Chapter 25: Phylogeny and Systematics


the evolutionary history of a species or group of related species


the study of biological diversity in evolutionary context, also name and classify species

fossil record

the ordered array in which fossils appear within layers, or strata, of sedimentary rocks that mark the passing of geologic time


the remains (or an impression) of a plant or animal that existed in a past geological age and that has been excavated from the soil

sedimentary rocks

form from layers of minerals that settle out of water or as dust and sand on land. Sedimentation is not continuous but occurs in intervals which leads to strata.

many fossils are not made of the actual remnants of animals but are made of...

minerals dissolved in water that filled the mold that the decaying organism left in the sediment

methods used by paleontologists to date fossils

relative dating, absolute dating (radiometric dating)

relative dating

because the strata correspond with the time in which the fossil lived, the strata tell the relative ages of fossils. the strata at one location can be correlated to a strata at another location by the presence of similar fossils (known as index fossils)

geologic time scale

established by geologists who studied many different sites. Has a consistent sequence of historical periods

Four eras of the historical periods in the geologic time scale

Precambrian, Paleozoic, Mesozoic, Cenozoic


finer intervals within each period of each era

the length of geologic eras and periods


Absolute dating

is not errorless but gives in years instead of relative terms

Radiometric dating

the measurement of certain radioactive isotopes in fossils or rocks, is the method most often used to determine the ages of rocks and fossils on a scale of absolute time

Why does radiometric dating work?

during their lifetime, organisms accumulate isotopes which have fixed rates of decay, so by measuring how much the isotope has decayed we can figure out how old the organism is

half life

the number of years it takes for 50% of the original sample (of isotope) to decay, is unaffected by temperature, pressure, and other environmental variables


Dating from changes that happen in amino acids during the process of fossilization. A chronometric dating technique that is NOT radiometric. Affected by temperature change

why is the fossil record incomplete

a substantial fraction of species that have lived probably left no fossils, most fossils that have formed have been destroyed and only a fraction of the existing fossils have been discovered. The fossil record is slanted in favors of species that lived a

what helps explain the current geographic distribution of species?

the history of Earth

on the global scale, what is the major geographic factor correlated with the spatial distribution of life and and with such evolutionary evolutionary episodes as mass extinctions and explosive increases of biological diversity

the shifting of the continents


the supercontinent formed near the end of the Paleozoic era, when plate movements brought all land masses together

the formation of Pangea had

tremendous environmental impact that reshaped diversity by causing extinctions and providing new opportunities for taxonomic groups that survived the crisis

when did Pangea break apart?

during the Mesozoic era, resulted in species isolation

why does a species become extinct?

its habitat has been destroyed or the environment has changed in a direction that is unfavorable to the species

mass extinctions are

inevitable and usually occur when global environmental change has been so rapid and disruptive that a majority of species has been swept away

2 most famous mass extinctions

Permian mass extinction (250 million years ago), and the Cretaceous mass extinction (65 million years ago)

Permian mass extinction

defines the boundary between the Paleozoic and Mesozoic eras,might have been caused by volcanism, which lead to global warming, and a decrease in oceanic oxygen

Cretaceous mass extinction

separates the Mesozoic from the Cenozoic; extinct include about half of all marine species and many terrestrial plants and animals, including most dinosaurs

impact hypothesis

due to unusually high iridium content (found mainly in asteroids) at the boundary between the Cretaceous and Cenozoic, theory is that a meteor impact tossed a huge cloud of dust in the air which stopped photosynthesis, caused severe climate change, and a

what is the "upside" of mass extinction?

the species that manage to survive become the stock for new radiations that fill the many biological roles vacated or newly created by extinctions


the naming and classification of species and groups of species

when did formal taxonomy start

in 18 th century with Linnaeus's book Systema naturae

two main characteristics of the Linnaean system

two part name for each species and a hierarchical classification of species into broader and broader groups of organisms


two part latinized name assigned to each species by taxonomists


first part of binomial, a species belongs in a genus, always capitalized and italicized

species epithet

second part of binomial, refers to one species within a genus, always italicized and lower case

order of hierarchical classification

Domain Kingdom Phylum Class Order Family Genus Species

taxon (plural taxa)

The named taxonomic unit at any given level. The specific name at any level.

phylogenetic trees

reflect the hierarchical classification of taxonomic groups nested within more inclusive groups

phylogenetic systematics

classification based on evolutionary history


a system of biological taxonomy based on the quantitative analysis of comparative data and used to reconstruct cladograms summarizing the (assumed) phylogenetic relations and evolutionary history of groups of organisms


a phylogenetic diagram based on cladisitics. represents the chronological sequence of branching during the evolutionary history of a set of organisms. This branching sequence is infered by analyzing homologies, identifying shared characteristis unique to


each evolutionary branch in a cladogram, consists of an ancestral species and all its descendants which is said to be monophyletic


Pertaining to a taxon derived from a single ancestral species that gave rise to no species in any other taxa.


pertaining to a grouping of species that consists of an ancestral species and some, but not all, of its descendants.


pertaining to a group of taxa derived from two or more different ancestors


likeness attributed to shared ancestry

convergent evolution

process by which unrelated organisms independently evolve similarities when adapting to similar environments


a similarity due to convergence


any feature that a particular taxon posseses

shared primitive character

A homology common to a taxon more inclusive than the one being defined.

shared derived character

an evolutionary novelty unique to a particular clade

outgroup comparison

a method used by systematists in cladistic analysis to differentiate shared characters that are derived from those that are primitive


a species or group related to the species being studied , but known to be less closely related than any study group members are to each other


In a cladistic study of evolutionary relationships among taxa of organisms, the group of taxa that is actually being analyzed.

is the chronology represented in a cladogram relative or absolute?


what are two uses of the cladogram?

placing species in the taxonomic hierarchy groups within groups; reflecting the nesting of clades within more comprehensive clades


simply names clade without the hierarchical tags, such as class, order, and family

At the molecular level, the evolutionary divergence of species parallels

the accumulation of differences in their genomes

molecular systematics

a scientific discipline that uses nucleic acids or other molecules in different species to infer evolutionary relationships

the four bases of DNA

A (adenine), G (guanine), C (cytosine), T (thymine)

after sequence alignment and the point by point comparisons of base sequences in homlogous DNA from a set species

systematists may convert the data to phylogenetic trees


states that a theory about nature should be the simplest explanation that is consistent with the facts

any phylogenetic diagram is

a hypothesis

molecular clocks

place the origin of taxonomic groups in time, are based on the observation that at least some regions of genomes evolve at constant rates

phylogenetic fuse hypothesis

perhaps the modern mammalian orders origninated about 100 million years ago, but did not proliferate extensively enough to be noticable in the fossil record until after extinctionof the dinosaurs almost 40 million years later.