UPCAT Biology — Taxonomy & ClassificationStudy Notes

Taxonomy is the science of naming, describing, and classifying organisms into groups called taxa (singular: taxon). This classification system is based on shared characteristics and evolutionary relationships between organisms. The main purposes of taxonomy include: organizing the vast diversity of life forms, providing a universal naming system that scientists worldwide can use, understanding evolutionary relationships between different species, and facilitating the study of biodiversity. The modern taxonomic system follows a hierarchical structure, meaning that organisms are grouped into increasingly specific categories. Each level of this hierarchy represents a different degree of relatedness between organisms.

The modern classification system consists of eight main taxonomic ranks arranged from the broadest to the most specific: Domain, Kingdom, Phylum, Class, Order, Family, Genus, and Species. Domain is the highest and broadest category, currently including three domains: Bacteria, Archaea, and Eukarya. Kingdom is the second highest rank, with six kingdoms: Eubacteria, Archaebacteria, Protista, Fungi, Plantae, and Animalia. Phylum groups organisms within a kingdom based on major structural characteristics. Class groups organisms within a phylum based on more specific similarities. Order groups organisms within a class based on even more specific characteristics. Family groups closely related organisms within an order. Genus groups very closely related species together. Species is the most specific category, representing organisms that can interbreed and produce fertile offspring.

The six-kingdom system is widely used to classify all living organisms based on their cellular structure, mode of nutrition, and other fundamental characteristics. Kingdom Eubacteria consists of prokaryotic organisms with peptidoglycan cell walls, including common bacteria like E. coli. These are mostly unicellular and can be autotrophic or heterotrophic. Kingdom Archaebacteria contains prokaryotic organisms with unique cell wall compositions and often live in extreme environments like hot springs or very salty water. Kingdom Protista includes mostly unicellular eukaryotic organisms like amoeba, paramecium, and various algae. They can be autotrophic or heterotrophic and show great diversity in form and function. Kingdom Fungi comprises heterotrophic organisms that obtain nutrition through absorption, including mushrooms, yeasts, and molds. They have cell walls made of chitin and can be unicellular or multicellular. Kingdom Plantae consists of multicellular, autotrophic organisms with cellulose cell walls that produce their own food through photosynthesis. Kingdom Animalia includes multicellular, heterotrophic organisms without cell walls that obtain energy by consuming other organisms.

Binomial nomenclature is the scientific naming system developed by Carolus Linnaeus that gives each species a unique two-part scientific name. The first part is the genus name, which is always capitalized, and the second part is the species name, which is always lowercase. Both names are written in italics or underlined when handwritten. This system ensures that each species has only one correct scientific name that is recognized worldwide, regardless of local common names. The genus name can be shared by closely related species, but the combination of genus and species names is unique to each species. Scientific names are usually derived from Latin or Greek words and often describe a characteristic of the organism or honor a person or place.

Organisms exhibit different types of life cycles based on when mitosis and meiosis occur during their development. There are three major types of life cycles. Diplontic life cycle occurs when mitotic division happens only in diploid cells. The organism spends most of its life in the diploid stage, and haploid cells exist only as gametes. Animals follow this type of life cycle. Haplontic life cycle occurs when mitosis happens only in the haploid stage. The zygote is the only diploid stage, and it immediately undergoes meiosis to produce haploid spores. Most fungi and some algae like Chlamydomonas follow this pattern. Haplo-diplontic life cycle, also called alternation of generations, occurs when mitotic division happens in both haploid and diploid stages. Organisms alternate between a multicellular haploid stage (gametophyte) and a multicellular diploid stage (sporophyte). All land plants and many algae exhibit this type of life cycle. In isomorphic alternation, the gametophyte and sporophyte look similar, while in heteromorphic alternation, they look different.

Modern classification goes beyond physical similarities and focuses on evolutionary relationships through phylogeny - the study of evolutionary history and relationships among organisms. Cladistic analysis uses cladograms to examine derived characters (new characteristics) that arise in evolutionary lineages. This approach groups organisms based on shared derived characteristics rather than overall similarity. The current system recognizes three domains: Bacteria, Archaea, and Eukarya. Domain Eukarya is further divided into four major supergroups: Excavata (includes diplomonads, parabasalids, and euglenozoans), SAR Clade (Stramenopila, Alveolata, and Rhizaria), Archaeplastida (red algae, green algae, and plants), and Unikonta (amoebozoans, fungi, and animals). This system better reflects evolutionary relationships and is continuously updated as new molecular and genetic evidence becomes available.