Chapter 18: The Tree Of Life

The Linnaean System of Classification Words to Know: Taxonomy, Taxon, Binomial Nomenclature, Genus, species

Linnaeus Swedish botanist Carolus Linnaeus introduced a scientific naming system in the 1750’s. Before that time, naturalists could not talk about what they had discovered because there was no universal naming system.

Taxonomy Taxonomy is the science of naming and classifying organisms. This gave scientists a standard way to refer to species and organize the diversity of living things. Linnaeus’s system classifies organisms based on their physical hierarchy. A group of organisms in a classification system is called a taxon (taxa) The basic taxon in the Linnaean system is the species.

Taxonomy From broadest to most specific Linnaean’s Taxonomy is: Kingdom King Broadest Phylum Phillip Class Came Order Over Family From Genus Germany Species Sunday Most Specific

Scientific Names Binomial Nomenclature is a system that gives each species a two-part scientific name using Latin words. The first part of the name is the Genus and the second name is the Species. The Genus is always CAPITALIZED and the species is lowercase. Binomial Nomenclature is ALWAYS written in Italics or Underlined. Ex: Quercus alba (white oak tree) and Puma concolor (mountain lion)

The Linnaean Classification System has Limitations Linnaeus created his classification system BEFORE technology allowed us to study organisms on the molecular level. Linnaeus’ system does not account for similarities that evolved this way. Scientists now use genetic research to help classify living things. Ex: The Giant Panda and Raccoon have been placed in the same family in the Linnaean system BUT the Panda is more closely related to bears genetically than raccoons. The Red Panda is actually more closely related to the raccoon.

Classification Based on Evolutionary Relationships Words to Know: Phylogeny, Cladistics, Cladogram, Derived Characteristics, taxon

Phylogeny To classify species according to how they are related, scientists must look at more than just physical traits. Modern classification is based on evolutionary relationships. The evolutionary history of a group of species is called a Phylogeny. Phylogenics can be shown as branching tree diagrams.

Cladistics The most common method used to make evolutionary trees is called cladistics. Cladistics is classification based on common ancestry. A Cladogram is an evolutionary tree that proposes how species may be related to each other through common ancestors. The traits that can be used to figure out evolutionary relationships among a group of species are those that are shared by some species but are not present in others. These traits are called Derived Characteristics.

Cladogram

Interpreting a Cladogram In a cladogram, groups of species are placed in order by the derived characters that have added up in their lineage over time Each place where a branch splits is called a node. Nodes represent the most recent common ancestor shared by a clade. You can identify clades by using the “snip rule”. Whenever you “snip” a branch under a node, a clade falls off. Ex: Tetrapod Cladogram All organisms in this clade have the derived character of 4 limbs. Embryo protected by amniotic fluid show where mammals branch off. Skull openings lead to turtles By the time you get to birds, they have every characteristic that comes before them.

Species’ Relatedness Today, new technology allows biologists to compare groups of species at the molecular level. In many cases, molecular data agree with classification based on physical similarities. Ex: Based on physical traits, most biologists considered segmented worms and arthropods to be more closely related than other species. Through molecular comparisons they discovered that round worms, NOT segmented worms should be more closely related to arthropods.

Molecular Clocks Words to Know: Molecular Clock, Mitochondrial DNA, Ribosomal RNA.

Molecular Clocks In the early 1960’s, biochemists Linus Pauling and Emile Zuckerkandl proposed a new way to measure evolutionary time. They compare amino acid sequences of hemoglobin to determine ancestry.

Molecular Evolution Molecular Clocks are models that use mutation rates to measure evolutionary time. Pauling and Zuckerkandl found that mutations tend to add up at a constant rate for a group of related species. The more time that has passed since two species have diverged from a common ancestor, the more mutations will have built up in each lineage, and the more different the two species will be at the molecular level.

Linking Molecular Data with Real Time To estimate mutation rates, scientists must find links between molecular data and real time. If scientists know when the species began to diverge from a common ancestor, they can find the mutation rate fro the molecule they are studying. Links can also come from fossil evidence.

Mitochondrial DNA and Ribosomal RNA. Mitochondrial DNA is DNA found only in mitochondria, the energy factories of cells. The mutation rate of mtDNA is about ten times faster than that of nuclear DNA, which makes mtDNA a good molecular clock for closely related species. It is ALWAYS inherited from the MOTHER of mitochondria in a sperm cell and are lost after fertilization. This helps trace cells back many generations. Ribosomal RNA is useful for studying distantly related species, such as species that are in different kingdoms or phyla. Mutations in Ribosomal RNA are at a much slower rate.

Domains and Kingdoms Words to Know: Bacteria, Archaea, Eukarya, prokaryote, eukaryote

Classification is Always a Work in Progress 1753 – classification has 2 kingdoms: plantae and animalia 1866 – classification has 3 kingdoms: protista, plantae, animalia. 1938 – classification has 4 kingdoms: monera, protista, plantae, animalia. 1959 – classification has 5 kingdoms: monera, protista, fungi, plantae, animalia 1977 – classification has 6 kingdoms: archae, bacteria, protista, fungi, plantae, animalia Now – Domains – broader than kingdoms: archae, bacteria, eukarya

The Three Domains The Three Domains are Bacteria, Archaea, and Eukarya The domain Bacteria includes single-celled prokaryotes in the kingdom Eubacteria. Eubacteria can be classified by many traits, such as their shape, need for oxygen, and whether they cause disease.

Archaea The domain Archaea are single-celled prokaryotes. However, the cell walls of archaea and bacteria are chemically different. Archaeabacteria can live in Extreme environments such as deep sea vents, hot geysers, Antarctic water, and salt lakes. All archaea are classified in the kingdom Archaeabacteria.

Eukarya The domain Eukarya is made up of all organisms with eukaryotic cells. Eukaryotic cells have a distinct nucleus and membrane-bound organelles. They can be single-celled, colonial, or multicellular. The domain Eukarya includes the kingdoms: Protista, Fungi, Plantae and Animalia

Classifying Bacteria and Archae Some scientists think that Bacteria and Archaea have NO true species. This is because many of these organisms transfer genes among themselves outside of typical reproduction. Our understanding of classifying bacteria is truly just beginning.