Learning Objectives

26.1 Taxonomy

• Compare and contrast organisms from each of the three domains of life.

• Explain why phenotypic similarity does not necessarily indicate close evolutionary relationship.

A. Chapter 26.1 Overview
 

     In chapter 26.1, the chapter goes over taxonomy and how scientists today name and classify animals into one of the three domains of life.  Taxonomy is the science of describing, naming, and classifying both extant and extinct animals.  Extant means that that the animal still exists and lives today.  Taxonomy results in the ordered division of species into groups based on similarities and dissimilarities in their characteristics.  Taxonomy itself has been going on for over 300 years since the first naturalist, John Ray, made the first attempt at broadly classifying all forms of life.  His ideas were then later continued by Carolus Linnaeus in the mid 1700's, starting the official beginning of taxonomy.

     Systematics is the study of biological diversity and the evolutionary relationships among organisms, both extant and extinct.  In the 1950's, Willi Hennig, a German entomologist, proposed that evolutionary relationships should be inferred from new features that are shared by the descendants of a common ancestor.  Since then, scientists have applied systematics to taxonomy, and researchers try to place new species into taxonomic groups based on evolutionary relationships with other species.  Also, previously established taxonomic groups are being revised as new data helps to shed more light on evolutionary relationships. 

     A Hierarchy is a system of organization that involves successive levels.  In biology, every species is placed into several different nested groups within a hierarchy.  For example, both leopards and flies are animals, despite their greatly different characteristics.  You can then take out the much smaller feline group out of the animals, which would include the leopards, lions, tigers, and many other cats.  The feline group, which is a subset of the animal group, has species that all share many similar traits.  The species that are placed together into smaller taxonomic groups are likely to share many of the same characteristics.  Biologists use these hierarchies to help separate and group different species.

B. The 3 Domains of Life

     In modern taxonomy, species are placed in progressively smaller hierarchical groups.  Each group at any level is called a taxa.  The taxonomic group called the kingdom was originally the highest and most inclusive group.  Linnaeus had classified all life into two kingdoms: plants and animals.  In 1969, ecologist Robert Whittaker proposed a five-kingdom system in which all life all life was classified into the kingdoms Monera, Protista, Fungi, Plantae, and Animalia.  However, as biologists began to find out more about the evolutionary relationships among these groups, they found out that these groupings didn't correctly reflect the relationships among them. 

     In the late 1970's, based on the information in the sequences of genes, biologist Carl Woese proposed the idea of creating a category called a domain.  Under this system, all forms of life are grouped within three domains: Bacteria, Archaea, and Eukarya.  The Domain of Bacteria consists of prokaryotes that  mainly have diacyl glycerol diseter lipids in their membranes, bacterial rRNA, and no nuclear membrane.  The Domain of Archaea consists of prokaryotes with no nuclear membranes, distinct biochemistry and RNA markers, possess unique evolutionary history, and are often characterized by living in extreme environments.  The Domain of Eukarya consists of all eukaryotes and have a nuclear membrane, unlike the other two domains.

     One of the original Kingdoms, Monera, was split into two domains, Bacteria and Archea, due to the major differences between the two types of prokaryotes.  The domain Eukarya previously consisted of four kingdoms, Protista, Fungi, Plantae, and Animalia.  However, researchers discovered that Protista isn't a separate kingdom but is rather a broad collection of organisms.  Taxonomists now place eukaryotes into seven groups called supergroups.  In the taxonomy of eukaryotes, a supergroup lies between a domain and a kingdom.  Kingdom Plantae is within the supergroup called Land Plants and the kingdoms Fungi and Animalia are within the supergroup Opisthokonta. 

Distinguishing Cellular and Molecular Features - Chart

C. Taxonomic Hierarchy

     It is helpful to categorize species into groups because subdividing the millions of species in the three domains of life makes it easier for biologists to appreciate the relationships between the large numbers of species.  The largest taxon is the domain, which is divided into supergroups among eukaryotes.  Below the domain and the supergroups is the kingdom, which is divided into phyla (singular phylum).  Each phylum is divided into classes, then orders, families, and genera (singular genus), each containing progressively fewer species that are more similar to each other.

Here is an example of the order scientists use when going down and naming a specific species.  For this example, the final animal goal is Homo sapiens or humans.    First, out of the domain, you have to chose one of the 3 kingdoms, Plantae, Fungi, and Animalia.  For this example, it's Animalia Next, you chose the phylum under Animalia, which in this case is Chordata Next Is the class under Chordata that we need, Mammalia Then the order after Mammalia, primates  After Primates is the family needed, which is Hominidae Then the genus after Hominidae, Homo  Finally, the species after the genus, sapiens

D. Binomial Nomenclature

     Binomial Nomenclature is the main method for naming species.  The scientific name of every species has two names, it's genus name and it's unique species name.  One example is the grey wolf, Canis lupis.  When naming a species based on it's binomial nomenclature, always capitalize the genus name and keep the species name not capitalized.  Also, the entire name for the species is always italicized.  You can also abbreviate the name and shorten the genus name down to the first letter once it has already been written previously in a passage.  For example, since Canis lupis  has already been mentioned, you can shorten it to C. lupis. 

     When naming a new species, genus names are always nouns or treated as such while the species can be either a noun or an adjective.  Many of then names have a Latin or Greek origin and can refer back to the species itself, either by it's appearance or its habitat.  There are certain rules that have to be followed when naming a new species that has been created by the International Commission of Zoological Nomenclature (ICZN), and as long as the rules are followed, anyone, not just the scientists, can name a new species. 

E. Phenotypic Similarity vs. Evolutionary Relationship

     As we know, there are many species that look alike and may act similarly too.  However, despite outward appearances, the two species may not be very similar when it comes to their evolutionary relationships with each other.  For example, two species that have adapted to the same environment may appear very similar physically.  However, this doesn't mean that they had the same descendant.  One way of determining just how close two species are in their evolutionary relationship is to compare just how recently they shared a common ancestor.  Take the baboon and the chimpanzee for instance.  They both appear very similar structurally and behaviorally at times.  But, when you go back in the phylogenic tree, you will see that the last time they shared the same descendant was at the Order, both being in the Order of Primates.  Form that point on, they separated and shared no common ancestors after that.  They both have successfully adapted to life in the forest, but have adapted to it differently.  Despite their great phenotypic similarities between the two, they are not too closely related once you consider their distance in their evolutionary relationships.  This can be seen in many other examples of species too, not just in this example.

     On the other hand, species that don't look very alike at all can be very closely related.  One example is the relationship between birds and lizards, which are surprisingly closely related when considering their great differences in phenotypic variation.

F. Useful Materials

G. Virtual Lecture

Grading Sheet (Victoria)