BIOEE 1780 Lecture Notes - Lecture 20: Cladogenesis, Species Complex, Convergent Evolution
7 May 2018
School
Department
Course
Professor
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)
