PHYLOGENY ANDPHYLOGENY ANDSYSTEMATICSSYSTEMATICSChapter 25Chapter 25

Sedimentary rocks are the richest Sedimentary rocks are the richest

source of fossilssource of fossils Fossils are the preserved remnants or impressions left by Fossils are the preserved remnants or impressions left by

organisms that lived in the past. organisms that lived in the past. Sedimentary rocks form from layers of sand and silt that are Sedimentary rocks form from layers of sand and silt that are

carried by rivers to seas and swamps, where the minerals settle to carried by rivers to seas and swamps, where the minerals settle to the bottom along with the remains of organisms. the bottom along with the remains of organisms.

The fossil record is the ordered array in which fossils appear The fossil record is the ordered array in which fossils appear within sedimentary rock strata. within sedimentary rock strata.

Fossils can be used to construct phylogenies only if we can Fossils can be used to construct phylogenies only if we can determine their ages. determine their ages.

The fossil record is a substantial, but incomplete, chronicle of The fossil record is a substantial, but incomplete, chronicle of evolutionary change. evolutionary change.

Fossils

Morphological and molecular Morphological and molecular similarities may provide clues to similarities may provide clues to phylogenyphylogeny

Similarities due to shared ancestry are called Similarities due to shared ancestry are called homologies. homologies.

Organisms that share similar morphologies or DNA Organisms that share similar morphologies or DNA sequences are likely to be more closely related than sequences are likely to be more closely related than organisms without such similarities. organisms without such similarities.

Similarity due to convergent evolution is called analogy. Similarity due to convergent evolution is called analogy. When two organisms from different evolutionary When two organisms from different evolutionary

lineages experience similar environmental pressures, lineages experience similar environmental pressures, natural selection may result in convergent evolution. natural selection may result in convergent evolution.

Homology vs. Analogy Homology vs. Analogy

For example, both birds and bats have adaptations that For example, both birds and bats have adaptations that allow them to fly. allow them to fly.

However, a close examination of a bat’s wing shows a However, a close examination of a bat’s wing shows a greater similarity to a cat’s forelimb that to a bird’s greater similarity to a cat’s forelimb that to a bird’s wing. wing.

Fossil evidence also documents that bat and bird wings Fossil evidence also documents that bat and bird wings arose independently from walking forelimbs of different arose independently from walking forelimbs of different ancestors. ancestors.

Thus a bat’s wing is homologous to other mammalian Thus a bat’s wing is homologous to other mammalian forelimbs but is analogous in function to a bird’s wing. forelimbs but is analogous in function to a bird’s wing.

Dating FossilsDating Fossils

Relative Dating estimates the order of prehistoric and geological events were determined by using basic stratigraphic rules, and by observing where fossil organisms lay in the geological record, stratified bands of rocks present throughout the world.

Absolute Dating is the process of determining a specific date for an archaeological or palaeontological site or artifact.

The Geologic Time Scale

Taxonomy employs a hierarchical Taxonomy employs a hierarchical

system of classificationsystem of classification

Under the binomial system, each species is assigned a Under the binomial system, each species is assigned a two-part Latinized name, a binomial. two-part Latinized name, a binomial.

The first part, the genus, is the closest group to which a The first part, the genus, is the closest group to which a species belongs. species belongs.

The second part, the specific epithet, refers to one The second part, the specific epithet, refers to one species within each genus. species within each genus.

The first letter of the genus is capitalized and both The first letter of the genus is capitalized and both names are italicized and Latinized. names are italicized and Latinized.

For example, Linnaeus assigned to humans the For example, Linnaeus assigned to humans the optimistic scientific name Homo sapiens, which means optimistic scientific name Homo sapiens, which means “wise man.” “wise man.”

Taxonomy contd.Taxonomy contd.

A hierarchical classification groups species into increasingly broad A hierarchical classification groups species into increasingly broad taxonomic categories. taxonomic categories.

Species to Genus to Family to Order to Class to Phylum to Species to Genus to Family to Order to Class to Phylum to Kingdom to Domain.Kingdom to Domain.

Classification and phylogeny are Classification and phylogeny are

linkedlinked Systematists explore phylogeny by examining various Systematists explore phylogeny by examining various

characteristics in living and fossil organisms. characteristics in living and fossil organisms. They construct branching diagrams called phylogenetic They construct branching diagrams called phylogenetic

trees to depict their hypotheses about evolutionary trees to depict their hypotheses about evolutionary relationships. relationships.

The branching of the tree reflects the hierarchical The branching of the tree reflects the hierarchical classification of groups nested within more inclusive classification of groups nested within more inclusive groups. groups.

Methods for tracing phylogeny began with Darwin, who Methods for tracing phylogeny began with Darwin, who realized the evolutionary implications of Linnaean realized the evolutionary implications of Linnaean hierarchy. hierarchy.

Darwin introduced phylogenetic systematics in On the Darwin introduced phylogenetic systematics in On the Origin of Species when he wrote: “Our classifications will Origin of Species when he wrote: “Our classifications will come to be, as far as they can be so made, genealogies.” come to be, as far as they can be so made, genealogies.”

Molecular clocks help track Molecular clocks help track

evolutionary timeevolutionary time Molecular clocks serve as yardsticks for measuring the Molecular clocks serve as yardsticks for measuring the

absolute time of evolutionary change. absolute time of evolutionary change. They are based on the observation that some regions They are based on the observation that some regions

of the genome evolve at constant rates. of the genome evolve at constant rates. For these regions, the number of nucleotide For these regions, the number of nucleotide

substitutions in orthologous genes is proportional to the substitutions in orthologous genes is proportional to the time that has elapsed since the two species last shared time that has elapsed since the two species last shared a common ancestor. a common ancestor.

In the case of paralogous genes, the number of In the case of paralogous genes, the number of substitutions is proportional to the time since the genes substitutions is proportional to the time since the genes became duplicated. became duplicated.