Definition of "Species"

[dihsmaj]

That makes no sense

Thanks Blue. Helpful as always

I was making a joke. It was innuendo. 'Woma balls'

 

[saximus]

lol oh man I can't believe I didn't get that. Sorry

 

[Cockney_Red]

A pony film, with a top sort in it...

 

[Bluetongue1]

To understand the concept of species and its limitations, I believe you need to have a broader appreciation of what taxonomy is about. When we look around at the natural world we see different types of organisms. These organisms reproduce to make young that are similar to the adults. Adult Grey Kangaroos produce baby Grey Kangaroos, Lemon Scented Gums produce seedling Lemon Scented Gums, Red Bellied Black Snakes produce baby Red Bellied Blacks Snakes and so on. So the notion that the natural world is populated by different types of organisms has been long established. Once humans had time to start thinking more about the world they lived in, instead of where their next meal was coming from, they started to notice that you could pretty much divide organisms into two groups. One group were stationary and put down roots to hold them in place. The other group were mobile. Upon closer inspection, early investigators found other consistent differences between these two groups. Then they started to look at each group. The closer they looked, the more they realised that within each group there smaller groups. Each smaller group was consistent – all those animals with internal skeleton vs those without, all those plants with flowers vs those without etc.

Over time science has built up a schema for grouping organisms, starting from some major groupings, with fundamental differences between them, down to individual types of organisms. There are seven levels of grouping they developed – Kingdom, Phylum, Class, Order, Family, Genus, Species. Groupings have been developed pretty much by looking at it from both ends and tweaking it to give the best fit. This, then, is a man-made system of grouping designed to reflect the natural order of things. It provide a workable way to deal with the multitude of living things on the planet. It summarise information into groups and helps facilitates the learning, as you need only remember the characteristics of the group and not those of each individual separately.
In reflecting the “natural order” I am referring to the fact that the system of classification reflects the phylogeny (evolutionary lineage) of organisms.

When the system was developed, they had no understanding of genetics and inheritance. That new species developed from previously existing species – they just did not know how. We now have a much better appreciation and recognise that the process of evolutionary change occurs gradually, rather than in discrete leaps, through accumulated genetic change in a population. So as Jonno pointed out, where on that continuum of change do you draw the lines – when does one species become a new species; or harder still, as Lace pointed out, what constitutes a sub-species? (if not certain what a species is?)

That’s your background (minus a fair wack of detail). Probably the most used definition of a species is: “A group of organisms that can, and do breed, to produce viable and fertile offspring under natural conditions”.

As the lady said, easy to distinguish when you have two groups occupying the same distribution (sympatric) - that includes an overlap in distributions. Basically, if two morphologically similar populations are sympatric and are not interbreeding, then they are two separate species. They may, in fact, be able to breed and produce viable and fertile offspring. However, if they are not doing that under natural conditions, then the species rule holds. It gets sticky with those groups that are allopatric (occupy different distributions).

For populations of organisms that don’t have overlapping populations, the question is what degree of genetic difference warrants allocation to another species? There isn’t a straight-forward answer to this. It gets down to experience, an “informed opinion” and using other forms of evidence to make the assessment. So someone with experience looking at genetic profiles of closely related species versus the degree of genetic variance with populations that are one species, is relied upon to make that call. Part of the problem is that then somebody of similar background may come along at a later date and revise that call.

Much of our reptile taxonomy has been developed on morphology. Certain features that one group possesses and others don’t is indicative of genetic isolation of the gene pools. In other words, they are not interbreeding with similar groups that have different morphological traits. Morphology is still considered important but it’s limitations are more recognised these days.

Last little complication to throw into the mix is the slack taxonomist. If you are going to reclassify a certain species what you should do is to examine all the specimens held in museums under that species name. One of theses will be the original type specimen and then there will often be supporting specimens held by the same museum. Problem is, there can also be supporting specimens held in Britain, Germany, South Africa and other museums that made collections in Australia. So a thorough taxonomic review of even one species can be rather expensive and time consuming. Some taxonomists have taken short cuts and we are left to wear the back wash. Because they got in first, they get full recognition and credence, whether deserving of it or otherwise. It does not happen too often but the legacy it leaves when it does is a bit of a nightmare.

Blue

 

[jpemery]

i would have to agree with lacy in that the definition of a species varies however the general idea is that its the capability on a two animals interbreeding and producing viable offspring in a natural environment. this does not always play true though as highlighted in Richard Dawkins book An Ancestors Tale where he takes two different "species" of cichlid fish which are genetically very similar and live side by side yet never interbreed in the wild yet when placed together in a tank they bred. Interesting though i must say, and as lacy said becomes very confusing.

 

[Bluetongue1]

I think you may have missed th point of clarification in Paragraph 5: "They may, in fact, be able to breed and produce viable and fertile offspring. However, if they are not doing that under natural conditions, then the species rule holds." A species is not defined solely on its ability to interbreed and produce viable and fertile offspring. The reason the definition of a species varies is that it is impractical and often impossible to determine if two organisms would in fact breed under natural conditions. so you need to use some other, more pragmatic criteria on which to base their taxonomic description. Under captive conditions is not at all uncommon for similar species to interbreed and some will in fact produce viable and fertile offspring i.e. fertile hybrids.

Plant taxonimist use a different set of criteria for delineating species. I cannot comment on taxony in the other kingdoms.

Blue

 

[longqi]

This is getting more interesting
A lot of crop plants such as tomatoes are hybrids that cannot be pollinated
This is done deliberately so that farmers have to buy new seedlings every crop

 

[saximus]

Ok so I think my original question has been answered to a reasonable degree. Does anyone have any idea about the speciation in dogs (or any other "pure breed" animals) thing? Is it just because it takes longer for speciation to occur than we have been pure breeding?
Also why can species reproduce in captivity when they otherwise wouldn't in the wild?

 

[Nighthawk]

The inability to interbreed is not a set diagnostic characteristic when definining what constitutes a valid species.

Taxonomy will never be an exact science. Science is forever trying to formally categorise what are simply points on a continuum. It will always be a matter of contention.

Exactly. It brings to mind the advice my lecturer gave us first day of macrobiology: "Your first year you'll find out 'what is'. Your second year you'll discover we're not sure. Your third year we'll finally let you know we don't have an effing (censored lol) clue and it's *your* job to find out. No life science is exact, it changes every day."

 

[saximus]

Haha I love lecturers like that

 

[Wallypod]

Dog and Dingo?

i think aussie cattle dogs have a little dingo in them, i am sure some one will correct me if i am wrong.

 

[lace90]

Different organism classifications speciate at different rates, due to differences in generation time, hox genes, if they are specialised or not etc ... there are so many factors. For mammals it takes many many years, most likely millions, for speciation to occur. Dogs from all around the world, whether domestic or wild, have been tracked back to a single 'type' from which then diverged across continents - and this happened relatively recently. Domestication occurred more recently than this still, and then the line breeding of 'breeds' even more recent. All these animals can be crossbred, and are all within the one species grouping, due to the number of fixed genes within all the dogs. There is not enough genetic distance (yet) to stop interbreering and allow speciation...and until their genes stop being mixed, which is seen so much in aus with dingo/domestic hybrids, speciation will not occur.

 

[Bluetongue1]

Does anyone have any idea about the speciation in dogs (or any other "pure breed" animals) thing? Is it just because it takes longer for speciation to occur than we have been pure breeding?

There is often an expectation that animals like dogs, which display such a range of different physical characteristics, should surely be made up of more than one species. I suspect this expectation is reinforced by using reptile field guides. For example, any group of species such as geckoes or skinks are distinguished by colour and pattern. From this, the obvious inference is that this is what distinguishes one species from the next. Then look at a group like dogs in which the same characteristics show massive variation yet you are told that are all the one species. That does not add up!

Most of the species of reptiles described in Australia were done from pickled specimens in jars. Consequently morphology is the prime taxonomic tool used. What the average reptile enthusiast does not know is the amount of detail that goes into the morphological description produced in describing a new species. A large range of features and substantial number of measurements is involved. In stark contrast, the list of features given in your standard reptile text come field guide is very small. Mostly pattern, colour, scales, pores and s-v length in adults. As a result, one naturally tends to infer that animals differing in these features should be separate species. The reality is that it requires a lot more than just those readily seen differences in appearance to warrant species status. The same applies to dogs. A quick example from the reptile world - Diplodactylus pulcher can have a pattern virtually identical to D. galeatus through to identical to D. vittatus – so how much emphasis would you put on pattern in delineating a new species?

The genes that control the characteristics in which domestic dogs differ are a small percentage of their total gene pool. Secondly, they still share the same set of base characteristics but vary in the form (shape, size, colour, length, etc) of these. I am unaware of any new characteristics having been developed as a result of a significant change in one or more genes but there may be some, but not many I’d warrant. Dogs have the longest history of domestication. Estimates range from 20,000 to 100,000 years. Through artificial selection, specific variations in the characteristics have been selected for, in exactly the same way as the different colours and patterns are line bred in snakes and lizards. Given the time man has been selecting specific forms of characteristics in dogs, the number and degree of variations is significantly more than for other domesticated animals. The bottom line is that all breeds of domestic dog are still capable of producing viable and fertile offspring if crossed. Size difference might prevent that occurring naturally but artificial insemination would give the results. So basically there simply hasn’t been sufficient genetic change for speciation. Those changes that have taken place are more apparent than substantial.

The rate of speciation in animals varies according to a number of things, such as the amount of selective pressureon differentiating the two populations, whether there is zero or limited gene flow beween the two populations, the rate of natural genetic mutations within those organisms, random chance of what mutations are produced, the length (time) of the reproductive cycle, the fecundity, the 'ability' of the organism to produce massive genetic change such as alteration in the number of chromosomes (almost exclusively plants but not always).

Blue

PS: Sorry! I did not realise others had posted before I put this up, so please forgive the repetition. I have left it because the majority is about why it seems a reasonable expectation that breeds of dogs could or should constitute separate species and yet it isn’t.

Also why can species reproduce in captivity when they otherwise wouldn't in the wild?

For speciation to occur, populations within a species must be separated and remain so for the time it take to become separate species (as Lace90 pointed out). If they come back together and remain genetically isolated by not inter-breeding, then they will remain as two species and will continue to evolve separately. What normally stops them from interbreeding are behavioural barriers. For example, amphibian populations which have developed a different mating call, lizards which have developed different mating displays, snakes that have developed altered pheromones. In a captive situation, if the animal they see is similar to them and they have done their calling, or displaying or laid down their pheromone trail, the drive to reproduce can easily over-come any reservations.
Brian Bush et al in Reptiles and Frogs in the Bush: Southwestern Australia, have a photo of a male Motorbike Frog in amplexus with a Western Banjo Frog (Pobblebonk) in the wild. I would imagine the Motorbike Frog had been calling for a female and the poor old Pobblebonk just happened to swim by in the dark at the wrong moment. In a captive situation where the partners to choose from are few, the chances of that happening are dramatically increased.

Blue

 

[GeckPhotographer]

Ok Bluetongue, your replies have been very informative (although mostly what I have already heard from my taxonomist father) and well written so a few points and then a few questions further delving into species related issues. I would love for Bluetongue to reply but others feel free to as well.

Pt1

Diplodactylus pulcher can have a pattern virtually identical to D. galeatus through to identical to D. vittatus – so how much emphasis would you put on pattern in delineating a new species?

Diplodactylus pulcher is very different to East coast through to Adelaide coastal pops of D.vittatus. These coastal pops (and a little line of less coastal pops) are what are currently defined as true D.vittatus. Other somewhat recently discovered (split) species that occur in WA can be extremely similar and identical in pattern. However the structural morphology is still different. (Anyone that looks at the nose of D.pulcher next to any D.previously vittatus or granariensis will instantly be able to tell the difference.) I realise you are speaking particularly about pattern, but that these species are still morphologically dissimilar is valid. A better example possibly is D.pulcher to D.klugei which in almost all singular (as in those not made up of many averaged specimens) respects are next to identical.

Pt.2

The genes that control the characteristics in which domestic dogs differ are a small percentage of their total gene pool. Secondly, they still share the same set of base characteristics but vary in the form (shape, size, colour, length, etc) of these. I am unaware of any new characteristics having been developed as a result of a significant change in one or more genes but there may be some, but not many I’d warrant. Dogs have the longest history of domestication. Estimates range from 20,000 to 100,000 years. Through artificial selection, specific variations in the characteristics have been selected for, in exactly the same way as the different colours and patterns are line bred in snakes and lizards. Given the time man has been selecting specific forms of characteristics in dogs, the number and degree of variations is significantly more than for other domesticated animals. The bottom line is that all breeds of domestic dog are still capable of producing viable and fertile offspring if crossed. Size difference might prevent that occurring naturally but artificial insemination would give the results. So basically there simply hasn’t been sufficient genetic change for speciation. Those changes that have taken place are more apparent than substantial.

So I would say all dogs, dingos and grey/brown/timber wolves are the same species C.lupus. I do not go into a subspecies level it gets to complex and hard to define. However the interesting thing to my mind is more based on defining speciation in a captive animal. If an organism is defined as a species based on its ability to and that it does produce fertile offspring naturally. Than how can we ever define an animal that is not in nature? (I realise dogs are not all captive thus the massive problem with feral dogs and this creates problems with the example.) But this should not take away from the point that captive animals can in a way be viewed as man made things and in a sense based on our definition of species being defined around a concept of the natural perhaps not even be able to count as a 'species'. Of course I above stated dogs are a species and they should be I am simply highlighting a complexity occurring when an animal has been in captivity for so long a time.

Now for a Question.
Firstly with genetics now forming a large change in our way to understand relationships and 'speciation' where do you think and based on what reasons do we genetically draw a species line?
Furthermore what are your opinions on cryptic species (i.e. species that seem obviously disjoint through genetic analysis and do not naturally interbreed to produce a genetic hybrid to the best knowledge of those defining them.). Obviously these species are extremely closely related with either no way to tell them apart without genetics, (or in some cases a pseudo-cryptic species which requires analysis of internal morphology.). But genetics says they are in fact 'different' and while I accept this is fair enough to name a new species, is it really in our best interest to do so? In further explanation of that I ask what is taxonomy. Someone will answer that it is our way of defining species to view them based on relationships and evolutionary divergence. Which is essentially true, but that is not all it is. Taxonomy is our way of defining what something is so 'we' can understand it. So we can look at two animal and answer a question 'what are they' and then use that knowledge to answer 'why and how?'. If we go to the point of defining these species that are so close morphologically we need genetics to tell them apart, then are we really trying to answer why and how they are different?
I do not believe we truly are I believe we are in many of these cases defining for the sake of defining and not for the sake of the greater perspective of science.



Another question and this one is more personally based not so much scientifically based. I would love to be a taxonomist when I am older. Understanding what animals are and why has been my passion for as long as I remember and my father (a geneticist, taxonomist and ecologist) has given me great grounding and experience in the area both of the specific and broader questions of these areas, even though I am yet to leave high school. My problem however is that I do not ethically like the concept of collecting and killing animals simply for humans to have an understanding of them. I realise there are approaches that go around this, (i.e. collecting the animal and keeping it captive thus also allowing other research on its behavior and husbandry). But as you I am sure will understand this is not a 'hat' where one 'size' or approach fits all animals. And thus I am not looking for a total answer of 'what I should do', but simply some creative thinking/discussion on ways to look at this problem.
I understand many say that 'the good coming of understanding these animals through science out-ways the effect on individuals of the species. However I have weighed this strongly and have come to the (probably cynical) view that this is not really the case but an excuse allwoing for the bad behvaior of humans ('we need to understand it so when we build for the good of our species we can protect one patch of it' instead of 'lets just keep our environment good and not threaten this species in the first place') as well as it being a view that we as 'humans' have the right to understand out environment, which although I respect I do want to understand our environment, doing this at the expense of the individual organism is selfishness on the part of us as humans.

(That posts not too long is it? )

 

[Elapidae1]

Not yet out of high school? WOW

 

[GeckPhotographer]

Not yet out of high school? WOW

Yr 12 so close and yet so much effort to go.
​

 

[lace90]

Hey Geck, I am currently doing taxonomic work to delineate between species of a cryptic-species complex. It takes a lot of work, I can tell you that. The reason it is important to differentiate between these particular species is because of their economic importance for the control of botanical pests - they each occupy different host species. Genetically, different. Enough so that across multiple gene regions in both nuclear and mitochondrial DNA the species are monophyletically grouped. In this case, I am also using morphology, but this is more for interest as statistically there are no differences.
It must be stated that the species line is drawn at different levels for different species, as long as you make your point clear to its importance and there is enough evidence, you can make new species. Or as happens a lot, collapse species or genus'. As long as you have the time and funding

 

[GeckPhotographer]

Hey Geck, I am currently doing taxonomic work to delineate between species of a cryptic-species complex. It takes a lot of work, I can tell you that. The reason it is important to differentiate between these particular species is because of their economic importance for the control of botanical pests - they each occupy different host species. Genetically, different. Enough so that across multiple gene regions in both nuclear and mitochondrial DNA the species are monophyletically grouped. In this case, I am also using morphology, but this is more for interest as statistically there are no differences.
It must be stated that the species line is drawn at different levels for different species, as long as you make your point clear to its importance and there is enough evidence, you can make new species. Or as happens a lot, collapse species or genus'. As long as you have the time and funding

That sounds interesting and in your case I can see the importance. Thanks for the reply.
​

 

[Bluetongue1]

Pt1

I simply chose a species that showed strong variation in a feature so frequently used in identification that many presume it is representative of / equates to a different species. I believe that over a long period of time we have subconsciously been conditioned to believe that different species are distinguished on their gross appearance (general external morphology). Pick up any modern field guide and there it is. The point to be taken out of the example used is that features such as colour and pattern and the like, do NOT define a species, whether it’s a gecko or a dog.

But this should not take away from the point that captive animals can in a way be viewed as man made things and in a sense based on our definition of species being defined around a concept of the natural perhaps not even be able to count as a 'species'.

It is possible for dogs to freely breed with wolves. The sorts of things that stop this from happening are that the dog must also be accepted by the pack. If you were to raise a dog and wolf cubs together under semi-natural conditions, it may well be accepted as part of a pack when the animals were released. So maybe it could fulfil the biological definition.

Firstly with genetics now forming a large change in our way to understand relationships and 'speciation' where do you think and based on what reasons do we genetically draw a species line?

The biological definition of a species can be thought of as the baseline. If you are sitting in a museum workshop with a jar full of pickled specimens, the biological definition becomes intractable. However, we don’t simply turf it out the window. Species groups have consistent sets of characteristics (same set of genes) which vary, often within consistent limits. This is shared by the population due to gene flow (biological species definition at work). Using as many specimens as they can access, the taxonomists job it to determine the degree of variation within a sub-set of the total characteristics. From this you can develop a description which defines the species. From that you can select the most useful diagnostic features to use in the field.

Genetic analysis on examples of established individual species, closely related species and more distant species provides samples of the degree of genetic separation indicative of separate species for those types of animals. I know they use reproductive and mitochondrial DNA, how they amplify DNA using the polymerase chain reaction and what electrophoresis is, but that’s about it. Once they start talking about long fragments versus short fragments, genetic markers and the like, I sit down and shut up (or try not to snore too loudly).

Obviously these species are extremely closely related with either no way to tell them apart without genetics, (or in some cases a pseudo-cryptic species which requires analysis of internal morphology.). But genetics says they are in fact 'different' and while I accept this is fair enough to name a new species, is it really in our best interest to do so?

Two species may well be morphologically indistinguishable. However, what you must bear in mind is that any species has a unique gene pool. As soon as we find any excuse to deliberately send any species to extinction we are saying it’s alright to reduce species diversity on earth, so long as you can come up with good excuse. I make no apologies that it is not alright!!! Biodiversity is what holds the natural world together. It is what gives it its robustness. It is what has allowed it to sustain itself for thousands of millions of years before we came along. It is what drives it forward to cope with physical changes. It is what keeps humanity alive. Unfortunately there are many who live in a fool’s paradise in the misguided belief that the human race is no longer dependent on nature for its continued existence. The sad part of that… they won’t be the ones to foot the bill! There are others reasons often cited, such as we don’t know what chemicals that they may carry that could be helpful to mankind.

My problem however is that I do not ethically like the concept of collecting and killing animals simply for humans to have an understanding of them.

Have you ever eaten fresh fish? Have eaten kangaroo meat or fed it to pets? These are examples of wild taking in which the animals are killed for human consumption or for human pets to do likewise. As far back as 1798, Reverend Thomas Malthus published an essay on populations in which he pointed out that many more offspring are produced than can possibly survive. Let’s illustrate Malthus’ conjecture using a simple example, a gecko with an extremely low reproductive rate – one clutch of 2 eggs per year over a reproductive life of only 3 years. Out of the 6 offspring produced, only 2 are required to “take the place” of the parents to maintain a stable population size. Out of out 6 offspring, 4 CANNOT survive. In other words 2/3 of all the offspring produced MUST perish.

Of course, in reality nearly all geckoes multi-clutch and three clutches per year is fairly common. They are also reproductively active for longer. So from 6 offspring in our example let’s go to a more realistic, yet still conservative number of 24 offspring produced as a result of three clutches per year over four years. So we now have a more realistic number of 22 individuals that MUST perish (instead of 4) for each 2 parents in the population. Geckoes have low a low reproductive rate. Most other groups/species of reptiles and frogs are lot more fecund. Yet the more offspring produced, the more individuals there will be that cannot survive.

If we do not know what species are out there, how can we effectively control or assist them as required, including affording those that need it the appropriate measures of protection?

If we go to the point of defining these species that are so close morphologically we need genetics to tell them apart, then are we really trying to answer why and how they are different?

It is not just about learning about them. It is also about managing them. Refer to the two answers above.

 

[GeckPhotographer]

[Two species may well be morphologically indistinguishable. However, what you must bear in mind is that any species has a unique gene pool. As soon as we find any excuse to deliberately send any species to extinction we are saying it’s alright to reduce species diversity on earth, so long as you can come up with good excuse. I make no apologies that it is not alright!!! Biodiversity is what holds the natural world together. It is what gives it its robustness. It is what has allowed it to sustain itself for thousands of millions of years before we came along. It is what drives it forward to cope with physical changes. It is what keeps humanity alive. Unfortunately there are many who live in a fool’s paradise in the misguided belief that the human race is no longer dependent on nature for its continued existence. The sad part of that… they won’t be the ones to foot the bill!There are others reasons often cited, such as we don’t know what chemicals that they may carry that could be helpful to mankind.

If we do not know what species are out there, how can we effectively control or assist them as required, including affording those that need it the appropriate measures of protection?

I agree with conservation as much as you. Defining species is certainly useful for protecting species and managing species. You do not have to lecture me that every brown skink out there is worth saving. However when we look at animals that are so morphologically similar only genetics can tell them apart. Than we are talking of species that are also extremely closely related in terms of genetics. We are not talking of each genetically separate species something that is going to produce a new chemical or structure that humans can use. In fact the variance between these in a quantifiable useful manner is probably so little that people uninterested in nature would argue, conserve one of the species conserve the all. Obviously I do not agree with that thought, but I do say the 'useful' difference between these species is probably none.
This is simply my (cynical) but unfortunately probably realistic view on how defining cryptic species is really going to impact on management, based on your point of use to humans.

Another point is what is our right to interfere with nature? Both in destroying and importantly in saving wildlife, even when we are the cause of its decline. To what degree do we take it upon ourselves to play 'god'. Because whether we accept it or not we are part of nature and our effects on it are natural. Should we just leave nature to adapt around us?
Once again I think not but it is a question that should definitely be asked to at least some level.

Now what I said originally and what I stick by is the effect of describing species on human selfishness, and this relates strongly to management of species. When we know we have described species and we know how we can build to leave that species in existence then that gives us the excuse to destroy nature. We lose the view of 'maybe there is something worth saving in this patch of bush' to the new view of 'these species occur here, they are not threatened and also occur there, therefore we can build here.'
While I can see that a perfect world where human selfishness stops overriding our desire to expand necessitating destruction of nature is pretty much impossible. I am simply arguing that our desire to find out more about nature and protect it is in a way producing the opposite effect in that we lose the bigger picture of nature and look to specifically at the genetic difference between two otherwise identical lizards.

That said you have many good points and I certainly have nothing against the protection of any species or such no matter how similar.

Have you ever eaten fresh fish? Have eaten kangaroo meat or fed it to pets? These are examples of wild taking in which the animals are killed for human consumption or for human pets to do likewise. As far back as 1798, Reverend Thomas Malthus published an essay on populations in which he pointed out that many more offspring are produced than can possibly survive. Let’s illustrate Malthus’ conjecture using a simple example, a gecko with an extremely low reproductive rate – one clutch of 2 eggs per year over a reproductive life of only 3 years. Out of the 6 offspring produced, only 2 are required to “take the place” of the parents to maintain a stable population size. Out of out 6 offspring, 4 CANNOT survive. In other words 2/3 of all the offspring produced MUST perish.

Thats a yes and a yes. But I view the taking of meat, (i.e. kangaroo or fish) from nature as a food source for the sustenance of life as different to the taking of specimens for their so called 'benefit'. This comes back to the 'god' view. What gives us the right to choose for a species which individuals of a species should be taken (even at random) for the benefit of a species? This as I am sure you will respect is different to taking individuals of the species for the benefit of ourselves. A selfish act as it may be, I view choosing something for ones own species as far more acceptable than taking individuals of another species for that species.
My ethical view of this perspective is highly unlikely to change I have seen a lot of arguments about it being for the greater good of the species and while I full accept the truth in these arguments and respect this perspective, my perspective cannot justify the taking of any individual for the 'good' of that species separate to our own. (Even with our own possible benefits as you mention from "chemicals" or such)

All your reasons are good and sound. The opinions I express are simply another view both from mine and other perspectives and I welcome any criticism of any of them as long as it is reasonable and constructive.