Why Evolution is True is a blog written by Jerry Coyne, centered on evolution and biology but also dealing with diverse topics like politics, culture, and cats.
Three young Amur tigers (aka Siberian tigers) and their mom in the Bronx Zoo, encountering their first deep snow in this week’s blizzard. This subspecies is adapted to snow—it’s now restricted to eastern Siberia—so they have the genes to tolerate it, but seemingly not the genes to recognize it!
You’ll know Eric MacDonald if you frequent this website or Butterflies and Wheels: in both places (and God knows where else) he leaves thoughtful and critical comments about religion. And Eric knows whereof he speaks: he’s an ex-Anglican priest and a strong advocate of assisted suicide. Many of us have urged him to start his own website, and now he has. Reflecting his interest in euthanasia, it’s called Choice in Dying, and is dedicated to his late wife, Elizabeth, who was afflicted with MS and chose to take her life at the Dignitas clinic in Switzerland (Eric tells the story on the website). I suspect that the backlash from his church and the Canadian authorities about this assisted suicide helped wean Eric from his faith.
Anyway, as you might guess from having read his posts, Eric doesn’t confine himself to assisted suicide, but deals as well with other problems of faith. The good news is that he’s posting regularly; the bad news is that he may now be too busy to leave his wonderful comments on our own websites. Bookmark his site, and leave some comments.
One of the most common problems that laypeople have with evolution is that there doesn’t seem to have been enough time for it. Given the idea that evolution is relatively slow, and yet there’s been an enormous amount of change since the first species a few billion years ago, how could natural selection (and other processes like genetic drift) have built all these exquisite, functioning organisms?
Part of the answer, of course, is that people fail to appreciate “deep time,” since we’re evolved to regard life over years and decades, not millions and billions of years. Evolutionists often demonstrate this by compressing all of evolution into a calendar year, showing how much evolution has occurred in a short segment of that time. Using this analogy in WEIT, for example, I show that the divergence between the ancestors of humans and chimps would have occurred only at 6 a.m. on December 31.
Another difficulty is that people assume that if one species evolves into another by changing many traits, it seems highly unlikely that they can all change at the same time by simultaneous fixation of adaptive mutations. If evolutionary change of a species involves gene substitution at L genes (with L being a number), and the proportion of all genes in each generation that are more favored than the “primitive” type is 1/K (this number is low because most mutations are deleterious), then the number of “trials” it takes to get adaptive mutations at all the genes is on the order of KL . In other words, each generation new mutations arise, and if adaptive ones aren’t there for every gene required to make a descendant from an ancestor, then that whole trial is discarded and the process starts the next generation. Finally, after about KL generations have passed, you’ll get the right type.
But that can take a huge amount of time. If you want to change 20,000 genes, for example, with only 1/40 of all segregating mutational variants being advantageous, then it would take 1034,040 “trials” (roughly the time it takes for a new adaptive mutation to become fixed) to effect this change. This could never occur, since even with an organism having 100 generations per year and with a “trial” equivalent to one generation, this would take a number of years equal to 10 followed by 34,038 zeroes. (Since life began there’s only been about 3 followed by nine zeroes years.) That’s not long enough!
As you’ve probably already guessed, evolution doesn’t work this way. As evolutionary change is occurring at one gene, it’s simultaneously occurring at other genes, affecting other traits, if there are adaptive mutations in the populations for those traits too. A “trial” doesn’t involve producing variants at every gene, with evolution occurring only if all of the genes have adaptive variants. Rather, in each trial the new adaptive mutations arise and begin their march toward fixation in some genes, leaving the rest to change during subsequent trials. In other words, evolution occurs in parallel rather than in series.
How does that change the speed of evolution? This is the topic of a new paper in PNAS by Herbert Wilf and Warren Ewens, a paper with the endearing title given above, “There’s plenty of time for evolution.” (I don’t know of another scientific paper whose title contains a contraction.) The point of Wilf and Ewens’s paper is to show mathematically that simultaneous substitution is much much faster than “serial” substitution, so that substantial evolutionary change can take place relatively quickly. This isn’t a new point, but the equations are new, and they show, as the title says, that there has been plenty of time for lots of evolution to have taken place.
Wilf and Ewens simply invoke the fact that at each gene, the substitution process takes place independently, with new adaptive mutations retained at each. If letters represent adaption mutations at each position in a word, with a “word” representing the number of genes differentiating an descendant from its ancestor, they propose the correct model:
But a more appropriate model is the following: After guessing each of the letters, we are told which (if any) of the guessed letters are correct, and then those letters are retained. [JAC: you don’t start the process over each generation, since if the right adaptive mutation is around for some genes, you needn’t consider those genes any longer.] The second round of guessing is applied only for the incorrect letters that remain after this first round, and so forth. This procedure mimics the “in parallel” evolutionary process. The question concerns the statistics of the number of rounds needed to guess all of the letters of the word successfully.
Here’s their complicated equation for the number of rounds of “guessing”, that is the number of rounds it takes to achieve adaptive evolution at every one of L genes:
The mean number of rounds that are necessary to guess all of the letters of an L letter word, the letters coming from an alphabet of K letters, is
[1]
with β(L) being the periodic function of log L that is given by Eq. 7 below. The function β(L) oscillates within a range which for K≥2, is never larger than .000002 about the first two terms on the right-hand side of Eq. 7.
Let’s put some biological numbers to this. Let’s assume that we have to change 20,000 genes to get from an ancestor to a descendant. (That’s a LOT of genes, since the whole human genome is only a tad bigger than this.) And let’s assume that at each gene only 1/40 of all gene variants are adaptive. (We’re assuming that if the population has as few as one “adaptive” variant, that one will sweep through the population. That’s not strictly correct since some of these will get lost by genetic drift and never contribute to evolution.) The 1/40 figure comes from assuming a population has a million births each generation, that there are 20,000 genes, that each generation of new births carries about 5 million new mutations in the genome—about 250 per gene—and that only one new mutation in 10,000 will be favored over the “resident gene type” (The mutation data are taken from humans, and assume that only a small percentage of new mutations arise in regions of the genome that actually do something.)
Using the formula, Wilf and Ewens calculate that complete gene substitution at all 20,000 genes would take about 390 “rounds” of guessing.
That compares to 1034,040 rounds of guessing if you ask for all the genes to change in a single “round”.
The difference occurs because under parallel evolution the number of trials (or mutational rounds that must occur to cause evolution) enters as K(log L) rather than KL. The first number is much smaller when L is large.
Of course we already knew that evolution works in parallel, but what impresses me is the huge shortening of time that occurs under realistic assumptions. This is one step towards dispelling the idea that Darwinian evolution works too slowly to account for the diversity of life on Earth today, even given the 3.5-billion-year history of life.
We need more models like this, for the idea that things are too complex to have evolved during Earth’s history is surprisingly common. For another useful example, see the model by Nilsson and Pelger (1994) on how rapidly a complex eye can evolve from a primitive eyespot, given reasonable assumptions about mutation rate and adaptiveness. (Dawkins also has a piece in the 1994 Nature highlighting this result.)
__________
Wilf, H. S., and W. J. Ewens. 2010. There’s plenty of time for evolution. Proc Natl Acad Sci USA 107:22454-22456.
Nilsson, D.-E., and S. Pelger. 1994. A pessimistic estimate of the time required for an eye to evolve. Proc. Roy. Soc. Lond. B 256:53-58.
Dawkins, R. 1994. The eye in a twinkling. Nature 368:690-691.
Check out the Japanese Alps in Honshu, Japan’s largest island, where plows open winter roads by digging through 56 feet of snow:
h/t: Matthew Cobb
Oh dear. One of ours—a University of Chicago cardiologist named Jalees Rehman, has written an article for HuffPo called “The parallel realities of modern science and Islam“. Rehman, a German Muslim, once spent a lot of time trying to “unify” science and Islamic spirituality, but eventually realized it was a mug’s game:
It was only when I became a scientist that I realized the challenge of actually unifying two bodies of knowledge that at their very core are completely distinct. Modern scientific knowledge consists of theories and models that are based on results of experiments which empirically test specific hypotheses. Spiritual knowledge is based on the study of sacred scriptures and metaphysical experiences. This fundamental disparity between modern science and spirituality results in a very different view of reality, as has been eloquently shown in Taner Edis’ excellent book An Illusion of Harmony, and unifying modern science and spirituality seems like trying to fit a square peg into a round hole.
That’s a promising start, but then Rehman goes off the rails:
These practical considerations have not deterred many contemporary Muslim scientists and philosophers and they are still actively trying to develop practical approaches to a modern day “Islamic Science”. However, there are also other voices that see modern science and religion as two distinct bodies of knowledge that allow us to view different but complementary aspects of reality. We do not advocate a unification of knowledge, but a form of mutual respect and dialogue so that each body of knowledge can draw from their partner’s strengths and wisdom.
That’s straight NOMA talk, but let’s reiterate what each magisterium can gain from “dialogue” with the other:
1. Religion. Religion gains but one thing from science: an increasing knowledge about the universe that makes mockery of religious doctrine, forcing the faithful to revise their dogma while claiming that it was consistent with science all along.
2. Science. Science has nothing to gain from religion, which is simply an annoyance that distracts us from our job.
If I’ve missed something, let me know.
As for science and religion being “complementary aspects of reality,” they’re no more complementary than astronomy and astrology, modern medicine and Christian Science, and evolution and creationism.
Jesus ‘n’ Mo ponder Ray ‘n’ Kirk:
Only a few of our entries mentioned the value of cats in helping one get through hard times. This story and photo come from Sal Bro and her cat Calista:
Calista was born on my birthday. I acquired her from a distraught hospital patient who’d left behind a beloved feline and her new litter of kittens. When conversations with my patient changed from returning home to nursing homes, I agreed to take in a scrawny white-and-tabby kitten and her sociable all-black sister. On adoption day, in place of the white tabby, they carted out a beautiful little calico. “Yep, that’s her,” I said and did not mention the mix-up.
Calista was a great comfort to me through the difficult end of a 10-year relationship. When suddenly the bed was only half-full, she dutifully warmed the other half. She sniffed my tears and snuggled with me on the couch. Later, she accepted a new house and two new step-siblings with only one attempt at suicide (failed, fortunately). Today, her pastimes include exploring the back yard, binge-eating, purging, and lounging atop a throne of sisal rope and carpet.
She defies photography, given a fear of anything that is pointed at her face, so I regret that I have only one photo of her in which she is looking at the camera and does not look crazed. It was taken with my web camera, during one of the dark, lonely evenings after the breakup. She’s been a good friend.
Thanks for the opportunity to share my kitteh.
Last September I highlighted a new book by journalist Isabel Wilkerson: The Warmth of Other Suns (subtitled The Epic Story of America’s Great Migration). I finally managed to get it from the library, and, because it was recalled by another reader (due today!), I polished off the 550-page book in a marathon reading stint over Christmas. I’m happy to report that, as all the reviews indicated, the book is superb, well worth reading for anyone interested in America’s racial divide, the civil rights movement, or simply the sociological consequences (which were huge) of the Great Migration. That term refers to the large-scale movement of blacks from America’s south to the north and west, starting around World War I and ending only when the Civil Rights Act was passed in 1964.
It’s a long book, but not a ponderous one. Wilkerson centers her story on three “migrants”: Ida Mae Gladney, a sharecropper who moved from Mississippi to Chicago in 1937, Robert Joseph Pershing Foster, a surgeon who left Louisiana for a new life in California in 1953, and George Swanson Sterling, a Florida fruit-picker who, after angering whites by trying organize his fellow pickers, fled from certain lynching to New York’s Harlem in 1945. You get to know these people in great detail: the infinite race-based humiliations and degradations that made them leave the South, their attempts (not wholly successful) to build new lives in supposedly discrimination-free parts of America, and their continuing and conflicted relationship with their southern birthplace.
The story is both heartening and sad: Wilkerson followed her subjects, all of whom she interviewed for many hours, from when she met them in the mid-1990s until they died. All three had terrific memories (I suppose that’s one reason Wilkerson chose them from all her subjects), so their biographies are full of engrossing detail. While they all achieved “success” of a sort in the North and West, it was mixed with more than a touch of tragedy (I won’t say more about that). She blends the stories of these three people, told in short, several-page snippets, with larger analyses of the demographics and sociology of the Great Migration. Her thesis is that this migration was the American equivalent of the migration of hopeful immigrants from Europe to America that took place beginning about the same time: both groups fled poverty and often discrimination for a new life and better jobs. The difference, of course, is that American blacks were already citizens; but, despite that, they couldn’t escape their origins for a simple reason: their pigmentation. While European immigrants could assimilate by marrying locals or changing their names, southern blacks could never pretend to be WASPs.
I give this book two enthusiastic thumbs up: you’ll not only learn a lot about this underappreciated part of recent America history (I see its remnants about me every day in Chicago, since I live on the South Side, perhaps the most famous destination of the Migration), but also become deeply involved in the lives of Ida Mae, George, and Robert. The ending is poignant and bittersweet, and will make you both proud of the migrants and sad about their fate. The writing is quite good (Wilkerson won a Pulitzer Prize for journalism—the first black woman to do so—for her work at The New York Times), and the scholarship, though thorough, is worn lightly. (The book was 15 years in the making and Wilkerson interviewed over 1200 people.) If there’s one flaw—and it’s a small one—the writing is occasionally awkward and more than occasionally repetitious, with the same facts repeated in different places. But that’s a trifle that should by no means put you off.
If you’re looking for a fat but fascinating book to start 2011, and you like history, sociology, and just plain good biography, this is the one for you. The hardback is only $16.95 at Amazon.
I’ve also finished Bomber County by Daniel Swift, one of Barnes and Noble’s picks for best nonfiction of 2010. This was okay, but did not in my estimation merit the rapturous reviews it garnered from Anthony Grayling and others. It was an ambitious and idiosyncratic undertaking: an attempt to unravel the story of Swift’s grandfather, an RAF bomber pilot killed in 1943; to recount the horrific life of those pilots, whose wartime longevity was measured in months; and to analyze the question of why there was so little war poetry in WWII compared to WWI. The book turned out to be a dog’s breakfast, with the back-and-forth between the topics making it hard to pay sustained attention. Swift’s insightful analysis of literature during the war, including works by Dylan Thomas, T. S. Eliot, and Virginia Woolf, was the best part, but in the end the center did not hold, and some of Swift’s lyricism seemed forced.
