These biological patterns seek an explanation: why do organisms come in groups and why are there groups of groups? Thus , there are two aspects to organizing organisms into groups: 1 a practical aspect, producing a way to manage all the variety of organismal life, to arrange its vast diversity, 2 a mechanistic process aspect, producing a system that will allow biologists to understand mechanisms that result in the patterns of diversity , e.
To a large extent these two aspects can be satisfied simultaneously, i. This book is primarily about organisms. We will be dealing with scientific names, and the groups that they describe, throughout the book and it is important to appreciate the basis and the limitations of the naming. But names are profoundly significant in ways that often are not appreciated. Like water to a fish and air to humans, we are so immersed in names that we rarely stop to consider them.
But names say much about humans, about how we think and what we think. Indeed, it is probably the case that names not only reflect how we think, they may actually dictate how we think. Names reflect the organization by which we view things and the way we process the information that we receive.
While classification is useful whenever one is faced with a large number of variable entities, we need to consider to what extent our classification is a reflection of our thought i.
Stated another way, names reflect an organization and it is important to consider whether the organization is inherent to what is being named or inherent to our minds.
Keep in mind that naming is a grouping process, i. Faced with diversity , humans lump things together into categories, putting similar things together into groups; this makes the diversity more manageable and this is what classification naming is all about.
Thus the fundamental question to address when naming groups of things is what criteria will be used to group them. For instance, if you are classifying motor vehicles one might group them based on color, on manufacturer , or on type of vehicle. When considering organisms deciding what criteria to group them on is a tough question: organisms are exceedingly diverse and they differ in myriad ways. Because living things have many, many characteristics, there are many different ways that they can be grouped.
One feature relates to the pattern of variation. Consider a group of organisms that has only one characteristic, or perhaps only one characteristic that might distinguish one organism from another, for example, a group of organisms that are all the same except for length. Figure 2 a and b show two such groups of organisms, one where a classification naming is an accurate reflection of reality and one where it is not. The difference between the two is in the pattern of variation.
Paraphyly non-monophyly : If a group of organisms includes an ancestor and only some of its descendants, that group is called paraphyletic or non-monophyletic. Scientists care about distinguishing paraphyletic and monophyletic groups because monophyletic groups provide information about how evolution has occurred which lineages emerge from which nodes whereas paraphyly does not.
Herpetology, the study of amphibians and non-bird reptiles - together known as "herpetofauna" or "herps" - is the study of a paraphyletic group because the group excludes mammals and birds, the latter of which are in fact reptiles. A group containing herps, birds, and mammals would be monophyletic, and this collection of animals would have a common ancestor at node 2 Figure 3.
This clade is known as the "Tetrapods. Polytomy: When an ancestral branch has just two descendants, we call that splitting pattern a dichotomy. If the ancestral branch has more than two descendants, it is a polytomy meaning cut into many parts.
A polytomy means that the relationships among these descendants are uncertain. Uncertainties in phylogenetic trees can exist because we have not yet been able to collect enough data to clearly disentangle or determine the relationships among those lineages. The way we classify lineages and clades within the Tree of Life into named groups is called a taxonomy.
Today, biologists generally agree that we should group organisms based on how they are related to each other through evolution. This means that the taxonomy we use should reflect shared ancestry that is, phylogeny , ideally by organizing individuals and species into monophyletic groups. Taxonomy is organized as a hierarchy. AmphibiaWeb predominantly uses four nested taxonomic levels that describe clades on the Tree of Life: order , family , genus , and species Figure 5.
When appropriate see Taxonomy considerations , we also use subgenus or subfamily names that provide additional evolutionary information regarding subsets of lineages within certain clades. A good example is the Northern Leopard frog , whose scientific name is Rana pipiens. This species falls into the broader family-level clade of Ranidae, known as the true frogs, which is within the order Anura Figure 5. The subgenus name of the Northern Leopard frog is Pantherana , which literally means "leopard frog"; Pantherana also includes other closely related species like the Pickerel frog Rana palustris and a recently-discovered species, the Atlantic Coast Leopard frog Rana kauffeldi.
Another well-known member of the family Ranidae is the American Bullfrog Rana catesbeiana , which falls into a different subgenus called Aquarana , which means "water frog".
Green frogs Rana clamitans are also in the subgenus Aquarana. A member of the genus Rana that currently has not been assigned a subgenus is the Wood frog Rana sylvatica. Nomenclature refers to the rules for how we curate names for lineages and clades. Nomenclature does not necessarily reflect evolutionary relationships or biology, but is simply a set of rules for maintaining stable taxonomy.
AmphibiaWeb promotes long-term stability in nomenclature and taxonomy because it helps easily organize information about species. This means that we prefer to keep names of lineages or taxonomic clades constant over time, even as we gather new information about them.
AmphibiaWeb chooses to accept or reject proposed taxonomic changes for specific lineages depending on whether the proposed changes both provide useful information for classifying organisms and promote the taxonomic stability of the group. A common misconception is that the newest taxonomy is the best taxonomy. Scientists are technically free to adopt or reject newly published taxonomic changes. AmphibiaWeb adheres to a number of criteria to help our community work with the most biologically-informed and useful taxonomy and nomenclature.
For an example of a decision tree for changing nomenclature, see Figure 6 adapted from Hillis We get more data we add information about lineages : Perhaps the most common change in phylogenies happens when we get new data about lineages that change our understanding of their relationships.
For example, phylogenies used to be based mostly on anatomy but they are now often based on combined assessments of DNA and anatomy, which have helped resolve previously unknown relationships within the Tree of Life. Species get added we find new lineages! Also the branch point that gives rise to organisms with legs is indicated at the common ancestor of mammals, reptiles, amphibians, and jawed fishes. It is easy to assume that more closely related organisms look more alike, and while this is often the case, it is not always true.
If two closely related lineages evolved under significantly different surroundings or after the evolution of a major new adaptation, they may look quite different from each other, even more so than other groups that are not as closely related.
For example, the phylogenetic tree in [Figure 4] shows that lizards and rabbits both have amniotic eggs, whereas salamanders within the frog lineage do not; yet on the surface, lizards and salamanders appear more similar than the lizards and rabbits.
Another aspect of phylogenetic trees is that, unless otherwise indicated, the branches do not show length of time, they show only the order in time of evolutionary events. In other words, a long branch does not necessarily mean more time passed, nor does a short branch mean less time passed— unless specified on the diagram. For example, in [Figure 4] , the tree does not indicate how much time passed between the evolution of amniotic eggs and hair. What the tree does show is the order in which things took place.
Again using [Figure 4] , the tree shows that the oldest trait is the vertebral column, followed by hinged jaws, and so forth. Remember that any phylogenetic tree is a part of the greater whole, and similar to a real tree, it does not grow in only one direction after a new branch develops. So, for the organisms in [link] , just because a vertebral column evolved does not mean that invertebrate evolution ceased, it only means that a new branch formed.
Also, groups that are not closely related, but evolve under similar conditions, may appear more similar to each other than to a close relative. Scientists continually obtain new information that helps to understand the evolutionary history of life on Earth. Each group of organisms went through its own evolutionary journey, called its phylogeny.
Each organism shares relatedness with others, and based on morphologic and genetic evidence scientists attempt to map the evolutionary pathways of all life on Earth. Historically, organisms were organized into a taxonomic classification system. However, today many scientists build phylogenetic trees to illustrate evolutionary relationships and the taxonomic classification system is expected to reflect evolutionary relationships.
How does a phylogenetic tree indicate major evolutionary events within a lineage? The phylogenetic tree shows the order in which evolutionary events took place and in what order certain characteristics and organisms evolved in relation to others. It does not generally indicate time durations. Skip to content Chapter Diversity of Life.
Learning Objectives By the end of this section, you will be able to: Discuss the need for a comprehensive classification system List the different levels of the taxonomic classification system Describe how systematics and taxonomy relate to phylogeny. Art Connection Figure 2: At each sublevel in the taxonomic classification system, organisms become more similar. Dogs and wolves are the same species because they can breed and produce viable offspring, but they are different enough to be classified as different subspecies.
This interactive exercise allows you to explore the evolutionary relationships among species. What is a phylogeny a description of? What do scientists in the field of systematics accomplish? Which statement about the taxonomic classification system is correct? There are more domains than kingdoms.
Kingdoms are the top category of classification. A phylum may be represented in more than one kingdom.
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