11.4С: Variety of living organisms

• The binomial system of names for species is universal among biologists and has been agreed and developed at a series of congresses
• When species are discovered they are given scientific names using the binomial system
• All organisms are classified into three domains
• Taxonomists classify species using a hierarchy of taxa
• The principal taxa for classifying eukaryotes are kingdom, phylum, class, order, family, genus and species
• In a natural classification, the genus and accompanying higher taxa consists of all the species that have evolved from one common ancestral species
• Taxonomists sometimes reclassify groups of species when new evidence shows that a previous taxon contains species that have evolved from different ancestral species
• Natural classifications help in identification of species and allow the prediction of characteristics shared by species within a gro

Binomial System

The binomial system of nomenclature is the formal system by which all living species are classified (taxonomy)

• It was initially developed by a Swedish botanist named Carolus Linnaeus in 1735
• It is periodically assessed and updated at a series of international congresses which occur every 4 years

The binomial system of nomenclature provides value because:

• It allows for the identification and comparison of organisms based on recognised characteristics
• It allows all organisms to be named according to a globally recognised scheme
• It can show how closely related organisms are, allowing for the prediction of evolutionary links
• It makes it easier to collect, sort and group information about organisms

3 Domains of Life

Classification of Humans

Sometimes new evidence shows that members o€ a group do not share a common ancestor, so the group should be split up into two or more taxa. Conversely species classifed in di€€erent taxa are sometimes €ound to be closely related, so two or more taxa are united, or species are moved €rom one genus to another or between higher taxa. The classifcation o€ humans has caused more controversy than any other species. Using standard taxonomic procedures, humans are assigned to the order Primates and the €family Hominidae. There has been much debate about which, i€ any, o€ the great apes to include in this €amily. Originally all the great apes were placed in another f€amily, the Pongidae, but research has shown that chimpanzees and gorillas are closer to humans than to orang-utans and so should be in the same amily. This would just leave orang-utans in the Pongidae. Most evidence suggests that chimpanzees are closer than gorillas to humans, so i humans and chimpanzees are placed in dierent genera, gorillas should also be in a separate genus.

A clade is a group of organisms that have evolved from a common ancestor

Cladistics is a method of classifying organisms into groups of species called clades (from Greek ‘klados' = branch)

• Each clade consists of an ancestral organism and all of its evolutionary descendants
• Members of a clade will possess common characteristics as a result of their shared evolutionary lineage

Clades can be organised according to branching diagrams (cladograms) in order to show evolutionary relationships. They can have different visual representations, but still show the same information.

Cladograms are tree diagrams that show the most probable sequence of divergence in clades

Cladograms are tree diagrams where each branch point represents the splitting of two new groups from a common ancestor

• Each branch point (node) represents a speciation event by which distinct species are formed via divergent evolution

Cladograms show the probable sequence of divergence and hence demonstrate the likely evolutionary history (phylogeny) of a clade

• The fewer the number of nodes between two groups the more closely related they are expected to be

Comparing DNA Sequences using DNA-DNA Hybridisation

• All organisms use DNA and RNA as their genetic material
  • Four bases (adenine, guanine, thymine, cytosine) form the basis of this code and undergo the same complementary pairing (A = T ; G = C)
  • Conserved genes have similar functions
• Common mechanisms for DNA replication and protein synthesis are used by all organisms (including similar enzymes for these processes)
  • The genetic code is (almost) universal – the same codons code for the same amino acids
• All living organisms use the same 20 amino acids for protein synthesis
  • These amino acids are all L-isomers and not D-isomers
• Large groups of organisms share certain proteins and metabolic pathways (e.g. haemoglobin, cytochrome c)
• The degree of difference between organisms in DNA / protein sequence demonstrates the level of relationship between organisms
  • The more similar the base / amino acid sequences are, the more closely related two species are likely to be

• Phylogeny refers to an evolutionary line of descent and can be determined by comparing similar molecules in different species (e.g. haemoglobin)
• Variations in these molecules (either base or amino acid sequence) will occur due to mutations
• The greater the differences between the common molecules, the longer the time span since the two species had a common ancestor
• Mitochondrial DNA is useful for tracing phylogeny as it is inherited via the maternal line, lacks recombination and has a known mutation rate

Constructing Cladograms

• A cladogram is a tree-like diagram where nodes represent the splitting of two new groups from a common ancestor
• Members of a clade typically share a set of features not found in more distant relatives
• Cladistics are most commonly based on molecular differences (base or amino acid sequences)
• Cladograms are calculated by software that depicts the cladogram with the fewest number of brances
• For rooting the cladogram, an outgroup (most distantly related species) is used

• The more nodes between two groups the less closely related they are expected to be
• Cladograms are based on probability, but improbable events (e.g. certain mutations) can occur, so relationships may be wrong

Example

Practice Questions