BIOEE 1780 Lecture Notes - Lecture 20: Cladogenesis, Species Complex, Convergent Evolution

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There are 1.9 million described species. It's estimated that there are 5-50 million species.
How do we decide that a group of organisms is a species?
1. Mostly morphological evidence
2. Sometimes DNA evidence
3. Occasionally evidence about breeding frequency
Species Concepts
Typological/Morphological/Phenetic Species Concept: species are defined using morphological
traits
o Recognition is based on overall similarity
o Can be determined with fossils
o Advantage: morphology can be readily observed
o Disadvantages:
Mistakes are easy to make. Males and females could be classified as separate species,
especially in the fossil species.
Cryptic Species: morphologically very similar, but are genetically distinct
Convergent evolution can cause species to look similar even when they're not closely
related, or they could've evolved from a common ancestor that looked similar.
This concept relies on human interpretation to define species, rather than allowing natural
mechanisms to determine species.
Biological Species Concept: species are groups of natural populations that are capable of
interbreeding and are reproductively isolated from other such groups
o Reproductive isolation defines the boundaries of species.
o Disadvantages:
Ecological isolation (2 populations may be capable of interbreeding, but they're separated
by geographic barriers so they're on their own evolutionary paths)
Behavioral isolation
What do you do with organisms that occasionally form hybrids with one another?
Some species can interbreed and produce viable offspring (dogs + wolves, lions + tigers,
lynx + bobcat, human-Neanderthal hybridization)
What do you do with asexual organisms?
What is meant by "potentially interbreeding?" Hard to test for viable reproduction
It can't be determined whether fossils were capable of interbreeding.
General Lineage Species Concept: species are metapopulations of organisms that exchange alleles
frequently enough that they comprise the same gene pool and the same evolutionary lineage
Ecological Species Concept: used to describe populations that are adapted to certain ecological
niches and because of their adaptations will form discrete morphological clusters
o Advantages: acknowledges the role the environment plays in controlling morphologic variation
o Disadvantages:
Can miss cryptic species
Niches tend to be assumed and are difficult to define completely.
Many taxa exploit overlapping resources, or can suddenly switch if a resource becomes
scarce
No single definition will ever fit all taxa. No one concept is "right," but species do exist.
Metapopulation: a group of spatially separated populations of the same species that interact at some
level (ex. exchange alleles)
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