Biological Classifications: Their Basis in Evolutionary

Biological Evolution

Biological Classifications: their Basis in Evolutionary Relationships

NSES Generalization

Biological classifications are based on how organisms are related. Organisms are classified into a hierarchy of groups and subgroups based on similarities which reflect their evolutionary relationships. Species is the most fundamental unit of classification.

The great diversity of organisms is the result of more than 3.5 billion years of evolution that has filled every available niche with life forms.

Further Description

Observations of the structural diversity of organisms lead to an understanding of the commonality of structure and function. The great diversity of organisms on Earth is classified into a hierarchy of groups and subgroups based on similarities of external and internal structure, chemical makeup, and evolutionary relationships. The most basic unit is the species, which in sexually reproducing populations consists of organisms that successfully reproduce among themselves. The classification system most commonly used today separates organisms into five major groups called kingdoms. The five kingdoms are: Monera, Protista, Fungi, Plantae, and Animalia. Kingdoms are further divided into subgroups.

The scheme used to classify organisms was devised by the eighteenth century Swedish naturalist Carolus Linnaeus. Linnaeus developed what he called the binomial system for naming organisms. According to his scheme, the scientific naming of organisms consists of two parts: the genus, which contains one species or a group of similar species, and the specific epithet (usually an adjective), which in some way describes the genus. Both names taken together denote the species name.

Biologically speaking, a species can be considered as (1) a reproductive unit whereby individuals of the same species are reproductively isolated from all other groups; (2) an ecological unit in that a species will interact with other species; and (3) a genetic unit consisting of inheritable traits and variations of those traits being selected from generation to generation.

The grouping of organisms into a hierarchical scheme is known as taxonomy. Each category in a taxonomic scheme defines an important set of characteristics unified according to phylogeny or common ancestry. Common descent should indicate similar genes, and hence, similar characteristics. This may not always be the case. Selective pressures of the environment may cause different species that are unrelated to have similar characteristics. Likewise, phylogenetically related organisms may have dissimilar characteristics.

Biological diversity can be understood in terms of the number of different kinds of organisms living in today’s world. This diversity of species can be grouped into five major categories, from bacteria and protists, Earth’s earliest life forms, to the diversification of multicellular plants, animals, and fungi. The large number of different kinds of organisms is the direct result of selective pressures on a great variety of body forms, behavioral responses, and physiological activities. Through selection, variations of individuals within a species have the opportunity to occupy, successfully or unsuccessfully, a wide range of habitats found on this planet. Successful variations compete better and will be maintained through reproductive success. Many millions of years of selection, driven by environmental pressures placed upon variations within a population, have resulted in the adaptation of millions of new forms. Because the environment is constantly in flux, variations are continually being tested and new species added to this tremendous catalog of life on Earth.

Much about the structure and function of life forms comes from comparative anatomy, comparative embryology, and comparative biochemistry.

Concepts Needed

Grade 9

Binomial nomenclature, genus, species, hierarchical scheme, classification, taxonomy, reproduction, kingdoms, inheritance, characteristics

Grade 10

Prokaryotic cells, eukaryotic cells, life cycles, haploid, diploid, meiosis, growth

Grade 11

Sporophyte, gametophyte, embryonic development, common descent, organization of body forms, alteration of generations, symmetry

Grade 12

Molecular taxonomy, cladogram, geologic time scale, plant and animal diversity

Empirical Laws or Observed Relationships

Observations of similar characteristics for organismsChomologies (characters similar due to shared descent) vs. analogies (characters similar because of parallel adaptation or chance); geostratification; increased diversity and complexity of living organisms over time; the rate of evolutionary change differs dramatically among different groups of organisms, and from one geologic era to another, as well as among genes of different function. Theories or Models

Systematics, monophylogeny, polyphylogeny, numerical phenetics, cladistics, amino acid sequencing, DNA sequencing, punctuated equilibrium