great Dane, for instance), they are all more closely related to one another genetically than any of them are to the more ancient dogs. They stem from controlled breeding practices that began in Europe only two or three centuries ago, when people began breeding them selectively to make specialists of them: game-hunters (spaniels, pointers, setters, bloodhounds, retrievers); hunters of “vermin” such as rats, mice, moles, and weasels (terriers, dachshunds); herding dogs (shepherds, collies); watchdogs (boxers, Dobermanns, Great Danes, Rottweilers), companionship dogs (poodles, Pomeranians), and many other kinds. These show-dogs, and mixed-breed dogs kept as pets, have largely supplanted the earlier village dogs in many countries of the Western world.
24.viii.16 Thanks to Ron Newman for his input.
5. Why are men hairier than women? – Answer: The immediate physiological cause is the level of testosterone, a hormone made primarily in the testis. The increase in testosterone at puberty in males leads to increases in bone mass and muscle mass; it also has indirect effects on aggression and blood-clotting – all of which likely proved advantageous for the males of our distant evolutionary ancestors, in strength, stamina, and aggressive hunting, in fighting each other for females and, once a family had been established, in protecting the female and the offspring. At some point our evolutionary ancestors lost most of their body hair, perhaps making cooling-by-sweating more effective as the species evolved a larger size and began hunting big game on the hot African plains – making us, their evolutionary descendants, the naked apes that we are.
But the increased testosterone at puberty in males also stimulates the growth of hair on the body and face. Why should there be any connection between testosterone level and the activity of hair follicles in the skin of males?
Here’s a plausible explanation, stemming from body- and face-hair’s being an additional and more obvious and certain signal to females of male testosterone level, fearlessness and strength. If females tended to choose higher-testosterone males (that is, hairier ones) for mating, they and their offspring probably ended up being better provisioned and better protected. Then, as one generation succeeded another, the higher-male-testosterone genes and the genes linking testosterone to hair growth would all have been passed on to increasing numbers of their offspring. In this way those genes and the female preference for males who carried them evolved together, giving the predominant hairy-male trait of the original Homo species, from which we Homo sapiens evolved.
On the flip side – one could speculate – the females’ smaller size, higher voice, relative hairlessness and smoother skin all tended to tap into the males’ protective instinct for his young offspring, so that the females with just those traits as well as their offspring tended to thrive in comparison with others. Those female traits and the males’ preference for them would again have evolved together, also becoming the predominant species-traits on that side.
These interlinked traits can be viewed as genetic holdovers from those ancient times a couple of million years ago. Perhaps to some degree those same features still make a difference in our mating practices today.
6. Why are flamingos pink? – Answer: There are two “why” questions here: (1) What is the mechanism by which flamingo feathers become pink? And (2) What evolutionary advantage might there be in having bright pink feathers?
The answer to (1) is the flamingo diet. Wherever the flamingos live (there are six species in the world, in east and west Africa, the Mediterranean, India, South America, and the Caribbean, and they are all pink), they feed on small shallow-water creatures and micro-organisms stirred up from the mud bottom, many of which contain carotenoid pigments. The flamingo’s body combines these pigments with feather proteins, which are then built into the feathers as they grow, turning them pink (young flamingos are gray). But what evolutionary advantage might there be in doing this?
The answer to (2) is the flamingo courtship and mating practices. Being a bright pink flamingo advertises that you are healthy and eating well, and have good genes. You are more likely to be chosen as a mate, and are therefore likely to have more offspring carrying those good genes.
You might think that being bright pink might be a disadvantage, making flamingos more conspicuous to predators. But their main predators are eagles, vultures, and storks, along with the occasional snake or wildcat or feral pig, which mainly eat flamingo eggs and chicks. So being bright pink as an adult is not really a disadvantage on that score. (Adults are already protected to a considerable degree by being large birds, by living in huge flocks of thousands of birds, and by living in places where there aren’t that many ground-level predators to begin with.) Adult coloration can then evolve to enhance mating success, just as it does for many other brightly colored birds.
24.x.17. Thanks to Ron Newman for the question.
A flamingo colony at Lake Nakuru, Kenya. A few immature flamingos (gray) in the foreground.
Wikimedia File: Large number of flamingos at Lake Nakuru.jpg
Date: 8/12/2007 Author: Syllabub License: CC BY-SA 3.0
7. Survival of the fit – “Survival of the fittest” is an expression that the philosopher Herbert Spencer used (1864) to describe Charles Darwin’s idea (1859) of how new species originate (Darwin himself later used the phrase). The basic idea is this: those individuals having traits that best adapt to their circumstances are the ones that most often survive and have the most offspring – assuming of course that the traits promoting survival are reproduced again at each generation. In this way, as the generations pass, a whole population gradually changes, trait by trait, until it eventually becomes a new species.
The phrase emphasizes the competition between the better adapted individuals and the not-so-well adapted ones. As for the mere persistence of a species, however, look around you: there are many ways to be healthy and have lots of children. You can do it with pale skin or dark skin, with a big nose or a small nose, with an IQ of 95 or 105, with a height of 5'6" or 5'10", with blood type A or blood type B. In the continuing generation of life there is not one winner and many losers, not one best type that evolution is always aiming for. Life is not a race; it is a surging forward into the future of many adapted forms. It could be called “survival (and successful reproduction) of the fit.”
8. Are humans going to evolve longer and stronger fingers and weaker lower extremities in the distant future because of the use of new technology? Answer: Probably not, even though both those evolutionary changes are theoretically possible. Longer and stronger fingers might be useful for long hours at the computer keyboard and for playing video games and sending rapid text-messages on hand-held mobile devices. But for such fingers eventually to become part of human nature, genetically determined, people with such fingers would have to have more children than people with ordinary fingers, and out-reproduce them on a global scale. That seems unlikely.
There is precedent for a “use-it-or-lose-it” process in evolution. Fish that spread into dark caves and reproduced there for many generations eventually lost their eyes. Mammals that moved from land back into the ocean and spent many generations there lost their limbs, evolving into whales. Birds that had few predators in a new environment or found other ways to escape them became flightless ostriches and kiwis. People and their descendants who hardly ever walk anymore, even as children, and go everywhere by riding, even up and down stairs in their own houses might, over the course of many generations, evolve weaker legs. But for that to happen, they would have to isolate themselves from everyone else in the world who still used their legs for walking and running and sports, and never have children with them, only with each other. That seems unlikely, unless they moved to the moon and they and their descendants never came back.
10.vii.18. Thanks to Parvin Ganjei-Azar for the question.
9. What is the evidence that human beings originated in Africa? Answer: (1) the location of other apes; (2) genetic diversity; (3) patterns of DNA variation; (4) fossils.
(1) Throughout the biological world it’s usual to find that closely related species—living species with a recent evolutionary ancestor in common—are also close geographically. Our closest living relatives, chimpanzees and gorillas, live in Africa.
(2) One would expect that where a species originated would be where the species’ oldest populations would be found; other populations spreading out from the place of origin would necessarily be younger populations. Being older, the populations at the place of origin would have had more time for gene variants to accumulate. It turns out that, as expected for the oldest human populations, there is more genetic diversity among the peoples of Africa than anywhere else on the planet.
(3) Changes in DNA sequences that happened when human populations spread across the globe, starting about 100,000 years ago or thereabouts, form a pattern something like this: abc --> Abc --> ABc --> ABC. Each new migration-destination is marked by one change that was simply added on to the just-previous pattern. Therefore the various DNA sequences seen in modern human populations can be assembled into a time-sequence, and which ones are the the earliest ones can be identified and located on a map. Those earliest ones, represented by the “abc” above, are located in Africa.
(4) Skeletal remains—fossils, hundreds of them—that have been dug up all over the world (Europe, Asia, Australia, the Americas, Africa) can be dated by chemical methods involving the measurement of atoms produced by very slow radioactive decay, or by certain kinds of slow physical changes that happen in underground materials. The oldest fossils that look like modern humans, Homo sapiens—from 200,000 to 300,000 years old—all come from Africa. Fossils that old that look like modern humans haven’t been found anywhere else. Some even older fossils have been found that are similar to the bones of modern humans but clearly not quite like us. They are Sahelanthropus, Ardipithecus, Australopithecus, and ancient species of Homo such as Homo habilis and Homo ergaster. They are all from Africa.
The strongest evidence is (3) and (4); (1) and (2) can be viewed as corroborating evidence.
The only way we could be wrong about the African origin of Homo sapiens would be if (a) our genetically closest relatives, the chimpanzees and gorillas, like us, migrated to Africa from somewhere else; (b) in spite of our having looked at the genes of thousands of people worldwide, our conclusions about genetic diversity are wrong because all the samples were biased in some way, and the most diverse peoples are not in Africa but elsewhere; or else African genetic diversity has some other cause besides the antiquity of its populations; (c) there’s something wrong with our analysis of the pattern of genetic diversity and its underlying assumption that DNA sequence-changes occur in succession; we’re wrong to think there’s an evolutionary process underlying such changes; (d) in spite of all our searching, we’ve missed all the truly oldest fossils of our species, which still lie hidden, buried and undiscovered perhaps in Europe or Asia or Australia; and (e) all the various methods of dating fossils are wrong and we don’t really understand the chemistry and physics behind them: what we think are young fossils are really old, and what we think are old fossils are really young.
For our species to have originated not in Africa but somewhere else, we would have to be wrong about (1) and (2) and (3) and (4). The fact that they all, independently, point to the same conclusion would have to be a mischievous coincidence. From the evidence, though, we haven’t much choice but to conclude that our species originated in Africa.
12 October 2019
10. Are humans still evolving? – Answer: Yes. Some people think that human evolution, having led us to our present state of relative perfection, is all in the past. Some people think, also, that the rise of civilization and especially of modern medicine and technology means that previously “unfit” individuals, those that couldn’t see well enough to detect the approaching lion or run fast enough to escape it, now survive and are able to pass on their genes to their children just as well as anybody else. We have good reasons to believe that this thinking is not correct, and that we’re still evolving.
Evolution is mainly just the natural result of some people’s having a genetic constitution that enables them to live longer and/or have more children than other people. That principle must always be true, even though nearsightedness and lions are no longer important factors in who has children and who doesn’t. Different factors must be operating now that we have civilization, dense human populations, and widespread travel and mixing of genes. The genes that make a difference these days might be genes that (a) give resistance to highly transmissible viral diseases such as AIDS and Covid-19, or a slightly better immune system overall, (b) give resistance to diseases transmitted from domestic animals, (c) reduce high blood pressure and the risk of heart disease in spite of high fat diets, (d) modify physiological traits so as to allow survival of periods of drought and starvation, (e) extend life by a few years so that we can help care for our grandchildren, or (f) alter our brains to make us more sociable. Some observations align with these possibilities, but it’s hard to get solid data because evolutionary change is such a long, slow, subtle process.
(In some cases evolution seems to entail a reduction or loss—perhaps because not having to make a particular structure saves a small amount of energy and material. Our little toes, the tail (coccyx), and the appendix seem to be on their way out.)
There must always be tendencies, some worldwide and some more local, for some people, because of their genes, to be slightly better adapted than other people, and so pass on those genes to more children than other people can. That’s evolution, at least on a minor scale: a change in gene frequencies. It’s that kind of evolution that accounts for the gradual appearance of different physical and physiological traits of different people (“races”) in different parts of the world. The changes in the genes responsible for such differences have occurred only in the last few thousand years; that’s “current” on an evolutionary timescale.
Theoretically, massive accumulation of gene changes in the face of major climate changes could lead to enough genetic change overall that we would no longer be Homo sapiens. That would be evolution on a grander scale, and would probably take a million years. Who knows? Stick around and see what happens.
1.v.20. Thanks to Steve Wetstein for the question.
11. What is “natural selection”? – Answer: The phrase “natural selection” was coined by Charles Darwin to refer to “selection that occurs in Nature”—in contrast to what might be called “human selection,” such as what a cattle breeder does in selecting particular cows for breeding, or what a wheat-farmer does in selecting certain seeds for next season’s crop. With natural selection, the selection of individuals that are most likely to survive and reproduce (and keep the species or population thriving) depends on Nature itself—the weather; the kind of food available; the abundance (or dearth) of water, shelter, and nesting places; other features of the local environment such as the extent of the grassland or the kinds of trees in the forest; the presence of harmful chemicals in the environment; the presence of infectious diseases; other organisms such as predators, prey, and parasites; and so on and on.
For example, in the Galápagos Islands during periods of drought, the hardness of seeds increases, and only the finches with more robust beaks do well under those conditions. They have better survival rates, and they produce more chicks for the next generation. Under those conditions, there is natural selection for stronger beaks.
Natural selection undoubtedly played a role in human evolution—not for beaks but for feet—specifically in the evolution of the musculo-skeletal changes that led to our ancestors’ being able to walk and run upright. Climate change in central Africa a few million years ago, with the thinning-out of forests and the expansion of grasslands, meant that there were new grasses and new grazing animals that could be added to the diet. Some of our ancient ape-like ancestors could better exploit these new food resources if they happened to lie toward one end of the natural range of variability in foot-, knee-, and hip anatomy, with bones, muscles, and tendons that allowed them to walk and run more efficiently in the open savannas. Over the span of hundreds of thousands of years, they gradually evolved an upright stance well adapted to the new ecology to which they were being exposed. They ate well and had many children. We modern humans are their natural descendants.
12. Is the rate of evolution related to generation time? – Answer: Yes. Evolution typically takes many generations. Old, common forms of genes may gradually be lost, and new and initially rare forms of genes may gradually become more frequent. This happens because at each generation offspring that are genetically better adapted to their environment may survive longer and have more offspring. Their genes are then preferentially represented in subsequent generations. Therefore the genetic composition of the population, or even the whole species, may gradually change. That’s evolution.
In absolute time, evolution can be more rapid when generation times are short. One hundred generations for mosquitoes is only five or six years. One hundred generations for bacteria may be only about a week. These organisms can evolve rapidly. For humans, with a generation time of 20 years (the number commonly used for such calculations), one hundred generations takes about two thousand years. Humans therefore evolve more slowly than mosquitoes or bacteria. (As a member of the human species, you won’t have evolved much, genetically speaking, from your great-grandparents’ time!)
If by “human” is meant any species in the genus Homo, the starting point of “humans” would have been about 125,000 generations ago (2½ million years), with the appearance of Homo habilis. (More ancient apes, such as Australopithecus afarensis (“Lucy”) and Ardipithecus ramidus, are not “human” but possible human ancestors.) A lot of evolution within the genus Homo has taken place during all those millennia, with the rise and fall of many different human species—Homo habilis, Homo erectus, Homo heidelbergensis, Homo neanderthalensis, Homo naledi, and others. Since those times we humans, Homo sapiens, have appeared. We probably differ from those other human species in several hundred of our genes—although we don’t know all the gene changes, nor to what extent they might have been important in the evolution of our distinctively modern human traits.
If by “human” is meant only our own species, Homo sapiens, we have been around for about 15,000 generations (300,000 years). Since that time we have undergone small-scale evolution. We are no longer exactly like the earliest Homo sapiens in Africa. Human populations have now spread to different continents and adapted genetically to different environments. We have lighter or darker skin, different digestive enzymes suitable for particular diets, susceptibility or resistance to locally prevalent diseases, body types adapted to cold or warm climates, and so on.
Genes giving lighter skin color probably arose about 3000 generations ago, when humans first migrated out of Africa into the Middle East and Europe. A gene allowing humans to continue digesting milk lactose even after weaning arose about 500 generations ago in Europe. Genes enabling people to live at low oxygen levels became common when people first migrated to the Tibetan plateau, perhaps as recently as about 300 generations ago. The sickle-cell gene, a “malaria-resistance gene” (but causing sickle-cell anemia when a person has two copies of the gene) became frequent in Africa from around 80 generations ago because of the increased spread of malaria.
These are examples of adaptive evolution on a small scale, over relatively short time-spans. They demonstrate that evolution typically takes place not all at once but piecemeal, affecting one gene or another depending on the environment. The same kind of process can, over longer time-spans and under the right circumstances, give rise to new species. Whether that will happen to us is uncertain.
14.vi.2021. Thanks to Steve Wetstein for the question.
13. Can genes change or adapt in just one generation? Answer: First, two clarifications: (1) A single gene in a single cell of a single organism changes (mutates) the instant it is mis-copied when the DNA molecule is replicated. That’s not the same thing as having a whole population of organisms undergo a gene change, which is what evolutionary change means. (2) A gene may mutate, and the result may be that the organism with the mutated gene can adapt better to its environment. Therefore, strictly speaking, it’s not genes that adapt, but organisms that carry certain genes.
Change of a gene in a whole population requires that repeated DNA replications produce many copies of a new (mutated) gene, which then, over the course of many generations, gradually spread throughout the population, replacing the old gene copies. Another way a whole population could have a gene change is that individuals carrying copies of the changed gene gradually become a separate population of their own. Either of those processes almost always takes many generations.
An exception to this general rule has occurred in some land snails whose shells are normally coiled in a right-handed helix. In several instances, over the course of evolution in several species, a gene mutation occurred that caused one snail’s offspring to have a shell that is coiled in a left-handed helix. These new snails can’t mate with any of the “righted-handed” snails in the original population; they can mate only with other “left-handed” snails. Voilà! – a new genetically distinct population in just one generation!
This is not the way evolution usually works, however. Evolution usually takes hundreds of generations (see question 12 above) for a new gene to replace an old one in a population.
16 June 2021. Thanks to Mariel and Diana for the question.
14. The 1% human-chimp DNA difference – what does it mean? Answer: Although no definitive fossil remains of the common ancestor have been found, according to other related fossils and according to the genetic evidence, modern humans (Homo sapiens) and modern chimpanzees (Pan troglodytes) evolved from a common ancestor that lived about seven million years ago. Since that time, both species have continued evolving, accumulating gene differences from that ancestor.
Careful base-by-base comparison (that is, comparison of each position in the DNA sequences of modern humans and modern chimpanzees) reveals a 1% DNA difference between our two species. That would seem to be a surprisingly small difference, given that DNA sequences of genes determine most of the differences between species, and given the rather large anatomical differences, to say nothing of behavioral differences, between humans and chimps. This seeming paradox has two explanations, both of which seem to apply here. One explanation is gene regulation; the other explanation is in how the DNA differences are distributed in the DNA, suggesting that the gene difference is closer to 5% than 1%. This makes our anatomical differences a little easier to understand.
First, much of the human-chimpanzee DNA difference lies not in the genes themselves but in their regulation during embryological and fetal development. Gene regulation does also depend on DNA, but “regulatory DNA” is a very small fraction of total gene DNA. A very few regulatory-DNA differences can make a big difference in how the genes are used, just as a few differences in people in power can make a big difference in how a government is run.
Second, how the 1% DNA difference is distributed in the DNA makes a difference. The differences could be concentrated in just a few genes, making only a few differences in a gene-by-gene comparison. Alternatively, the differences could be spread out over many genes, making many gene differences. For example, only three words out of thirty are affected by eight instances of the letter “o” in this sentence: “It is a foolproof plan: set the dynamite at eleven-thirty, enter the bank at noon, shoot the guard, and get away in the hearse, which they will never suspect.” In contrast, a different eight instances of the letter “o” in another thirty-word sentence affect a total of eight words: “She felt herself an elder daughter of the beloved old father, as she breathed the sea air in full volume from the billowy west one morning very early after sunrise.”
According to an indirect estimate, about a thousand genes—5% of the total—are slightly but significantly different in our two species. Among the genes that appear to be different in their functions in humans and chimps are genes affecting the skin, genes of the immune system, genes affecting some aspects of anatomy, genes involved in the development of the brain, and genes that regulate other genes.
Remember also, in spite of the differences, humans and chimps are indeed very similar, compared to the differences between, say, humans and sheep.
3.xi.21 Thanks to Gene Stern for the question.