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Evolution
is the core of modern Taxonomy (Systematics) Taxonomy
– classifying living organisms according to their evolutionary
relationships (phylogeny). Includes
describing, identification, naming (nomenclature), and classification (a
hierarchy of group relationships) [Taxonomy may also be called systematics
or pylogenetics]
Put simply, taxonomy is the study of biological diversity Levels of
biological diversity (biodiversity): Biodiversity
studies that involve the public are currently underway in many parts of
the world including nearby Tennessee. These efforts to document the
all of the species found within a particular area are called ATBI's or All
Taxa Biodiversity Inventories. See the following web site for more
information: http://www.dlia.org/atbi/ Carolus Linnaeus 1750’s publication “System of Nature” in which he created most of these categories or groups below (recall a taxon is a group, e.g. a species is a taxon or a group of individuals of the same kind; a genus is a group of related species, a family is a group of related genera, an order is a group of related families, a class is a group of related orders, a phylum is a group of related classes, and a kingdom is a group of related phyla): Linnaeus also is also credited for consistently using binomial nomenclature to name species. The name of a species is a two word name, the first word is the genus (capitalized first letter of genus name) and the second word is the specific epithet (written in lower case). Both parts of the species name are set off in different font (italics or underlined) to indicate that they are Latin words. How did species form? Linneaeus considered each species specially created and unchanging. Today, modern science explains new species formation through a process that can be called speciation. Speciation is the formation of new species from preexisting species. What is a species? Here are two concepts:
For purposes of our coverage of evolutionary biology the biological species concept is most important to understanding modes of speciation. 2 modes of Speciation: 1. Allopatric Speciation - speciation resulting from geographic isolation. A geographic barrier prevents gene flow between separated populations. Over time given different situations in the the separated regions, the reproductively and geographically isolated populations accrue new mutations resulting in differences leading to different kinds of organisms. For example, consider the Nene, a goose endemic to the Hawaiian Islands thought to be derived from Canada Geese long ago blown off course. http://www.thewildones.org/Animals/nene.html
Back to the Linnaean Hierarchy: The
Hierarchy of
Classification
(the
bold names are the ranks of various taxa). A taxon is a
group (taxa is plural, i.e. groups). An example of a taxon is the
species Quercus alba (a group of interbreeding populations of white
oak trees). Another taxon is the genus Quercus (a group of
related species of trees that produce acorns). It should be noted that the application of names is artificial, a human
imposition on nature, a human construct. But we advance our state of knowledge through
such tools as naming and the sharing of ideas of human construction.
While it can be said that species are real, those white oak trees really
do exists, the higher taxa, genus and above, are theoretical ideas about
relationships. Also, it should not go unsaid that
modern
classifications are viewed as evolutionary hypothesis about the
relationships of organisms.
[To recall the hierarchy of classification, consider the mnemonic "King Phillip Came Over From Genoa, Spain" where each word begins with the same letter as a taxon, King is for Kingdom and so on] The hierarchy of classification ideally
reflects our understanding of the evolutionary relationships among the
diverse forms of life. But
the idea that dogs evolved from bacteria-like life forms of long ago seems
impossible at best. To begin
to close the impossible gap that exists between dog and bacteria we must
borrow from the geologic theory of uniformity:
small changes can accrue (via natural selection operating on random
mutations) over vast amounts of time resulting in major changes.
Thus, through the process of natural selection, changes in the
genetic makeup of populations may ultimately result in new species and new
genera and new families, etc. Continuing
along this line of reasoning, closely related species may continue to
change becoming ever more different until they are no longer considered
very similar, thus there is a common ancestor shared between trees we call
oaks and trees we call maples but oaks and maples are clearly different at
both the genus and family level.
With continued change from the earliest life forms, life has
diversified into very distantly related groups we call Kingdoms.
Dogs are in the kingdom Animalia and bacteria are in a separate
kingdom thus reflecting this ancient, shared ancestry. Study figure 14.4, p. 222-223, "Milestones in the history of life" 6 kingdoms: Eubacteria, Archaebacteria, animals, fungi, protists, plants. [Thinking of life as either plant or animal is deeply engrained in our psyche. Yet we now know the animal-plant dichotomy is wholly inadequate to characterize most of biological diversity. As an example, consider bacteria. Bacteria all vastly different from all other life. Bacteria lack a nucleus. We learn from an early age in school that the nucleus is "command central," it is the structure that holds all genetic information, it is a structure generally taken for granted as present in living cells. Yet, bacteria are without this seemingly crucial cellular structure that is present in all plants and animals. Thus, bacteria can't be considered in any logical classification scheme as either plant or animal. Additional kingdoms have been erected for nucleated forms of life which lack an embryonic stage. The lack of an embryo during the maturation of progeny produced by sexual reproduction precludes their inclusion within either the plant or animal kingdom for both plants and animals are characterized by the presence of an embryo.] Using the figure 14.4 as a guide, consider these four main points that illustrates the complexities of evolution. v Many of the earliest forms of life remain highly successful today. v Evolution is (messy) not a linear progress towards human perfection. Consider the cross linkages below. v Endosymbiosis Theory [early nucleated cells engulfed prokaryotic cells that become the organelles mitochondria (see light brown cross link) and chloroplasts (see light green cross link)] Ø evidence: mitochondria and chloroplast have their own bacterial-like DNA, they have bacteria-like ribosomes, they replicate independent of cell replication. Secondary endosymbiosis (between a heterotrophic eukaryote and another photosynthetic eukaryotic that it engulfed) explains the multiple membrane bound chloroplasts found in various protistan groups (see dark green cross link in fig. 14.4) v Classification systems are subject to change as new information discovered: ex. We have changed from a two kingdom system to a 5 kingdom system, to a 6 kingdom system, and some propose even more kingdoms that would further divide the relationships among living organisms. New ideas about relationships between organisms are reflected in the push to recognize a new category (Domain) above the rank of Kingdom. Molecular microbiologists have proposed three Domains: Archaebacteria, Eubacteria, and Eukaryotes (see p. 213). The idea is that the two bacterial domains are very distantly related to one another, they diverged very early in the history of life (see Fig. 14.4) and the eukaryotic line soon developed, presumably off of some early archaebacterial type and later through endosymbiosis gained mitochondria derived from eubacteria through endosymbiosis. Cladistics dominates modern approaches to phylogeny Shared, novel features (derived traits) are used to define various groups, or in this case, clades--branches; a group consists of a branch or branches. The branching diagrams from your lab book show shared, derived traits as bars along the branches. The eukaryote clade is defined in part by the shared, novel trait of possessing mitochondria. Cladistics does not require the use of the formal categories of the Linnaean hierarchy. |