Introduction to Evolutionary Biology

Homologous and Homoplasia Traits

• Homologous traits are those inherited from a common ancestor.

• Example: Camera eye evolved independently in different lineages, indicating homoplasia.

• Gain and loss of traits (e.g., green and yellow circles) can illustrate the adaptive changes on a phylogenetic tree.

• The camera eye example indicates that it evolved twice, thus requiring fewer evolutionary changes.

• Homoplasia (similar traits arising independently) is more parsimonious than shared ancestry with subsequent loss of traits.

Convergent Evolution

• Example: Bats and butterflies share wing structures due to convergent evolution.

• Natural selection led to the evolution of wings in both groups independently.

• This is differentiated from homologous traits, like a bat’s forelimb and a bird’s forelimb, which share a common ancestry as tetrapods.

Phylogeny vs. Taxonomy

Taxonomy

• Definition: The practice of classifying and naming organisms.

• Involves determining what delineates individual species.

• Determining genus, family, and class can be subjective based on perceived similarity.

Phylogeny

• Focuses on evolutionary relationships and shared ancestry.

• Organizes species into clades, often disregarding similar appearances.

• Emphasizes derived traits versus superficial similarities in classification.

Taxonomic Groups

Monophyletic Groups

• Definition: Includes a common ancestor and all its descendants.

• Example: Mammals share a common ancestor leading to all mammalian taxa.

• Ideal classification reflects complete lineages without missing branches.

Paraphyletic Groups

• Definition: Includes a common ancestor but excludes one or more descendant branches.

• Example: Reptiles are often considered a paraphyletic group as they exclude birds, which share a common ancestor with certain reptiles.

• Misleading classifications may occur when based solely on appearance rather than evolutionary relationships.

Creating Monophyletic Groups

• To convert a paraphyletic group (e.g., reptiles) into a monophyletic group:

  • Include all descendants, such as birds (e.g., bluejay), crocodiles, and turtles.

  • Ensure that the grouping encompasses all branches from the common ancestor, thereby creating complete representation.

Monophyletic groups reflect common
ancestry and thus evolutionary history

Paraphyletic names may be helpful when
discussing groups that share (or lack)
some key feature, but are avoided in strict
taxonomical ranks.

Polyphyletic groups do not reflect
evolutionary history and should be re-
organized or their use discontinued.