Evolution and "jump theories"

Factual Questions
1

A friend and I were discussing evolution the other day. Out of the blue, my friend said that “nobody I’ve discussed this with can tell me how a random process can add genetic information” (or something along those lines) and that there are still massive open questions regarding evolutionary development, requiring “jump theories” and such to explain these gaps.

Now, I’ve only got a rudimentary understanding of all this–the sort you’d get from popular science books–but my understanding is that mutations are simply an error in the copying of the DNA and there’s no need to increase the complexity of the DNA strand itself. So I guess the DNA of an ant is as complex as the DNA of a human, but our codes are quite different. Is my understanding correct here?

Also, does anyone know what this “jump theory” business is about?

2

Nope – Human DNA is significantly more complex than most insects’ and less complex than a lot of other things. Flies have eight chromosomes, for example, while humans have 46. On the other hand, the domesticated cow, one of the stupidest animals on the planet, has 60. Not all organisms have diploid chromosomes like us sophisticated mammals, either. Many plants tend to have five or six or even seven copies of some chromosomes. Genetics is ridiculously complicated. This makes sense – if all DNA were equally complex, that would seem to imply that it just magically appeared that way all at once. But we know that self-replicating molecules started off rather simple, and got more complex over time through random errors in copying.

I’m guessing he’s referring to punctuated equilibrium, the fact that organisms tend not to evolve at a steady rate over time, but experience somewhat rapid spurts of change followed by longer periods of stability. That’s not really hard to explain: nature tends to produce equilibriums, and stable populations aren’t pressured to adapt until there’s a sudden change of environment: an ice age, for example, or a volcanic eruption that separates them from their favorite food source, etc.

3

OK, so then how do chromosomes get added during the copying process?

Are there two possible errors: either the wrong “letter” appears in the copied code, or an extra “letter” is added to the code itself?

(Very helpful answers, BTW. Thanks.)

4

Chromosomes aren’t nucleotides.

TGAC are the 4 letters, which correspond to nucleotides, and eachother. T couples A, G with C. Likewise, A couples with T, C with G.

So if one side of a sequence would be TGAC the other side would be ACTG. An error that went AATG, instead of ACTG in the copy process, it might change the pigment of our skin, but would not add a chromosome. Likewise, if it was ACTG was changed to ACATG through a copying error, it still wouldn’t add a chromosome, I think.
Italicised part italicised because I’m not sure of it, I’m like 60%, and that’s bad odds for me… so if someone with an education on this topic would care to confirm / deny, I’d be grateful.

ETA: Of course, that’s an insanely simplified version, because the real sequence is like a bagilionty nucleotides long…

5

Yeah, that distinction occurred to me after I posted.

Does that mean, then, that a copying error could result in an extra chromosome, sort of as a “side effect”? In other words, while the sequence of nucleotides is just as long, can the resulting number of chromosomes be increased?

But this then goes back to my original question: How can a DNA sequence itself get longer as it’s passed from one generation to the next?

6

I’m afraid I cannot answer these questions. Although I am very interested to read them.

I do have something of a completely non-scientific, ‘Ill try to give you a believable answer’ though.

Chromosomal Disorders, like Down Syndrome, aren’t uncommon.

Donkeys have 62 chromosomes, horses have 64.

We also know that Horses and Donkeys can breed, to produce Mules. So it’s possible for animals with differing numbers of Chromosomes to have offspring.

From the article about Ligers on Wikipedia…
While it has been said that hybrid animals are unable to breed, they actually can. If a male liger with uneven chromosomes mates with a female liger with uneven chromosomes, the inherited number of chromosomes will be nn.5, making a whole diploid number[citation needed] Because of the lion’s gene for being dominant, hybrid breeding decreases the dominance of the male liger, making it able to breed, but not easily.[citation needed] The fertility of hybrid big cat females is well-documented across a number of different hybrids. This is in accordance with Haldane’s rule: in hybrids of animals whose sex is determined by sex chromosomes, if one sex is absent, rare or sterile, it is the heterogametic sex (the one with two different sex chromosomes e.g. X and Y).

So it’s also possible for these hybrids to be fertile, although it’s rare.

This is as much as my mind can cobble together as a theoretical possibility…

7

Humans are sometimes born with extra chromosomes, it happens because it’s not a perfect process.

Again, it’s not a perfect process so it can certainly get longer or shorter.

8

Do we understand how this occurs, though?

Thanks again for everyone’s answers.

9

You need to clarify your terminology, first.

The genome is the sum total of genetic information that defines an individual, and consists of strands of DNA.

The genome is divided into chromosomes. In humans, there are 26 chromosome pairs (diploid) - other species can have more copies of chromosomes (up to hexaploid).

Chromosomes contain individual genes and noncoding material. Gene expression relies on the copies contained on the chromosomes and other regulatory factors.

So what causes genetic variability during DNA replication?

Transcription errors: This could be as simple as a single codon transposition, as described above. Longer sequences can be accidently repeated, or foreign DNA inserted (such as viral DNA). All these increase the information contained in the DNA. Of course, genes can also be chopped down and reduced in size.

Repair: DNA can get damaged, by ionising radiation or chemicals. Enzymes whose job it is to repair DNA can get confused, relinking unrelated bits of DNA, or changing the gene order.

Cell replication: During cell division, the paired chromosomes separate (whether meiosis for germ cells or mitosis for normal division). During this process, things can go wrong. Chromosomes can fail to separate, leading to cells with additional identical chromosome copies (XYY, XXY). This impacts on gene expression and replication. Chromosomes can simply break, or they can tangle, leading to chromosomes containing a mixture of genes, and impacted gene expression.

All these things can increase or decrease genetic information. They may or may not lead to viable individuals that have increased fitness. But they are happening all the time, and time (in conjunction with selection pressure) is what makes these things work out for the improvement of the species.

Si

10

Great reply! Thanks.

11

Ditto - very informative!

12

I’d just like to add another possible mechanism of “adding genetic material”: Polyploidy. I wouldn’t have known the first thing about this process if my SO didn’t happen to be doing a PhD in evolutionary genetics, and took the time to sit and explain things to me in words of three syllables or less, but it’s fascinating stuff.

Polyploidy seems to be especially important in plants, and occurs when two species cross-breed to produce individuals with twice (or more) the usual number of chromosomes. Voila! Instant doubling of genetic material.

13

Additionally, those arguing that mutation, as a random process, just adds noise, not information to the genome generally just don’t understand information very well, since it’s actually the case that random noise maximizes information. Look at it this way: A perfectly non-random string, like AAAAA…A only contains very little information – it can be equally well expressed by nA, where n is the amount of times A shows up. A string that is completely random, on the other hand, can’t be compressed at all (since compression relies on finding patterns in the data, and a random string has no patterns by definition), and is thus very high in information content – so, having a random process act on non-random data will generally increase information content, not decrease it, somewhat contrary to intuition, but I think rather plausible if you look at the compressibility, i.e. that a totally determined string can be maximally compressed without losing information, while no compression is possible for a random string.

14

I am relatively ignorant on the subject. I don’t remember hearing the term “jump theory”. Is that something like the plot to the science fiction book Darwin’s Radio?

15

I guess your friend never discussed this with Charles Darwin? :smack:

This is what the theory of Natural Selection sets out to explain, and it succeeds very well. Genes and DNA are how it works here on Earth, but the idea applies to every circumstance where imperfect replication occurs. If there is alien life elsewhere, Darwin’s theory explains it as well as it explains terrestrial life.

I don’t know what ‘gaps’ your friend is talking about. We may be missing some details, but the basic process was laid out 150 years ago by a man who recently celebrated his 200th birthday. It is not exactly an obscure idea.

16

One common method is gene duplication. During DNA synthesis, the copying mechansim slips (this is well understood), and accidentally inserts another copy of a gene into the genome. Under the right conditions, this extra copy is then available to be tinkered with by evolution - it’s sort of a new blank template that can be changed into something new and useful without damaging the organism.

This is just one thing that happens - there are lots more I don’t have time to get into, many of which have already been discussed. Suffice it to say your friend doesn’t know what he’s talking about.

17

One problem here is that there are multiple definitions of ‘information’, and creationists generally refuse to rigorously define which definition they are using. In a strictly mathematical sense, the amount of information in a string of digits (let’s keep it binary for simplicity) can be defined by how long the string is when it is maximally compressed. Thus a string 010101010101… can be ‘compressed’ as “write 01 and then repeat forever” and thus contains less information than an equally long strand of random 0’s and 1’s which cannot be ‘compressed’ in the same manner.

Using that definition, it is obvious to see how mutations can add information to a genome; the simplest example is a point insertion, in which a single nucleotide is inserted to make the genome one base longer: AAATTT --> AAACTTT.

But creationists tend to use ‘information’ in a much more nebulous sense which defies quantification. They use examples along the lines of "randomly changing one letter in the sentence ‘now I lay me down to sleep’ to ‘nof I lay me down to sleep’ destroys information because the sentence no longer makes sense in English. Or “A mutation that changes ‘I saw a cow’ to ‘I saw a crow’ destroys information because ‘cow’ gives you information about the sex of the animal but ‘crow’ does not.” These are bad examples because I can claim that in my personally-invented language, Englishh, ‘crow’ means a redhaired male left-handed middle-aged Baptist pedophile, and therefore the change from cow to crow has introduced quite a lot of information.

They also say things like ‘two identical copies of a newspaper don’t have more information than one copy of that newspaper’. Assuming for the moment that that was true (I disagree), the moment that one copy is distinguishable from the other, there is more information present. They will also claim that any change in a protein’s function due to mutation of its corresponding gene represents a loss of function of the protein because the original status quo has been disturbed; they therefore claim that loss of function means that the mutation must have resulted in loss of information.

I will be much more sympathetic to creationist arguments about ‘information’ when they can give a definition that is internally consistent and allows one to calculate the amount of information in a given system.

18

This highlights the contradictory nature of their conception of information perfectly, because it implies that, if two identical copies make for the same information content, and if information content decreases with random changes, then one newspaper plus a flawed copy of itself actually must contain less information than just the original newspaper, despite that original newspaper still existing untouched! (And thus, the flawed copy must, on its own, actually contain negative information!)

19

I would disagree with your post here. Darwin concluded or even postulated nothing regarding genetics or how the underlying molecular mechanism of evolution occurs. He couldn’t possibly have known anything about it. What natural selection seeks to explain is how evolution occurs at the level of individuals and species. The underlying details were not discovered until much later, and I believe the OP is addressing the details at the molecular level.

20

When the chromosomes pairs line up during meiosis , they cross over and swap pieces. Errors in this process can lead to stretches of DNA being duplicated in the resulting gametes. Some viruses can integrate into DNA and add their genes to the genome. Transposons are jumping genes that can move around in the DNA and produce multiple copies of themselves. Each of these processes can duplicate genes in the genome, and these multiple copies can then diverge by accumulating mutations and acquire specialized functions. Having multiple copies of a gene allows natural selection to drive the specialization of these two genes towards different functions.