The Morphological Species Concept encompasses groups of populations descended from a common ancestor that are as similar in appearance to one another as are members of a biological species and are separated from other such groups by a morphological gap as great as that between biological species.
Although often used to define species of organisms that reproduce asexually, fossil species usually are morphological species since data required to assess reproductive isolation seldom are present.
The portion of the definition that compares differences to those between biological species is somewhat problematical in that cryptic biological species sibling species show few morphological differences and thus must be ignored for purposes of definition.
Cryptic species normally require genetic information for recognition. In practice, we often use the biological species concept but assess the likelihood of reproductive isolation based on morphological criteria.
For example, if two populations have the opportunity to interbreed but show no or rare morphologically intermediate individuals, it generally is assumed that little or no gene flow is occurring between them.
The species especially biological species is considered the most objective of rank of the taxonomic hierarchy. Even if we don't know whether two populations would interbreed if given the chance, there is some actual state of reproductive status between them. Other ranks, such as genera and families are more subjective, depending more on the weight that taxonomists place on the similarities and differences.
One taxonomist might consider a group of species to represent one genus, while another having the same data might consider them to represent two or more genera. Generally, an unstable concensus is reached that may eventually break down because of greater biological knowledge or because of changes in philosophy between generations.
In the latter case, the tendency has been for the formation of larger units such as genera followed by later taxonomists subdividing these into smaller units followed by later lumping these once more into larger units. Those that advocate larger units often are called lumpers and those favoring smaller units splitters. Lumping and splitting because of philosophical differences are different from those required by advances in biological knowledge.
A species, for example, may be found to be more closely related to members of a genus different than the one it has been placed in; movement into the other genus is required by the new biological knowledge. Many species have been shuffled about through the years, sometimes shuffled back and forth between genera as more and more data accrue.
Relationships essentially are hypotheses and, as data accumulate, hypotheses may be strengthened or weakened. In the early days of mammalian research, places where mammals were collected tended to be scattered about, with lots of gaps between collection stations.
It often was found that two stations had kinds of mammals that were similar, but yet recognizably different. A sample of one kind of mice, for example, might be overall similar to one from another station, but with some differences in color, in length of tail, size of ears, etc.
Since these were recognizably different they often were named as separate species, usually within the same genus. As geographic gaps were filled in, however, not uncommonly the mammals from the intermediate sites were intermediate in character states. Eventually, then, these animals were recognized as actually being members of the same species despite some differences. This means that the same species living in two different areas will start to have different traits.
Furthermore, it can be extremely difficult to apply the same definition across all of biological life, from bacteria to complex mammals, which is why the concept of a subspecies was introduced. As you can imagine, a subspecies is subordinate to a species.
This is where things start to get even murkier. Join the ZME newsletter for amazing science news, features, and exclusive scoops. More than 40, subscribers can't be wrong. It all started with population variation. Biologists studying mammals saw that across the same species, individuals could be very different.
For example, you could find two mammals of the same species that are different colors, or that behave differently. Take dogs, for example. The main reason for this is that dogs originated from domesticated wolves, and there is much less genetic variation between dog breeds than between wolves — even if they look so different.
Basically, a species is the largest group within which interbreeding produces viable offspring. S o, it can be extremely difficult to say when a new subspecies starts to emerge or when a subspecies becomes so distinct that it is a brand new species.
What about Kanye and Taylor versus two chimps? It may surprise you to learn that penguins have twice as much genetic variation as humans do. And this subspecies of chimp has more genetic variation than all the humans on earth. Humans haven't been around as long as other animals. We haven't had enough time to develop much variation in our DNA. Humans also migrate and reproduce a lot. Different groups of humans are never apart long enough for genetic differences to add up.
Human DNA is too similar to split us into subspecies or races. So we can't use biology to sort people into groups like we can with animals. Even though our DNA is similar, humans look very different from each other. But the traits we use to guess someone's race don't always work well. Think about skin color. There aren't just a few colors: there are more shades than you can ever count. The traits we use are also independent of each other.
No matter which traits we use, there is no good way to group humans using appearance or DNA. Instead, everyone groups people into races based on the traits they think are most important.
The way we categorize people into races changes over time, too. Think about people you know who are Irish or Italian. Today we might categorize them as white, like many people from Europe.
But years ago, they were not considered the same race as Americans with European origins. Humans love to organize things into groups. But when it comes to race, these groups tell us more about our culture than they do about our biology. Have a different answer or more to add to this one? Send it to us.
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