Predictions of natural selection

[Roboramma]

Hi guys,

I've been hanging out with some of the "evolution is just a theory" crowd, and it's driving me a bit nuts. I try to avoid those discussions because I can go crazy and we really antagonise each other.

Anyway, one thing I find quite useful is talking about the real and accurate predictions made by the theory of evolution. When it comes to common decent I like to bring up things like bats, flying animals that fill the same niche as birds, but share so many properties with other mammals - why, for instance, do bats have fur rather than feathers? That's something well explained by common decent, but not by, for instance, creationism.

Anyway, I'm looking for predictions made not just by common decent, but by natural selection. My favourite example is sex-ratios: the way that in most sexually reproducing species, the sex ratio is very close to 50/50. I think that the exceptions to that are even more interesting, because they are also predicted by natural selection, and so differentiate it from other theories that maybe predict a 50/50 sex ratio.
If anyone can link to a good explanation of such exceptions, and why they are predicted (such as ants?) I'd love that.

Any others? For instance, if a new species of mammal were discovered, what could we say about it without having seen it? (above and beyond things predicted by common decent)

I'll probably try to find some more of my own to post. Hopefully this will turn in to a productive thread.

 

[Fiona]

I cannot help in the way you ask, Roboramma, but this is something I would like to know more about because it is something I have not been able to understand and it bothers me. I would like it if, in the course of replying to your request, people could maybe say a little more about just how those predictions are made, if they are. How does natural selection predict sex ratios,as you say it does? Can this be incorprated in this thread without derail?

 

[Vorpal]

Some of the key predictions of natural selection rather than common descent are actually of what we shouldn't find:
(1) A completely unique feature that is not similar to those found in other species.
(2) A feature that exists solely for the benefit of a parasite (that is not similar to useful features in other species).
Either of these would constitute potentially falsifying evidence for evolution.

The sex ratio tending to 0.5 is a prediction of genetics. However, in the Hymenoptera family, males have a lone X chromosome, while females XX. This deviation from the usual XY/XX pattern can be taken be taken to be a loose prediction of the deviation of the sex ratios in ants, etc., at least in the sense that the main reason for it to be 0.5 no longer applies. I don't know of any studies on this specific issue.

 

[Hokulele]

The sex ratio tending to 0.5 is a prediction of genetics. However, in the Hymenoptera family, males have a lone X chromosome, while females XX. This deviation from the usual XY/XX pattern can be taken be taken to be a loose prediction of the deviation of the sex ratios in ants, etc., at least in the sense that the main reason for it to be 0.5 no longer applies. I don't know of any studies on this specific issue.

This is called haplodiploidy and it explains gender ratios in creatures such as bees based on inheritance. Here is a pretty decent intro to haplodiploidy. I am sure you can find plenty more on the web.

 

[rjh01]

I seem to remember another thread on the same subject. I will make a similar post. One prediction is that you will get many animals that are related. For example primates have many similar characteristics and same with cats.

Another prediction is that certain viruses will change over time so that it can infect the same person several times. I am thinking of the flu.

 

[Byzantine Magpie]

A couple of things come to mind here:

1. Back in the early days of antibiotics, one of their discoverers (was it Fleming?) warned that if we overused/misused them, bacteria would become resistant to them. This is now becoming a serious problem; the tiny proportion of bacteria with natural resistance to the antibiotics passed on this immunity to their "offspring", and that characteristic spread through the population. The same thing happened with rabbits in Australia - most were wiped out by the Myxomatosis virus, and only a small proportion survived - the proportion which had a natural immunity. Now virtually all feral rabbits in Australia are immune to Myxo, and the same thing has happened/is happening with the Calicivirus. I don't know whether anyone out-and-out said as such, but it shouldn't have been a surprise to anyone.

2. Not so much a prediction, but a sign of common ancestry, is that we have some bizarrely useless muscles in our body. There's one muscle in our arms, in particular, which is very important for birds in allowing them to fly, but which has no use for humans. As a result, although it's still there (well, for most humans - something like 10% of humans don't actually have the muscle and don't suffer for that) it's just a long thin piece of spaghetti-like muscle with no ability to be used like a muscle. Someone who knows a bit more about human anatomy might care to name it and explain it in a bit more detail. There are other examples in other animals, such as vestigial limbs in snakes and whales.

3. I understand someone accurately described the naked mole rat before it was discovered, simply by idly hypothesising what a communally-living mammal would be like.

 

[rjh01]

I have added a couple of tags to this thread. One of them included the thread I was thinking about Need help - please give examples of predictive power of evolution

 

[blobru]

I seem to remember another thread on the same subject...

This? [ETA: yup]

My favorite ToE prediction is from the man himself (sort of an unknown species prediction -- predicting they exist):

Some of the most ancient animals, as the Nautilus, Lingula, &c., do not differ much from living species; and it cannot on our theory be supposed, that these old species were the progenitors of all the species belonging to the same groups which have subsequently appeared, for they are not in any degree intermediate in character. Consequently, if the theory be true, it is indisputable that before the lowest Cambrian stratum was deposited, long periods elapsed, as long as, or probably far longer than, the whole interval from the Cambrian age to the present day; and that during these vast periods the world swarmed with living creatures. ... To the question why we do not find rich fossiliferous deposits belonging to these assumed earliest periods prior to the Cambrian system, I can give no satisfactory answer. ...I wrote in 1859, about the existence of living beings long before the Cambrian period... The case at present must remain inexplicable; and may be truly urged as a valid argument against the views here entertained.
-- chapter 10, Origin of Species

He admits no Precambrian fossils would pretty much be it for Evolution. He predicts, if his theory is true, that they'll show up. And so they do, under a microscope in 1953, all sorts of 'em... 94 years after Darwin's make-or-break prediction! Whoa. I mean, I can't predict what I'll find when I open the refrigerator, let alone buried 500 million years deep beneath it. That's what I call a smokin' theory. Nice one, Chuck.

 

[biomorph]

This is called haplodiploidy and it explains gender ratios in creatures such as bees based on inheritance. Here is a pretty decent intro to haplodiploidy. I am sure you can find plenty more on the web.

I enjoyed reading that, thanks for the linky.

 

[Roboramma]

2. Not so much a prediction, but a sign of common ancestry, is that we have some bizarrely useless muscles in our body. There's one muscle in our arms, in particular, which is very important for birds in allowing them to fly, but which has no use for humans. As a result, although it's still there (well, for most humans - something like 10% of humans don't actually have the muscle and don't suffer for that) it's just a long thin piece of spaghetti-like muscle with no ability to be used like a muscle. Someone who knows a bit more about human anatomy might care to name it and explain it in a bit more detail. There are other examples in other animals, such as vestigial limbs in snakes and whales.

This seems to be evidence of common decent, but not natural selection, per se.

3. I understand someone accurately described the naked mole rat before it was discovered, simply by idly hypothesising what a communally-living mammal would be like.

That's awesome! Any links or references for that? I'd love to learn more about it - it's perfect for what I'm looking for.

 

[Roboramma]

Some of the key predictions of natural selection rather than common descent are actually of what we shouldn't find:
(1) A completely unique feature that is not similar to those found in other species.
(2) A feature that exists solely for the benefit of a parasite (that is not similar to useful features in other species).
Either of these would constitute potentially falsifying evidence for evolution.

I'm having a hard time understanding this - could you explain why?

 

[Jorghnassen]

Well, there's this little moth (actually, it's pretty big) in Madagascar, with this funny name: Xanthopan morganii something...

 

[Roboramma]

I cannot help in the way you ask, Roboramma, but this is something I would like to know more about because it is something I have not been able to understand and it bothers me. I would like it if, in the course of replying to your request, people could maybe say a little more about just how those predictions are made, if they are. How does natural selection predict sex ratios,as you say it does? Can this be incorprated in this thread without derail?

As for sex ratios, I'll give you my understanding. A lot of my understanding comes from Richard Dawkins: here's an example of him explaining it.

The simplest way to understand how sex ratios evolve, or are selected, is to look at it from the point of view of a parent, trying to maximize the number of grandchildren it has.

In that video dawkins points out that every child born has both a mother and a father. Okay.

So, let's say I live in a population where there are more females than males. Should I have more sons, or more daughters?
Because there are more females than males, I should have more sons.
Why? Because if we look at future generations they are decendants of those individuals who reproduced in this generation. Exactly half of those ancestors will be male, exactly half female.
But of those individuals alive, more are female. Thus, the percentage of females who become ancestors is less than the percentage of males. So, when females are in the majority any particular male has a better chance of becoming an ancestor than any particular female.
And from my perspective, if I have sons, I have a greater chance of becoming a grandparent than if I have daughters.
This means that if a situation arose where there were more females than males, producing more males would be selected for, and vice versa - and that selection pressure stabalises around a 50/50 ratio.

Now, let's consider an apparent example where this shouldn't be the case - species of harem forming animals, like elephant seals, for instance.
Amoung elephant seals, the sex ratio is 50/50, yet any particular male has only very small chance of reproducing. One male may have (for instance, I'm making up this number) a dozen different females with whom he reproduces, and thus a dozen children in a particular year. Most males, on the other hand, have none.
Let's say, for simplicity, that 1/10 males reproduces, but does so with 10 females. (the fact that these numbers are reciprocals is a product of the fact that the sex ratio is 50/50 - think about it, it will make sense).
So, in this case, why not have many more daughters than sons, since daughters are almost gauranteed a mate, and thus you are almost garanteed grandchildren, but sons are almost garanteed not to have a mate?

Let's say daughters each produce, say, 3 children, 1 of which dies for some reason.
My sons, then, have a 10% chance of producing 30 children, 10 of whom will die young, and a 90% chance of none.
The average between them is the same. If I have ten daughters, I have 30 grandchildren. If I have 10 sons, I have 30 grandchildren.
And my nieghbor, who has a mix, has the same number. (on average).

On the other hand, any deviation from a 50/50 sex ratio will affect this population in the same way it affected the other one mentioned above.

 

[Roboramma]

Well, there's this little moth (actually, it's pretty big) in Madagascar, with this funny name: Xanthopan morganii something...

Is that the moth whose existence Darwin predicted by seeing the flower that it feeds on?
Even to point of predicting the exact length of it's tongue?

If so, I was hoping someone would bring that up, and yeah, I think it qualifies nicely.

 

[Kotatsu]

I cannot help in the way you ask, Roboramma, but this is something I would like to know more about because it is something I have not been able to understand and it bothers me. I would like it if, in the course of replying to your request, people could maybe say a little more about just how those predictions are made, if they are. How does natural selection predict sex ratios,as you say it does? Can this be incorprated in this thread without derail?

NB: the story outlines below is based on memories of a number of papers I read some years ago; even then, it was somewhat peripheral to the subject I was studying, and thus I may misremember, or mix things up. Nevertheless, even if some specific detail may wrong, it may still serve as an example of how such a prediction might occur.

Leeches (Hirudinoidea) were, I understand, for a long time something of a problem. They were undoubtedly annelids, as evidenced by several characteristics (1), but how did they fit in? The most common placement was as a sister group to all of Oligochaeta (earthworms, potworms, blackworms, and so on), with this pair, in turn, placed as a sister to the Polychaeta.

Cladistic studies gradually showed this placement to be unsatisfactory. It became apparent that the origin of the leeches was to be found within the Oligochaeta, rather than outside it. But where to find a suitable location for them? Many factors, of which some will be outlined below, obscured matters.

One of them is diet. While leeches are largely (or entirely?) actively carnivorous or parasitical, oligochaetes are detritivores, herbivores, suspension feeders, or filtrators. No carnivorous oligochaetes were known. Was this transition to carnivory an apomorphic trait, the initial stages of had left no descendants?

During the 1980s, there are a number of papers published, if my memory serves me, by Hrabe and Brinkhurst (both eminent scholars), which discuss the Lumbriculidae. These worms --- of which the California Blackworm, on which I did my thesis, is perhaps the best known --- are suitable candidates for the origin of the leeches. At least some of them are more active swimmers, and they have the size and, I believe, certain features of the nervous system and perhaps ultrastructure of the sperm (though here I am very likely to be mixing it up with something else) in common with the leeches.

However, they, too lacked carnivorous members. Nevertheless evidence was piling up that suggested that the Lumbriculidae truly were the sister group of the leeches, making the latter nested within the Oligochaeta (2).

In 1994, however, McKey-Fender and Fender published a paper on the new lumbriculid genus Phagodrilus from Washington and/or Oregon (I can't remember), and finally, science knew about a carnivorous oligochaete outside the leeches, sealing the case for sister group relationship, at least, between these and the lumbriculids.

The story doesn't end there; there are now 14 species of Phagodrilus described, and I believe there is evidence that the leeches are actually nested within the Lumbriculidae, rather than as a sister group (3). However, as is often the case among the invertebrates, the Lumbriculidae and the leeches still await molecular phylogenies that can solve whether they are sister groups of nested together. Some work is being carried out by my former supervisor, Christer Erséus, and I think Patrick Martin in Belgium is interested as well, but neither of them work primarily with these groups, so --- strange as it may seem --- it is not highly prioritised.

Nevertheless, errors and mix-ups aside, I believe this illustrates quite well how predictions can be made. I realise now, of course, that the question was about natural selection, and that this whole passage is largely irrelevant to your question (and perhaps to the thread as such), but it took too long to write this to just erase it and pretend nothing happened damn it.

---
(1) The details are irrelevant and largely uninteresting, but they have a segmented structure similar to that of oligochaetes and polychaetes, and I believe there are similarities in the organisation of the genitalia.
(2) The Hirudonoidea and the Oligochaeta together were, at that time, called the "Clitellata", and this new placement of the Hirudinoidea as a part of Oligochaeta made "Oligochaeta" and "Clitellata" synonymous. At least the aquatic oligochaeta community of the world has decided to adopt "Clitellata", suppressing "Oligochaeta", in their papers, though "Oligochatea" is still often used, especially by "megadrilologians" (those who work with the various kinds of earthworms), and as the International Code of Zoological Nomenclature does not govern names at this level, either term is acceptable, as long as you are clear with what you mean. Strictly speaking, however, "Oligochaeta" without the Hirudinoidea, is a paraphyletic group.
(3) This, too, has caused quite a lot of controversy in the clitellatological world, where some very esteemed authors moving for the acceptance of paraphyletic groups in systematics, as they do not want to rework the existing order too much. Rüdiger Schmelz and Tarmo Timm are the main proponents of this view.

 

[Roboramma]

That was a great post, kotatsu. Slightly off-topic or not, thanks for it.

 

[Earthborn]

My favourite example is sex-ratios: the way that in most sexually reproducing species, the sex ratio is very close to 50/50.

I don't think that is much of a prediction. A prediction in the sense of the "scientific method" is something that hasn't been observed before the theory was formulated but is a logical consequence if the theory is true. After the formulation of the theory observing the thing as it was described by the theory is support for the theory.

But the sex-ratios in most sexually reproducing species was known since ancient times, especially to anyone raising animals. It is not a prediction scientists made on the basis of the theory of natural selection that they later confirmed.

 

[Kotatsu]

That was a great post, kotatsu. Slightly off-topic or not, thanks for it.

Thank you. If nothing else it'll teach to read before I answer.

However, let me try again. This example, too, is based on something I heard about many years ago, and I may mix it up. It is slightly more on topic, though, and is about sexual selection; one step closer to natural selection, of which cannot think of a single good example. It is a bit outside my field, so I have to make do with what I have heard from colleagues and such like; the following is based on a seminar I attended some years ago, and the same qualification applies as in the last example.

Sexual selection suggests that if some part of the morphology of a member of one sex reflects its over-all conditions, then the opposite sex may use this feature in order to select a mate. Ideally, the selector should chose a displayer that appears to be in as good condition as possible, as that, to some extent at least, suggests that the offspring, too, will be likely to have good genes. Also, it suggests that the displayer is apt at finding food, protect itself, or that it is in possession of an area suitable for the rearing of offspring.

To some extent, it is in the "interest" of these displaying individuals --- especially those with less than top quality genes, which are less likely to be selected --- to counter this effect by cheating in some way. Naturally, that seldom works out in the long run, but it may in the short perspective, which is where evolution operates.

In widowbirds, weavers, and bishops, we have an example of this. Females are attracted by the males by their yellow, orange, or red colouration (1). Males acquire this colouration by transporting carotenoids to their feathers, which is a passive process. The depth of the redness is directly related to the amount of carotenoids circulating in the bird's blood. As the birds get the carotenoids from their diet, the depth of the redness is an indirect measure of how good the individual is at collecting food (2), and thus how good it would be at providing food to their offspring and to the female when she is brooding.

However, as by-products of these carotenoids, which are apparently somewhat unstable, are toxic to the bird, there is another level to it. The depth of the colouration of the plumage is also a measure of the over-all fitness of the bird. The redder he is, the higher is the level of toxins he can tolerate in his blood, and, thus, the better is his general condition.

The person holding the seminar had predicted that this would be an area where some method of "cheating" the females would evolve. Since the female react on red colouration, any method to increase the depth of colouration without also increasing the concentration of toxic carotenoids in the blood would be selected for.

And here comes the interesting part, because exactly this had been found. In one branch of the phylogenetic tree of this group, a mutation had occurred in the DNA coding for a protein (I don't remember what this protein originally did), which altered the by-products of the carotenoid in some fashion, making them non-toxic. This enabled these birds to get very red, and thus get all the females, without thereby increasing the concentration of the toxic products in the blood stream.

So the prediction held out. But, unless I misremember entirely, it doesn't end here. You see, as this method for the females to select the best possible mate was now disconnected from any real skill in the males, a new one was expected to evolve. The females couldn't rely on the colouration any longer, as they had evolved to react to deep red, and now all males were deep red, whether they were in a good condition or not! This is precisely the sort of situation that doesn't work in the long run.

Well, the obvious solution was for the females to evolve a reaction to another stimuli, and that's precisely what happened: the group of birds that have these altered proteins that make the toxic by-products of the carotenoids harmless and thus get all the females are the ones known as widowbirds, the weavers with extremely long tails! So when one signal for suitability was disabled, another evolved! And, as some of you may know, the females are still locked on this one, so much that if you cut off feathers from one male and glue them to another, these males with artificially elongated tail feathers will attract the most females.

This new signal, I assume, is more stable, though, as it is not as easy to cheat as the old one.

Sadly, I shamefully cannot support this tale either with specific papers; I can't even recall the name of the person who held the seminar. If necessary, I will try to ask around at work, or search for articles about this, but I will go on vacation to Germany tomorrow, so that'll have to wait.

---
(1) These are often splendid birds. If you are unfamiliar with them, look them up. The belong to the genus Euplectes, family Passeridae or Ploceidae, depending on the taxonomy used.
(2) They live communally, so it is not a measure of the quality of their territory.

 

[paximperium]

Thanks for the post Kotatsu. I learned something new today.

 

[ohms]

Is that the moth whose existence Darwin predicted by seeing the flower that it feeds on?
Even to point of predicting the exact length of it's tongue?

If so, I was hoping someone would bring that up, and yeah, I think it qualifies nicely.

I found a short video about that on YouTube here.