Lecture 7 How classification works .

Lecture 7How classification works

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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.

Amount of divergence

Tim

e

Importance of recognizing species

• Survival depends on our ability to recognize species

How do species arise?

1) speciation by cladogenesis

2) speciation by anagenesis between a

& ba

b

anagenesis

Anagenesis: no cladogenesis

cladogenesis

Cladogenesis: speciation

What is a species?

• Biologists do not agree on ONE way to define a species – thus,

“species concepts”

Why?

At least 22 different definitions have been proposed to explain what a “species” is.

Three main ones:1. Morphological2. Biological3. Phylogenetic

Species concepts

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

Fertile Hybrids: Violations of both the Morphological v. Biological

species Concepts

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