A number of factors help biologists decide whether an organism belongs to a new species. In the case of Homo floresiensis, the new hominid presented a unique combination of primitive and more recently evolved (derived) features not found in any other taxon. Some of the important characteristics used to differentiate among hominids are: brain size (earlier hominids had brains with volumes around 400-450 cm3, while modern humans have brains averaging 1,300 cm3); jaw shape (during human evolution, jaws have become less elongated, with the development of more pronounced chins); and bipedal posture (whether or not they walked on two legs). Homo floresiensis presents a small brain volume, but has facial and dental features more similar to Homo erectus, the closest known relative to modern humans. In addition, Homo floresiensis appears to have walked on two legs.

The Family Hominidae contains humans, great apes and their extinct relatives (http://tolweb.org/tree?group=Hominidae). Members of this family also are referred to as "hominids." The Tribe Hominini consists of several, related genera (Homo, Ardipithecus Australopithecus and Paranthropus) with bipedal posture, among other shared, derived characteristics. Members of this tribe are called "hominins." Current evidence now points toward three species of the genus Homo: Homo sapiens (modern humans), Homo erectus and Homo floresiensis.

In sympatric speciation, reproductive isolation is said to arise within a single, freely, and randomly interbreeding population in the absence of any spatial segregation. Thus, gene flow is initially restricted by biological features of organisms rather than by geography or distance. Because even low levels of gene exchange can swamp out the build up of genetic differentiation that is required for speciation, the occurrence of sympatric speciation is highly debated and controversial. Models of sympatric speciation propose various evolutionary forces and processes as the driving force behind the genetic divergence required for reproductive isolation in sympatry, including diversifying selection (a form of natural selection) and polyploidy (when multiple, duplicate copies of the genome are present within individuals). 

When populations inhabit environments with multiple resources and microhabitats, some individuals may possess traits or characteristics that allow them to use one of the resources and/or microhabitats more efficiently. Over time, diversifying selection can cause the population to split into genetically distinct groups that are adapted to discreet niches or the use of different resources within the environment. If selection pressures are strong enough to overcome gene exchange in the population, speciation can occur in sympatry.

An example of sympatric speciation resulting from diversifying selection may be found in flies of the genus Rhagoletis. These flies exhibit strong fidelity to the host plants in which they mate and leave their offspring to develop. Until the mid-nineteenth century, Rhagoletis in the northeastern United States used hawthorns exclusively as their host plant. However, when apples were introduced in some areas approximately 150 years ago, a new "race" of Rhagoletis appeared that inhabits apples rather than hawthorns. Because hawthorns and apples are often found in the same geographic area, the hawthorn and apple-maggot flies can exist in trees that are only yards apart. Many consider that the hawthorn flies are the parent species of the apple flies and that the speciation event was initiated by genetic variations that caused some members of the original population to be attracted to new hosts. This argues for sympatric speciation in Rhagoletis. However, this claim has come under debate. For example, some argue that the hawthorn and apple-maggot flies descended from distinct, independent lineages, through allopatric speciation.

Even if there are no temporal or behavioral cues to keep individuals of two species from hybridizing, it may simply be physically impossible for mating to take place. Mechanical isolation occurs when two species have significant anatomical differences that prevent them from mating.

For example, many species of the fly genus Drosophila are virtually indistinguishable except for differences in the male and female genitalia. Similar to the workings of a lock and key, males cannot copulate successfully with females from the wrong species. .

Mechanical isolation is a premating isolating barrier as well as a prezygotic isolating barrier.

When hybrid zygotes are formed, they frequently die at some point during their development due to genetic incompatibility between the two parent species. This isolation mechanism, known as hybrid inviability, prevents the genomes of the two species from mixing. In this way, the gene flow between the two populations is impeded. One example of hybrid inviability occurs in the frog genus, Rana. Interbreeding between some species of Rana results in the formation of hybrid tadpoles, but the tadpoles die before they become reproductive adults. Thus, the populations corresponding to different species remain genetically isolated.

One example of hybrid inviability occurs in the frog genus, Rana. Interbreeding between some species of Rana results in the formation of hybrid tadpoles, but the tadpoles die before they become reproductive adults. Thus, the species remain distinct.

Hybrid inviability is a postmating isolating mechanism and also a postzygotic isolating mechanism.

In some cases, hybrid zygotes successfully develop into adults but the adult individuals do not produce viable gametes. This isolating barrier, known as hybrid sterility, restricts the amount of genome mixing that can occur across two species.

A classic example of a sterile hybrid is the mule, the offspring of a female horse and a male donkey. All mules typically are sterile. However, within other taxonomic groups, hybrid sterility is displayed in only one sex. For example, in the fly genus Drosophila, crosses between different species produce sterile male hybrids and female hybrids that usually are fertile.

When hybrid sterility affects one sex preferentially, it is generally the heterogametic sex (the sex with two different sex chromosomes) that is sterile, while the homogametic sex (the sex with a matching pair of sex chromosomes) is fertile. This is known as "Haldane's rule."  In most mammals and insects, including flies, the males are heterogametic and the females are homogametic. Birds and reptiles are the opposite. That is, the males are homogametic and the females are heterogametic.

Hybrid sterility is both a postmating and a postzygotic isolating barrier. 

Yeast are living organisms that belong to the Kingdom Fungi. Species of the yeast genus Saccharomyces are used in baking and brewing. These yeasts metabolize sugars and produce carbon dioxide as a waste product. In fresh bread dough, the accumulated carbon dioxide makes the bread rise and become spongy. When yeast break down sugar in an environment without oxygen, ethyl alcohol also is produced as a waste product.

This activity begins by asking students to predict what might happen when yeast are “fed” table sugar (sucrose, C₁₂H₂₂O₁₁). Sucrose consists of two smaller sugar molecules (monosaccharides): glucose and fructose.

The complete Food and Fitness Activities Guide for Teachers may be downloaded as a PDF file from the Teacher Resources menu on BioEd Online: http://www.bioedonline.org/resources/nsbri.cfm

Classification systems attempt to solve the problem of providing meaningful groupings of organisms.  The Swedish scientist, Carolus von Linnaeus, is credited with introducing binomial nomenclature and hierarchical classification as an organized way of naming and describing organisms and their relationships to one another.  Binomial nomenclature refers to the use of a two-part name for each species (one name designating genus and one designating species).

Linnaeus described a hierarchical classification system using seven taxonomic categories, or taxa (Kingdom, Phylum, Class, Order, Family, Genus, Species).  Beginning with species, each category becomes progressively more comprehensive.  For example, while the leopard, tiger and domestic cat all belong to different genera, they are grouped together in the same family.

Taxonomy is the science of classification.  When taxonomic systems include hypothesized evolutionary relationships among groups, the field generally is referred to as Phylogenetics.  Systematics is a larger field involving classifying organisms based on their phylogenetic relationships.  Systematics can be thought of as the study of biological diversity and how that diversity evolved.  In a sense, Charles Darwin introduced systematics in his revolutionary work, The Origin of Species.  He wrote, "The natural system is founded on descent with modification; that the characters which naturalists consider as showing true affinity between any two or more species, are those which have been inherited from a common parent, and, in so far, all true classification is genealogical" (Darwin, 1859).

Early naturalists identified plants and animals by observable structural similarities and referred to organisms using long complicated phrases.  This was known as the "polynomial system."  In this system, a plant might be described by phrases of 12 or more words.  It is not surprising that polynomial names could become very complex and were often misinterpreted when translated from one language to another.

In the 1700s, Carolus von Linnaeus, sometimes referred to as the Father of Classification, described a binomial system, which was published in his early work, System Naturae (1735).  Although he created the two-word system as a short-cut for users of this work, the system was rapidly adopted as a manageable way of naming species.  
 
In the binomial nomenclature system, genus and species-just two names-replace the long string of words used in the polynomial system.  The meaning of words can differ from language to language and from country to country.  For example, in Great Britain, the word "buzzard" refers to an organism Americans call a hawk.  For this reason, scientific names are written in Latin to maintain a uniform system of naming across all languages.

In the binomial system, genus is always a noun, underlined (or italicized), and capitalized; species is a descriptive term, underlined (or italicized), and not capitalized.  Some examples of binomial names include: Quercus rubra (red oak), Panthera pardus (leopard), or Homo sapiens (human).

Carolus von Linnaeus created a hierarchical classification system using seven taxonomic categories, or taxa (Kingdom, Phylum, Class, Order, Family, Genus, Species).  These categories are based on shared physical characteristics, or phenotypes, within each group.  Beginning with kingdom, each successive level of classification becomes more and more specific.  Organisms within the same order have more in common with one another than organisms within the same class. For example, all species of bears are mammals, but not all mammals are bears.  A useful pneumonic tool to help students remember the hierarchical classification system is: "King Phillip Came Over For Green Soup," with the first letter of each word representing each category, beginning with kingdom and ending with species.

Staphylococcus is a genus of gram positive bacteria (approximately 0.5–1.0 µm in diameter), of more than 30 species. Members of this genus are sphere-shaped and grow in clusters, pairs, and occasionally short chains. MRSA (Methicillin-Resistant Staphylococcus aureus) is a strain of Staphylococcus aureus bacteria that has developed resistance to an entire class of antibiotics (called beta lactams), including methicillin, penicillin, amoxicillin, and oxacillin. In addition, S. aureus is often found to be resistant to antiseptics and disinfectants. Over the past four decades in the US, it has evolved from an easily controlled microbe to a major public health problem.