Planet of the horses

16.7 – 15.8 million years

Horses have probably been the single most important domesticated animal in human history. Also, more than with other livestock, people get attached to horses as individuals. I’m guessing that in history and literature there are more horses with individual names than any other animal. (Alexander the Great’s horse was Bucephalus, “Ox-head”; Muhammed’s was al-Buraq*; Charlemagne’s was Tencendur; Don Quixote’s was Rocinante; Gandalf’s was Shadowfax.) We’ll be hearing a lot more about horses and horse folk on Logarithmic History once we get to human history.

Being so charismatic, horses have featured in a big way in arguments over evolution. Thomas Henry Huxley (1825-1895), “Darwin’s bulldog,” knew he needed to find good evidence for evolution. When he visited the United States in 1876, he was ready to give a lecture based on horse fossils from Europe. But visiting Yale, he was so impressed with O. C. Marsh’s collection of horse fossils from the western United States, that he rewrote his lecture around it.

Henry Fairfield Osborn (1857-1935) was director of the American Museum of Natural History and a huge presence in American paleontology. He was active at a time when most scientists accepted evolution, but many weren’t so keen on Darwin’s theory of natural selection. He thought horses were a fine example of “orthogenesis,” the tendency of species to follow a fixed line of evolution, reflecting internal forces, maybe related to willpower. He thought that humans shared a migratory spirit with horses, so that anywhere horse fossils were found would be a good place to look for human fossils. This theory didn’t pan out too well. A massive AMNH expedition to Central Asia led by Ray Chapman Andrews found all sorts of wonders – dinosaur eggs, baluchitheres – but no fossil “pro-men.” Orthogenesis leant itself naturally to diagrams showing evolution from early to modern horses going in a straight line.

George Gaylord Simpson (1902-1984), paleontologist, was one of the great figures in the evolutionary Modern Synthesis that brought together Darwin’s theory of natural selection and Mendel’s genetics. There was no room for orthogenesis in the Modern Synthesis, and Simpson emphasized that the evolution of horses was a matter of adaptation to a changing environment – especially the spread of grasslands. Also that horse evolution looked more like a bush than a ladder.

Stephen Jay Gould (1941-2002) was the most widely recognized American evolutionary biologist of recent times. (For example had a spot on The Simpson’s — “Lisa The Skeptic,” Season 9.) Gould had his own take on the modern synthesis, taking the “bushes not ladders” theme for horses and other animals (including human ancestors), and pushing it a step further. According to the theory of “punctuated equilibrium” (formulated in collaboration with Niles Eldredge), species mostly change relatively little during the time they exist (evolutionary stasis). Most evolutionary change happens when a small population buds off to form a new species and reproductive isolation allows it to conserve any evolutionary novelties it has developed. This opens up the possibility of “species selection.” Applied to horses, for example, this could mean that horses were evolutionarily successful for some time not so much because individual horses were well-adapted, but because something about horses collectively (their harem mating system, maybe) made one horse species especially likely to generate new species. Both horses and primates seem to be especially prone to bud off new species:

Speciation and chromosomal evolution seem fastest in those genera with species organized into clans or harems (e.g., some primates and horses) or with limited adult vagility and juvenile dispersal, patchy distribution, and strong individual territoriality (e.g., some rodents). This is consistent with the … hypothesis … that population subdivision into small demes promotes both rapid speciation and evolutionary changes in gene arrangement by inbreeding and drift.

 * Richard Dawkins doesn’t believe that Muhammed’s horse, al-Buraq, carried him (i.e. Muhammed) to heaven and back.

The Burgess Shale and “Wonderful Life”

507 – 480 million years ago

The Burgess Shale (about 510 Mya) is not the oldest Cambrian deposit known. There are deposits from China (Chengjiang) closer to the beginning of the era. But it is particularly rich and well studied. It also featured in debates about some Big Questions: How important are evolutionary laws versus historical accidents? Has the living world become more or less diverse over time?

Stephen Jay Gould (1941-2002) was one of the most widely known evolutionary biologists of his time. In 1989 he wrote a book about the Burgess Shale, called “Wonderful Life.” The title alluded to Frank Capra’s movie, “It’s a Wonderful Life,”* starring Jimmy Stewart. In the movie, the Stewart character, who thinks he’s wasted his life stuck in his home town, has a chance to see how things would have turned out if he had died young in an accident. He learns that his existence made a huge difference to his town.

Similarly, Gould argued that accidents of which Cambrian species survived and which went extinct made a huge difference to the later evolution of life. To make his case, he developed a subsidiary argument: that the Cambrian fauna displays a radical diversity of body plans in comparison with later eras. Gould made an analogy with the early development of automobiles, which featured diesel engines, steam engines, and electrical engines, before settling down on mostly just gasoline engines. (He was writing before Priuses and Teslas, of course.)

This subsidiary argument has not fared well. Most of the supposedly radically different forms from the Burgess Shale turn out to be not that radically different from one another, or from modern forms. Most notorious was the case of a specimen called Hallucigenia. Paleontologists thought that this creature was like nothing that ever lived before. But later more complete finds of related forms made it clear they were looking at it upside down; its “legs” are actually defensive spikes, the “tentacles” on top are actually legs. It’s probably related to the ancestor of velvet worms, a group related to vertebrates and still living in Australia. A different perspective comes from Simon Conway Morris one of the experts on the Burgess Shale, in in his book “Crucible of Creation.”

And here, from a nice review of more recent work on contingency and determinism in evolution:

Gould would be pleased that his thought experiment of replaying life’s tape has been transformed into an empirical research program that explores the roles of historical contingency and natural selection at multiple levels. However, his view of historical influences as the central feature of evolution remains debatable. Laboratory replay experiments show that repeatable outcomes are common, at least when defined broadly (e.g., at the level of genes, not mutations). Moreover, convergence in nature is more common than many biologists would have wagered not long ago. On the other hand, as evolving lineages accumulate more differences, both experimental and comparative approaches suggest that the power of selection to drive convergence is reduced, and the contingent effects of history are amplified.

*Not to be confused with Jerome Bixby’s short story “It’s a Good Life,” about a very different small town.

Central Dogma blues

3.40 – 3.23 billion years ago

[T]he Darwinian process may be described as a chapter of accidents. As such, it seems simple, because you do not at first realize all that it involves. But when its whole significance dawns on you, your heart sinks into a heap of sand within you. There is a hideous fatalism about it, a ghastly and damnable reduction of beauty and intelligence, of strength and purpose, of honor and aspiration, to such casually picturesque changes as an avalanche may make in a mountain landscape, or a railway accident in a human figure. If it be … a truth of science, then the stars of heaven, the showers and dew, the winter and summer, the fire and heat, the mountains and hills, may no longer be called to exalt the Lord with us by praise; their work is to modify all things by blindly starving and murdering everything that is not lucky enough to survive in the universal struggle for hogwash.

.George Bernard Shaw. Back to Methusaleh. Preface iv

A broken symmetry lies at the heart of life, ruling out Bernard Shaw’s preferred mechanism of change, Lamarckian evolution through the inheritance of acquired traits. “Lamarck … held as his fundamental proposition that living organisms changed because they wanted to. As he stated it, the great factor in Evolution is use and disuse.” (Back to Methusaleh, preface ii)

Shaw was not attacking Darwin himself. He knew that Darwin accepted the inheritance of acquired traits as one mechanism of evolution. In fact Darwin put a lot of ingenuity into trying figuring out how the process might work. But this was all wasted effort on Darwin’s part. In Shaw’s day, August Weismann had countered Lamarck, arguing that the inheritance of acquired traits was ruled out by one great asymmetry: the separation of germ plasm (destined to pass on to the next generation) from soma (destined to perish).

With the discovery of the DNA double helix, biologists came to recognize an even more profound, more ancient asymmetry. This asymmetry is codified as the Central Dogma of molecular biology: information passes from nucleic acids (the basis of genetic inheritance) to proteins, but not vice versa. 

The Central Dogma may be a predictable, inescapable result of selection acting at multiple levels, both within and among protocells, at the very origin of life, assigning some molecules the necessary but dead-end job of catalysis, giving others a shot at immortality as replicators. 

We developed a model consisting of a population of protocells, each containing a population of replicating catalytic molecules. The molecules are assumed to face a trade-off between serving as catalysts and serving as templates. This trade-off causes conflicting multilevel selection: serving as catalysts is favoured by selection between protocells, whereas serving as templates is favoured by selection between molecules within protocells. This conflict induces informatic and catalytic symmetry breaking, whereby the molecules differentiate into genomes and enzymes, establishing the central dogma. 

The origin of the central dogma through conflicting multilevel selection

Earlier on Logarithmic History, I noted that broken symmetries – female and male, predator and prey, ruler and ruled – are often morally fraught. This applies to the great asymmetry at the center of natural selection. Darwin put a positive spin on it: “From the war of nature, from famine and death, most exalted object which we are capable of conceiving, namely the production of the higher animals, directly follows,” but a many people have found it hard to accept this view of life. In Shaw’s day it was perhaps still possible to hold out hope that Lamarckian evolution provided a gentler path for progressive evolution. And for much of the twentieth century, social scientists tried to quarantine natural selection, allowing it a role in human physical evolution, while sticking to a blank slate view of mind and culture. But there is one more twist in the story: in the twenty first century, the rules of the game may change entirely, as new reproductive technologies present the promise, and peril, of finally overturning the Central Dogma. 

A family tree for the Sun

7.45 – 7.06 billion years ago

Recently, some astronomers teamed up with some evolutionary biologists to produce a “family tree” of our Sun and some of its neighbors. The tree is based on the abundances of different chemical elements; these abundances don’t change much over the lifetime of a star, and can be thought of as a kind of inherited trait, something like DNA. The tree groups stars roughly according to their ages, with younger stars having more “metals” (elements other than hydrogen and helium), but only roughly, since other processes affect stellar chemistry.

Drawing a family tree for stars might seem like an odd thing to do. Stars aren’t really related as parent and offspring. On the other hand, we might at least call some bunches of stars “siblings,” if they originate from the same stellar neighborhood, and consequently have similar chemical makeup. As with galactic or mineral “evolution,” we’ve got something that borders on evolution, even if it’s not quite what gets biologists all het up.

Evolution and broken symmetries

8.34 – 7.89 billion years ago.

No big news in the universe today. Some evolutionary thoughts: Species evolve. Do planets? stars? galaxies?

Charles Darwin didn’t use the word “evolution” often. But he did write a lot about “descent with modification,” which is pretty much what biologists mean by evolution. For example, the usual definition of genetic evolution is “change in gene frequency,” i.e. descent with (genetic) modification.

However, people sometimes talk about evolution that doesn’t involve descent with modification, in contexts that have nothing much to do with biological evolution – cosmic evolution or stellar evolution in the history of the universe, for example, or mineral evolution in the history of the earth. Another Victorian writer, the sociologist and philosopher Herbert Spencer, offered a definition of evolution that might cover these cases.

Evolution is an integration of matter and concomitant dissipation of motion; during which the matter passes from an indefinite, incoherent homogeneity to a definite, coherent heterogeneity.

It’s easy to make fun of this definition. It’s the sort of abstract word pile that style manuals tell you to avoid, and that gives sociology a bad name. For that matter, it’s easy to make fun of Herbert Spencer. He may be some of the inspiration for the character of Mr. Casaubon, the dried up, impotent pedant in George Eliot’s “Middlemarch.” (Spencer probably turned down a chance to marry George Eliot = Mary Ann Evans. You should be careful about offending a writer.) But it may be that Spencer was groping toward the important modern concepts of symmetry and symmetry breaking.

A simple example: imagine you’re holding a bicycle exactly upright. The bicycle is pretty much bilaterally (mirror image) symmetrical. (OK, not really, the gears are on the right side, so it’s not a perfect mirror image. But just pretend …) Now let go of the bike. It will fall to one side or the other. The symmetry is broken, and you need one extra “bit” of information to tell you which side the bicycle is on.

Symmetry breaking is a fundamental concept in physics. In the very early history of the universe, the four forces of nature — gravitational, strong, weak, and electromagnetic – were united, but then as the universe cooled, one by one, these forces broke the symmetry and turned into separate forces. More symmetry breaking generated elementary particles, and nuclei, and atoms. When atoms first formed, they were distributed symmetrically through the universe as a diffuse gas. But gravitation pulled atoms and other particles together into clumps, leaving other parts of space emptier, and the spatial symmetry was broken (a “translational” symmetry in this case).

Symmetry breaking will keep showing up throughout the history of the universe. Consider sexual reproduction. A simple early form of sex involved two equal sized gametes (sex cells) joining to produce a new organism. Some species still do it this way. But more commonly the symmetry is broken – some organs or organisms produce little gametes that move around easily (sperm or pollen), others produce big gametes that don’t move around so easily (eggs or ovules). We call the first sort of organs or organisms male and the second sort female. Sex in most multi-cellular organisms is a broken symmetry. This broken symmetry will go on to have a dramatic consequences for human social evolution. It entails, for example, that patrilineages can expand their size much more rapidly than matrilineages.

Or consider the rise of political stratification, the move from small-scale societies where “every man is a chief over himself” to large-scale societies of chiefs and commoners, rulers and ruled. Another broken symmetry. It may be more or less an accident (good or bad luck, Game of Thrones style) who ends up being king, but it’s not an accident that somebody is, past a certain social scale.

We don’t attach much moral significance to broken symmetries where the physical world is concerned. You’re being way too sensitive if you feel sorry for the poor weak nuclear force that missed its chance to be the strong nuclear force, or for the dwarf Gaia-Enceladus galaxy that got cruelly torn apart and cannibalized by the Milky Way. Broken symmetries in social life – males and females, kings and commoners – are another matter …

Central Dogma blues

3.79 – 3.60 billion years ago

[T]he Darwinian process may be described as a chapter of accidents. As such, it seems simple, because you do not at first realize all that it involves. But when its whole significance dawns on you, your heart sinks into a heap of sand within you. There is a hideous fatalism about it, a ghastly and damnable reduction of beauty and intelligence, of strength and purpose, of honor and aspiration, to such casually picturesque changes as an avalanche may make in a mountain landscape, or a railway accident in a human figure. If it be … a truth of science, then the stars of heaven, the showers and dew, the winter and summer, the fire and heat, the mountains and hills, may no longer be called to exalt the Lord with us by praise; their work is to modify all things by blindly starving and murdering everything that is not lucky enough to survive in the universal struggle for hogwash.

George Bernard Shaw. Back to Methusaleh. Preface iv

A broken symmetry lies at the heart of life, ruling out Bernard Shaw’s preferred mechanism of change, Lamarckian evolution through the inheritance of acquired traits. “Lamarck … held as his fundamental proposition that living organisms changed because they wanted to. As he stated it, the great factor in Evolution is use and disuse.” (Back to Methusaleh, preface ii)

In Shaw’s day, August Weismann countered Lamarck, arguing that the inheritance of acquired traits was ruled out by one great asymmetry: the separation of germ plasm (destined to pass on to the next generation) from soma (destined to perish). In modern parlance, this asymmetry is codified as the Central Dogma of molecular biology: information passes from nucleic acids (the basis of genetic inheritance) to proteins, but not vice versa. 

The Central Dogma may be a predictable, inescapable result of selection acting at multiple levels, both within and among protocells, at the very origin of life. 

We developed a model consisting of a population of protocells, each containing a population of replicating catalytic molecules. The molecules are assumed to face a trade-off between serving as catalysts and serving as templates. This trade-off causes conflicting multilevel selection: serving as catalysts is favoured by selection between protocells, whereas serving as templates is favoured by selection between molecules within protocells. This conflict induces informatic and catalytic symmetry breaking, whereby the molecules differentiate into genomes and enzymes, establishing the central dogma. 

The origin of the central dogma through conflicting multilevel selection

Earlier on Logarithmic History, I noted that broken symmetries – female and male, predator and prey, ruler and ruled – are often morally fraught. This applies to the great asymmetry at the center of natural selection. Darwin put a positive spin on it: “From the war of nature, from famine and death, most exalted object which we are capable of conceiving, namely the production of the higher animals, directly follows,” but a many people have found it hard to accept this view of life. In Shaw’s day it was perhaps still possible to hold out hope that Lamarckian evolution provided a gentler path for progressive evolution. And for much of the twentieth century, social scientists tried to quarantine natural selection, allowing it a role in human physical evolution, while sticking to a blank slate view of mind and culture. But there is one more twist in the story: in the twenty first century, the rules of the game may change entirely, as new reproductive technologies present the promise, and peril, of finally overturning the Central Dogma. 

A family tree for the sun

7.44 – 7.04 billion years ago.

No big news in the universe today. Some evolutionary thoughts: Species evolve. Do planets? stars? galaxies?

Charles Darwin didn’t use the word “evolution” often. But he did write a lot about “descent with modification,” which is pretty much what biologists mean by evolution. For example, the usual definition of genetic evolution is “change in gene frequency,” i.e. descent with (genetic) modification. And Darwin argued that all living things belong to one or a few family trees linked by recent or remote common descent.

Recently, some astronomers teamed up with some evolutionary biologists to produce a “family tree” of our Sun and some of its neighbors. The tree is based on the abundances of different chemical elements; these abundances don’t change much over the lifetime of a star, and can be thought of as a kind of inherited trait, something like DNA. The tree groups stars roughly according to their ages, with younger stars having more “metals” (elements other than hydrogen and helium), but only roughly, since other processes affect stellar chemistry.

Drawing a family tree for stars might seem like an odd thing to do. In what sense are stars related as parent and offspring? Evolutionary biologists face a similar situation where group selection is concerned. Suppose you have a population of organisms. Those organisms form groups that last for a time and disband, with their members “seeding” the population at large, and influencing group formation in the next generation. Although there are ancestor-descendant relations between individual organisms, you can’t identify any one group in generation t+1 as the descendant of any particular group in generation t. Are the groups in this case meaningful evolutionary units? It depends on which biologist you talk to.

Evolution and broken symmetries

8.81 – 8.34 billion years ago.

No big news in the universe today. Some evolutionary thoughts: Species evolve. Do planets? stars? galaxies?

Charles Darwin didn’t use the word “evolution” often. But he did write a lot about “descent with modification,” which is pretty much what biologists mean by evolution. For example, the usual definition of genetic evolution is “change in gene frequency,” i.e. descent with (genetic) modification.

However, people sometimes talk about evolution that doesn’t involve descent with modification, in contexts that have nothing much to do with biological evolution – cosmic evolution or stellar evolution in the history of the universe, for example, or mineral evolution in the history of the earth. Another Victorian writer, the sociologist and philosopher Herbert Spencer, offered a definition of evolution that might cover these cases.

Evolution is an integration of matter and concomitant dissipation of motion; during which the matter passes from an indefinite, incoherent homogeneity to a definite, coherent heterogeneity.

It’s easy to make fun of this definition. It’s the sort of abstract word pile that style manuals tell you to avoid, and that gives sociology a bad name. For that matter, it’s easy to make fun of Herbert Spencer. He may be some of the inspiration for the character of Mr. Casaubon, the dried up, impotent pedant in George Eliot’s “Middlemarch.” (Spencer probably turned down a chance to marry George Eliot = Mary Ann Evans. You should be careful about offending a writer.) But it may be that Spencer was groping toward the important modern concepts of symmetry and symmetry breaking.

A simple example: imagine you’re holding a bicycle exactly upright. The bicycle is pretty much bilaterally (mirror image) symmetrical. (OK, not really, the gears are on the right side, so it’s not a perfect mirror image. But just pretend …) Now let go of the bike. It will fall to one side or the other. The symmetry is broken, and you need one extra “bit” of information to tell you which side the bicycle is on.

Symmetry breaking is a fundamental concept in physics. In the very early history of the universe, the four forces of nature — gravitational, strong, weak, and electromagnetic – were united, but then as the universe cooled, one by one, these forces broke the symmetry and turned into separate forces. More symmetry breaking generated elementary particles, and nuclei, and atoms. When atoms first formed, they were distributed symmetrically through the universe as a diffuse gas. But gravitation pulled atoms and other particles together into clumps, leaving other parts of space emptier, and the spatial symmetry was broken (a “translational” symmetry in this case).

Symmetry breaking will keep showing up throughout the history of the universe. Consider sexual reproduction. A simple early form of sex involved two equal sized gametes (sex cells) joining to produce a new organism. Some species still do it this way. But more commonly the symmetry is broken – some organs or organisms produce little gametes that move around easily (sperm or pollen), others produce big gametes that don’t move around so easily (eggs or ovules). We call the first sort of organs or organisms male and the second sort female. Sex in most multi-cellular organisms is a broken symmetry. This broken symmetry will go on to have a dramatic consequences for human social evolution. It entails, for example, that patrilineages can expand their size much more rapidly than matrilineages.

Or consider the rise of political stratification, the move from small-scale societies where “every man is a chief over himself” to large-scale societies of chiefs and commoners, rulers and ruled. Another broken symmetry. It may be more or less an accident (good or bad luck, Game of Thrones style) who ends up being king, but it’s not an accident that somebody is, past a certain social scale.

We don’t attach much moral significance to broken symmetries where the physical world is concerned. You’re being way too sensitive if you feel sorry for the poor weak nuclear force that missed its chance to be the strong nuclear force, or for the dwarf Gaia-Enceladus galaxy that got cruelly torn apart and cannibalized by the Milky Way. Broken symmetries in social life – males and females, kings and commoners – are another matter …

Planet of the horses

18.3 – 17.4 million years ago

We’re now running through Big History at the rate of 1 million years per day.

Horses have probably been the single most important domesticated animal in human history. Also, more than with other livestock, people get attached to horses as individuals. I’m guessing that in history and literature there are more horses with individual names than any other animal. (Alexander the Great’s horse was Bucephalus, “Ox-head”; Muhammed’s was al-Buraq*; Charlemagne’s was Tencendur; Don Quixote’s was Rocinante; Gandalf’s was Shadowfax.) We’ll be hearing a lot more about horses and horse folk on Logarithmic History once we get to human history.

Being so charismatic, horses have featured in a big way in arguments over evolution. Thomas Henry Huxley (1825-1895), “Darwin’s bulldog,” knew he needed to find good evidence for evolution. When he visited the United States in 1876, he was ready to give a lecture based on horse fossils from Europe. But visiting Yale, he was so impressed with O. C. Marsh’s collection of horse fossils from the western United States, that he rewrote his lecture around it.

Henry Fairfield Osborn (1857-1935) was director of the American Museum of Natural History and a huge presence in American paleontology. He was active at a time when most scientists accepted evolution, but many weren’t so keen on Darwin’s theory of natural selection. He thought horses were a fine example of “orthogenesis,” the tendency of species to follow a fixed line of evolution, reflecting internal forces, maybe related to willpower. He thought that humans shared a migratory spirit with horses, so that anywhere horse fossils were found would be a good place to look for human fossils. This theory didn’t pan out too well. A massive AMNH expedition to Central Asia led by Ray Chapman Andrews found all sorts of wonders – dinosaur eggs, baluchitheres – but no fossil “pro-men.” Orthogenesis leant itself naturally to diagrams showing evolution from early to modern horses going in a straight line.

George Gaylord Simpson (1902-1984), paleontologist, was one of the great figures in the evolutionary Modern Synthesis that brought together Darwin’s theory of natural selection and Mendel’s genetics. There was no room for orthogenesis in the Modern Synthesis, and Simpson emphasized that the evolution of horses was a matter of adaptation to a changing environment – especially the spread of grasslands. Also that horse evolution looked more like a bush than a ladder.

Stephen Jay Gould (1941-2002) was the most widely recognized American evolutionary biologist of recent times. (For example had a spot on The Simpson’s — “Lisa The Skeptic,” Season 9.) Gould had his own take on the modern synthesis, taking the “bushes not ladders” theme for horses and other animals (including human ancestors), and pushing it a step further. According to the theory of “punctuated equilibrium” (formulated in collaboration with Niles Eldredge), species mostly change relatively little during the time they exist (evolutionary stasis). Most evolutionary change happens when a small population buds off to form a new species and reproductive isolation allows it to conserve any evolutionary novelties it has developed. This opens up the possibility of “species selection.” Applied to horses, for example, this could mean that horses were evolutionarily successful for some time not so much because individual horses were well-adapted, but because something about horses collectively (their harem mating system, maybe) made one horse species especially likely to produce new species. Both horses and primates seem to be especially prone to bud off new species:

Speciation and chromosomal evolution seem fastest in those genera with species organized into clans or harems (e.g., some primates and horses) or with limited adult vagility and juvenile dispersal, patchy distribution, and strong individual territoriality (e.g., some rodents). This is consistent with the … hypothesis … that population subdivision into small demes promotes both rapid speciation and evolutionary changes in gene arrangement by inbreeding and drift.

 * Richard Dawkins doesn’t believe that Muhammed’s horse, al-Buraq, carried him (i.e. Muhammed) to heaven and back.

A family tree for the Sun

7.44 – 7.04 billion years ago.

No big news in the universe today. Some evolutionary thoughts: Species evolve. Do planets? stars? galaxies?

Charles Darwin didn’t use the word “evolution” often. But he did write a lot about “descent with modification,” which is pretty much what biologists mean by evolution. For example, the usual definition of genetic evolution is “change in gene frequency,” i.e. descent with (genetic) modification. And Darwin argued that all living things belong to one or a few family trees linked by recent or remote common descent.

Recently, some astronomers teamed up with some evolutionary biologists to produce a “family tree” of our Sun and some of its neighbors. The tree is based on the abundances of different chemical elements; these abundances don’t change much over the lifetime of a star, and can be thought of as a kind of inherited trait, something like DNA. The tree groups stars roughly according to their ages, with younger stars having more “metals” (elements other than hydrogen and helium), but only roughly, since other processes affect stellar chemistry.

Drawing a family tree for stars might seem like an odd thing to do. In what sense are stars related as parent and offspring? Evolutionary biologists face a similar situation where group selection is concerned. Suppose you have a population of organisms. Those organisms form groups that last for a time and disband, with their members “seeding” the population at large, and influencing group formation in the next generation. Although there are ancestor-descendant relations between individual organisms, you can’t identify any one group in generation t+1 as the descendant of any particular group in generation t. Are the groups in this case meaningful evolutionary units? It depends on which biologist you talk to.