The Daily Bucket is a regular feature of the Backyard Science group. It is a place to note any observations you have made of the world around you. Insects, weather, fish, climate, birds and/or flowers. All are worthy additions to the bucket. Please let us know what is going on around you in a comment. Include, as close as is comfortable for you, where you are located.
Names are important. It is very difficult to communicate if you don't use names. Here on the Daily Bucket diaries each diary and comment are full of names.
But names also tell us how we think. What sets of things get lumped under a single name?
The first diary focused on the rules governing taxonomy, most notably the rule of precedence which governs which scientific name is the correct one. We also discussed two different ways that this can result in a name change for a species with a well known and long-used scientific name.
What we are going to do now is talk about species. In particular we're going to address this question: why do the species names change all the time? Why is the slate-colored junco now the dark-eyed junco? Why are winter wrens on the west coast now called Pacific wrens? We'll get to that shortly but first a bit of background.
All organisms in the Linnean taxonomy are classified into a series of hierarchical groups, Kingdom, Phylum, Class, Order, Family, Genus, and Species. In many cases additional ranks are used as well (e.g. Suborder or Superfamily). We'll discuss these in more detail in a third diary but what I want to point out now is that the ranks are, in at least one sense, arbitrary human constructs. For example the plant family Cactaceae (cacti) is an easily recognizable group of organisms, clearly distinct from all others. It is composed of three distinct groups of plants ('typical' cacti, prickly pear and cholla, and a small group of tropical trees with the typical cactus flower and spine structure). There is no reason that each of those groups (now subfamilies) could not be made families and Cacti become a superfamily. It is a matter of judgement. Another way to think about it is that different groups with the same rank are not equivalent (it is not even clear how you would measure equivalence). The Cactaceae and the Asteraceae are not both families because they represent evolutionarily equivalent units. Botanists decided that they were groups that were reasonable to consider as families. The lowest main taxonomic rank is species. Species are thought to be biologically 'real' in a sense that the higher taxa are not.
An examination of organisms in general reveals that we don't exist as a big group. Organisms tend to exist in groups of very similar individuals with small differences between them and then 'gaps' in form until you get to another group of very similar organisms. Humans vary in any number of characteristics and so do chimpanzees. However we don't tend to see individuals with characteristics halfway between the two. A nice, concise but somewhat more technical way to phrase this is that phenotypic variation is discontinuous across all living things.
The conclusion is that a general trend in evolution is not just change but separation into discrete groups which we call species. In other words, as things change they tend to separate into groups which become more and more distinct from one another over time. Unlike a genus, a species has objective biological reality (as a rank). There is a reason that Darwin called his book 'On the Origin of Species' and not On the Origin of Orders or something else.
Given that, it is surprising that not a lot of attention was paid to defining species or consider how species formed (despite his title Darwin doesn't really talk much about details of how species form) until about the 1940s. Before that time (and to a large extent to this day) what was used to define species is what we now know as the Morphological Species Concept. Species were/are defined by a series of observable characteristics of their structure (morphology). Everything that fit the description was a member of that species.
The problem with this approach is that it ignores potentially important characteristics and it also makes the definition somewhat arbitrary. Many species, particularly wide ranging species, vary somewhat over their ranges and the decision about where to draw lines is a judgement call in this system. This approach produces species that may end up being just as arbitrary as higher taxa such as families.
Ernst Mayr introduced the Biological Species Concept in the 1940s and it has proved enormously influential (I'm guessing many readers were taught it in high school). Mayr argued that the process of speciation (i.e. one species becoming two) was a process of evolving reproductive isolation. In other words,all members of a species are capable of interbreeding with one another in nature and the process of speciation involves evolution so that a single species splits into two species in which members of one species can no longer breed with members of the other species.
Mayr's concept has been enormously popular with evolutionary biologists who are interested in studying process (i.e. whose primary interest in this case is understanding the mechanisms that drive the formation of new species). Adoption of the Biological Species Concept to define bird species was responsible for combining Red-shafted and Yellow-shafted Flickers in a single species. Also the creation of the Dark-eyed Junco from several different species. And so on. In case it was decided that there was sufficient interbreeding between the different forms that they could be considered the same species.
It has been less popular with evolutionary biologists who are more interested in studying evolutionary pattern, for whom the question of species definition is more important. If you are interested, for example, in studying the rate of species formation in the different subfamilies of Cacti, you would need to have some kind of generally applicable way to define species. As a way of universally defining species the Biological Species Concept has its problems.
1) It can't be applied to species that only reproduce asexually such as bacteria.
2) It can't be applied to extinct species
3) It requires fairly detailed field observations and/or lab experiments that may not be feasible in all cases
4) Even in some sexually reproducing groups, reproductive isolation doesn't seem to evolve but clearly distinct forms do. For example in oaks there are many species and lots of hybridization between them in nature. But they remain distinct.
5) If you have geographically isolated populations (i.e. populations on two islands) that have no opportunity to ever interbreed how do you decide if they are the same species or two different species?
For this reason there has been some push back against the biological species concept, specifically relating to its use in defining species. A number of different species concepts have been proposed over the years. The above link lists only a few of them, look here for a more exhaustive list.
The most influential of these concepts are probably the phylogenetic and evolutionary species concepts. I'm not enough of an expert in this area to give you the technical difference between them but the emphasis in both is on differences in evolution leading to definable differences among groups. For example imagine three populations A, B, and C. A and B have high rates of movement of individuals between them. Although they exist in somewhat different environments and thus experience somewhat different natural selection the movement of individuals keeps the two populations similar. Population C is further away and movement of individuals between it and populations A and B is very rare. When that happens those individuals can and do interbreed but it isn't happening often enough to prevent population C from evolving in a different direction than the others.
The application of this concept leads to the creation of many new species. Hence Pacific Wren, Oak and Juniper Titmice, etc. The situation is even more extreme in sedentary organisms. The Slimy Salamander (Plethodon glutinous) has been split into a large number of different species in eastern North America based on genetic differences among populations.
My personal opinion is that a lot of these changes are arbitrary. There are two problems with trying to apply some sort of universal species concept. The first is that although speciation seems to be a more or less universal phenomenon the way in which it is manifested may vary a lot among different organisms. The second is that speciation is usually not an instantaneous process. There are bound to be lots of organisms out there that are part way through the process of splitting to form two different species. Where you want to draw the line is going to be a bit arbitrary. My preference would be for an acceptance that the lines are always going to be a bit arbitrary and only change them when there is new information that really changes our biological understanding of what is going on.