Are pinnipeds “living transitional” animals?

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Discussion by: draymunoz
I have a question. Would an animal like a pinniped (seal) be considered a living transitional animal? That is to say an animal that harbors physical traits of two vastly different species, in the seals case fins of marine animals and a skull more reminiscent to land mammals than to marine mammals such as whales. Some here might give me the technical answer and say “every living animal is a transitional animal since they are are all links in the chain of evolution.” Although I agree with this, there is of course an obvious reason why we do not classify all fossils as transitional fossils, only certain ones such as tiktaalik get label that because of their distinct anatomy. I want to know why or why not a pinniped may be regarded as such. Thank you 

23 COMMENTS

  1. It depends on your historical perspective. Tiktaalik can be said to be transitional because we know that the tetrapod descendants of it or its relatives eventually gave rise to legged land animals. Seals, however, may all go extinct and not give rise to anything.

  2. There are four options for all extant species. Time will tell what happens.

    1) Radically adapt to changing conditions. Change as needed, perhaps ultimately become a transitional species.
    2) Become optimised to conserved conditions. Do this long enough and become a “living fossil”.
    3) Fail to adapt, or be out competed in your niche.  Become extinct.
    4) Have the population cleaved and separately follow one or more of the above in different ways.  Become a basal species.

    Of course, 1 and 2 are not mutually exclusive.  Some traits may be conserved while others diverge.

  3. All species are transitional. 
    It’s just that certain ones like  tiktaalik, illustrate a particular lineage. – in that particular case the transition from lobed fins for walking on the sea-bed, to the legs and arms of amphibians and land animals.
    As evolution has no “objectives” beyond adapting to changes for gene survival, such transitions are always made with hindsight.
    The evolution of marine mammals is well illustrated for whales, here:- http://ngm.nationalgeographic….

    There is no reason to believe that seals will follow the same route:  – only that such things are possible.

    The fins of early fish evolved into the arms, legs and wings of later vertebrate animals. Even today there are “transitional” uses of fins in the amphibious fish for walking on land – Mudskippers.

    http://ihatetheocean.blogspot….

    http://www.netmeister.org/blog

  4. Alan4 is dead on (as usual).  Every living thing you see and every fossil you find is transitional.  When scientists talk of transitional species, they usually are speaking about a form that illustrates a certain, specific point between two groups of organisms.

    When creationists speak of transitional species, they are usually demonstrating just how much they do not understand anything biological.

  5. What I really want to know is how cetaceans developed an up-down swimming motion. Dolphins that have lost their flukes tend to go to a side to side swimming motion, which can cause deformities to the spine. Sea lions don’t use their hind flippers at all in swimming – they just pull with long front flippers. Polar bears also just use their front feet for propulsion. Seals, on the other hand, move their hind flippers from side to side in order to propel themselves forward. It seems like that motion would come more naturally than the swimming motion of cetaceans.

  6. I’d disagree that any living species could be described as transitional. Extinct species that were the last species of an evolutionary branch were not transitional, because they never evolved into anything else. And we cannot say which species that exist today will evolve into new species. I think that the only species that can be described as transitional are those that evolved into other species.

  7. Jumped Up Chimpanzee – I’d disagree that any living species could be described as transitional.
    Extinct species that were the last species of an evolutionary branch were not transitional, because they never evolved into anything else.
    And we cannot say which species that exist today will evolve into new species.

    The argument is largely semantic.
    All living species are transitional until they go extinct and are no longer living. 
    While changes from one generation to the next are small, because of the pace of evolution over long time scales, no species remains unchanged (even if some “living fossils” only have slight changes) through the generations.
    The term “species” is also rather fuzzy around the edges in the real world , where human classification systems for convenience and labelling, only approximately fit the variations within a gene pool.  Species vary over time as well as over geographical distribution.

    Classifications of sub-species and varieties, only partially address this, once you get down to fine details, – which is why there are arguments between botanists/zoologists about where boundaries and lumper/splitter demarcations lie. 

    I think that the only species that can be described as transitional are those that evolved into other species.

    That is where the term “transitional species”, is useful in debate for highlighting major links to evolutionary branches, but these are only the larger more obvious transitions between wider spaced groups.

  8. I don’t think you could call seals transitional in the way I think you mean. There would have to be an evolutionary link between the various groups concerned. Whales did not evolve from land mammals via the ansectors of modern seals. Even if they were related, odern seals would more accurately contain transitional features. Probably most well known “transitional” fossils were not direct ancestors of new taxa, but probably offshoots of the real ancestral species. 

  9. The argument is largely semantic.All living species are transitional until they go extinct and are no longer living. 

    I agree it is a semantic argument, so I’m looking for a meaningful use of the term “transitional species”.

    I don’t see how it is a meaningful use of the term “transitional species” to apply it to all living species or, indeed, to ANY living species.The only meaningful use of the term “transitional species” is to use it to label a species that sits somewhere on an evolutionary branch between 2 other species, as a way of demonstrating a particular course of evolution.

  10. That’s exactly the point – all species sit on an evolutionary branch between two other species! Unless they go extinct with no descendants but even then the fossils of such a species will share traits with the descendants of it’s cousins. 
     I think the use of the term ‘transitional’ in the initial post leaves a bad taste in my mouth because it hints at evolution toward a predetemined point. Whether pinnipeds eventually evolve into creatures with a very different morphology is irrelevant when you consider that, at the moment, they are as perfectly adapted to their environment as they can be. 

  11. Jumped Up Chimpanzee
      I don’t see how it is a meaningful use of the term “transitional species” to apply it to all living species or, indeed, to ANY living species.

    If you took the ancestry of humans (or any other species) from the time we were fish to the present day and sampled the population every 100,000 (or 50,000, or 20,000) years, you would produce a chain of “transitional species”.

    The only meaningful use of the term “transitional species” is to use it to label a species that sits somewhere on an evolutionary branch between 2 other species, as a way of demonstrating a particular course of evolution.

    What you are missing is the change in species over time (which is the nature of “transitional species”).
    We are not the same “species” as our distant fish ancestors.
    The changes in evolution are continuous ( as is the usual diversity within gene-pools), but classifying them in separate boxes requires SAMPLING. 
    That is why in botany there is a TYPE SPECIES – ( http://en.wikipedia.org/wiki/T… ) from within the diversity of the gene-pool, as a reference example.  Species are NOT made up of identical genetic clones!

    An illustration of the genetic continuum is RING SPECIES ( http://en.wikipedia.org/wiki/R… ) where the branching of evolution can be seen taking place. http://en.wikipedia.org/wiki/F… –
    http://en.wikipedia.org/wiki/F

    Sometimes Natural Selection has removed “intermediate transitional forms”, some times there is a whole string of closely related genera, species, sub-species, and varieties illustrating the continuous branching as genetic diversity fans out. 
    In any case there is usually a diversity and geographical separation of forms, within a gene-pool, where selection pressures are different.  (Such as mountain, lowland, or coastal varieties.)

    Jumped Up Chimpanzee – I agree it is a semantic argument, so I’m looking for a meaningful use of the term “transitional species”.

    Martin_C – Unless they go extinct with no descendants but even then the fossils of such a species will share traits with the descendants of it’s cousins.

    In studying fossils, because of rarity, many examples are “transitional genera” rather than “species”.
    Much detailed modern field-work is in studying these present-day variations at the level of transitional sub-species or varieties, in the tracking of their evolution.

    (It is rather like asking a science-duffer to identify African animals in an ecosystem!  You can bet that ALL they identify, will be big enough to fall over, or they get attention because they bite!)

    In the study of evolution the fine detail, is just as important as the big and blatantly obvious!

  12.   Martin_C
    I think the use of the term ‘transitional’ in the initial post leaves a bad taste in my mouth because it hints at evolution toward a predetemined point.

    “Transitional” is about where their lineage has come from, not about where it is going. http://en.wikipedia.org/wiki/P…  – We can however plot present trends in selective pressures and track recent local or global changes.

    Whether pinnipeds eventually evolve into creatures with a very different morphology is irrelevant when you consider that, at the moment, they are as perfectly adapted to their environment as they can be. 

    If they were “perfectly adapted”, Natural Selection would not be causing the diversification of species which is shown – and on-going in this chart.

    http://upload.wikimedia.org/wi

  13. Because cetaceans are mammels, and have the bone structure in the hips to facilitate running originally with legs, as for example a dog does. Cetaceans are not fish, who do not have a decendancy originating in running on the land.

    Dolphins that have lost their flukes (I guess you mean in an accident or some such) have to still get about, and just like a dog (for example) that loses its back legs in an accident uses the next most efficient means to do so. This causes a side to side motional effect that causes perhaps deformities due to them not being evolved to use that method.

    Sea lions do use their hind flippers, just not so obviously so, more for steering, still however the mammilian backbone and hip structure force a verticle swimming method, although it is not so obvious as in dolphins.

    Polar bears also do not use a side to side motion, and do use their hind legs, to swim, mainly to tread water.

    Seals and sea lions use the flippers as individual items like the legs/feet their ancestors possessed, and the vertical movement is less obvious, and not the same motion entirely as dolphins etc, as the dolphins et al have lost the individual feet/leg derivatives from their ancestors, whereas seals and sealions have some of this feature still present…

    Hope that helps, I think I’ve got that somewhere right….

  14. Jumped Up Chimpanzee –
    I don’t see how it is a meaningful use of the term “transitional species” to apply it to all living species or, indeed, to ANY living species.The only meaningful use of the term “transitional species” is to use it to label a species that sits somewhere on an evolutionary branch between 2 other species, as a way of demonstrating a particular course of evolution.

    Just to clarify the point I made earlier. –

    There are two different, but valid meanings of “transitional species”.

    The first is the common ancestor at the branch in the tree of life where two or more species diverge – as illustrated in this comment: – http://richarddawkins.net/disc

    The other meaning, is that even if the line of descent of a particular species did not diverge into a tree of branching separate species, the line of ancestral species would not remain unchanged, so there would be a whole series transitional palaeontological fossil species, which would be quite different from their present-day descendants.

    Changes from one generation to the next will only be in small steps, but over longer time, ancient species will change into a series of new species separated by time. – As I explained here:- http://richarddawkins.net/disc

    All life does this, so all life-forms are transitional, between the past ancestral species they have been, and the future species they will become – if they survive.

  15. Alan,

    Thanks for your explanations.

    I understand the very gradual process of evolution and the fact that, as you say,

    all life-forms are transitional, between the past ancestral species they have been, and the future species they will become – if they survive

    It’s just that it has always been my understanding, as a non-biologist, that in general discussion the term “transitional species” is commonly used to label an extinct species that helps fill a significant gap in a particular branch of evolution, in order to provide further evidence for that particular path of evolution.

    So while all life-forms are transitional (and equally so) in a very strict sense, I still think the meaningful use of the term in general discussion is when it’s applied to the species that help fill the large gaps.

  16. “there is of course an obvious reason why we do not classify all fossils as transitional fossils”

    yes, because their line ended. if pinnipeds evolve into something else, maybe more whale like then their fossils will be called transitional. if they just die out then no.

    i suspect the reason you’ve picked this example is because in human pigeonholeing terms, they’re not one thing or the other (not fully land mammal, not fully equatic) but that is quite a narrow view. you’re right that ALL species can be considered transitional, but the only thing i can think that singles out pinnipeds as different is human bias towards catagorizing animal types.

    whether on land or water, they use the same limbs to get about. compare that with the ancestors of insects whos breathing apparatus evolved into a means of propulsion (gills to wings).

    also, consider the idea that nothing is determined. the descendants of modern pinnipeds may well go back to land and their limbs re-evolving into some sort of feet

    [edit correction thanks to Sjoerd Westenborg]

  17. If you watch how cetaceans swim, the movement comes from the hip area. Think of the hunching of a running animal, the bunching and extension of the spine near the back legs at the pelvis. 

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