Carol Eunmi Lee 11/16/17 Title goes here 1 Speciation (1) Geographic Mechanisms of Speciation (What circumstances lead to the formation of new species?) (2) Species Concepts (How are Species Defined?) Today’s OUTLINE: Mechanisms of Speciation Last Time: Genetic Models: The roles of: Mutations Natural Selection Genetic Drift This Time: Geographic Models: Allopatric Model (difference place) Sympatric Model (same place) Parapatric Model (adjoining) (1) Mechanisms of Speciation Last Time: Genetic Models: How do Genetic Drift, Natural Selection, Mutations, etc. create new species? Are there “speciation” genes? This Time: Geographic Models: How does speciation occur in Nature? Is geographic isolation required? Mechanisms of Speciation Geographic (Ecological) Models: Allopatric Model (different place) Disperse to Another Location Vicariance: a barrier is formed This geographic split could lead to Dobzhansky-Müller incompatibilities Sympatric Model (same place) Polyploid speciation Mate Choice Niche Partitioning (e.g. different food source, Host Plant) Parapatric Model (adjoining) Geographic Models of speciation ■ Allopatric speciation: geographic isolation ■ Sympatric speciation: no geographic isolation ■ Parapatric speciation: geographic separation (or gradient), but not isolation
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Speciation - · PDF fileNatural Selection Genetic Drift This Time: ... Involves Geographic Isolation Dispersal ... except for one population within the clade
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Carol Eunmi Lee 11/16/17
Title goes here 1
Speciation
(1) Geographic Mechanisms of Speciation(What circumstances lead to the formation of new species?)
(2) Species Concepts(How are Species Defined?)
Today’s OUTLINE:
Mechanisms of SpeciationLast Time: Genetic Models:
The roles of: MutationsNatural SelectionGenetic Drift
This Time: Geographic Models:Allopatric Model (difference place)Sympatric Model (same place)Parapatric Model (adjoining)
(1) Mechanisms of Speciation
Last Time: Genetic Models: How do Genetic Drift, Natural Selection, Mutations, etc. create new species? Are there “speciation” genes?
This Time: Geographic Models: How does speciation occur in Nature? Is geographic isolation required?
Mechanisms of SpeciationGeographic (Ecological) Models:
Allopatric Model (different place)Disperse to Another LocationVicariance: a barrier is formedThis geographic split could lead to Dobzhansky-Müller incompatibilities
Sympatric Model (same place)Polyploid speciationMate ChoiceNiche Partitioning (e.g. different food source, Host Plant)
Parapatric Model (adjoining)
Geographic Models of speciation
■ Allopatric speciation: geographic isolation
■ Sympatric speciation: no geographic isolation
■ Parapatric speciation: geographic separation (or gradient), but not isolation
Carol Eunmi Lee 11/16/17
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Allopatric Models
Involves Geographic Isolation
Dispersal
Vicariance
Allopatric Models•Following geographic separation between populations,
Dispersal
Vicariance
• This geographic separation provides the setting that allows speciation at the molecular level to occur (last lecture)
Allopatric ModelsDispersal
Vicariance
• Random Mutations would arise in the separated populations, and then selection or genetic drift would lead to fixation of those mutations
• If different mutations are fixed in the different populations, reproductive isolation could arise through Dobzhansky-Müller incompatibilities (last lecture)
Allopatric SpeciationExamples (see book)■ Dispersal:
◆ Colonization of islands◆ Colonization of lakes
■ Vicariance: ◆ Highway going through a forest◆ Fragmentation of habitats◆ Formation of Panama splitting the Caribbean &Pacific Oceans
Sympatric modelsSpeciation with no geographic separation Speciation despite gene flow
(1) Formation of polyploids (discussed in previous lecture)
(2) Natural Selection due toNiche PartitioningSexual Selection
Sympatric Model(1) Formation of Polyploids
(covered in previous lecture)
Important mechanism for plantsOccurs rarely in animals
■ So, when hybrids are formed between different species, they are often costly and maladaptive because of hybrid breakdown (the hybrids are maladaptive)
Reproductive isolation could occur at different stages of reproduction
■ Prezygotic Reproductive Isolation (before egg is fertilized)◆ Failure to Mate
✦ Genetic drift and divergence in bird songàwon’t mate✦ Selection on coat coloràdon’t recognize each other
◆ Sperm-egg incompatibility
■ Postzygotic Reproductive Isolation (after egg is fertilized)◆ DM incompatibilities cause embryo to not develop
(Example: enzymes don’t work together)
Reinforcement
■ So, the prediction is that in sympatry (when two different species are in the same place), mechanisms to avoid mating (prezyotic isolation) would be strong
■ Whereas in allopatry, prezygotic isolation would not be needed because the different species would not come into contact
Carol Eunmi Lee 11/16/17
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(2) How are Species Defined?
How are species defined?
So, what criterion should be used?
Historically, the most common criteria had been using morphological characters (external phenotype)
Speciation is a messy process
■ Rates of molecular, phenotypic (morphological) evolution and reproductive isolation are not necessarily concordant, but often discordant
■ Speciation is a jagged messy idiosyncratic process, where species boundaries are often difficult to define
The smallest monophyletic group is called a species
There is a derived character that is shared by the 4 populations
A A A
BBB
C C C
DDD
E E E
FFF
G G G
Group IIIGroup II
Group I
(a) Monophyletic group (clade) (b) Paraphyletic group (c) Polyphyletic group
A monophyletic clade consists of an ancestral taxa and all its descendants
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Carol Eunmi Lee 11/16/17
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Phylogenetic Species Concept
Easy to see evolutionary relationships on large and small taxonomic scales
It can be used on any species (sexual, asexual) for which there is phylogenetic information (molecular, morphological, biochemical data) on extant or fossil species
■ Easiest. Easier than constructing phylogeny or intermating
Phenetic (often Morphological)Species Concept
Strengths
Problems:■ Different species can look similar due to
convergent evolution
■ Populations that look distinct sometimes belong to the same species
■ Speciation can occur without changes in morphology or other traits (cryptic species)
Phenetic Species ConceptWhich species concept to use?■ Whenwediscussanimalsweoftenusethebiologicalspeciesconceptasthegoldstandard...complementedwiththephylogeneticandpheneticspeciesconcepts
1. Which of the following is a species according to the biological species concept?
(A) All hominin species (most are fossil species).
(B) A population of bacteria for which 80% of their DNA sequences are identical.
(C) All allopolyploid plants.
(D) A group of beetles that can intermate and produce offspring for multiple generations.
2. Which of the following is NOT a reason that Species are difficult to define?
(A) Many plants that are genetically divergent are able to mate
(B) Many organisms that are morphologically similar are genetically distinct
(C) Many organisms are asexual
(D) Sometimes groups split off from within a monophyletic group (such as birds splitting off from the reptiles)
(E) Sometimes sexual populations that are unable to interbreed could still be the same biological species
3. Which of the following is most likely to be a "species" according to the Phylogenetic Species Concept?
(a) A population of bacteria that has a gene that allows glucose metabolism
(b) Bird populations, which share a unique heritable feather structure
(c) Spider populations that can interbreed and produce fertile offspring
(d) Crustacean populations that form a clade (genetically-related group), except for one population within the clade that colonized land and became insects
(e) Populations of deer that share similar antler shape
4. Under which of the following scenarios is reinforcement most likely to evolve?
(a) Different fish species, with each living in a separate pond
(b) Two snail species, where each lives on opposite sides of a freeway
(c) Different species of crickets living together in a park, where hybrids between them have low survival rates
(d) Different insect species, each living on a different species of fruit in a forest
(e) Different species of allopolyploid plants living in a field