Andrew Chung
Chapter 25
Phylogeny
and
Systematics
To begin, phylogeny is the evolutionary
history of a species or group of species.
Biologists construct phylogenies by using
fossils of extinct organisms, systematics,
and the relationships between organisms.
Many different technologies have been
developed in order to further our
knowledge in evolutionary history.
25.1 Phylogenies are based on
common ancestries inferred from
fossil, morphological, and molecular
evidence.
The Fossil Record
Fossils can be used to further our
knowledge of phylogeny. The age of fossils
can be determined by the fossil record, the
order that fossils have accumulated in
layers of sedimentary rock. However, fossils
are a limited source of information as many
organisms were not fossilized, and fossils
could have been destroyed over time.
Morphological and Molecular Homologies
Anatomical and
molecular similarities
between organisms
also help us to learn
more about
phylogeny.
Homologies are similarities inherited from a
common ancestor.
Morphological homology is when
organisms have similar anatomical features
because of a shared ancestor, for example,
the forelimbs of humans, bats, cats and
whales. Analogy is when organisms possess
similar phenotypes but is of different origin.
For example, bats and birds both have
wings, but are of completely different
ancestry.
25.2 Phylogenetic Systematics
connects classification with
Evolutionary History.
Taxonomy is the division of organisms into
categories based on characteristics linking
similar organisms to each other.
Binomial Nomenclature
Binomial Nomenclature consists of two
parts, the genus and the specific epithet, or
species. For example, the leopard is called
Panthera pardus.
Hierarchical Classification
Carolus Linnaeus developed a system in
which he could group species into broader
categories. Related genera were place into
families, the families into orders, etc. Any
name at any level of hierarchy is called a
taxon.
25.3 Phylogenetic systematics
informs the construction of
phylogenetic trees based on shared
characters.
A cladogram is a diagram depicting
patterns of shared characteristics among
species. A clade is a group of species that
includes an ancestral species and all its
descendants. Cladistics is the study of
Andrew Chung
resemblances among clades.
Shared Primitive Characters and Shared
Derived Characters
A shared primitive character is a character
that is common outside the taxon or clade
that is being defined. A shared derived
character is an evolutionary feature that is
unique to a particular clade.
Outgroups
Outgroups are species that are closely
related to the ingroup, which is the group of
species being studied. The outgroup must
be less related to the ingroup than the
species in the ingroup are to each other.
Phylogenetic Trees and Timing
In a phylogram, the length of a branch is
related to the number of changes in a DNA
seuquence in that lineage.
the evolutionary changes occurred,
according to the diagram.
25.4 Much of an organism’s
evolutionary history is documented
in its genome.
Gene Duplications and Gene Families
Gene duplication is important because it
increases the number of genes in the
genome, allowing for a higher frequency of
evolutionary change. Orthologous genes are
homologous genes that are passed in a
straight line from one generation to the next
but ended up in different gene pools due to
speciation. Paralogous genes are a result of
gene duplication, so they are found in more
than one copy in one genome. Orthologous
genes must wait after speciation to diverge,
while paralogous genes can diverge while
in the same gene pool since there is more
than one copy.
25.5 Molecular clocks help track
evolutionary time.
Phylograms are relative. They present the
sequence of events/mutations relative to
one another.
Ultrametric trees are useful because from
looking at the diagram, you can see when
Molecular Clocks
A molecular clock is an evolutionary timing
method based on the observation that
regions of genomes evolve at constant rates.
However, genes evolve at different rates,
some a million times faster than others, and
only some genes are clock-like.
Neutral Theory
The theory is that most evolutionary change
has no effect on the probability of survival,
and therefore is not influenced by
Darwinian selection. The theory states that
neutral mutations are responsible for the
constant rate of evolution. If there is a low
amount of neutral mutation, there is a slow
rate of evolution. If there is a high amount
of neutral mutation, a high rate of evolution.