Phylogeny and Systematics

By: Ashley Yamachika

Biologists use systematicsThey use systematics as an analytical approach to understanding the diversity and relationships of organisms, both present-day and extinct

Systematists use morphological, biochemical, and molecular comparisons to infer evolutionary relationships

Phylogenies are based on common ancestries inferred from fossil morphological and molecular evidence.

The Fossil RecordSedimentary Rocks…

Are the richest source of fossilsAre deposited into layers called strata

Based on the sequence in which fossils have accumulated in such strataFossils reveal characteristics that may have been lost over time

Radiometric Dating

Measures the decay of radioactive isotopes in terms of half-life

Half-life is the amount of time it takes for ½ the amount of a radioactive isotope to decay

Morphological and Molecular HomologiesPhylogenetic history can be inferred

from certain morphological and molecular similarities among living organisms

Organisms that share very similar morphologies or similar DNA sequences

Are likely to be more closely related than organisms with vastly different structures or sequences

Convergent Evolution occurs when similar environmental pressures and natural selection produce similar (analogous) adaptations in organisms from different evolutionary lineages

Analogous Structures or molecular sequences that evolved independently are also called homoplasies

Parallel EvolutionWhen both descendants are similar in a particular respect, evolution is defined as parallel if the ancestors considered were also similar, and convergent if they were not

Divergent Evolution

The diversification of an ancestral group into two or more species in different habitats is called divergent evolution.

When it involves the formation of a large number of species to occupy different niches is called an adaptive radiation.

Phylogenetic systematics connects classification with evolutionary history

TaxonomyIs the ordered division of organisms into categories based on a set of characteristics used to assess similarities and differences.

Binomial NomenclatureIs the two-part format of the scientific name of an organismWas developed by Carolus Linnaeus

The binomial name of an organism or scientific epithet

Is latinizedIs the genus and species

Hierarchical Classification

Systematists depict evolutionary relationships

In branching phylogenetic trees

Phylogenetic systematics informs the construction of phylogenetic trees based on shared characteristicsA cladogram

Is a depiction of patterns of shared characteristics among taxa

A clade within a cladogramIs defined as a group of species that includes an ancestral species and all its descendants

CladisticsIs the study of resemblances among clades

Clades• Valid Clade is monophyletic• Signifying that it consists of the ancestor

species and all its descendants

• Paraphyletic Clade• Is a grouping that consists of an

ancestral species and some, but not all, of the descendants

PhylogramsIn a phylogram

The length of a branch in a cladogram reflects the number of genetic changes that have taken place in a particular DNA or RNA sequence in that lineage

Orthologous genesAre genes found in a single copy in the genomeCan diverge only once speciation has taken place

Paralogous genesResult from gene duplication, so they are found in more than one copy in the genomeCan diverge within the clade that carries them, often adding new functions

Neutral Theorystates that

Much evolutionary change in genes and proteins has no effect on fitness and therefore is not influenced by Darwinian selectionAnd that the rate of molecular change in these genes and proteins should be regular like a clock

Eubacterial• Most numerous organisms on earth • Earliest life forms (fossils date 3.5 billion years old) • Microscopic prokaryotes (no nucleus nor membrane-bound organelles)

• Have only one circular chromosome • Have small rings of DNA called plasmids • Most are unicellular

• Found in most habitats • Main decomposers of dead

organisms so recycle nutrients • Some cause disease