Yet the myth persists. It persists for three reasons:
- It plays into the popular misconception that natural selection is synonymous with evolution. If a species isn't adapting by obvious changes over time then it isn't evolving. Another way of saying this is that some species can be so perfectly adapted to their environment that all changes are selected against and negative selection prevents evolution.
- External morphological changes are the only evidence of evolution.
- The so-called "living fossils" show no evidence of morphological change over millions of years when, in fact, all of the popular examples show plenty of evidence of such change. In other words, the facts are misrepresented.
Why is this important? Because it's wrong to promote incorrect versions of evolutionary theory.
Today's New York Times Book Review has a review of a new book by Richard Fortey called "Horseshoe Crabs and Velvet Worms." The review [Some Things Should Be Dead] praises the writing style and readability of the book but, like most science journalism, does not get into details about the accuracy of the text.1
While preparing this post, I discovered that Jerry Coyne had also read the review [Two New Biology Books]. Coyne has met Fortey and thinks him a "lovable bear of a man, infectiously excited about biology." However, Coyne wonders what explanation Fortey will offer to support his claim of fossil species.
Fortey has a new book, and it’s about “living fossils,” those plants and animals that have persisted for millions of years without much change in their morphology (think ginkgo tree, coelocanth, and horseshoe crab). To evolutionists, these species are a mystery: why have they remained unchanged so long? One explanation—that they simply lack genetic variation that fuels evolution—is probably wrong: work ages ago by Bob Selander and Dick Lewontin showed that horseshoe crabs are just as genetically variable in their DNA as more malleable species. Another classic explanation is that these species simply live in unchanging environments, so that they arrived at their optimal morphology eons ago and there’s nothing new to adapt to. That’s an appealing but largely untestable explanation, especially because some creatures that live in similar habitats (like the shallow marine habitats of the horseshoe crab) have undergone substantial evolutionary change.I'm not going to buy the book 'cause the only explanation I could accept would be that there's no such thing as a living fossil. I might be interested in a lengthy discussion about the different between natural selection and random genetic drift and/or a discussion about the kinds of morphological changes that have been observed recently among the four living species of horseshoe crabs but I doubt that would be in Fortey's book. Maybe Jerry Coyne will read it and prove me wrong.
At any rate, Fortey’s new book is "Horseshoe Crabs and Velvet Worms: The Story of the Animals and Plants that Time has Left Behind," and it was reviewed in Thursday’s NYT. Reviewer Dwight Garner gives it two thumbs up, and I’ll be reading it for sure, if for no other reason to see Fortey’s explanation for unchanging species.
1. This is a pet peeve of mine. The top three most important criteria of good science writing are: accuracy, accuracy, and accuracy. If a review doesn't tell me about the quality of science in a book then the review is completely useless. I don't care if the book wins a Pulitzer Prize (given out by non-scientists) for being an enjoyable read that sounds convincing to most readers. I don't judge science writing by style as the first criterion, nor do I judge it by the personality of the writer.
34 comments :
It seems to me acceptable to use terms such as 'living fossils' as long as its clear that evolution does continue in the specials, just that the changes are so minimal or of such a nature as to not be superficially noticeable. At any rate, I'd be hard-pressed to come up with readable alternatives... have you got any suggestions?
I agree with you that the changes that did take place, and what variations occurred among close relatives would be among the more fascinating things to have in such a book.
I write as the author of `Horseshoe crabs and velvet worms' which has attracted attention on this site. I am shocked that such comments should be made by a supposedly serious scientist who has not even read the book. In the book I repeatedly make the point that morphological lack of change does NOT imply no change. Here I am in Chapter 1: "Modifications are happening at the genomic level all the time. There really is no such thing as 'no change'..." I make this point again and again.I am a scientist who has published more than two hundred papers, mostly on palaeontological topics. I write my popular books to reach a wider audience, but I try very hard not to short change the science. I would like to think that the book would have been read before being the object of such strongly worded opinions.
Why Larry Moran does not think that morphological conservatism is at least as real and as interesting as changing allele frequencies is a mystery to me. But regardless of the answer, I fail to see why it annoys him to the point of attacking a book on the subject that he has not read. Gingko looks pretty much the same now as it did in the Mesozoic while the Eudicots have diversified in less time to produce plants as different as an Oak tree and a Buttercup. This is a real issue and it is interesting and important. It's also weird to hear this vitriol from a fan of Stephen Jay Gould, who was all about stasis in the fossil record.
I'm delighted to hear that you make such a point in your book. I assume you begin by pointing out that the subtitle "The Story of the Animals and Plants that Time Has Left Behind" is misleading since lots of evolutionary change has happened over the course of millions of years.
I assume you go on to point out that evolution is much more than just morphological change so that it's quite wrong to say that these species have not evolved for millions of years. Do you give your readers a proper textbook definition of evolution in those first few pages?
If so, then what's the point of the book? If horseshoe crabs have evolved just like all other living species then why single them out for special attention?
While you're here, perhaps you could answer another question. Your "Survivors" TV show focuses on the reason why certain species survived mass extinctions. For example, according to the Natural History Museum Website ...
What adaptations helped the horseshoe crab?
So, what is it about horseshoe crabs that enabled them to survive? ‘Being able to feed on almost any organic matter helped,’ says Fortey.
'And, they have a special kind of blood, which is blue! It coagulates when it encounters bacteria. They can 'wall up' any wounds they receive.'
Another key to their survival seems to be their tolerance of habitats that fluctuate in salinity (levels of salt). When environmental changes happen, they can move to safety.
An ability to live with low levels of oxygen is also important. Fortey adds, ‘The horseshoe crab was able to cope with periods of oceanic deoxygenation that were fatal to many marine organisms.’
Does that mean you reject David Raup's view that surviving mass extinctions may just be a question of chance?
Do you have any scientific evidence showing that the survivors were better adapted than the losers?
I'd also like to point out a potential problem with the Natural History Museum website because they say the following about "Survivors": "Discover the history of life on Earth through the stories of living organisms that have survived unchanged for millions of years." You might want to contact them to correct that statement since you now say that lots of change has taken place.
There also seems to be a problem with the way your museum explains How Does Evolution Work. They seem to have neglected to mention any mechanism other than natural selection. Do you agree with their definition of evolution?
You make a good point about my criticism of a book I have not read. On the other hand, we have to make decisions all the time about whether to buy a book or not (the price is $35). In this case, I based my decision on information I've heard about your TV show and your other writings. I also based it on the subject matter as described in the title and subtitle of the book.
Because you took the time to comment, I'll order a copy as soon as it becomes available in Canada. Like Jerry Coyne, I'll blog more about your scientific explanations of evolution and morphological change after I've read it.
Dear Sir
Very nice article.As I am a blogger and use to write some science articles in my blog i need your permission to translate this article and present it in my blog.
Thank you.
Why Larry Moran does not think that morphological conservatism is at least as real and as interesting as changing allele frequencies is a mystery to me.
There's no mystery here. The first step is admitting that "morphological conservation" does not mean that there have been no changes in morphology. It means that the changes haven't been as remarkable as in some other lineages.
The second step is admitting that evolution affects lots more than just external morphology so you also have to look at other changes as well. The data shows that genes and proteins have evolved just as much in so-called "living fossils" as in all other species.
The third step is understanding what kind of stasis Gould was referring to in punctuated equilibria. He's talking about speciation and stasis that lasts only for the life of a species.
The conclusion is that all living species have evolved over time and there's no such thing as a species that hasn't evolved for a hundred million years or more.
I have no problem with a discussion of why some species look superficially the same in the fossil record as species that lived a long time ago. You could begin with various bacteria, such as cyanobacteria. Such a discussion could be very informative as long as it's not confined to adaptationist explanations that have no supporting evidence.
I don't think you're characterizing Dave Raup's ideas very well. The options aren't really "bad genes or bad luck". Pure bad luck is the "field of bullets" view, which Raup doesn't hold. A better way to say it is that the selection conditions during mass extinctions are quite different from those that pertain at other times, so the luck involved is to have the proper pre-adaptations. As Fortey says, one of those might be a tolerance for low-oxygen environments. Or, in the K/T extinction, not being too big. Possession of those characters that are advantageous at odd times, while lucky, is probably the consequence of prior selection, though obviously not selection for survival of mass extinctions.
Pure bad luck is the "field of bullets" view, which Raup doesn't hold.
Yes he does, although he's careful to suggest that the truth may often lie somewhere between bad genes and bad luck. The point is that it's not necessary to postulate adaptive explanations for every survivor of mass extinctions. Sometimes the tape of life may replay entirely differently.
I look forward to seeing how Richard Fortey discusses these alternative viewpoints in his new book.
I've always hated the "tape of life" metaphor. But if we acknowledge that there's a stochastic element in any extinction, Raup's book (the one alluded to above) shows many instances of non-random patterns in mass extinctions, which would not happen in the "field of bullets" case. When he says "bad luck" he is indeed generally talking about the bad luck of not being adapted to conditions that only recur at timescales beyond those selection can track. Read that book again. In fact a field of bullets is highly unlikely to wipe out any speciose clade. We need nonrandom extinction to explain any taxonomic extinction patterns.
On the other hand, Raup uses "bad genes" to refer to competition or failure to adapt in ordinary circumstances.
Larry said: "The third step is understanding what kind of stasis Gould was referring to in punctuated equilibria. He's talking about speciation and stasis that lasts only for the life of a species."
I'm not sure exactly what point you are trying to make. But if you are saying that Gould and Eldridge were were not using "stasis" and "punctuated equilibrium" to refer to the rate or amount of evolution, then you are wrong.
From the abstract of their 1977 paper (first 2 sentences): "... evolution is concentrated in very rapid events of speciation (geologically instantaneous...). Most species, during their geologcal history, either do not change in any appreciable way, or else they fluxtuate mildly in morphology, with no apparent direction."
It is quite clear that they use the term stasis to refer to the pace/speed of evolution, and that their measure of evolution is "change in morphology".
They explicitly contrast their theory of punctuated equilibrium with that of phyletic gradualism, the idea that changes in morphology occur relatively continuously and gradually.
"A change in morphology over time" was the first meaning of the term "evolution" in the field of biology, and still acceptable (and common) today.
"If so, then what's the point of the book? If horseshoe crabs have evolved just like all other living species then why single them out for special attention?"
As Anonymous (apparently Dr. Fortey) says above: "I write my popular books to reach a wider audience…”. I think that was the point of the book. But to do that successfully, and be able spread knowledge and interest in biology and evolution, there seems to be a need for catchy titles and subtitles as well as an interesting overall perspective. A book saying that "horseshoe crabs have evolved just like all other living species" is unlikely to sell and spread the knowledge. Isn’t that right Dr. Fortey?
Thanks to all those who have replied to my initial 'squeal' about being taken to task for things I did not actually say in what I hope is a carefully written, but still entertaining book.I'm afraid television does cut out 'boring' caveats, and perhaps I should draw the lesson to avoid such entanglements in the future.But most TV presenters know practically nothing, so I truly hope some viewers will at least have the sense of engagement with a proper scholar. I have also written elsewhere about why the 'tape of life' metaphor has nothing to do with science and everything to do with metaphysics (so thank you, John Harshman). I do know Raup's "bad genes or bad luck" paper, of course, but I am not sure that he dealt with the idea that persistence of certain habitats might have been the way of cheating the statistics! And I do reassert the contention that endurance of morphology IS remarkable, and indeed even more so if genomic changes are boiling away underneath. In the book I do quote work on the genome of the tuatara, for example, which shows how change has been happening and happening at that level while the animal itself is almost inertia encapsulated! I think - as my old biology teacher would have said - that this is "jolly interesting". If I might add another comment from a personal angle, for many years I have been coping with attacks from intelligent designers (I originally coined the term IDiots) and naturally it makes me wince horribly to think that somebody might, possibly, think that I could offer succor to this mindset.
Sorry for throwing in yet another metaphor. What is some species have arrived at their morphological optimum in the adaptive landscape (the peak in morpho-space)? The genetic or physiological change boiling on underneath will then be all for maintaining that morphology (as long as this remains the optimal morph). Thus change at the lower levels and stasis at the morphology will not even be contradictions
How about? "In terms of their biochemical, molecular, and cellular functions, horseshoe crabs have evolved and adapted just like all living species. The appearance of modern species of horseshoe crabs make them easily distinguishable from the fossil species. However, in terms of external morphology, they have not changed as much as some other species."
Doesn't it all depend on what point you're trying to make with the general public? In my case, I would emphasize the point that evolution is inevitable and that there's much more to evolution than just adaptation and external morphology. Those are the important points about evolution that the general public doesn't understand.
I've always hated the "tape of life" metaphor.
Of course you do. The tape of life metaphor emphasizes accident and contingency in the history of life and those aren't your favorite explanations.
When he says "bad luck" he is indeed generally talking about the bad luck of not being adapted to conditions that only recur at timescales beyond those selection can track. Read that book again.
Raup wants us to think about as many possibilities as possible and this includes the idea that a species may survive a mass extinction just because it was lucky. For example, a species that was confined to the Yucatán Peninsula may have had no chance of surviving the K-T extinction event while one that happened to be living on the bottom of the ocean in the South China Sea may have survived.
Here's where he makes that point (p. 105).
Extinction is apparently selective to some degree and in some instances, but proving this is devilishly difficult. Whatever the actual level of selectively, it is never very prominent. At every turn in the search for good causes, we are blocked by ignorance. Or we run the risk of letting our enthusiasm for answers cloud good sense. But this adds to the fun and challenge of the whole enterprise. How nice it is to be in one of the most difficult sciences—as opposed to one of those commonly called the hard sciences!
My goal is to try and make sure that the public is exposed to the fun and challenge of science. That means resisting efforts to dumb it down and allowing enthusiasm for a particular explanation to cloud good sense.
Richard Fortey says,
I have also written elsewhere about why the 'tape of life' metaphor has nothing to do with science and everything to do with metaphysics ...
I'd be interested in seeing that article and reading about the alternative that you support.
And I do reassert the contention that endurance of morphology IS remarkable, and indeed even more so if genomic changes are boiling away underneath.
I agree that it's interesting. I'm looking forward to reading your explanation. (BTW, I don't like your choice of metaphors to describe all the adaptations that took place in the cells, internal organs, and molecules of these species.)
I originally coined the term IDiots.
That's really interesting. We've been using the term on talk.origins since the mid 1990s. When did you start using it?
... it makes me wince horribly to think that somebody might, possibly, think that I could offer succor to this mindset.
You're probably referring to my earlier posting when I criticized you for saying that evolution allows some species to remain basically the same. Evolution is defined as a change in the frequency of alleles in a population so what you're saying is that no change is also an example of evolution.
I can understand why you wince! :-)
“Doesn't it all depend on what point you're trying to make with the general public? In my case, I would emphasize the point that evolution is inevitable and that there's much more to evolution than just adaptation and external morphology.”
Yes, it does, but that approach might be more limited: there are only a few truths out there and it requires lots of imagination and writing skills to make it interesting. However, your suggestion in this particular case is great: ‘Evolution is Inevitable.’ A book with this title and content might outsell any horseshoe crabs books regardless of their catchy subtitles.
What about experimenting with ‘Evolution is Inevitable' using a novel writing approach, the blogging approach: you outline the book on your blog, and invite colleague bloggers to jump in as coauthors.
The tape of life metaphor emphasizes accident and contingency in the history of life and those aren't your favorite explanations.
Are you joking? What is there accidental or contingent about rewinding a tape? No matter how many times you rewind, another play still gives you the same result. It's a bad metaphor because it doesn't resemble the thing he's trying to describe. And who says I don't like accident or contingency? You seem to be doing what a lot of annoying talk.origins posters do, squishing the opinions of everyone who disagrees with them on anything into a huge, amorphous ball.
Your Raup quote is fine, but doesn't support your point. Raup uses "bad luck" for too many things, including the bad luck of being poorly adapted to the brief conditions of an asteroid impact. That is, some bad luck is a form of bad genes. But he reserves "bad genes" solely for the idea of poor adaptation to long-term conditions. And I think you're confused by his terminology into thinking that he advocates the "field of bullets" scenario. Obviously there's a stochastic component to all extinctions. My point is that Raup is clear that's not the only component, and I would argue he thinks it isn't even the dominant component.
I too oppose dumbing down science, but then again I accept that education is a process of controlled lying. You have to give the big picture before you can get to the qualifiers and exceptions.
What is there accidental or contingent about rewinding a tape? No matter how many times you rewind, another play still gives you the same result. It's a bad metaphor because it doesn't resemble the thing he's trying to describe.
From Wonderful Life page 48 ...
I call this experiment 'replaying life's tape.' You press the rewind button and, making sure you thoroughly erase everything that actually happened, go back to any time and place in the past—say, to the seas of the Burgess Shale. Then let the tape run again and see if the repetition looks at all like the original.
I too oppose dumbing down science, but then again I accept that education is a process of controlled lying. You have to give the big picture before you can get to the qualifiers and exceptions.
I agree. The big picture is that the history of life is a product of evolution and many other factors. The net result is a haphazard and, largely accidental, collection of species that just happen to include us.
It's really cool to focus on a few strange animals that may look a lot like their ancient ancestors but let's not forget that the vast majority of species are microscopic. Let's also not forget that evolution cannot be stopped and it may be very misleading to focus on a few minor characters and imply that this represents all of evolution.
John, do you think that an emphasis on living fossils is a good way to teach evolution to a general public that is mostly creationist? Do you think it helps them understand the correct big picture?
There is more to evolution than natural selection. Even if there were species where every single enzyme is perfectly adapted to the current conditions, there would still be almost as much evolution going on as before the species reached the adaptive peak.
If you don't understand this then you don't understand the problem. Read Evolving Humans.
“However, your suggestion in this particular case is great: ‘Evolution is Inevitable.’ A book with this title and content might outsell any horseshoe crabs books regardless of their catchy subtitles.”
Well, I might have to second guess my comment that a bloggers’ book with the title ‘Evolution is Inevitable’ might outsell the book under discussion here, "Horseshoe Crabs and Velvet Worms: The Story of the Animals and Plants that Time has Left Behind," by Richard Fortey.
I had to open my big mouth this morning, telling my wife, Becky, about the comment I made. She said:
“You don’t know what you are talking about!! Richard Fortey is a genius!! By implying that some animals and plants do not evolve, he hit the jackpot!! Don’t you know that the most people don’t believe in evolution!! Not only will they buy the book for themselves and their grandchildren, but for all their friends and neighbors!!”
I did mumble something about the content of the book, which is about evolution (the slow kind, apparently), but it was too late… my confidence was dissipating, so I quickly changed the subject… and it worked! What a great idea! (I hope she doesn’t read this comment).
""A change in morphology over time" was the first meaning of the term "evolution" in the field of biology, and still acceptable (and common) today."
Nope. This debate is just emphasizing inter alia that this very popular meaning of the term "evolution" is no longer scientifically acceptable. It had it's time at an epoch one had little idea (if any) about the genetic basis of morphological changes, by now that meaning has already turned outdated and misleading. Evolution is the (continuous) process of changes in alleles frequencies over time; since external morphology is essentially determined by a subset of the genes, any change in morphology implies a change in the genome (-> evolution), but the reverse is obviously not true.
The fossils usually cannot give us a hint about changes in the whole genome but only about those changes with visible morphological effects. Nevertheless, if we want to guesstimate a kind of "evolution speed" over the aeons, we have no choice but to rely on morphological data retrieved from fossils: it has to be taken "as is" (the alternative would be to say we have no idea about how slow/fast evolution proceeded). From the data, one can infer a rate of morphological changes, that is a rate of meaningful change in a very particular restricted subset of an evolutionary unit's genome. On the overall, considering larger number of species and/or longer periods of time, the statistical factor partially cancels out the uncertainties and the compared rates of morphological changes might be carefully taken as pretty loose measures of evolution speeds: it's the best we can do at present and the only fairest meaning we can attach to the determined "speed of morphological changes over time".
As a side note, I'll stress that Gould and Eldredge wanted to put the emphasis on the apparently dramatic variations of morphological changes rate over time, it was their "scoop" theory and it needed to sound very shaky: the word "evolution" stands for it. For the same reasons, they also might have neglected to some extent in their arguments that evolution is more than simple change in morphology (due to subsequent change in a part of the genome). Anyway and with all due respect, the fact that Gould and Eldredge wrote something which could loosely being interpreted as equating evolution speed with morphological change rate does not imply the absolute truth and infallibility of the statement.
John, do you think that an emphasis on living fossils is a good way to teach evolution to a general public that is mostly creationist? Do you think it helps them understand the correct big picture?
No. But is anyone suggesting such a thing?
And how does quoting Gould's original statement of the tape metaphor address its inappropriateness?
John, I quoted Gould's original version because he specifically says that you "thoroughly erase everything that actually happened" then start over.
You said, "No matter how many times you rewind, another play still gives you the same result."
Now I'm confused. What did you mean?
"... this very popular meaning of the term "evolution" is no longer scientifically acceptable." ... "Evolution is [exclusively] the (continuous) process of changes in alleles frequencies over time"
Just cause you say this (and want this) does not make it so. Go read the scientific literature in the broad field of biology and see how it is used, and then get back to me. (You will find that the common element in all uses is "change over time", whether the topic is changes in allele frequency, morphology, molecular structure, accumulation of mutations, etc.)
There are three elements required in any accurate definition of evolution.
1 change over time
2, the thing that changes must be heritable (genetic change)
3. it's the populatiion that changes not the individual
A change in morphology over time could be an example of evolution and, in practice, we often assume that the change is due to underlying changes in allele frequencies. However, you can get fooled. For example, Europeans have gotten taller and more robust over the past several hundred years but that's almost certainly not evolution.
I meant what I said: the tape metaphor is a bad one, exactly because it doesn't fit what Gould is trying to communicate. Rewinding a tape doesn't erase it. And if you do erase a tape and play it again, you get nothing, because you erased the tape, dammit. Now you could erase a tape and re-record something else on it, but that isn't a good metaphor either. There really is nothing about tape that could be arranged to fit what Gould wants.
This is a stupid rhetorical point, irrelevant to any real evolutionary question, but it does annoy me that you don't get it.
gah! He's referring to the way that tapes actually work. If you rewind a tape, making sure to erase it, and then replay it, you get nothing but static. That's why he thinks it's a silly metaphor, not because it's "his favorite explanation." It's embarrassing to read your stuff sometimes. Check the beam in your own eye before you go accusing others of riding hobbyhorses.
"If horseshoe crabs have evolved just like all other living species then why single them out for special attention?"
Because [he explained patiently] they have not evolved "just like all other living species;" rather they show a quite remarkable and unusual conservatisim in external morphology.
"However, in terms of external morphology, they have not changed as much as some other species."
There you go! Except that they have changed much less than nearly all kinds of animals for which we have adequate fossil records to judge. And, as Dr. Fortey correctly points out, that's interesting, at least to those of us who think organisms are interesting. (I realize that your enthusiasm is reserved for molecular phenomena.)
"In my case, I would emphasize the point that evolution is inevitable and that there's much more to evolution than just adaptation and external morphology. Those are the important points about evolution that" are my personal hobbyhorses.
fixed it
"all the adaptations that took place in the cells, internal organs, and molecules of these species"
How about some examples of adaptive changes in the cells and internal organs of horseshoe crabs for which you have scientific evidence?
Thanks. Now I get it. When Gould says, "Then let the tape run again ..." he should have said "Then let the tape record again ..."
Is that it? Is that what all the fuss is about?
Don't you have anything better to do?
Is that it? Is that what all the fuss is about?
It's what that particular bit of fuss is about, sort of. As I said, erasing and recording again is also a poor metaphor. Best really to forget all about the tape thing.
Of course I have something better to do. But this is what the internet is for. And you chose to focus on the tape metaphor to the exclusion of anything else we were arguing about. I personally think the "living fossil" trope is fine as long as you add a list of caveats (not the same species as ancient ones, applies only to morphology, not molecular evolution, we tend not to notice differences in taxa the farther they are from us). Some groups appear to be considerably more morphologically conservative than others, and it's an interesting question why that is.
From Wonderful Life page 48
One of my favorite books of all time, and I hope (though I lack the education to know) an example of how accurate writing about evolution can be popular.
Among innumerable other fascinating topics in the book Gould communicates very well his great interest in the subject of contingency as applied to evolution.
"Just cause you say this (and want this) does not make it so. Go read the scientific literature in the broad field of biology and see how it is used, and then get back to me."
It's not my personal view but a very largely shared opinion you should be aware of by now. Yes, when speaking generalities, the term "evolution" is usually presented basically as a change over time:
"evolution The gradual process of change that occurs in populations of organisms over a long period of time. It manifests itself as new characteristics in a species, and the formation of new species." (E. Owen & E. Daintith - Dictionary of Evolutionary Biology, Facts on File 2004)
... but if one wants to refine the explanations, one has to mention genetics:
"Biological evolution is the process of change and diversification of living things over time, and it affects all aspects of their lives—morphology (form and structure), physiology, behaviour, and ecology. Underlying these changes are changes in the hereditary materials. Hence, in genetic terms evolution consists of changes in the organism’s hereditary makeup." (J. P. Rafferty ed. - New thinking about evolution, Britannica Educational Publishing 2011)
... and the new definition and its popularity are crystal clear:
"Evolutionary change is a populational process: it entails, in its most basic form, a change in the relative abundances (proportions or frequencies) of individual organisms with different genotypes (hence, often, with different phenotypes) within a population." (D. Futuyma - Evolution, Sinauer 2005)
or:
"Most biologists define evolution as a change in the proportion of alleles (different forms of a gene) in a population." (J. Coyne - Why Evolution Is True, Oxford U.P. 2009)
or, if you prefer French:
"La génétique des populations a développé une formalisation de l’évolution à l’échelle du génome en la considérant comme la variation des fréquences alléliques au cours du temps, au sein d’une population." (F. Brondex - Evolution. Synthèse des faits et théories, Dunod 2003)
On another hand, I did not use the word "exclusively" and were I to use a determinant, it would be rather "ultimately" or "essentially". You should stick with me on this.
"A change in morphology over time could be an example of evolution and, in practice, we often assume that the change is due to underlying changes in allele frequencies. However, you can get fooled."
Of course. I assumed we were speaking about inherited (-> genetically determined) traits, otherwise one could call "evolution" the accumulation of individuals with broken legs in a population. :-)
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