How many species?

| 44 Comments

I’ve got a little question to keep people busy over the long weekend.

There are three populations of an organism. The populations are physically separated from each other as a result of geographical factors. Geographically, they are arranged in a more or less linear fashion. The geographic details are as follows:

Population A is the northwestern population. Population B lies to the southeast, and is eparated from population A by a minimum of ~14km. Population C is southeast of population B, and separated by about 50km.

Populations A & B are identical to each other in appearance and in a key reproductive characteristic. Population C differs very slightly in appearance, but is substantially different in the reproductive characteristic.

The organisms (flying insects) were captured and bred in the laboratory. Experimental crosses were made for the different combinations of these three populations, with the following results:

Read more (at The Questionable Authority):

44 Comments

I’d say one. There is still significant gene flow in this case, since all the females offspring are fertile (although still not as fertile). Species are seperate genetic units, as long as there is gene flow I don’t think it could be said they are distinct species. Subspecies definitely, but not true species. This is backed up by the fact that species A and species C, which are the furthest apart and thus should be the most distinct (if you compare to, say, ring species). However, A and C breed more successfully than B and C (even though there is gene flow to some extent), indicating A and C must still be sharing enough genetic information to be compatible.

Simply based on fertility, I’ll say two species.

Incredibly high fertility between A and B and low (or non-existent) between A & C and B & C.

I’m not a biologist nor did I stay at a Holiday Inn Express.

B and C produce no fertile offspring, so I’d say that would qualify them as two different species. So I would say there are two species (A/B and C).

My question for you is this: How many different species should these three populations be grouped in? Provide an explanation for your answer. Oh, and if you hadn’t guessed, this isn’t a hypothetical case. I’ve removed the names so that you can’t see what the “right” answer is.

Wild guess: you have one hybrid swarm, and the idea that “species” will always be clearly defined is a holdover from pre-Darwinian typological thinking.

If evolution is true, species will not always be distinct, and therefore will not always be clearly definable.

well, i must say that additional evidence is warranted before placing species status in any case.…laboratory breeding is not sufficient evidence. A+B are quite similar in reproductive capacity, and the interpopulation breeding is even higher than A’s intrapopulation breeding success (though no error is given), so is indicative of a potentially interbreeding unit. C appears relatively distinct, but again this is difficult to say without addiditional evidence. laboratory crosses are notoriouosly difficult to interperet, in insects particularly. i have had issues keeping similar looking species from crawling into each others’ cages on occaison. molecular data from multiple markers (eg STRs) would facilitate an understanding of potential inbreeding/isolation in the field, which in my opinion is a better measure of species status in morphologically cryptic groups. my guess, unless this is a trick question which I am missing is that they should be treated as a single species until other evidence supports a split. though i am a bit of a holistic taxonomist, and am a bit conservative in species delineation unless multiple lines of evidence support it.

Upon further reflection, an additional random thought. Considering the percentage fertility of male progeny as a selective factor, assuming no other selective factors, this is the situation:

A does best if it mates with B B does best if it mates with B C does best if it mates with C

So, C and A+B will continue to diverge, while A will be “chasing” B and B will “running away” from A, setting up a mild red queen situation.

So, the number of species depends upon who you ask. If species could talk,

C would say two: C and A+B A would say two: C and “B”, and A would include itself in B B would say three: B, C, and that sneaky little A

Of course the right answer is always “E”, all of the above.

;-)

Nick

Posted by ihateaphids on May 26, 2006 10:41 PM

Why, oh why, do you hate the poor aphids?

i don’t hate aphids :) i just am frustrated by them sometimes because they misbehave in the lab by dying and getting themselves squished by my fingers.

OT:

Nick -

I posted a request for some info in the thread you recently made.

thanks

Clearly there is no biblical justification for dividing them into more than one species, or should I say, ‘kind’. The lower fertility is a consequence of the Fall. You think Noah had room on the Ark for all these piddling little variants?

Oh, BTW, they also all have four legs.

For the non-biologists like me this is a facsinating hypothetical. I’ll enjoy the discussion and the answer.

c Wrote:

B and C produce no fertile offspring, so I’d say that would qualify them as two different species. So I would say there are two species (A/B and C).

Yes they do. Some percentage of the females born were fertile in the cases where the males were sterile, including the B/C crosses (the possibility exists that the females were sterile in some cases where the males weren’t, they do not explicitly rule this out). And even if they didn’t, gene could still move between B and C by way of A (similar to a ring species).

Although they may be geographically isolated, they are not reproductively isolated. If any geographic barriers present were to disappear in the near future and the groups were to become continuous, the fertility of the A/C and B/C crosses would mean that the genetic differences between the populations would most likely disappear through interbreeding and thus mixing of the gene pools. Since they still have the potential to recombine into one population I would say that definitely means they are still one species.

That is, of course, assuming the experiment was not flawed as ihateaphids suggested.

This is one of the questions that keeps philosophers of biology awake at night.

Basing my discussion on the biological species concept, I note that A is more fertile with B than it is with A. So, if we treat A and B as distinct species on basis of reproduction, then we must also treat A as a distinct species from itself, which is absurd. What ever our answer, it must treat A/B as a single species.

Turning to C, I notice that it is completely infertile with B (on available data). It is also largely infertile with A. More interestingly, it is most fertile with the group that is geographically most isolated from it (ie, A at 64 km rather than B at 50 km). This suggests that this is NOT a ring species, and indeed that explicit isolating mechanisms have been selected for to isolate B and C. Consequently, I would say that C is a distinct species from B, and hence from A/B.

So, while being happy to revise my opinion in light of further data (say the discovery of groups E, F, G, H joining A with C); on current data I would say two species.

Todd:

Although they may be geographically isolated, they are not reproductively isolated. If any geographic barriers present were to disappear in the near future and the groups were to become continuous, the fertility of the A/C and B/C crosses would mean that the genetic differences between the populations would most likely disappear through interbreeding and thus mixing of the gene pools. Since they still have the potential to recombine into one population I would say that definitely means they are still one species.

Ignoring the data we don’t have, but which apparently follows the same general tendency, an A female who mates with a C male thereby loses the opportunity of mating with an A male or B male in that season. The result is a 22 to 23% reduction in their reproductive fitness for that season. The result is that discrimination by A and B females against mating with C males will rapidly be selected for. The end of geographic isolation would result in two distinct breeding populations, not one.

Todd Wrote:

If any geographic barriers present were to disappear in the near future and the groups were to become continuous, the fertility of the A/C and B/C crosses would mean that the genetic differences between the populations would most likely disappear through interbreeding and thus mixing of the gene pools.

I’m curious whether other people agree with this statement. The tension zone model suggests that a parapatric hybrid zone can persist indefinitely under some conditions without genetic differences being lost. If so, what level of hybrid disadvantage would be required to sustain genetic differentiation among these populations when they hybridize in parapatry?

Secondly, many recognised species can and do produce fertile hybrids in the lab, but rarely interbreed in nature. Part of the restriction on natural gene flow may result simply from the fact that they naturally live in parapatry, so opportunities for hybridization are rare. They’re not separated by any environmental barrier or variation, and unless something changes radically, they will gradually acquire further isolating mechanisms. Is their species status a taxonomic mistake or a recognition that some more essential species criterion has been met despite occasional gene flow? If so, what might that essential criterion be?

ihateaphids Wrote:

i don’t hate aphids :) i just am frustrated by them sometimes because they misbehave in the lab by dying and getting themselves squished by my fingers.

I also study aphids (the pea aphid, to be specific). Which species do you work on? :)

Ok, I didn’t see your link.

We would be foolish to try to answer this question with only the information given.

We can’t make any judgement without information on species ecology (particularly phenology and host range): Ok, so they interbreed in the lab (so do lions and tigers, in zoos), but ARE THEY LIKELY TO DO SO IN THE WILD? If in nature, they mate at different times, or on distinct hosts, then the fact that they hybridize in the lab is somewhere between irrelevant and misleading.

I’m guessing that this is a trick question involving host races. (Guy Bush’s tephritids Rhagoletis pomonella?)

Also, nick is correct that the concept of species as a pigeonhole is predarwinian typology.

And timothy is wrong ( :) ). This type of problem does not give philosphers nightmares: it gives cladistics adherents nightmares!

Just as a hypothesis, and because I feel like writing something, even if it shows my ignorance, I’d say the scenario suggests that A/B and C are on different evolutionary tragectories. Whether C is a distinct species is more a function of each of our biases than an objective evaluation of the taxonomy of these anonymous organisms.

There! Lot’s of commas. I feel better now.

I imagine these critters are unable to cover by flight the distances between their respective geographic distribution areas (let’s say - hypothetically - that these are separate islands).

In my completely pragmatic opinion, the ability to intercross in captivity can be informative, but is just not meaningful enough or decisive, and certainly not a feasible way to attribute species status.

So, my very modest conclusion would be to say that if two populations are a) clearly genetically isolated in nature (for whatever reason, even just geographical) and b) sufficiently and recognizably distinct genetically and/or phenotypically, they might as well be called species, with the clear understanding that our taxonomic nomenclature is not definitive or fixed.

It is in the nature of evolution that species have to be both real and, in at least some instances, fuzzy and dynamic. So, let’s just embrace the idea instead of whining about it. Long live D. dunfordi.

Long live D. dunfordi.

In the immortal words of the People’s Front of Judea: “Splitter!”

Bob

The answer is always 42, it’s just a matter of choosing the proper units.

This is only the latest of many exampes (e.g. those seabirds around the pole) that show that the human concept of species is not well-defined. This is only occasionally of concern, but it is important to note in resisting the tendency of reifying the concept.

Seems to me that delineating or naming species based on laboratory interbreeding alone is on shaky ground both biologically and philosophically. a pragmatic consideration is ‘can you tell them apart in your hand or under the scope?’ i realize that morphological based spp concepts may leave much to be desired as they ignore crucial biogeographical or molecular information but unless systematists are willing to make a molecular paradigm shift (and deal with the accompanying philosophical issues that, on the whole, would simply shapeshift from extant issues on delineation using other concepts) then it’s fruitless to make these decisions based on molecular data alone (i work on trichoptera, and let me speak for the group: dna species id would be a nightmare for taxonomy, much as the recent dilemma in plethodontid salamanders P. glutinosus complex).

So, someone asserted the monist position, albeit jokingly. How about it? What does anyone think about the Anti-Realist position on species? I have been following the issue from afar and it seems that pluralist stances are logically reduced to an anti-realist perspective on the Existence of species as a real entity. If there is no crisp definition for ‘species’, considered across all taxa, does this in fact mean it is a projection of human cognition? If so, then why do morphological species concepts often coincide with what we can measure in reality?

Erasmus,

Genetics trumps morphology everytime. If the molecular work makes the situation seem complex, that’s probably because it is complex.

Stepping into the murky waters with little data I would first propose that the number of species is dependent upon:

1. Funding level of the PI. 2. Tenure. 3. Number of graduate students. 4. Interdepartmental politics (interruptions to field season).

The questions I want to ask in this study are:

What isolating mechanism(s) is/are at work here and what are their effects? How have the flying insects changed morphologically and genetically? What secondary genetic changes have occurred in these populations as a result of these isolating mechanisms and how have these further isolated these populations leading to possible speciation?

With that, some questions arise.

1. Populations A and B do not differ with respect to the ability to interbreed, since no information in given about the ecology (location or food resources) it is impossible to tell if these are truly 2 isolated populations and not an artifact of collections. Where are these flying insects found and what do they feed on? What evidence is there that they are truly isolated populations? Is there any genetic divergence between the populations to support the assertion they are isolated populations? 2. What were the number of successful matings between the populations and how did they compare with the controls? Are there pre mating (behavioral) isolating mechanisms at work separating these populations? Percentages of fertile offspring don’t tell us anything about population interbreeding success rates. 3. The only cross with significant numbers of offspring occurs between Male from C x Female from A: 69% of male progeny fertile. Was this value the percentage progeny that hatched or progeny that survived pupation or progeny that survived to maturity? If the value is progeny that survived to maturity, then combined with the reported overall 10% survival rate, 69% = 0. 4. Was there differential mating success between the fertile male offspring (from the above cross) with females from the 2 parental populations? 5. You note: Female offspring were fertile in the cases where the males were sterile, but the number of offspring surviving to maturity was greatly reduced in those crosses overall (around 10% of what was seen in the control crosses) and the fertility of the female hybrids was reduced compared to the control crosses. This indicates 2 different effects, reduced fertility and reduced survival. Reduced fertility, is this reduction in fertility the result of a pre or post mating mechanism? Is it behavioral or a sperm gamete problem? Reduced survival, at what stage are the offspring between the different populations of these flying insects dying? Is it at hatching, larval stage, pupation? It is important to separate these 2 effects.

I would like to see data on:

1. Chromosome cytology. Any gross chromosomal alterations between populations A,B and C. 2. Some information on genetic distance, either enzyme or DNA based. Quick and dirty, before Tuesday as you promised. 3. Information on mating behavior. 4. Information on resource utilization. 5. Descriptive ecology of the areas where these flying insects are found. 6. Specifics on survival between the different crosses.

Crosses are not enough to confer species status. Supporting ecological, cytological, and genetic data must be collected before any conclusions can be drawn. I would also keep in mind that this is not an either/or proposition, while there may be evidence of significant isolation it may not warrant species status but rather subspecies status.

Delta Pi Gamma (Scientia et Fermentum)

I’m still developing a model on how this relates to the development of the Pandas thumb. Since no location is given I’m assuming these flying insects may occur within the range of Pandas and I want to be prepared with a possible explanation as to the relationship between flying insects and the development of Pandas thumbs. I’m thinking that the Pandas thumb may have some utility in impaling these flying insects so Pandas can eat them like little buffalo wings while drinking beer. I’ve already established the utility of the thumb for attaching taps on kegs so this would be an added plus. Little buffalo wings would also provide an additional protein source to the Pandas vegetarian diet. I’ve not worked out the barbeque or the dipping sauce recipes yet.

‘genetics trumps morphology every time’

sounds like you are one of the folks that advocate a genetic species concept. please, enlighten us with an explanation of why there aren’t the same problems of delineation that are already exist with any other species concept. i’d like to hear it, i suspect that what you are considering is just as arbitrary as any other species concept.

there aren’t enough data, yet, to do this with most groups of organisms. thankfully. it would be clusterfark for sure, for an example look up the petranka et al problem with Plethodon glutinosus, where one or two good species became a zillion based on molecular, with little regard to distributions or ecology. so much for ‘genetics trumps morphology’.

To answer some of Bruce’s questions, the PI=the grad student=me, but I’ve got sufficient funding for some preliminary work. I also should be able to get enough funding to take me through the end of my PhD. (My department has a tradition of allowing and encouraging grad students to design and set up their own projects.)

I will have access to undergrads doing senior theses. The department politics are good, and the field season is year-round. I’ll be providing details on the genetic (chromosomal, enzymatic, and DNA) studies that have been published in a later post, along with some information on mating behavior.

To answer your question (#5) on the reduced fertility, the effect I’ve reported is a post-mating effect, determined by examination of testes and ovaries from the offspring of the crosses. There are also some pre-mating differences, but I don’t have the paper on that at home, so it will have to wait until Tuesday.

Ecologically, the insects are restricted to native forests above 1000’ here in the Hawaiian Islands. The feeding behaviors are similar in all three populations, but there is a major difference in ovipositional behavior. Population C will only ovideposit in the presence of rotting bark from a single genus of plants. Larvae from populations A and B have been raised from rotting plant parts (stems and bark) from (IIRC) 14 plant families, including 2 not native to Hawaii.

OK Mike.

I give up.

How many?

Have you looked at rates of migration in the wild? Population C seems to be so far away from A that gene flow between the two seems pretty unlikely anyway (unless they hitch rides on field workers’ boots), so the lab results may be irrelevant that way. Like any good population geneticist, I want to see the F_ST!

I remember a long time a go (the ’80s) there was a cartoon in New Scientist accompanying a letter about speciation. The cartoon had a couple of penguins on a small ice floe, separated from the main ice. One was sayng to the other “Hey baby, fancy going off to form a new species?”.

Bob

Not to be snarky, but is this a veiled attempt to recruit members of this blog community in your data analysis? If so, whoopee, Delta Pi Gamma is on its way to Hawaii. All we need to set up shop is a constant beer supply. The engineer poet has a figured out a way to run everything with wind power, so we don’t need power for the keg cooler or the computers.

All you have to do is come with some explanation for your advisor as to why all these funny looking drunk frat guys are messing with your experiments, arguing over your data, and generally telling each other to piss off with “you’ll full of crap that’s not the way it works here let me at the blackboard and I’ll show you.….”

Delta Pi Gamma (Scientia et Fermentum)

Stuart - I’ll let everyone know on Tuesday.

Bob - I’d be shocked if there’s much (if any) gene flow between pop C and any of the others; I also doubt that there’s been much recent flow between A and B. I will, of course, be looking at gene flow as I work on these taxa. I can’t give you Fst values right now, but I do have Nei’s genetic distances for the combinations: Pop A - Pop B: 0.050 Pop A - Pop C: 0.245 Pop B - Pop C: 0.306 The distances were calculated from a 13-locus allozyme study conducted in 1977.

Bruce - unless he recognizes the unfamiliar faces, my advisor would have a hard time seeing the difference between that and what goes on in the lab now. But I’m not trying to drum up analysis help - that’s just a side benefit. I just thought it would be fun to show people a real-life evolution problem.

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Erasmus,

Most of the criticisms of the ‘genetic species concept’ seem to boil down to the perception that genetic tools find too many evolutionarily independent lineages. Admittedly, this makes for messy taxonomy. But evolution IS messy. Some ‘species’ may be composed of myriads of genetically isolated populations. Do we ignore what this says about the evolutionary process because it’s messy?

Plethodon glutinosus (pretend I knew how to italicize) is a beautifully complex group with an amazing evolutionary history. I would like to see this history understood, not glossed over in the interest of a simplified taxonomy.

I’ll say it again, genetics trumps morphology every time. Ignore the molecular information if you will, but if you do, your taxonomy won’t reflect the evolutionary relationships of your organisms.

PS

I hope this isn’t a double post

The answer is always 42, it’s just a matter of choosing the proper units.

You might be closer to the truth than you think.

Odds are that the question originally made it to your computer in the form of ASCII encoded text, and in ASCII, the value 42(hex) is the code for a capital letter “B”

With the additional information on host range (although further info would be useful: is the genus to which C is restricted, a subset of the hosts accepted by A and B?), hybrid viability (as opposed to fertility) of 0.1*control; and Nei genetic distances, I am confident in concluding that you have at least 2 species: (A+B) and C. Further info might also split A from B.

I’d say you still have one species overall, but with one group–C– being an incipient species.

As any land use lawyer (ahem) can tell you, the legal definition of “species” includes “subspecies” and (my favorite) “distinct population segment”. 16 USC 1532(?)

as it’s one am California time, i’m not inclined to go find and provide a link to the US Fish and Wildlife Service’s policy on the recognition of DPS’s. If there’s any interest I’ll do it Tuesday afternoon.

So, as a matter of law, the answer is 1.

please, put how many species there are and what they are.….….….….….….….….….….….….….….….….….….….….….….…thank you

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This page contains a single entry by Mike Dunford published on May 26, 2006 8:46 PM.

Chelifores, chelicerae, and invertebrate evolution was the previous entry in this blog.

Intelligent Thought: Science versus the Intelligent Design Movement is the next entry in this blog.

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