Definition of evolution and the distinction between micro/macro


(Mervin Bitikofer) #62

Well, since I haven’t yet officially retracted my own complicity, you can all still use me as your chew toy if you aren’t done getting things off your chests or there’s more good education potential.


#63

I would have made the same assumption too – I’m sure there are many scientific terms that I first learned only from creationist sources, so it’s sometimes hard to parse the accuracy levels of all of their usages. This has been good for me to read through even if I don’t understand all the details. :smiley:


(Jeff Schloss) #64

Actually “changes in allele frequency” is a widespread but increasingly inadequate definition of evolution for at least three reasons:
(1) you can have dramatic changes in genotype frequency as a result of stringent changes in selection , without changes in allele (or gene) frequency (e.g., balancing to disruptive selection)
(2) you can have inversions, translocations, changes in ploidy that are associated with heritable phenotypic changes and even speciation due to reproductive isolation, that don’t change allele or genotype frequency. For this and the above reasons a more adequate definition is “change in the genetic constitution of a population over time.” However, even this comes up short in the view of some because:
(3) there are other informational replicators besides genes and other modes of phylogenetically heritable change that many include in evolution which some do and some don’t continue to view as fully Darwinian

Moreover, although I am not a fan of “micro” vs. “macro” evolution (given demarcation issues), a number of evolutionary biologists make this distinction. And the “change in allele frequency” is not the single, standard definition given even in textbooks. [E.g, the leading intro bio text by evolutionary biologists George Johnson & Peter Raven] The term continues to be widely used in at least three contexts: heritable change over time (contra Steve Meyer’s change of time); common descent; and causal explanation of 1 & 2.


(Curtis Henderson) #65

I’d have to double-check to make sure, but I’m fairly certain I saw a reference to microevolution and macroevolution in the freshman biology text we use. Don’t hold me to that, though :stuck_out_tongue: I just remember seeing the terms somewhere I didn’t really expect.


(Chris) #66

“heritable changes”: True, I didn’t mention heritable changes explicitly although I think that is part of natural selection. A problem with natural selection by itself is that any allele that reaches 0% is lost forever and so natural selection tends to run out of variation to select. The view of the Neo-Darwinian Theory of Evolution is that mutations supply new variations (perhaps even resurrect an extinct one) for NS to work on. That’s why I included mutations in my definition.
“environmental gauntlets.”: Protective gloves? Just say natural selection, and add genetic drift if you think it’s necessary.


(Chris) #67

Sorry Paul but in this case I will take even Wikipedia as a better source than Yahoo Answers. The terms were in use before Behe wrote his book. Endler for instance used it in 1968 which was well before Behe published Darwin’s Black Box in 1996.


(Paul Nelson) #68

Chris, I was being sarcastic (please read farther back in the thread), quoting the confused individual on the Yahoo discussion board (linked above). Of course Mike Behe didn’t make up the terms.


(Chris) #69

Definition of the Theory of Evolution
Overall in this discussion there seems to be a general rejection of “a change in allele frequency in a population over time” as a definition for the modern Theory of Evolution. I sure Population Geneticists will continue to use the term “evolution” but for the sake of avoiding confusion I think they would be better using a different word.

Overall it seems most people would include common descent as part of a definition. I doubt however that we would ever get a single definition that would satisfy everyone. I will repeat the definition I gave above:

Evolution (the Neo-Darwinian Theory of) = All life on Earth is descended from a Universal Common Ancestor and the main mechanism by which this has happened is by Mutation and Natural Selection.

I have capitalised what I think are key words. I think this is close to the definitions by Kerkut and Coyne that I also quoted.


(Lynn Munter) #70

No, mutations are heritable changes. The fact that you are this confused already tells me I was right to reword, because you didn’t really understand the words you were using (the capitals were a hint).

This would be an issue with any type of selection, but luckily it only comes up when a population is dying anyway so we don’t have to worry about it as long as normal reproduction outpaces mortality.

This statement confuses me. Is there an alternative view that mutations do not result in variations? It seems pretty tautologically obvious to me that they do. Is anybody claiming that mutations don’t exist, or that resulting variations are all equally likely to survive? What is the meaning of the way you phrased this as a doubtful statement?

And yet you seem unaware that mutations is just another word for changes or variations. That’s why I played with it.

“…Gauntlets” may have been a titch too playful, but the point is that environmental pressures are the same thing as natural selection.

Since you carelessly chose to repeat this without adjusting it in any way, I’m not ashamed to point out that it would be better to say, “All life on Earth is descended from a Universal (sic) Common Ancestor and the main mechanism by which this has happened is by Meiosis and Mitosis.”


(Chris) #71

You should be, because you’ve now omitted hereditary, mutations, and natural selection. Without those common descent would be impossible.


(Chris) #72

My apologies.


(Lynn Munter) #73

They’re not really main mechanisms of descent, though, are they? Natural selection hasn’t got anything to do with selecting life based on whether it was descended from a Universal Common Ancestor.

All that’s necessary for descent from an ancestor is…ordinary reproduction.

(If you’re still confused about what point I’m trying to make, review and compare the edited version I suggested: “All life on Earth is descended from a common ancestor and current diversity is due to heritable changes filtered by the environment.”)


#74

Every 1 or 2 sentence description of evolution is going to be inadequate. It takes entire books to describe evolution, and even then there are going to be exceptions and mechanisms that may be left out. It’s a bit like trying to describe christians in one sentence, or describe western culture in one sentence.


#75

The point is that you can’t fully describe evolution in a sentence or two. Changes in allele frequencies is a good general description, but like any simplified description of any theory in science there are exceptions to the general definition, as well as nuances that have to be kept out of a short description.

That description doesn’t work, either. Most of the genetic differences between species is due to neutral drift, not natural selection. Again, any short description of evolution is bound to fail because the theory can’t be fully described in a sentence or two.


#76

This is why mutation is included as a mechanism because mutations produce new variations. For humans, each person is born with 50 to 100 mutations with a handful of those occurring in functional DNA. Continuing on with the math for the human genome, there are 3 billion bases for the haploid genome and 3 possible substitution mutations at each base for a total of 9 billion possible mutations. If each person is born with 100 mutations that means you need 90 million births to have an even chance of having all possible substitution mutations at every position in the human genome. This is just a rough calculation since there are indel variations in the human population and other differences that would slightly change the math, but that is the rough estimate. The number of possible indels is nearly infinite, as are the possible recombination events, and those happen in each generation as well.

In other words, there is a new supply of variation in each generation.


(Haywood Clark) #77

I would hope so. Change in allele frequencies over time is the definition of evolution (a phenomenon), not any theory.

There is no single theory of evolution. TheorieS of evolution are about the mechanisms that produce those changes in allele frequencies over time.


(Haywood Clark) #78

Really? Don’t the first two create new alleles?

Is there even a single case of a change in ploidy not changing any allele frequencies?

And your claim of speciation puzzles me the most! How, in your mind, might allopatric speciation possibly work, if allele frequencies are not changing independently in the separated populations?

Are there any such cases of speciation to which you can point me?


(Haywood Clark) #79

Double Hmmm…“punctuated equilibrium,” 144 results.

You do realize, don’t you, that most papers published on evolutionary biology are empirical and not theoretical, and one would expect far more mentions of theories in the latter, smaller category?


#80

True, but “a change in allele frequency in a population over time” is an extremely decent short description. And it was given by Professor Mohamed Noor and Professor Brian Hare in some online courses I took. Both are professors at Duke and they’re active researchers in evolutionary biology. Not too shabby. We can expect a robust theory like the TOE to accumulate new facts.


#81

I agree. “Descent with modification” is my other favorite, and one of the older definitions. The problem is that some people (e.g. the Third Way group) try to act as if these definitions are set in stone and completely describe the entirety of the theory of evolution. They find a willing strawman in these textbook definitions. Nonetheless, I think we should continue to use these definitions because they still communicate the core principles of the theory to those who are truly interested in learning what the theory is about.