February 12, 2007 9:12 PM   Subscribe

Evolutionary Mechanisms Filter: Help me argue evolution with a friend, there's

My friend stopped 'believing' in evolution when she took high school biology. She says that because 2 organisms who are different by 2 chromosomes (or possibly more than a 2 chromosomal difference, i'm not that up to speed on my biology) can't breed, she finds it's highly unlikely evolution could have occurred. She says she agrees that 2 organisms could mate and produce an offspring that isn't capable of breeding with it's parents because it has a mutation and 2 chromosomes are now different. But she thinks the chances of the same mutation happening twice, close enough to allow the two offspring to breed and propagate a new species just isn't probable enough to let her believe in evolution. So my question is, how can evolution occur? Given this scenario, it does seem improbably that evolution could occur. I remember vaguely reading that sometimes organisms could breed if they had 3 or 4 or more chromosomes different, but I can't remember where I read that. Direct answers are preferred, but I'd be appreciative of links to sites that explain specific points in detail, I'm a fairly quick learner. I did try to google this, but I mostly pull up sites arguing evolution vs creationism. I'm not really looking for a huge debate, just an answer to this specific problem. Thanks! (Also, if I didn't make myself clear enough, let me know and I'll try to explain better)
posted by sepsis to Science & Nature (24 answers total) 5 users marked this as a favorite
i would simply point out the vast number of animals and plants that have been developed via cross-breeding.

maybe send a photo of a different one every day, until they admit that maybe, just maybe, if high school biology disproved it, that somebody would have noticed by now.
posted by Tacos Are Pretty Great at 9:28 PM on February 12, 2007

Also, I could point at things like the differences between animals and people of even a few centuries ago and today, to show the obvious differences.
posted by Tacos Are Pretty Great at 9:29 PM on February 12, 2007

First of all, you are both labouring under the intense misconception that all reproduction is sexual. This is far far far from the case.

Secondly, in some organisms, such as plants with two chomosomes total, the mutation to four has been observed and does indeed happen often enough that the new species can sometimes find a mate, while unable to breed with the parents.

Lastly, this whole concept that the dividing line between species is whether they can interbreed with the parent species, is an oversimplification and really a fallacy. For example, there are species (or sub-species or varients - the point here is that interbreeding can't tell you if they're species or varients) where species A can breed with species B, but not with C. Species B can breed with C but not with D. C can breed with D but not with A, and D can breed with A, but not with B.

One final note: Evolution takes a long time precisely because these things are improbable. If they were probable, it would happen a million times faster. And even happening a million times faster, it would still seem so slow as to be static to most human observers.
posted by -harlequin- at 9:31 PM on February 12, 2007 [2 favorites]

Perhaps another misconception going on here - it sounds like she thinks that interbreeding with the parent only ceases to be possible (and thus a new species arises) when the chomosome number changes. This is false. Pick any number of chomosomes, say 30, there probably thousands and thousands of species with that number of chromosomes. Some of them plants, some of them animals. They can't interbreed - they're completely different lifeforms.

- Evolutionary changes don't need to involve a change in the number of chromosomes.
- Having the same number of chromosomes does not mean two species can interbreed.
- While evolution _can_ coincide with changes in chromosome numbers, it doesn't have to. There are many many other ways it can occur.

Chromosomes are taught in highschool because they're an easy concept, and highschool biology is really just an over-simplication designed to outine the bigger picture, but which is not particularly accurate, and leaves plenty of holes.
posted by -harlequin- at 9:44 PM on February 12, 2007 [1 favorite]

The only known living Wholphin, a bottled-nosed dolphin/false-killer-whale hybrid, had a baby. I can't find any info on chromosome count there, but it's definitely a fertile cross-bred mammal.
posted by 0xFCAF at 10:09 PM on February 12, 2007

More importantly you can point it out working in the real world with a bottle of antibiotics and a strep throat. Just ask her what happens when you don't take all the antibiotics. She knows the answer, which is that all the bacteria aren't killed and a drug-resistant strain comes about. Its not that improbable at all. Evolution at work. Natural Selection.
posted by Ironmouth at 10:13 PM on February 12, 2007

I think a lot of people here are missing the point. I think what she's saying is that the Creationist concept of "kinds", irreducible lines of animals who may evolve slightly, but who had unique origins, is implied by the fact that they have different numbers of chromosomes.

What I think she is thinking is that there's no way for the number of chromosomes to change with time, and thus species who have different numbers of chromosomes cannot possibly be descended from a common ancestor.

Polyploidy is mentioned above as a counter example. It works in plants; it's possible to induce it using a chemical called colchicine. It generally makes plants very robust, but it turns out to be fatal in chordates.

However, there are other ways for the number of chromosomes to change, especially in order to rise. That's as a result of reproductive mistakes where one additional copy of a chromosome is included in a gamete (i.e. in humans to include 24 instead of 23).

In some cases doing that causes negative consequences, as a function of which chromosome ends up tripled in the resulting zygote, who has 47 chromosomes instead of 46. That's known as "trisomy", one of the cases of the more general term "aneuploidy", which refers to any case where the number of chromosomes is wrong.

It can result in miscarriage (trisomy 16 in humans). It can cause non-fatal defects; Down Syndrome is trisomy 21 e.g. the result of having an extra copy of chromosome 21. It can have unusual effects that don't represent handicaps. (See this recent thread.) And in some cases it can have almost no effect at all, to where it isn't detectable in the phenotype.

Aneuploidy breeds true, by the way, and follows the inheritance pattern of "dominance", which is to say that half the children of an aneuploidy parent will have the same chromosome extra or missing. (When it come sex chromosome mistakes in a male parent it's a bit more complicated than that. A "supermale" XYY will have half his kids normal, a quarter XXY (Klinefelter's syndrome) and a quarter XYY ("supermale").

Current theory is that the majority of speciation branching events are caused by geographical isolation, usually of a small breeding group, and that part of that usually is the fact that there will be, purely by chance, a substantial genetic difference on average between that small group and the parent stock from which they came. If they're isolated from the parent stock for long enough, then evolutionary pressures can cause the species to diverge enough so that when they once again come into contact they don't interbreed, which by definition means they're separate species. But your friend would claim that they would still have the same number of chromosomes, so they would still be part of the same "kind".

Not necessarily. If a benign case of aneuploidy is present in that small group (which can be as small as a single breeding pair) then it can breed true in most of the offspring and become established. A single extra chromosome eventually becomes two of them, and mutation eventually makes them change from the other pair they were originally copied from. Now the species has one more pair of chromosomes than the parent stock.

Or it can go the other way: a missing chromosome (monosomy) can become a missing pair, if by chance all essential genes on it managed to get copied onto other chromosomes so that nothing essential got lost. And then you have one less pair of chromosomes than the parent stock.

Missing chromosomes don't necessarily lead to death -- and don't necessarily lead to sterility. Donkeys have 31 pairs of chromosomes; horses have 32 pairs. They can crossbreed. When the father is a donkey the offspring is known as a "mule". When the father is a horse, the offspring is known as a "hinny". In both cases the animal has 63 chromosomes; 31 pairs plus one extra.

What many people don't realize is that a small percentage of mules can breed. Most are sterile, but not all. In that particular case they're not very good breeders, but in other cases of aneuploidy they might well be normally fertile.

So it is possible evolutionarily speaking for the number of chromosomes, and thus the number of chromosome pairs, in a line of animals to change, and therefore the fact that different species of animals have different numbers of chromosomes doesn't prove they're not related and don't descend from common ancestors.
posted by Steven C. Den Beste at 10:26 PM on February 12, 2007 [3 favorites]

It also sounds like your friend thinks that mutations consist [solely or at least primarily] of changes in chromosome number. Those changes do occur [and they occur with some frequency in certain kinds of plants] but much of evolution consists of much smaller changes: changes in a single base ("letter") or in several bases of DNA, which alter the way a given gene works, or whether it works at all. During the production of eggs & sperm or during the division of single-celled critters, certain cellular processes can also change which versions of a given gene get passed on to the offspring; this also affects the process of evolution.

Sometimes these changes and mutations are fatal or cause illness [diseases like sickle-cell anemia], or result in offspring that can't breed. But there's a lot of cases where this doesn't happen: sometimes we only need one working copy of a gene, sometimes changes are mostly neutral [like hair color] or even beneficial, etc. In many genes, the alleles (versions) of the genes don't need to match. Remember, we've got two copies of each gene [excepting certain genes found on the X & Y chromosomes in men.] Lots of mutations occur on recessive genes or in DNA that doesn't currently code for genes - these mutations may eventually result in evolutionary changes as well. Similarly, extra copies of certain chromosomes in a given organism may not necessarily hurt, which is why some plants with extra chromosomes survive. Mutations that happen on those extra chromosomes might end up creating some cool new altered genes without affecting the overall viability of the plant. Nature makes good use of the redundancy of our genetic info! Finally, as -harlequin- says, this stuff can happen on a short timescale [days and months for bacteria] or on a very long timescale [millions of years for many large & complex species]: plenty of time for small changes to accumulate and unlikely things to happen.

It sounds like your friend had a crappy high school bio course and didn't really understand what genes are and how they're passed on or altered. A decent textbook [I used Campbell's in high school, though it's probably overkill here] would help explain these things to her. I suspect that clear illustrations of the cell division process & an explanation of the nature of DNA and of simple Mendelian genetics would really help.
posted by ubersturm at 10:31 PM on February 12, 2007 [1 favorite]

Sites with a lot more useful info.

Darwin's famous book The Origin of Species makes the case for evolution of new traits through natural selection by comparing it (at length) to the breeding of new traits in domestic animals (pigeons, livestock...) by artificial (human-controlled) selection. Maybe she could take a look at that.

Also, for the love of mike, can't her high school bio teacher answer this?
posted by LobsterMitten at 10:55 PM on February 12, 2007

sorry, that sounded testy when I didn't mean it to at all; reading The Origin of Species and talking to the teacher were sincere suggestions, not snark
posted by LobsterMitten at 11:01 PM on February 12, 2007

Sort of summing up the other excellent answers you've received, what she doesn't understand is that speciation is a much more gradual process than she is guessing. If an offspring overnight has a mutation that gives it two extra chromosomes, it's most probably not going to find a mate it can successfully breed with. However these additional chromosomes aren't added overnight -- they are the sum of smaller scale mutations over time. These more easily spread through a population, gradually over time giving the population more or less chromosomes and constituting a new species. Very long term spatial separation could even isolate that group from another group so the new species and its ancestor species could co-exist for a time. At that point, the new and old won't be able to breed, but there will be thousands of generations between them, not just one.
posted by ontic at 11:17 PM on February 12, 2007

So my question is, how can evolution occur?

Not only by mutation. One way it has occured is by acquisition of biological structures with incidentally their own genetic material from other species. Another way is populations being separated and gradually diverging. I don't think that necessarily implies mutation.
posted by Listener at 11:30 PM on February 12, 2007

Listener, eventually divergence of separated populations is indeed due to mutation.

Acquisition of genetic material from other species is virtually unknown in chordates. (That's primarily something that happens in bacteria.)
posted by Steven C. Den Beste at 11:39 PM on February 12, 2007

OK, not to insult your friend, but she may want to ask herself what the chances are that she spotted the Achilles' heel of evolutionary biology in high school science class, and that this weak point eluded the Stephen Jay Goulds of the world? Without even looking at her question my immediate assumption would be that the material was not presented in enough depth at the high school level to answer her question, and as others have said, that why one should ask questions in class...
posted by mattholomew at 2:51 AM on February 13, 2007

This whole discussion reminds me that a 'little' knowledge is dangerous. Firstly your friend is either misunderstanding her biology teacher or her biology teacher is munging her explanation. A biology teacher is unlikely to have pointed out any weaknesses in evolutionary theory.

Arguing the facts when you have close to none is rather pointless, and will quickly become frustating. A quick google may not be enough to settle this. I would recommend "The Blind Watchmaker" by Richard Dawkins as the evolutionary biologist's reply to alternative theories. In fact virtually any book by Richard Dawkins will probably deal with any 'supposed' flaws in the theory of evolution. Particularly our own inability to comprehend what is 'improbable' when you factor in the immensely vast timescales that are involved, and some of the mis-statements of 'fact' by creationists and others.

Hope that's a pointer anyway.
posted by sdevans at 3:55 AM on February 13, 2007

So let me get this straight... she doesn't understand it so she quit believing it? Whoo.. here's an argument for Darwinism. That is going to really be a handy habit in the information age.

Try this.... genes seldom act alone. It's not necessary for TWO genes to simultaneously mutate to get a morphological expression of a trait. (e.g.; green eyes). Her sticking point seems to be related to the unlikely simultaneous paired identical mutation of a single gene. I agree, that would be LOW probablility... non-zero,but low. SHe is under the impression that both have to change for a trait to be expressed. Apparently, so are you.

Species don't spontaneously come into being. The most 'fit' are gradually 'chosen' through environmental pressures and become dominant. Sometimes 'most fit' is a really subtle thing. Over time, a trait becomes dominant in a population. Over millions of generations (sometimes a lot less) piles of these subtle characteristics add up and you get something substantially different than what existed at the arbitrary point where you started keepint track and WE call it a new species.

Does your friend also think that driving from NY to LA happens instantly or does she think it takes any one of an infininte number of paths to get there?

The best thing you can do is to encourage your friend to study harder until she understands it. This topic really is not up for serious debate anywhere unless one party has some sort of religious axe to grind. It is so clearly demonstrable by thousands of experiments that show it in action TODAY (e.g.; drug resistance in bacteria) that CHOOSING to not believe it is like choosing to believe the newscaster is INSIDE your TV. Failing that, get smarter friends. Comprehension of speciation is a pretty good discriminant in that task.
posted by FauxScot at 4:19 AM on February 13, 2007

Since other people have already answered the question...

This whole discussion reminds me that a 'little' knowledge is dangerous. Firstly your friend is either misunderstanding her biology teacher or her biology teacher is munging her explanation. A biology teacher is unlikely to have pointed out any weaknesses in evolutionary theory.

That's the first thing which came to my mind. After I graduated with a biochem degree, I came to the conclusion that explaining biology in a HS classroom to sufficiently answer misconceptions and questions about evolution is damn-near impossible. In lower-level sciences, evolution tends to be taught in its own, isolated chapter with few references throughout the texts (unless things have changed since then). When I get to my upper-level sciences, evidence of evolution soon became intertwined throughout everything we studied as I saw how easy it is to "doubt" evolution when it's taught separately from other parts of biology.
Hence, I think part of the issue which arises in examples like this is due to evolution being taught as a scientific theory in one spot, but then the issue is dropped when a more comprehensive examination of evolution is required (as in intertwining evolution into other parts of the biology lectures besides the one chapter on evolution).
posted by jmd82 at 5:52 AM on February 13, 2007

So your friend's problem is basically about speciation. TalkOrigins has a bunch of examples of change-of-number-of-chromosomes speciation (aka aneuploidy) here, which I've quoted at the bottom of this post. Also, please forgive me if I repeat anything said above. If your friend wants to email me with specific questions, my email is in my profile, and I've taught evolution for several semesters, so hopefully I can help.

For your friend to really understand evolution, she needs to understand the actual mechanisms of speciation, which generally occurs in animals by isolation of a segment of the population followed by changes in the genome of the isolated segment by genetic drift and mutations. Eventually (over MANY generations) the isolated population will be different. Keep in mind, though, that there's not a hard line delineating species. There is an incredible diversity of phenotype within "dogs", yet they can still interbreed. So can wolves and dogs, which we identify as different species.

Now that said, most mutations that occur are simple point mutations. Maybe your bio teacher didn't cover the fact that each chromosome is made up of millions of pairs of bases. A point mutation is a change in ONE of those base pairs. Far from a complete chromosomal change. Most of those point mutations are deleterious, those base pairs are responsible for building amino acids, and a point mutation would change the amino acid produced. Sometimes there is no effect, and sometimes it's beneficial. Sometimes those beneficial ones are kept in the genome, and genetic drift eliminates other alleles, and BAM! speciation occurs down the line.

Really, though, speciation is a very minor part of evolution. The biological species concept is useful, but it breaks down when hybrids of "different" species have viable offspring, which happens sometimes. The small changes (variation in genome by genetic drift, point mutations, etc) are the important ones. They have a gradual effect that is cumulative and real. Identifying "species" is a vestige of a past paradigm of classification and a result of humans' propensity to order and classify into types and kinds.

Finally, if what she points out actually did disprove evolution, does she not think that somebody somewhere would have realized that? I mean, thousands of biologists, geneticists, and anthropologists study this stuff daily. If they haven't seen a problem with one of the most basic points of the genome, so much so that it's taught in a high school, there really isn't a problem. Somebody would have seen that and pointed it out, but no one ever has. Because it's not actually a problem, and she should do more research before forming uninformed opinions about things that make her look uneducated and easily fooled.

The aforementioned quote from TalkOrigins:
5.1.1 Plants

(See also the discussion in de Wet 1971). Evening Primrose (Oenothera gigas)

While studying the genetics of the evening primrose, Oenothera lamarckiana, de Vries (1905) found an unusual variant among his plants. O. lamarckiana has a chromosome number of 2N = 14. The variant had a chromosome number of 2N = 28. He found that he was unable to breed this variant with O. lamarckiana. He named this new species O. gigas. Kew Primrose (Primula kewensis)

Digby (1912) crossed the primrose species Primula verticillata and P. floribunda to produce a sterile hybrid. Polyploidization occurred in a few of these plants to produce fertile offspring. The new species was named P. kewensis. Newton and Pellew (1929) note that spontaneous hybrids of P. verticillata and P. floribunda set tetraploid seed on at least three occasions. These happened in 1905, 1923 and 1926. Tragopogon

Owenby (1950) demonstrated that two species in this genus were produced by polyploidization from hybrids. He showed that Tragopogon miscellus found in a colony in Moscow, Idaho was produced by hybridization of T. dubius and T. pratensis. He also showed that T. mirus found in a colony near Pullman, Washington was produced by hybridization of T. dubius and T. porrifolius. Evidence from chloroplast DNA suggests that T. mirus has originated independently by hybridization in eastern Washington and western Idaho at least three times (Soltis and Soltis 1989). The same study also shows multiple origins for T. micellus. Raphanobrassica

The Russian cytologist Karpchenko (1927, 1928) crossed the radish, Raphanus sativus, with the cabbage, Brassica oleracea. Despite the fact that the plants were in different genera, he got a sterile hybrid. Some unreduced gametes were formed in the hybrids. This allowed for the production of seed. Plants grown from the seeds were interfertile with each other. They were not interfertile with either parental species. Unfortunately the new plant (genus Raphanobrassica) had the foliage of a radish and the root of a cabbage. Hemp Nettle (Galeopsis tetrahit)

A species of hemp nettle, Galeopsis tetrahit, was hypothesized to be the result of a natural hybridization of two other species, G. pubescens and G. speciosa (Muntzing 1932). The two species were crossed. The hybrids matched G. tetrahit in both visible features and chromosome morphology. Madia citrigracilis

Along similar lines, Clausen et al. (1945) hypothesized that Madia citrigracilis was a hexaploid hybrid of M. gracilis and M. citriodora As evidence they noted that the species have gametic chromosome numbers of n = 24, 16 and 8 respectively. Crossing M. gracilis and M. citriodora resulted in a highly sterile triploid with n = 24. The chromosomes formed almost no bivalents during meiosis. Artificially doubling the chromosome number using colchecine produced a hexaploid hybrid which closely resembled M. citrigracilis and was fertile. Brassica

Frandsen (1943, 1947) was able to do this same sort of recreation of species in the genus Brassica (cabbage, etc.). His experiments showed that B. carinata (n = 17) may be recreated by hybridizing B. nigra (n = 8) and B. oleracea, B. juncea (n = 18) may be recreated by hybridizing B. nigra and B. campestris (n = 10), and B. napus (n = 19) may be recreated by hybridizing B. oleracea and B. campestris. Maidenhair Fern (Adiantum pedatum)

Rabe and Haufler (1992) found a naturally occurring diploid sporophyte of maidenhair fern which produced unreduced (2N) spores. These spores resulted from a failure of the paired chromosomes to dissociate during the first division of meiosis. The spores germinated normally and grew into diploid gametophytes. These did not appear to produce antheridia. Nonetheless, a subsequent generation of tetraploid sporophytes was produced. When grown in the lab, the tetraploid sporophytes appear to be less vigorous than the normal diploid sporophytes. The 4N individuals were found near Baldwin City, Kansas. Woodsia Fern (Woodsia abbeae)

Woodsia abbeae was described as a hybrid of W. cathcariana and W. ilvensis (Butters 1941). Plants of this hybrid normally produce abortive sporangia containing inviable spores. In 1944 Butters found a W. abbeae plant near Grand Portage, Minn. that had one fertile frond (Butters and Tryon 1948). The apical portion of this frond had fertile sporangia. Spores from this frond germinated and grew into prothallia. About six months after germination sporophytes were produced. They survived for about one year. Based on cytological evidence, Butters and Tryon concluded that the frond that produced the viable spores had gone tetraploid. They made no statement as to whether the sporophytes grown produced viable spores.
5.1.2 Animals

Speciation through hybridization and/or polyploidy has long been considered much less important in animals than in plants [[[refs.]]]. A number of reviews suggest that this view may be mistaken. (Lokki and Saura 1980; Bullini and Nascetti 1990; Vrijenhoek 1994). Bullini and Nasceti (1990) review chromosomal and genetic evidence that suggest that speciation through hybridization may occur in a number of insect species, including walking sticks, grasshoppers, blackflies and cucurlionid beetles. Lokki and Saura (1980) discuss the role of polyploidy in insect evolution. Vrijenhoek (1994) reviews the literature on parthenogenesis and hybridogenesis in fish. I will tackle this topic in greater depth in the next version of this document.
posted by The Michael The at 5:59 AM on February 13, 2007

I can understand where your friend is coming from. After high school, evolution sounded like another form of creationism to me. Giraffes want to eat the leaves at the top of the tree so they grow longer necks? Ya right! I didn't believe in evolution, either.

Then I took a physical anthropology class where we learned about human evolution. Things started to make sense, but the concepts can be difficult. It can be really hard to envision these mechanisms. What helped me was starting to think of evolution as a theory about death, rather than about birth. Here is an example:

In England, there are butterflies that live on trees with a light grey bark. Birds eat them. The butterflies are mostly light grey, same colour as the bark. Sometimes a brown or black or dark grey butterfly is born. This change isn't really a big change, in terms of mating. However, it is a big change in terms of dying, since they stand out on light grey bark really well and birds catch them easily. There are always some butterflies of colours other than light grey in the population, but they never get very high numbers because their lives tend to be short.

Then the industrial revolution, fuelled by coal, came along. Coal soot turned the tree trunks black. All of a sudden, the white butterflies were the ones that stood out, and with in a very short time (a few years), the proportions had reversed: the majority were dark butterflies and the minority were white butterflies. The black butterflies were now surviving and increasing their numbers, whereas the white ones were dying and not increasing their numbers. It's not that a mutation arises and then that organism is able to out-mate everybody else. It's more that small mutations develop in populations but don't become widespread unless they offer some increased chance of survival.

In terms of how evolution works, there are basically four mechanisms: mutation, gene flow, genetic drift, and natural selection (the above example is natural selection acting on a population that has experienced mutation sometime in the past). Gene flow is basically migration (new genes are added or removed through the physical movements of individuals in the population), and genetic drift is the random loss of genes to a population through something other than natural selection (like an avalanche that kills 25% of the population). Because of these other mechanisms, small mutations can, over time, develop large results.

I know you didn't want links, but this one is pretty good and it has pictures, which are really useful for modelling gene changes in populations over time. I've linked the contents page, because the whole thing looks great, but chapter 3, the Mechanisms of Change, might be of particular interest to you.
posted by carmen at 6:31 AM on February 13, 2007 [2 favorites]

Yet another eplanation of how chromosome number and evolution relate can be found here.
posted by TedW at 7:14 AM on February 13, 2007

So let me get this straight... she doesn't understand it so she quit believing it?

Worked for Michael Behe.
posted by Lentrohamsanin at 8:54 AM on February 13, 2007

Pertinent article from today's LA Times (registration required):

Dumbing down evolution to kill it
posted by FauxScot at 9:33 AM on February 13, 2007

Previous AskMe on a similar topic.
posted by DevilsAdvocate at 10:04 AM on February 13, 2007

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