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Showing posts with label Evolutionary Biology. Show all posts
Showing posts with label Evolutionary Biology. Show all posts

Wednesday, February 07, 2024

Philip Ball's new book: "How Life Works"

Philip Ball has just published a new book "How Life Works." The subtitle is "A User’s Guide to the New Biology" and that should tell you all you need to know. This is going to be a book about how human genomics has changed everything.

Friday, August 11, 2023

What is the Modern Synthesis?

Serious criticisms of evolutionary theory have been floating around for half a century. The main focus is over the Modern Synthesis and whether it's the best explanation of evolution. That requires a throrough understanding of what the Modern Synthesis actually means and how it's understood by most evolutionary biologists.

One view is that the Modern Synthesis is almost exclusively about natural selection. If that's true, then Stephen Jay Gould makes a good case when he argues that the Modern Synthesis is effectively dead—it was killed off by the neutral theory and the recognition that random genetic drift is a major player in evolution [Is the Modern Synthesis effectively dead?].

Tuesday, August 01, 2023

Help fix the Wikipedia article on evolution

The Wikipedia article on evolution [Evolution] is a "Featured article," which means two things: (1) it is one of the best articles Wikipedia has to offer, and (2) it was voted a featured article by Wikipedia editors and that means they will resist any changes.

You will be shocked to learn that the article isn't perfect. It could use some serious updating and revision but my first attempt was reverted by an editor named Efbrazil who has vowed to revert any edits I make unless I can get his approval. So I thought I'd give it a try and you can see the result on the Talk:Evolution pages. My intitial objective is to edit the introductory paragraphs in the lead to eliminate the reference to expression of genes and to introduce the term "allele," which is covered in the main part of the article. Here's the current opening paragraphs of the lead,

In biology, evolution is the change in heritable characteristics of biological populations over successive generations.[1][2] These characteristics are the expressions of genes, which are passed on from parent to offspring during reproduction. Genetic variation tends to exist within any given population as a result of genetic mutation and recombination.[3] Evolution occurs when evolutionary processes such as natural selection (including sexual selection) and genetic drift act on this variation, resulting in certain characteristics becoming more or less common within a population over successive generations.[4] It is this process of evolution that has given rise to biodiversity at every level of biological organisation.[5][6]

The theory of evolution by natural selection was conceived independently by Charles Darwin and Alfred Russel Wallace in the mid-19th century and was set out in detail in Darwin's book On the Origin of Species.[7] Evolution by natural selection is established by observable facts about living organisms: (1) more offspring are often produced than can possibly survive; (2) traits vary among individuals with respect to their morphology, physiology, and behaviour; (3) different traits confer different rates of survival and reproduction (differential fitness); and (4) traits can be passed from generation to generation (heritability of fitness).[8] In successive generations, members of a population are therefore more likely to be replaced by the offspring of parents with favourable characteristics for that environment. In the early 20th century, other competing ideas of evolution were refuted as the modern synthesis concluded Darwinian evolution acts on Mendelian genetic variation.[9]

I'm also thinking that we should modify the following sentences that don't seem to be appropriate in a "Featured article,"

According to the now largely abandoned neutral theory of molecular evolution most evolutionary changes are the result of the fixation of neutral mutations by genetic drift.[101] In this model, most genetic changes in a population are thus the result of constant mutation pressure and genetic drift.[102] This form of the neutral theory is now largely abandoned since it does not seem to fit the genetic variation seen in nature.[103][104]

Editor Efbrazil seems to be he only editor willing to discuss these problems and he is hard to convince. If anyone else is interested in improving this Wikipedia article, I invite you to participate in the discussion on the Talk pages.


Sunday, February 12, 2023

Happy Darwin Day! 2023

Charles Darwin, the greatest scientist who ever lived, was born on this day in 1809 [Darwin still spurs tributes, debates] [Happy Darwin Day!] [Darwin Day 2017]. Darwin is mostly famous for two things: (1) he described and documented the evidence for evolution and common descent and (2) he provided a plausible scientific explanation of evolution—the theory of natural selection. He put all this in a book, The Origin of Species by Means of Natural Selection published in 1859—a book that spurred a revolution in our understanding of the natural world. (You can still buy a first edition copy of the book but it will cost you several hundred thousand dollars.)

Thursday, October 13, 2022

Macroevolution

(This is a copy of an essay that I published in 2006. I made some minor revisions to remove outdated context.)

Overheard at breakfast on the final day of a recent scientific meeting: "Do you believe in macroevolution?" Came the rely: "Well, it depends on how you define it."
                                                                         Roger Lewin (1980)

There is no difference between micro- and macroevolution except that genes between species usually diverge, while genes within species usually combine. The same processes that cause within-species evolution are responsible for above-species evolution.
                                                                         John Wilkins

The minimalist definition of evolution is a change in the hereditary characteristics of a population over the course of many generations. This is a definition that helps us distinguish between changes that are not evolution and changes that meet the minimum criteria. The definition comes from the field of population genetics developed in the early part of the last century. The modern theory of evolution owes much to population genetics and our understanding of how genes work. But is that all there is to evolution?

The central question of the Chicago conference was whether the mechanisms underlying microevolution can be extrapolated to explain the phenomena of macroevolution. At the risk of doing violence to the positions of some of the people at the meeting, the answer can be given as a clear, No.
               Roger Lewin (1980)

No. There's also common descent—the idea that all life has evolved from primitive species over billions of years. Common descent is about the history of life. In this essay I'll describe the main features of how life evolved but keep in mind that this history is a unique event that is accidental, contingent, quirky, and unpredictable. I'll try and point out the most important controversies about common descent.

The complete modern theory of evolution encompasses much more than changes in the genetics of a population. It includes ideas about the causes of speciation, long-term trends, and mass extinctions. This is the domain of macroevolution—loosely defined as evolution above the species level. The kind of evolution that focuses on genes in a population is usually called microevolution.

As a biochemist and a molecular biologist, I tend to view evolution from a molecular perspective. My main interest is molecular evolution and the analysis of sequences of proteins and nucleic acids. One of the goals in writing this essay is to explain this aspect of evolution to the best of my limited ability. However, another important goal is to show how molecular evolution integrates into the bigger picture of evolution as described by all other evolutionary biologists, including paleontologists. When dealing with macroevolution this is very much a learning experience for me since I'm not an expert. Please bear with me while we explore these ideas.

It's difficult to define macroevolution because it's a field of study and not a process. Mark Ridley has one of the best definitions I've seen ...

Macroevolution means evolution on the grand scale, and it is mainly studied in the fossil record. It is contrasted with microevolution, the study of evolution over short time periods., such as that of a human lifetime or less. Microevolution therefore refers to changes in gene frequency within a population .... Macroevolutionary events are more likely to take millions, probably tens of millions of years. Macroevolution refers to things like the trends in horse evolution described by Simpson, and occurring over tens of millions of years, or the origin of major groups, or mass extinctions, or the Cambrian explosion described by Conway Morris. Speciation is the traditional dividing line between micro- and macroevolution.
                                                                         Mark Ridley (1997) p. 227

When we talk about macroevolution we're talking about studies of the history of life on Earth. This takes in all the events that affect the actual historical lineages leading up to today's species. Jeffrey S. Levinton makes this point in his description of the field of macroevolution and it's worth quoting what he says in his book Genetics, Paleontology, and Macroevolution.

Macroevolution must be a field that embraces the ecological theater, including the range of time scales of the ecologist, to the sweeping historical changes available only to paleontological study. It must include the peculiarities of history, which must have had singular effects on the directions that the composition of the world's biota took (e.g., the splitting of continents, the establishment of land and oceanic isthmuses). It must take the entire network of phylogenetic relationships and impose a framework of genetic relationships and appearances of character changes. Then the nature of evolutionary directions and the qualitative transformation of ancestor to descendant over major taxonomic distances must be explained.
                                                                     Jeffrey S. Levinton (2001) p.6

Levinton then goes on to draw a parallel between microevolution and macroevolution on the one hand, and physics and astronomy on the other. He points out that the structure and history of the known universe has to be consistent with modern physics, but that's not sufficient. He gives the big bang as an example of a cosmological hypothesis that doesn't derive directly from fundamental physics. I think this analogy is insightful. Astronomers study the life and death of stars and the interactions of galaxies. Some of them are interested in the formation of planetary systems, especially the unique origin of our own solar system. Explanations of these "macro" phenomena depend on the correctness of the underlying "micro" physics phenomena (e.g., gravity, relativity) but there's more to the field of astronomy than that.

Levinton continues ....

Does the evolutionary biologist differ very much from this scheme of inference? A set of organisms exists today in a partially measurable state of spatial, morphological, and chemical relationships. We have a set of physical and biological laws that might be used to construct predictions about the outcome of the evolutionary process. But, as we all know, we are not very successful, except at solving problems at small scales. We have plausible explanations for the reason why moths living in industrialized areas are rich in dark pigment, but we don't know whether or why life arose more than once or why some groups became extinct (e.g., the dinosaurs) whereas others managed to survive (e.g., horseshoe crabs). Either our laws are inadequate and we have not described the available evidence properly or no such laws can be devised to predict uniquely what should have happened in the history of life. For better or worse, macroevolutionary biology is as much historical as is astronomy, perhaps with looser laws and more diverse objectives....

Indeed, the most profound problem in the study of evolution is to understand how poorly repeatable historical events (e.g., the trapping of an endemic radiation in a lake that dries up) can be distinguished from lawlike repeatable processes. A law that states 'an endemic radiation will become extinct if its structural habitat disappears' has no force because it maps to the singularity of a historical event.
                                                                 Jeffrey S. Levinton (2001) p.6-7

In conclusion, then, macroevolutionary processes are underlain by microevolutionary phenomena and are compatible with microevolutionary theories, but macroevolutionary studies require the formulation of autonomous hypotheses and models (which must be tested using macroevolutionary evidence). In this (epistemologically) very important sense, macroevolution is decoupled from microevolution: macroevolution is an autonomous field of evolutionary study.
     Francisco J. Ayala (1983)

I think it's important to appreciate what macroevolutionary biologists are saying. Most of these scientists are paleontologists and they think of their area of study as an interdisciplinary field that combines geology and biology. According to them, there's an important difference between evolutionary theory and the real history of life. The actual history has to be consistent with modern evolutionary theory (it is) but the unique sequence of historical events doesn't follow directly from application of evolutionary theory. Biological mechanisms such as natural selection and random genetic drift are part of a much larger picture that includes moving continents, asteroid impacts, ice ages, contingency, etc. The field of macroevolution addresses these big picture issues.

Clearly, there are some evolutionary biologists who are only interested in macroevolution. They don't care about microevolution. This is perfectly understandable since they are usually looking at events that take place on a scale of millions of years. They want to understand why some species survive while others perish and why there are some long-term trends in the history of life. (Examples of such trends are the loss of toes during the evolution of horses, the development of elaborate flowers during the evolution of vascular plants, and the tendency of diverse species, such as the marsupial Tasmanian wolf and the common placental wolf, to converge on a similar body plan.)

Nobody denies that macroevolutionary processes involve the fundamental mechanisms of natural selection and random genetic drift, but these microevolutionary processes are not sufficient, by themselves, to explain the history of life. That's why, in the domain of macroevolution, we encounter theories about species sorting and tracking, species selection, and punctuated equilibria.

Micro- and macroevolution are thus different levels of analysis of the same phenomenon: evolution. Macroevolution cannot solely be reduced to microevolution because it encompasses so many other phenomena: adaptive radiation, for example, cannot be reduced only to natural selection, though natural selection helps bring it about.
     Eugenie C. Scott (2004)

As I mentioned earlier, most of macroevolutionary theory is intimately connected with the observed fossil record and, in this sense, it is much more historical than population genetics and evolution within a species. Macroevolution, as a field of study, is the turf of paleontologists and much of the debate about a higher level of evolution (above species and populations) is motivated by the desire of paleontologists to be accepted at the high table of evolutionary theory. It's worth recalling that during the last part of the twentieth century evolutionary theorizing was dominated by population geneticists. Their perspective was described by John Maynard Smith, "... the attitude of population geneticists to any paleontologist rash enough to offer a contribution to evolutionary theory has been to tell him to go away and find another fossil, and not to bother the grownups." (Maynard Smith, 1984)

The distinction between microevolution and macroevolution is often exaggerated, especially by the anti-science crowd. Creationists have gleefully exploited the distinction in order to legitimate their position in the light of clear and obvious examples of evolution that they can't ignore. They claim they can accept microevolution, but they reject macroevolution.

In the real world—the one inhabited by rational human beings—the difference between macroevolution and microevolution is basically a difference in emphasis and level. Some evolutionary biologists are interested in species, trends, and the big picture of evolution, while others are more interested in the mechanics of the underlying mechanisms.

Speciation is critical to conserving the results of both natural selection and genetic drift. Speciation is obviously central to the fate of genetic variation, and a major shaper of patterns of evolutionary change through evolutionary time. It is as if Darwinians—neo- and ulra- most certainly included—care only for the process generating change, and not about its ultimate fate in geological time.
     Niles Eldredge (1995)

The Creationists would have us believe there is some magical barrier separating selection and drift within a species from the evolution of new species and new characteristics. Not only is this imagined barrier invisible to most scientists but, in addition, there is abundant evidence that no such barrier exists. We have numerous examples that show how diverse species are connected by a long series of genetic changes. This is why many scientists claim that macroevoluton is just lots of microevolution over a long period of time.

But wait a minute. I just said that many scientists think of macroevolution as simply a scaled-up version of microevolution, but a few paragraphs ago I said there's more to the theory of evolution than just changes in the frequency of alleles within a population. Don't these statements conflict? Yes, they do ... and therein lies a problem.

When the principle tenets of the Modern Synthesis were being worked out in the 1940's, one of the fundamental conclusions was that macroevolution could be explained by changes in the frequency of alleles within a population due, mostly, to natural selection. This gave rise to the commonly accepted notion that macroevolution is just a lot of microevolution. Let's refer to this as the sufficiency of microevolution argument.

At the time of the synthesis, there were several other explanations that attempted to decouple macroevolution from microevolution. One of these was saltation, or the idea that macroevolution was driven by large-scale mutations (macromutations) leading to the formation of new species. This is the famous "hopeful monster" theory of Goldschmidt. Another decoupling hypothesis was called orthogenesis, or the idea that there is some intrinsic driving force that directs evolution along certain pathways. Some macroevolutionary trends, such as the increase in the size of horses, were thought to be the result of this intrinsic force.

Both of these ideas about macroevolutionary change (saltation and orthogensis) had support from a number of evolutionary biologists. Both were strongly opposed by the group of scientists that produced the Modern Synthesis. One of the key players was the paleontologist George Gaylord Simpson whose books Tempo and Mode in Evolution (1944) and The Major Features of Evolution (1953) attempted to combine paleontology and population genetics. "Tempo" is often praised by evolutionary biologists and many of our classic examples of evolution, such as the bushiness of the horse tree, come from that book. It's influence on paleontologists was profound because it upset the traditional view that macroevolution and the newfangled genetics had nothing in common.

Just as mutation and drift introduce a strong random component into the process of adaptation, mass extinctions introduce chance into the process of diversification. This is because mass extinctions are a sampling process analogous to genetic drift. Instead of sampling allele frequencies, mass extinctions samples species and lineages. ... The punchline? Chance plays a large role in the processes responsible for adaptation and diversity.
        Freeman and Herron (1998)

We see, in context, that the blurring of the distinction between macroevolution and microevolution was part of a counter-attack on the now discredited ideas of saltation and orthogenesis. As usual, when pressing the attack against objectionable ideas, there's a tendency to overrun the objective and inflict collateral damage. In this case, the attack on orthogenesis and the old version of saltation was justified since neither of these ideas offer viable alternatives to natural selection and drift as mechanisms of evolution. Unfortunately, Simpson's attack was so successful that a generation of scientists grew up thinking that macroevolution could be entirely explained by microevolutionary processes. That's why we still see this position being advocated today and that's why many biology textbooks promote the sufficiency of microevolution argument. Gould argues—successfully, in my opinion—that the sufficiency of microevolution became dogma during the hardening of the synthesis in the 1950-'s and 1960's. It was part of an emphasis on the individual as the only real unit of selection.

However, from the beginning of the Modern Synthesis there were other evolutionary biologists who wanted to decouple macroevolution and microevolution—not because they believed in the false doctrines of saltation and orthogenesis, but because they knew of higher level processes that went beyond microevolution. One of these was Ernst Mayr. In his essay "Does Microevolution Explain Macroevolution," Mayr says ...

Among all the claims made during the evolutionary synthesis, perhaps the one that found least acceptance was the assertion that all phenomena of macroevolution can be ‘reduced to,' that is, explained by, microevolutionary genetic processes. Not surprisingly, this claim was usually supported by geneticists but was widely rejected by the very biologists who dealt with macroevolution, the morphologists and paleontologists. Many of them insisted that there is more or less complete discontinuity between the processes at the two levels—that what happens at the species level is entirely different from what happens at the level of the higher categories. Now, 50 years later the controversy remains undecided.
                                                                         Ernst Mayr (1988) p.402

Mayr goes on to make several points about the difference between macroevolution and microevolution. In particular, he emphasizes that macroevolution is concerned with phenotypes and not genotypes, "In this respect, indeed, macroevolution as a field of study is completely decoupled from microevolution." (ibid p. 403). This statement reiterates an important point, namely that macroevolution is a "field of study" and, as such, its focus differs from that of other fields of study such as molecular evolution.

If you think of macroevolution as a field of study rather than a process, then it doesn't make much sense to say that macroevolution can be explained by the process of changing alleles within a population. This would be like saying the entire field of paleontology can be explained by microevolution. This is the point about the meaning of the term "macroevolution" that is so often missed by those who dismiss it as just a bunch of microevolution.

The orthodox believers in the hardened synthesis feel threatened by macroevolution since it implies a kind of evolution that goes beyond the natural selection of individuals within a population. The extreme version of this view is called adaptationism and the believers are called Ultra-Darwinians by their critics. This isn't the place to debate adaptationism: for now, let's just assume that the sufficiency of microevolution argument is related to the pluralist-adaptationist controversy and see how our concept of macroevolution as a field of study relates to the issue. Niles Eldredge describes it like this ...

The very term macroevolution is enough to make an ultra-Darwinian snarl. Macroevolution is counterpoised with microevolution—generation by generation selection- mediated change in gene frequencies within populations. The debate is over the question, Are conventional Darwinian microevolutionary processes sufficient to explain the entire history of life? To ultra-Darwinians, the very term macroevolution suggests that the answer is automatically no. To them, macroevolution implies the action of processes—even genetic processes—that are as yet unknown but must be imagined to yield a satisfactory explanation of the history of life.

But macroevolution need not carry such heavy conceptual baggage. In its most basic usage, it simply means evolution on a large-scale. In particular, to some biologists, it suggests the origin of major groups - such as the origin and radiation of mammals, or the derivation of whales and bats from terrestrial mammalian ancestors. Such sorts of events may or may not demand additional theory for their explanation. Traditional Darwinian explanation, of course, insists not.
                                                              Niles Eldredge (1995) p. 126-127

Eldredge sees macroevolution as a field of study that's mostly concerned with evolution on a large scale. Since he's a paleontologist, it's likely that, for him, macroevolution is the study of evolution based on the fossil record. Eldredge is quite comfortable with the idea that one of the underlying causes of evolution can be natural selection—this includes many changes seen over the course of millions of years. In other words, there is no conflict between microevolution and macroevolution in the sense that microevolution stops and is replaced by macroevolution above the level of species. But there is a conflict in the sense that Eldredge, and many other evolutionary biologists, do not buy the sufficiency of microevolution argument. They believe there are additional theories, and mechanisms, needed to explain macroevolution. Gould says it best ....

We do not advance some special theory for long times and large transitions, fundamentally opposed to the processes of microevolution. Rather, we maintain that nature is organized hierarchically and that no smooth continuum leads across levels. We may attain a unified theory of process, but the processes work differently at different levels and we cannot extrapolate from one level to encompass all events at the next. I believe, in fact, that ... speciation by splitting guarantees that macroevolution must be studied at its own level. ... [S]election among species—not an extrapolation of changes in gene frequencies within populations—may be the motor of macroevolutionary trends. If macroevolution is, as I believe, mainly a story of the differential success of certain kinds of species and, if most species change little in the phyletic mode during the course of their existence, then microevolutionary change within populations is not the stuff (by extrapolation) of major transformations.
                                                         Stephen Jay Gould (1980b) p. 170

Naturalists such as Ernst Mayr and paleontologists such as Gould and Eldredge have all argued convincingly that speciation is an important part of evolution. Since speciation is not a direct consequence of changes in the frequencies of alleles in a population, it follows that microevolution is not sufficient to explain all of evolution. Gould and Eldredge (and others) go even further to argue that there are processes such as species sorting that can only take place above the species level. This means there are evolutionary theories that only apply in the domain of macroevolution.

The idea that there's much more to evolution than genes and population genetics was a favorite theme of Stephen Jay Gould. He advocated a pluralist, hierarchical approach to evolution and his last book The Structure of Evolutionary Theory emphasized macroevolutionary theory—although he often avoided using this term. The Structure of Evolutionary Theory is a huge book that has become required reading for anyone interested in evolution. Remarkably, there's hardly anything in the book about population genetics, molecular evolution, and microevolution as popularly defined. What better way of illustrating that macroevolution must be taken seriously!

Macroevolutionary theory tries to identify patterns and trends that help us understand the big picture. In some cases, the macroevolution biologists have recognized generalities (theories & hypotheses) that only apply to higher level processes. Punctuated equilibria and species sorting are examples of such higher level phenomena. The possible repeatedness of mass extinctions might be another.

Remember that macroevolution should not be contrasted with microevolution because macroevolution deals with history. Microevolution and macroevolution are not competing explanations of the history of life any more than astronomy and physics compete for the correct explanation of the history of the known universe. Both types of explanation are required.

I think species sorting is the easiest higher level phenomena to describe. It illustrates a mechanism that is clearly distinct from changes in the frequencies of alleles within a population. In this sense, it will help explain why microevolution isn't a sufficient explanation for the evolution of life. Of course, one needs to emphasize that macroevolution must be consistent with microevolution.

I have championed contingency, and will continue to do so, because its large realm and legitimate claims have been so poorly attended by evolutionary scientists who cannot discern the beat of this different drummer while their brains and ears remain tuned to only the sounds of general theory.
        Stephen Jay Gould (2002)

If we could track a single lineage through time, say from a single-cell protist to Homo sapiens, then we would see a long series of mutations and fixations as each ancestral population evolved. It might look as though the entire history could be accounted for by microevolutionary processes. This is an illusion because the track of the single lineage ignores all of the branching and all of the other species that lived and died along the way. That track would not explain why Neanderthals became extinct and Cro-Magnon survived. It would not explain why modern humans arose in Africa. It would not tell us why placental mammals became more successful than the dinosaurs. It would not explain why humans don't have wings and can't breathe underwater. It doesn't tell us whether replaying the tape of life will automatically lead to humans. All of those things are part of the domain of macroevolution and microevolution isn't sufficient to help us understand them.


Friday, October 29, 2021

Do scientists write books for the "casual reader"?

I just read a review in Science of Douglas Futuyma’s new book How Birds Evolve: What Science Reveals About Their Origin, Lives, and Diversity. [Contextualizing avian origins and evolution]. Many of you will be familiar with Futuyma because he’s the author of one of the best textbooks on evolution.

Right after reading the review, I signed on to Amazon intending to buy the book but when I saw the price ($95 Cdn) I had second thoughts. Much as I’d like to see how Futuyma handles a complex topic like bird evolution, I don’t think I want to spend that much money.

But there are parts of the review that I find intriguing enough to address because they relate to my concern about science writing. Here’s the first thing that caught my eye. The reviewer, Alan Feduccia, writes,

Although casual readers might find the text somewhat advanced and laborious, the chapters are composed in well-written conversational prose, with expositions on multifarious evolutionary phenomena that are infused with scientific explanations.

This highlights an issue that I’ve been writing about recently. "Casual readers" are not going to buy this book and I’m confident that Futuyma is not writing for casual readers. What’s the point of saying that such readers might find the book "advanced and laborious"? What I want to know is whether the actual intended audience would find the book laborious.

The reviewer goes on to describe some of things that are in the book.

Futuyma’s discussion begins with a section that explains Charles Darwin’s transformative ideas—from natural selection and fitness to brood parasitism to gene flow and genetic drift—and thematic chapters elaborate on the relationship between these ideas and bird lineages. The book describes complex evolutionary issues in understandable terms, ...

That sounds like the kind of science writing that I admire. We should aim for explaining complex issues in terms that are understandable to an audience that is prepared to buy the book. This may mean that the casual readers - who will never buy the book - are left out but that’s okay. It may mean that Futuyuma’s book is not going to win a Pulitzer Price for general nonfiction but that’s okay too since good science books are not high on the list of previous award winners.1 I’m pretty sure that scientific accuracy isn’t a prominent criterion in selecting award winners (in any category).

The reviewer is somewhat critical of the science in the book and takes Futuyma to task for promoting “just-so” stories and for not fully explaining the speculative nature of some of his conclusions. The reviewer notes that “controversy, not consensus, is grist to the mill of good science” and that strikes me as insightful. The problem is that writing about controversy and attempting to explain both sides of an issue are very hard to do and often in conflict with the emphasis on style that is promoted by science writers and editors.


1. Previous Pulitzer Prize winning books that might be counted as science books are: The Emperor of All Maladies by Siddhartha Mukherjee (2011); Guns, Germs and Steel: The Fates of Human Societies, by Jared Diamond (1998); The Beak Of The Finch: A Story Of Evolution In Our Time, by Jonathan Weiner (1995); The Ants, by Bert Holldobler and Edward O. Wilson (1990); and On Human Nature, by Edward O. Wilson (1979).

Monday, April 06, 2020

The Function Wars Part VII: Function monism vs function pluralism

This post is mostly about a recent paper published in Studies in History and Philosophy of Biol & Biomed Sci where two philosophers present their view of the function wars. They argue that the best definition of function is a weak etiological account (monism) and pluralistic accounts that include causal role (CR) definitions are mostly invalid. Weak etiological monism is the idea that sequence conservation is the best indication of function but that doesn't necessarily imply that the trait arose by natural selection (adaptation); it could have arisen by neutral processes such as constructive neutral evolution.

The paper makes several dubious claims about ENCODE that I want to discuss but first we need a little background.

Background

The ENCODE publicity campaign created a lot of controversy in 2012 because ENCODE researchers claimed that 80% of the human genome is functional. That claim conflicted with all the evidence that had accumulated up to that point in time. Based on their definition of function, the leading ENCODE researchers announced the death of junk DNA and this position was adopted by leading science writers and leading journals such as Nature and Science.

Let's be very clear about one thing. This was a SCIENTIFIC conflict over how to interpret data and evidence. The ENCODE researchers simply ignored a ton of evidence demonstrating that most of our genome is junk. Instead, they focused on the well-known facts that much of the genome is transcribed and that the genome is full of transcription factor binding sites. Neither of these facts were new and both of them had simple explanations: (1) most of the transcripts are spurious transcripts that have nothing to do with function, and (2) random non-functional transcription factor binding sites are expected from our knowledge of DNA binding proteins. The ENCODE researchers ignored these explanations and attributed function to all transcripts and all transcription factor binding sites. That's why they announced that 80% of the genome is functional.

Wednesday, February 12, 2020

Happy Darwin Day! 2020

Charles Darwin, the greatest scientist who ever lived, was born on this day in 1809 [Darwin still spurs tributes, debates] [Happy Darwin Day!] [Darwin Day 2017]. Darwin is mostly famous for two things: (1) he described and documented the evidence for evolution and common descent and (2) he provided a plausible scientific explanation of evolution—the theory of natural selection. He put all this in a book, The Origin of Species by Means of Natural Selection published in 1859—a book that spurred a revolution in our understanding of the natural world. (You can still buy a first edition copy of the book but it will cost you several hundred thousand dollars.)

Friday, February 07, 2020

The Function Wars Part VI: The problem with selected effect function

The term "Function Wars" refers to the debate over the meaning of 'function,' especially in the context of junk DNA.1 That debate intensified in 2012 after the ENCODE publicity campaign that tried to redefine function to mean anything they want as long as it refutes junk DNA. This is the sixth in a series of posts exploring the debate and why it's important, or not. Links to the other five posts can be found at the bottom or this post.

The world is not inhabited exclusively by fools and when a subject arouses intense interest and debate, as this one has, something other than semantics is usually at stake.
Stephen Jay Gould (1982)
Much of the discussion seems like quibbling over semantics but I'm reminded of a similar debate over the mode of evolution: is it gradual or punctuated? As Gould pointed out in 1982, there's a serious issue underlying the debate—an issue that shouldn't get lost in bickering over the meaning of 'gradualistic.' The same warning applies here. It's important to determine how much of the human genome is junk and that requires an understanding of what we mean by junk DNA. However, it's easy to get distracted by focusing on the exact meaning of the word 'function' instead of looking at the big picture.

Tuesday, January 14, 2020

The Three Domain Hypothesis: RIP

The Three Domain Hypothesis died about twenty years ago but most people didn't notice.

The original idea was promoted by Carl Woese and his colleagues in the early 1980s. It was based on the discovery of archaebacteria as a distinct clade that was different from other bacteria (eubacteria). It also became clear that some eukaryotic genes (e.g. ribosomal RNA) were more closely related to archaebacterial genes and the original data indicated that eukaryotes formed another distinct group separate from either the archaebacteria or eubacteria. This gave rise to the Three Domain Hypothesis where each of the groups, bacteria (Eubacteria), archaebacteria (Archaea), and eukaryotes (Eucarya, Eukaryota), formed a separate clade that contained multiple kingdoms. These clades were called Domains.

Sunday, December 15, 2019

The evolution of citrate synthase

Citrate synthase [EC 2.3.3.1] is one of the key enzymes of the citric acid cycle. It catalyzes the joining of acetyl-CoA and oxaloacetate to produce citrate.
acetyl-CoA + H2O + oxaloacetate → citrate + HS-CoA + H+
We usually think of this reaction in terms of energy production since acetyl-CoA is the end product of glycolysis and the citric acid cycle produces substrates that enter the electron transport system leading to production of ATP. However, it's important to keep in mind that the enzyme also catalyzes the reverse reaction.

Monday, October 21, 2019

The evolution of de novo genes

De novo genes are new genes that arise spontaneously from junk DNA [De novo gene birth]. The frequency of de novo gene creation is important for an understanding of evolution. If it's a frequent event, then species with a large amount of junk DNA might have a selective advantage over species with less junk DNA, especially in a changing environment.

Last week I read a short Nature article on de novo genes [Levy, 2019] and I think the subject deserves more attention. Most new genes in a species appear to arise by gene duplication and subsequent divergence but de novo genes are genes that are unrelated to genes in any other clade so we can assume that they are created from junk DNA that accidentally becomes associated with a promoter causing the DNA to be transcribed. A new gene is formed if the RNA acquires a function. If the transcript contains an open reading frame then it may be translated to produce a polypeptide and if the polypeptide performs a new function then the resulting de novo gene is a new protein-coding gene.

The important question is whether the evolution of de novo genes is a common event or a rare event.

Sunday, September 08, 2019

Contingency, selection, and the long-term evolution experiment

I'm a big fan of Richard Lenski's long-term evolution experiment (LTEE) and of Zachary Blount's work in particular. [Strolling around slopes and valleys in the adaptive landscape] [On the unpredictability of evolution and potentiation in Lenski's long-term evolution experiment] [Lenski's long-term evolution experiment: the evolution of bacteria that can use citrate as a carbon source]

The results of the LTEE raise some interesting questions about evolution. The Lenski experiment began with 12 (almost) identical cultures and these have now "evolved" for 31 years and more than 65,000 generations. All of the cultures have diverged to some extent and one of them (and only one) has developed the ability to use citrate as a carbon source. Many of the cultures exhibit identical, or very similar, mutations that have reached significant frequencies, or even fixation, in the cultures.

Several other laboratory evolution experiments have been completed or are underway in various labs around the world. The overall results are relevant to a discussion about the role of contingency and accident in the history of life [see Evolution by Accident]. Is it true that if you replay the tape of life the results will be quite different? [Replaying life's tape].

Thursday, August 22, 2019

Reactionary fringe meets mutation-biased adaptation.
7. Going forward

This the last of a series of posts by Arlin Stoltzfus on the role of mutation as a dispositional factor in evolution. Arlin has established that the role of mutation in evolution is much more important than most people realize. He has also built a strong case for the influence of mutation bias. How should we incorporate these concepts into modern evolutionary theory?

Click on the links in the box (below) to see the other posts in the series.



Reactionary fringe meets mutation-biased adaptation.
7. Going forward

by Arlin Stoltzfus

Haldane (1922) argued that, because mutation is a weak pressure easily overcome by selection, the potential for biases in variation to influence evolution depends on neutral evolution or high mutation rates. This theory, like the Modern Synthesis of 1959, depends on the assumption that evolution begins with pre-existing variation. By contrast, when evolution depends on the introduction of new variants, mutational and developmental biases in variation may impose biases on evolution, without requiring neutral evolution or high mutation rates.

Thursday, August 15, 2019

Reactionary fringe meets mutation-biased adaptation.
5.5 Synthesis apologetics

This is part of a continuing series of posts by Arlin Stoltzfus on the role of mutation as a dispositional factor in evolution. In this post, Arlin explains how defenders of the Modern Synthesis react in the face of serious challenges to the theory that was formulated in the 1940s and 50s. Rather than reject the theory, they engage in various forms of "synthesis apologetics."

Click on the links in the box (below) to see the other posts in the series.




Reactionary fringe meets mutation-biased adaptation. 5.6 Synthesis apologetics
by Arlin Stoltzfus

Tuesday, August 06, 2019

Reactionary fringe meets mutation-biased adaptation.
5.4. Taking neo-Darwinism seriously

This is part of a continuing series of posts by Arlin Stoltzfus on the role of mutation as a dispositional factor in evolution. In this post Arlin discusses his view of neo-Darwinism and why it is inconsistent with macromutations and lateral gene transfer. He equates neo-Darwinism with the Modern Synthesis (1959 version), a comparison that might be challenged. Click on the links in the box (below) to see the other posts in the series.




Reactionary fringe meets mutation-biased adaptation. 5.4. Taking neo-Darwinism seriously
by Arlin Stoltzfus

The Modern Synthesis is often described as the result of combining Darwinism and genetics. This description, in my opinion, is concise and historically accurate: the Modern Synthesis of 1959 is a sophisticated attempt to arrange the pieces of population genetics to justify a neo-Darwinian dichotomy in which variation merely supplies raw materials, and selection is the source of initiative, creativity and direction.

Friday, August 02, 2019

Reactionary fringe meets mutation-biased adaptation.
6. What "limits" adaptation?

This is part of a continuing series of posts by Arlin Stoltzfus on the role of mutation as a dispositional factor in evolution. In this post Arlin discusses the role of adaptation and what determines the pathway that it will take over time. Is it true that populations will always adapt quickly to any change in the environment? (Hint: no it isn't!) Click on the links in the box (below) to see the other posts in the series.




Reactionary fringe meets mutation-biased adaptation.
6. What "limits" adaptation?

by Arlin Stoltzfus
According to the hatchet piece at TREE, theoretical considerations dictate that biases in variation are unlikely to influence adaptation, because this requires small population sizes and reciprocal sign epistasis.

Yet, we have established that mutation-biased adaptation is real (see The empirical case and Some objections addressed). If theoretical population genetics tells us that mutation-biased adaptation is impossible or unlikely, what is wrong with theoretical population genetics?

Adaptation, before Equilibrium Day

Wednesday, July 31, 2019

Reactionary fringe meets mutation-biased adaptation.
5.3. How history is distorted.

This is the ninth in a series of guest posts by Arlin Stoltzfus on the role of mutation as a dispositional factor in evolution. Click on the links in the box (below) to see the other post in the series.


Reactionary fringe meets mutation-biased adaptation.
5.3. How history is distorted.

by Arlin Stoltzus
In his famous Materials for the Study of Variation, Bateson (1894) refers to natural selection as "obviously" a "true cause" (p. 5). Punnett (1905) explains that mutations are heritable while environmental fluctuations are not, concluding that "Evolution takes place through the action of selection on these mutations" (p. 53). De Vries begins his major 1905 English treatise by writing that ...
"Darwin discovered the great principle which rules the evolution of organisms. It is the principle of natural selection. It is the sifting out of all organisms of minor worth through the struggle for life. It is only a sieve, and not a force of nature" (p. 6)
Morgan (1916), in his closing summary, writes:
"Evolution has taken place by the incorporation into the race of those mutations that are beneficial to the life and reproduction of the organism" (p. 194)

Monday, July 22, 2019

Reactionary fringe meets mutation-biased adaptation.
5.2. The Modern Synthesis of 1959

This is the eighth in a series of guest posts by Arlin Stoltzfus on the role of mutation as a dispositional factor in evolution.


Reactionary fringe meets mutation-biased adaptation. 5.2. The Modern Synthesis of 1959
by Arlin Stoltfus

As we learned in What makes it new?, the newness of the effect of biases in the introduction process results from a classical assumption that evolution can be understood as a process of shifting the frequencies of existing alleles. How did this position emerge? Was it a technical, mathematical issue?

Friday, July 19, 2019

Reactionary fringe meets mutation-biased adaptation. 5.1. Thinking about theories

This is the seventh in a series of guest posts by Arlin Stoltzfus on the role of mutation as a dispositional factor in evolution.


Reactionary fringe meets mutation-biased adaptation. 5.1. Thinking about theories
by Arlin Stoltzfus

A wikipedia page disambiguating "Modern Synthesis" defines neo-Darwinism as
"the state-of-the-art in evolutionary biology, as seen at any chosen time in history from the 1890s to the present day."
Because "neo-Darwinism" and the "Synthesis" are conflated with whatever is widely accepted, they are now regularly attacked on grounds that are completely unrelated to genuine neo-Darwinism or the original Modern Synthesis, e.g., as when a network of life (rather than a tree) is invoked as a contradiction of Darwinism. The attack by Noble (2015) on the
"... conceptual framework of neo-Darwinism, including the concepts of "gene," "selfish," "code," "program," "blueprint," "book of life," "replicator" and ˜"vehicle."
is entirely a critique of late-20th-century reductionism à la Dawkins, and addresses neither neo-Darwinism (selection and variation as the potter and the clay), nor the original Modern Synthesis, which is simply not reductionistic, but positively invokes emergent phenomena (population-level forces, the gene pool as dynamic buffer) in the service of selection as a high-level governing principle.

"The state of the art" is a phrase that needs no modification. Nothing good can come from linking it to the name of a dead person.