AR103 - Readings pgs. 93-104, 117-151 Week 2 Notes - Sep. 19, 2011
Chapter Five: Macroevolution: Processes of Vertebrate and Mammalian
Pages #93 - 104
The Human Place in the Organic World
- Classiﬁcation is in biology, the ordering of organisms into categories, such as orders,
families, and genera, to show evolutionary relationships.
- Multicellular organisms that move about and ingest food are called animals. Within the
kingdom Animalia, there are more than 20 major groups called phyla.
- Chordata is the phylum of the animal kingdom that includes vertebrates.
- It includes all the animals with a nerve cord, gill slits (at some stage of development)
and a supporting cord along the back.
- Most chordates are vertebrates - so called because they have a vertebral column.
- Vertebrates are animals with segmented bony spinal columns; includes ﬁshes,
amphibians, reptiles (including birds), and mammals.
- Vertebrates also have developed brain and paired sensory structures for sight,
smell, and balance.
- They are subdivided into 5 classes: cartilaginous ﬁshes, bony ﬁshes,
amphibians, reptiles / birds, and mammals.
- By putting organisms into increasingly narrow groupings this hierarchical arrangement
organizes diversity into categories. Making statements about evolutionary and genetic
relationships between species and groups of species.
- At each succeeding level ﬁner distinctions are made between categories until at the species
level, only those animals that can potentially interbreed and produce viable offspring are
Principles of Classiﬁcation
- Taxonomy is the ﬁeld that specializes in establishing the rules of classiﬁcation
- Organisms are classiﬁed ﬁrst, and most traditionally, according to their physical similarities.
- Basic genetic regulatory mechanisms are highly conserved in animals; that is, they’ve been
maintained relatively unchanged for hundreds of millions of years.
- This is the essential genetic foundation for most macroevolutionary change.
- Large anatomical modiﬁcations, therefore, don’t always require major genetic
- Homologies are similarities between organisms based on descent from a common ancestor.
Structures that are shared by a species.
- Homologies alone are reliable indicators of evolutionary relationships, but we have to
be careful not to draw to hasty conclusions from superﬁcial similarities.
- Analogies are similarities between organisms based strictly on common function, with no
assumed common evolutionary descent. Similarities based on independent functional adaption
and not on shared evolutionary descent. (i.e. Wings in birds and butterﬂies).
- Homoplasy (homo meaning ‘same’ and plasy meaning ‘growth’) is the separate evolutionary
development of similar characteristics in different groups of organisms. The process that leads
to the development of analogies (i.e. Wings in birds and butterﬂies).
Constructing Classiﬁcations and Interpreting Evolutionary Relationships
- There are 2 major approaches or “schools” when interpreting evolutionary relationships with
the goal of producing classiﬁcations:
1 AR103 - Readings pgs. 93-104, 117-151 Week 2 Notes - Sep. 19, 2011
- 1. Evolutionary Systematics is a traditional approach to classiﬁcation (and
evolutionary interpretation) in which presumed ancestors and descendants are traced
in time by analysis of homologous characters
- 2. Cladistics is an approach to classiﬁcation that attempts to make rigorous
evolutionary interpretations based solely on analysis of certain types of homologous
characters (those considered to be derived characters).
- In recent years cladistic methodologies have predominated among
- Features shared by both evolutionary systematics and cladistics:
- Both schools are interested in tracing evolutionary relationships and in constructing
classiﬁcations that reﬂect these relationships
- Both schools recognize that organisms must be compared using speciﬁc features
(called characters) and that some of these characters are more informative than others
- Both approaches focus exclusively on homologies
- The differences between evolutionary systematics and cladistics are:
- Cladistics more explicitly and more rigorously deﬁnes the kinds of homologies that yield
the most useful information
- Some homologous characters are much more informative than others.
- Ancestral is referring to characters inherited by a group of organisms from a remote ancestor
and thus not diagnostic of groups (lineages) that diverged after the character ﬁrst appeared;
also called primitive.
- In most cases, analyzing ancestral characters doesn’t supply enough information to make
accurate evolutionary interpretations of relationships between different groups.
- Cladistics focuses on traits that distinguish particular evolutionary lineages; such traits are far
more informative than ancestral traits.
- Clade is a group of organisms sharing a common ancestor. The group includes the common
ancestor and all descendants
- Lineages that share a common ancestor
- When identifying a clade, the characteristic of interest are called derived
- Derived (modiﬁed) is referring to characters that are modiﬁed from the ancestral
condition and thus diagnostic of particular evolutionary lineages.
- Theropods are small-to medium-sized ground-living dinosaurs, dated to approximately 150
million years ago and thought to be related to birds
- The possibility of homoplasy must always be considered, as it can add considerably to the
complexity of what seems like a straightforward evolutionary interpretation
- Strict cladistic analysis assumes that homoplasy is not a common occurrence; if it were,
perhaps no evolutionary interpretation could be very straightforward
- Shared derived is relating to speciﬁc character traits shred in common between two life-forms
and considered the most useful for making-evolutionary interpretations.
- Cladistic analysis emphasizes that several characteristics should be examined, since
homoplasy might muddle an interpretation based on just one or two shared traits
- Phylogenetic tree is a chart showing evolutionary relationships as determined by evolutionary
systematics. It contains a time component and implies ancestor-descendent relationships.
- Traditional evolutionary systematics illustrates the hypothesized evolutionary
relationships using phylogenetic tree
- Also called a phylogeny
- Incorporates the dimension of time
- Usually attempt to make some hypothesis regarding ancestor-descendent relationships
(for example, theropods are ancestral to modern birds)
2 AR103 - Readings pgs. 93-104, 117-151 Week 2 Notes - Sep. 19, 2011
- Cladogram is a chart showing evolutionary relationships as determined by cladistic analysis.
It’s based solely on interpretation of shared derived characters. It contains no time component
and does not imply ancestor-descendant relations