Chapter 25 – Phylogeny and Systematics

 

Phylogeny is the evolutionary history of a species or group of related species.

 

A.  Fossil record and geologic time

            1.  Sedimentary rocks are the richest source of fossils

                        a.  The fossil record refers to the order in which fossils appear within layers of rock that mark the passing of geologic time.

                       

                        b.  Organic substances in dead organisms typically decay rapidly.  Parts that are rich in minerals (eg. teeth, bones) may become fossils.

 

                        Figure 25.1 (p. 485) – A gallery of fossils.

 

            2.  Paleontologists use many methods to date fossils

                        a.  Relative dating

                                    i.  Fossils near the surface are relatively recent, while those that are deeper are relatively older.

 

                                    ii.  Geologists have established a geologic time scale that reflects a consistent sequence of historical periods. 

                                                - Those periods are grouped into four eras:  Precambrian, Paleozoic, Mesozoic, and Cenozoic. (**Know these!**)

 

                                    Table 25.1 (p. 487) – The Geologic Time Scale

                       

                        b.  Absolute dating – age is given in years, instead of relative terms (before/after, early/late)

                                    i.  Radiometric dating is the measurement of radioactive isotopes found in fossils and rocks, to determine age.

                                                - The half-life of an isotope is the number of years it takes for 50% of the original sample to decay. 

 

                                    Figure 25.2 (p. 488) – Radiometric dating.

           

            3.  The fossil record is substantial, but does not provide a complete evolutionary history

                        a.  The fossil record usually tells us about abundant, widespread organisms with hard shells or skeletons.

 

            4.  Phylogeny has a biogeographic basis in continental drift

                        a.  Moving continents isolate populations, allowing for evolution to occur.

 

                        b.  250 million years ago all continents were connected as Pangaea.

 

                        c.  Pangaea “broke” apart about 180 million years ago.

 

                        Figure 25.4 (p. 490) – The history of continental drift.

 

            5.  History is punctuated by mass extinctions

                        a.  There has been a general increase in species diversity over time.

                       

                        Figure 25.5 (p. 491) – Diversity of life and periods of mass extinctions.

 

                        b.  Permian extinction

                                    i.  90% of marine species went extinct

 

                                    ii.  Pangaea formed and some species began competing with each other for the first time.

 

                                    iii.  Mass extinction was caused by volcanic eruptions and climate changes

                       

                        c.  Cretaceous extinction

                                    i.  Dinosaurs went extinct

 

                                    ii.  An asteroid (or comet) hit the earth and created a cloud of debris that blocked out sunlight for months.  Temperatures dropped and plants died.

 

B.  Systematics:  Connecting classification to phylogeny

            - Systematics is the study of biological diversity in an evolutionary context, including taxonomy and phylogenetics.

 

            1.  Taxonomy uses a hierarchical classification system

                        a.  Review the Linnaean (binomial) system of classification:  genus and species.

 

                        b.  Review hierarchical classfication:  Kingdom, Phylum, Class, Order, Family, Genus, Species

                                    - A named taxonomic unit at any level is called a taxon.

 

                        Figure 25.7 (p. 493) – Hierarchical classification.

 

                        c.  Phylogenetic trees are used to place different taxonomic shemes together, and to show connection between classification and phylogeny. 

 

                        Figure 25.8 (p. 494) – The connection between classification and phylogeny.

 

            2.  Modern phylogenetic systematics is based on cladistic analysis

                        a.  A phylogenetic diagram (tree) is also called a cladogram. 

 

                        b.  Each branch in the tree is called a clade.

 

                        c.  Monophyletic pertains to a taxon that is derived from a single ancestral species.

 

                        d.  Polyphyletic pertains to a taxon whose members were derived from two or more ancestral forms not common to all members.

 

                        e.  Paraphyletic pertains to a taxon that excludes some members that share a common ancestor with members included in the taxon.

 

                        Figure 25.9 (p. 495) – Monophyletic versus paraphyletic and polyphyletic groups.

 

            3.  Constructing cladograms

                        a.  Identify homologies à shared characteristics derived from one ancestor.

 

                        b.  The greater the number of homologous parts between two organisms, the more closely related they are.  (Refer to Figure 25.8)

 

                        c.  Classification scheme reflects these similarities

 

                        Figure 25.11 (p. 497) – Constructing a cladogram.

 

                        Figure 25.12 (p. 498) – Cladistics and taxonomy.

 

            4.  Phylogeny can be inferred from molecular data

                        a.  DNA and RNA sequences can be compared to determine phylogeny.

 

            5.  The principle of parsimony helps systematists reconstruct phylogeny

                        a.  Phylogenies can be extremely complicated.

 

                        b.  The principle of parsimony states that a theory about nature should be the simplest explanation that is consistent with facts.

                                    - “Keep it simple.”

                                    - Sometimes called “Occam’s Razor.”

 

                        c.  A phylogenetic tree is a hypothesis.  There may be many possible trees, but the simplest one is probably the most accurate.

 

                        Figure 25.14 (p. 500) – Simplified version of a four-species problem in phylogenetics.

 

                        Figure 25.15 (p. 500-501) – Parsimony and molecular systematics.

 

                        Figure 25.16 (p. 502) -  Parsimony and the analogy-versus-homology pitfall.

 

            6.  Modern systematics is flourishing with debate

                        a.  Lots of debate when comparing major groups

                                    - Orders of mammals

 

                        Figure 25.19 (p. 505) – When did most major mammalian orders originate?