Ch 20.1 Phylogeny - Phylogeny and Evolutionary Relationships

Phylogeny showcases evolutionary relationships among species.
The glass lizard exemplifies the continuum of life, from early organisms to current species diversity.
Biologists trace phylogeny to understand the evolutionary history of species.
Phylogeny of lizards and snakes indicates that legless conditions evolved independently in glass lizards and snakes from different lineages of legged lizards.
Phylogenies are reconstructed using systematics, which classifies organisms and determines their evolutionary relationships.

Common names can be confusing due to multiple species sharing names or names being inaccurate.
Examples include jellyfish (anidarian), grayfish (crustacean), and silverfish (insect).
Scientific names are used to avoid ambiguity.
Binomial nomenclature, a two-part naming system, was introduced by Carolus Linnaeus.
The first part is the genus name, and the second part is the specific epithet unique to the species within the genus.
- Example: Panthera pardus (leopard).
The genus name is capitalized, and the binomial is italicized.
Newly created scientific names are Latinized.
- Example: Naming an insect after a friend requires a Latin ending.
Many binomials assigned by Linnaeus are still in use, including Homo sapiens (wise man) for humans.

Species are grouped into a hierarchy of categories.
Closely related species are grouped into the same genus.
Taxonomists use progressively more comprehensive categories:
- Family.
- Orders.
- Classes.
- Phyla.
- Kingdoms.
- Domains.
Biological classification is like a postal address, identifying a person's location.
Each taxonomic unit at any level of the hierarchy is called a taxon.
- Example: Panthera is a taxon at the genus level; Mammalia is a taxon at the class level.
Taxa broader than the genus are not italicized.
Classifying species is a way to structure our human view of the world.
Higher levels of classification are defined by particular characters chosen by taxonomists.
Characters useful for classifying one group may not be appropriate for others.
Larger categories are often not comparable between lineages (e.g., an order of snails vs. an order of mammals).
Historical placement of species does not necessarily reflect evolutionary events.
Taxonomy is always evolving as more is learned about evolutionary relationships.

Evolutionary history is represented in a phylogenetic tree.
The branching pattern often matches how taxonomists classify organisms.
Misclassifications can occur if a species loses a key feature shared by close relatives.
Reclassification may be necessary to accurately reflect evolutionary history based on new evidence (e.g., DNA).
The Linnean system does not show evolutionary relationships between groups like amphibians, mammals, and reptiles.
Difficulties in aligning Linnean classification with phylogeny have led to proposals for basing classification entirely on evolutionary relationships.
Names are assigned only to groups that include a common ancestor and all its descendants.
- Example: Birds are considered a subgroup of Reptilia because they evolved from reptiles.
Phylogenetic trees represent hypotheses about evolutionary relationships.
Relationships are depicted as dichotomies (two-way branch points).
Each branch point represents the common ancestor of the two lineages diverging from it.
An evolutionary lineage is a sequence of ancestral organisms leading to a descendant taxon.

Each tree has a branch point representing the common ancestor of chimpanzees and humans.
Chimpanzees and humans are sister taxa, sharing an immediate common ancestor.
Each branch point has a sister group.
Members of a sister group are each other's closest relatives.
Branches can be rotated without changing the relationships shown in the tree.
The order of taxa at the right does not represent a sequence of evolution.
The tree is rooted, with a branch point representing the most recent common ancestor of all taxa in the tree.
A basal taxon diverges early in the history of the group.
The basal taxon is not necessarily more primitive or less evolved but is less closely related to other taxa.

Phylogenetic trees show patterns of descent, not phenotypic similarity.
Closely related organisms may not resemble one another if they have evolved at different rates or faced different environmental conditions.
- Example: Crocodiles are more closely related to birds than to lizards but look more like lizards.
Phylogenies show hypothesized patterns of descent inferred from data like molecular genetic sequence similarity.
The ages of taxa or branch points cannot necessarily be inferred.
The tree does not indicate that chimpanzees evolved before humans, only that they share a recent common ancestor.
Unless branch lengths are specified, the diagram should be interpreted solely in terms of patterns of descent.
A taxon on a phylogenetic tree did not evolve from the taxon next to it.
- Example: Humans did not evolve from chimpanzees.
The ancestor of humans and chimpanzees was neither a human nor a chimpanzee.
A phylogeny is a hypothesis of our understanding of evolutionary relationships.
Uncertainty about a branch point is illustrated as a polytomy (two or more lineages stemming from a single branch point).
A polytomy signifies a lack of sufficient evidence about the pattern of divergence.

Understanding phylogeny has practical applications.
Phylogenetic analysis of maize identified two wild grass species that may be maize's closest living relatives.
These relatives can be reservoirs of beneficial alleles for cultivated maize.
Phylogenetic trees can infer species identities by analyzing the relatedness of DNA sequences.
Researchers used this approach to investigate illegal whale harvesting.

Researchers purchased whale meat samples from Japanese fish markets.
They sequenced part of the mitochondrial DNA (mtDNA) from each sample.
They compared the results with mtDNA sequences from known whale species.
A gene tree was constructed to show patterns of relatedness among DNA sequences.
The mtDNA sequences of six unknown samples matched those of illegally harvested whales.

Based on a given tree: Are leopards more closely related to badgers or wolves? Explain.
Which of the three trees depicts a different evolutionary history? Explain.
Draw It: The bear family (Ursidae) is more closely related to the badger/otter family (Mustelidae) than to the dog family (Canidae).