How classification works Lecture 7 How classification works www.goldiesroom.org

Modern classification Darwin’s ideas about descent with modification have given rise to the study of phylogeny or evolutionary relationships among organisms The goal of phylogeny classification or evolutionary classification, is to group species into larger categories that reflect lines of evolutionary descent, rather than overall similarities and differences Biologists now group organisms into categories that represent lines of evolutionary descent or phylogeny, not just physical similarities Species within a genus are more closely related to each other than a species in another genus because all members of a genus share a recent common ancestor

The higher the level of the taxon the , further back in time is the common ancestor of all the organisms in the taxon Organisms that appear very similar may not share a recent common ancestor To refine the process of evolutionary classification, many biologists now prefer a method called cladistic analysis which identifies and considers only those characteristics of organisms that arise as lineages evolve over time

Characteristics that appear in recent parts of a lineage but not in its older members are called derived characters A derived character is a trait that arose in the most recent common ancestor of a particular lineage and was passed along to its descendants A cladogram can be constructed to show derived characters A cladogram links groups of organisms by links showing how evolutionary lines, or lineages, branched from common ancestors

A speciation event, in which an ancestral lineage branches into two new lineages, is the basis for each branch point, or node. Each node represents the last point at which the new lineages shared a common ancestor. Time Amount of divergence

Importance of recognizing species All human cultures recognize and name species. Because we eat other species of living organisms it is very important that we can properly recognize species with different characteristics and that we have names for them, so we can communicate about the characteristics of the different species. Also in order to avoid parasites its very important to know which species may be harmful to us Survival depends on our ability to recognize species

anagenesis between a & b How do species arise? 2) speciation by anagenesis between a & b a 1) speciation by cladogenesis

Anagenesis: no cladogenesis

Cladogenesis: speciation

What is a species? Biologists do not agree on ONE way to define a species – thus, “species concepts” Why?

Species concepts Three main ones: Morphological Biological At least 22 different definitions have been proposed to explain what a “species” is. Three main ones: Morphological Biological Phylogenetic

Morphological species concept : Two organisms that display “substantial” and consistent morphological differences are different species Advantages: Easy to apply, can be used with fossil species Disadvantages: It is not testable, the definition of “substantial” is subjective. Problems with convergence, cryptic species, hybridization (= gene flow) & intermediate phenotype

Morphological species concept : Chronospecies: a morphology that can be identified as a stage in an evolving lineage (anagenesis) Note increasing number of pleura (or “ribs”) on trilobites

Biological species concept “Species is a group of interbreeding natural populations that are reproductively isolated from other such groups” Advantages: testable in many cases, objective Disadvantages: difficult to apply and test (impractical), cannot be used with fossils, irrelevant to asexual populations

Violations of both the Morphological v. Biological species Concepts Fertile Hybrids: Violations of both the Morphological v. Biological species Concepts A wholphin or wolphin is a rare hybrid, born from a mating of bottlenose dolphin. Polar-Grizzly Bear Hybrid. An example of a sheep-goat chimera. A Cama is a hybrid between a camel and a llama. A jaglion, Jaguar-Lion Hybrid. Würdemann's Heron, a great blue heron white egret hybrid.

Phylogenetic species concept : Lineages with different evolutionary histories are different species – these lineages must be the smallest units of evolution Advantage: it is testable, objective and can be applied to living and fossil species Disadvantage: it requires comprehensive phylogenetic analyses = need a cladogram!

Phylogenetic species concept : Lineages with different evolutionary histories are different species – these lineages must be the smallest units of evolution Instead of depending on reproductive isolation, this concept revolves around fixed differences between populations Species are the smallest population that you don’t have any reason to divide into even smaller clades or populations