What makes a species a species?

[Top Trilo]

What makes a species a species?

Why are these two both the same species, canis familiaris,

And these two completely different

[gigantoraptor]

As far as I understand (as teached in school), two species cannot produce fertile offspring. Dogs can mate in between breeds (although the two in the picture might be a stretch), and produce fertile offspring. Horses (Equus caballus) and donkeys (Equus africanus asinus) for example cannot do this and would only produce a sterile hinny or mule. 

[Top Trilo]

Thanks gigantoraptor I was thinking something like that but then how do you with fossils? Is there a way to know if one species is for sure different than another?

[FranzBernhard]

2 hours ago, Top Trilo said:

how do you with fossils? Is there a way to know if one species is for sure different than another?

No, it isn´t. That´s one reason why there are so many synonyms.

Among paleontologists, there are two poles: lumpers and splitters. No explanation necessary.

But this also hold true for extant species, just stumbled across Ostrea edulis:

And this isn´t worst case...

Franz Bernhard

 

[jpc]

Biologists and paleontologists have been discussing thisa question for 150 years and more.  The answer can be quite complicated but gigantorapor's answer is pretty good.  

[Top Trilo]

Thanks guys

[Kane]

56 minutes ago, gigantoraptor said:

As far as I understand (as teached in school), two species cannot produce fertile offspring. Dogs can mate in between breeds (although the two in the picture might be a stretch), and produce fertile offspring. Horses (Equus caballus) and donkeys (Equus africanus asinus) for example cannot do this and would only produce a sterile hinny or mule. 

Actually, there have been a few cases where a non-sterile offspring is produced. Rare, but it does happen (I'll have to fish out the paper on that one). The chromosome count becomes the average of the two (63). When something like this happens, it challenges the definition of species, which is already problematic as it is not prescriptive, and Darwin's use of it was not to be taken literally (and so there is really no truly operational definition as one would find in physics). Like jpc said, it's a long-running debate! If technically two members of the same species have the capacity to produce offspring, there are interesting cases when we consider a species of bird that gets separated geographically for some time and develop different birdsong. Technically they *could* reproduce if brought together, but they don't recognize each other's song, and so don't. 

 

What a species is, and the name assigned to it, may be two separate things. But with some fossils, the species name assigned to a specimen to make it distinct from a morphologically similar one is far from perfect. For example, with trilobites we still don't have evidence of sexual dimorphism, which means it is possible that we are giving different species names to the same species that differ on the basis of their sexually-based characteristics. Without their DNA, we might never know!  

[jpc]

1 hour ago, Kane said:

If technically two members of the same species have the capacity to produce offspring, there are interesting cases when we consider a species of bird that gets separated geographically for some time and develop different birdsong. Technically they *could* reproduce if brought together, but they don't recognize each other's song, and so don't. 

 

 

Look into speciation in red crossbills for an example of this.

[RuMert]

Dog breeds are isolated artificially with a lot of effort (while dogs themselves are more then eager to mate no matter the breed). Maybe with enough time dog breeds will transform into true species, but I doubt humanity will last long enough to witness it. Species is a functional concept, it turns around isolation and specialization, not visible characteristics. Fossils different enough beyond sexual dimorphism and caste system (as in ants) are surely different species isolated naturally for a long time unlike dogs. Reproductive barrier is probably the main characteristic of a species: thus reproduction defines almost anything: species, castes, dimorphism, lifespan, generation differences and so on

[FossilDAWG]

The definition given by gigantoraptor is the standard one: each species can be thought of as a gene pool that cannot interbreed with (i.e. exchange genes with) other gene pools (=species).  This definition works well in most cases but there are lots of exceptions.  If individuals of two species can interbreed and produce a viable but sterile offspring, that fits the definition because the mixed genome of the hybrid is not transmitted any further.  However there are also examples where the hybrids are fertile and can cross with other hybrids, or back-cross to either parent species.  I just read about an extreme example of some salamanders (Ambystoma) that form all-female species in which females can mate with multiple males of different species, use some genes from each of the males, and produce female offspring that carry genes from several fathers.  There are other oddities such as ring species, which is " a connected series of neighbouring populations, each of which interbreeds with closely sited related populations, but for which there exist at least two "end" populations in the series, which are too distantly related to interbreed, though there is a potential gene flow between each "linked" population. " (definition from Wikipedia).  Examples include herring and lesser black-backed gulls in the Arctic Circle, and Ensatina salamanders in California.  Among insects, there are examples of non-interbreeding populations that behave as distinct species, but the cause is a symbiotic bacteria (Wolbachia) with different bacterial strains in each population.  When the symbiont is removed with antibiotics the populations become fully interfertile.

 

Apart from the "gene flow" definition of species, the "old school" definition, still useful today, is that each species will be characterized by a unique set of features that might include morphology (shape, color, etc i.e. features you can see and measure) or even behaviors that prevent interbreeding with related species.  For example, one of the best examples of an actual speciation event involves Rhagoletes flies.  Originally these flies developed in Hawthorn fruits.  When European settlers brought apple trees to North America, some populations shifted to using apples as a food for their larvae.  Since hawthorns and apples produce fruit at different times, the population that shifted evolved a different life cycle with mating and egg laying synchronized with the availability of apple fruit.  As a result they no longer interbreed with the hawthorn-using species.  Over time, as the two populations accumulate unique mutations, they will evolve morphological features that can also be used to distinguish them.

 

When it comes to fossils we can't measure gene flow or behavior, so we are left with morphology.  This means that some paleo-species may include multiple biological species that had not yet accumulated enough genetic differences to have distinct morphologies.   Another problem is that it can be hard to assess biological variation in fossil species if they are rare and we don't have a large enough sample size.  Since we have only the option of comparing morphology between individuals, we run into the issue of "splitters" and "lumpers".  Splitters will use the smallest difference (number of spines, convexity of the shell, etc) to name a new species.  Lumpers will assume such variation reflects biological variation, sometimes to an extreme.  The ideal would be to have a large enough sample size to do a statistical analysis on various samples and show that they can be reliably separated by quantifiable characters.  That situation is not common though so paleontologists often have to make "educated assumptions" about how much variation occurs between individuals in a species versus how much variation occurs between species.  Another complication is that we know that many modern species vary depending on the environment.  For example mollusc shells can vary in thickness and spinyness depending on wave energy or predation pressure.  Individuals can vary a lot, but those individuals with thinner shells will be killed and removed from the population in places where waves crash down with enough energy to smash thin shells so all you will see remaining is thick-shelled individuals.  In deeper/quieter water more of the thin shelled individuals may survive.  In fossil samples the two populations will look quite different even though they are the same species.  It is useful to consider the environment, which may be reflected in sedimentary structures or rock type/sediment grain size etc, when trying to decide if two sets of fossils represent distinct species, but it is rare that this is explicitly done in my experience.

 

Well enough about that for now.  Time to get back to work.

 

Don

[Huntonia]

2 hours ago, Kane said:

there are interesting cases when we consider a species of bird that gets separated geographically for some time and develop different birdsong. Technically they *could* reproduce if brought together, but they don't recognize each other's song, and so don't

Conversely, another interesting similar occurrence; as with the greenish warbler after two populations were separated and developed different adaptations and were considered a perfect example of the evolution of species. Then it was discovered the two can successfully hybridize in nature. If I understand correctly they are still considered distinct as the hybrid offspring is not well equipped for survival and is limited to a very small range. 

Link:

https://www.google.com/amp/s/phys.org/news/2014-05-epic-evolution-species.amp

[Top Trilo]

This is a lot of great information. Thank you for the help I will have to look into some of these examples you guys provided

[Rhiguita]

The species concept also gets brought up sometimes in biology in regards to the number of species and how they are measured, as in a lot of amphibians and reptiles the number of species is likely less due to morphologically similar animals being considered the same species even though they are genetically distinct, as with water monitors being traditionally considered one species with several subspecies now being considered a species complex, where similar species were all lumped together as one, although another issue is that species is somewhat subjective as the biological species definition is that it can be hard to tell in the wild whether or not 2 individuals are the same species. also this does not cover plants, since plants are considered very messy in terms of species since they have the bad habit of cross breeding cross family and having fertile offspring.

 

In paleontology a thing that gets brought up in lumping and splitting is ontogenetic variation, which is where a organism changes how it looks as it grows up, and this becomes a issue in fossils some times as juvenile members of some species get separated due to looking different. a way this gets corrected is often finding enough specimens to cut down the number of species. for example dracorex  and stygimoloch were considered separate genera but overtime they were reclassified as juvenile forms of pachycephalosaurs, so in paleontology age of a fossil animal is also very important. another interesting example is in fossil plants, where sometimes multiple names can be given to the same organism, such as one for the leaf and one for the flower and one for the stem, as unless they are all connected we cant tell in paleontology if they were all from the same organism.