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Species & Speciation
Four Major Evolutionary Forces - Review
Genetic drift
Gene flow Selection
Mutation
Natural Selection Review
• Overproduction & competition
• Phenotypic variation
• Some variation is heritable
• Differential survival & reproduction
*Leads to changes in allele frequencies over time
Natural Selection Review
What is a species?Species - a latin word meaning “kind” or “appearance”
Similarity between species Diversity within a species
Species concepts:• Morphological species concept (earliest)• Biological species concept (Ernst Mayr 1942)• Phylogenetic species concept (1960’s)...and many, many, more!
What is a species?
Morphological Species Concept
Species are: groups of organisms that are similar in appearance & habit to one another
Test: measure the phenotypic features of organisms, groups organisms that are similar phenotypically
Issues: phenotype may reflect environment and not genealogy
Biological Species ConceptSpecies are: “groups of interbreeding populations that are
reproductively isolated from other such groups” (Mayr 1942)
Test: can members of two populations mate and produce viable and fertile offspring?
Issues: • can’t apply to organisms that reproduce asexually• can’t apply to fossils• difficult to test if potential species never have chance to interbreed naturally
Phylogenetic Species ConceptSpecies are: the smallest monophyletic groups and thus share an exclusive common ancestor
phylogeny – the evolutionary history of a speciesphylogenetic tree – branches reflect common descent of taxamonophyletic – taxa share an exclusive common ancestor
A BC Outgroup
Asian elephants African forestelephants
African savannahelephants
Phylogenetic Species Concept
Same species
Test: do populations share unique derived traits that cause them to cluster together on the tips of a phylogenetic tree?
Issues: need a really, really good phylogenetic tree (rare)
3-Step model of Speciation
1. Isolation (often physical)
2. Divergence(independent evolution)
3. Secondary contactbut no genetic exchange
ancestralpopulation
1
2
3
Types of isolationAllopatry• greek meaning “other country”• physical or geographic barrier to gene flow
Sympatry• greek meaning “together country”• no physical or geographic barrier
Allopatry Sympatry
Pre-mating barriers – prevent fusion of egg & sperm• temporal isolation • spatial isolation• behavioral isolation• mechanical isolation• gametic isolation
Post-mating barriers – young with reduced fitness produced• hybrid inviability• hybrid sterility
Reproductive isolating mechanisms
Pre-mating barriers – Temporal isolationMating takes place at different times
0
100
200
300
400
Return date
Sock
eye
salm
on (1
,000
s)
Sockeye salmonPhoto credit: A. Wolfe
Sock
eye
salm
on (1
,000
s)
Date of return
Pre-mating barriers – Spatial isolationMating takes place in different locations
Soapberry bugs
Pre-mating barriers – BehavioralMating relies on recognition of proper cues & signals
Satin bowerbird MacGregors bowerbird
Post-mating barriers – hybrid inviability, sterility
Hybrids are produced, but are sterile or inviable due to incompatibilities between the chromosomes
or genes of parent species
Zebroids Mules
X
3-Step model of Speciation
1. Isolation (often physical)
2. Divergence(independent evolution)
3. Secondary contactbut no genetic exchange
ancestralpopulation
1
2
3
Speciation is a processCan be “fast” (1,000-10,000s yrs).....
But often takes millions of yrs
Speciation can occur in sympatry....But more commonly allopatry
Higher levels of diversity/taxonomy arise from the isolation of populations
and the evolution of species
Adaptive Radiation
Galapagos Finches
Adaptive Radiation
Lake Malawai Cichlids Hawaiian Honeycreepers
