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BioA414
Population Genetics
Handout X
Speciation
• Process  new species are formed from
previously existing ones
• Multiplication of species  Darwin
• Species  like individual organisms
– "Parental" species  "daughter" species
– Reproduction  individual level or species
level
Species concepts
• Biological Species Concept (BSC)
• Morphological/Phenetic/Typological Species Concept
• Phylogenetic/ Cladisic Species Concept
• Evolutionary Species Concept
• Genetic Species Concept
• Microspecies Concept
• Ecological Species Concept
• Mate-Recognition Species
Species concepts
• Biological Species Concept (BSC)
– Advanced by Ernst Mayr in 1942
– A group of interbreeding organisms that can
produce viable offspring
– Advantages  based on a mechanism
– Disadvantages  Fossils certainly can’t
mate any longer, bacteria don’t have sex, etc.
What is a Species?
Species concepts
• Morphological/Phenetic/Typological Species
Concept
– A morphospecies  on overall similarity
– Individuals  not exactly the same
– All biologists make some use of this species concept
– Advantages  morphology can be readily observed
– Disadvantages  ♂ and ♀ individuals of the same
(biological) s pecies into separate species
Appearance isn’t everything
Sturnella magna
Sturnella neglecta
Members of different species - eastern (left) and western (right) meadowl ark
Appearance isn’t everything
Two ants of the s pecies Pheidole barbata  different roles in the same colony
(a) Similarity between different species
(b) Diversity within a species
Species concepts
• Phylogenetic/ Cladisic Species Concept
– Considers the evolutionary relationships
– Common ancestry and shared evolutionary history
 a distinct branch of an evolutionary tree
– Advantages
 recognizes the role of history
 objective and can use nearly any sort(s) of data
Species concepts
• Genetic Species Concept
– Genetic similarity or distance
– Genetic analyses can uncover cryptic species
that morphological studies would not
– Advantages  independent evidence for
morphological and biological species
– Disadvantages  relies on human judgment
Species concepts
• Evolutionary Species Concept
– Genealogical basis of the phylogenetic
species concept + genetic basis of the BSC
 interbreeding organisms
 reproductively isolated
 a beginning, an end
 a distinct evolutionary trajectory
Species concepts
• Microspecies Concept
– Uses DNA
– Covers organisms that reproduce asexually
– Advantages  clear criteria and useful for
microorganisms and some plants
– Disadvantages  some organisms may
reproduce asexually, with ↑ mutation rates
Species concepts
• Ecological Species Concept
– Describe populations adapted to certain
ecological niches  clusters
– Advantages  acknowledges the role of the
environment  morphological development
– Disadvantages  can miss cryptic species,
not a very robust species concept
Species concepts
• Mate-Recognition Species
– Observe which individuals they mate with
– Differs from BSC  a mating attempt!
– Advantages  observational concept
– Disadvantages
 not very clear what advantages!
 applies to sexually reproducing organisms
Northern spotted owl (left) and barred owl (right)
Strix occidentalis
Strix varia
Biological species concept
• Speciation
– Populations acquire reproductive isolating
mechanisms
• Barriers
– Prevent fertilization  prezygotic isolation
 changes in location or timing of breeding
 changes in courtship displays
– Hybrids are unviable or sterile  postzygotic isolation
 act after mating to prevent hybrids from
backcrossing into eithe r pare ntal species
How many species are there?
About 2 million species have been described
Estimates range from 4x106 to 108
How do species arise?
The key to speciation is
reproductive isolation of
populations
There are extrinsic and
intrinsic reproductive
isolating mechanisms
Geographic isolation is
the primary extrinsic
reproductive isolating
mechanism
Reproductive isolation may occur with or
without geographic isolation
Extrinsic mechanis m
Intrinsic mechanis ms
Modes of Speciation
• Allopatric speciation
– Most widely accepted mode of speciation
– Has wide support from studies on
geographic variation among species
– Championed by Ernst Mayr
– Geographic separation of two populations 
the first step in the process
Allopatric speciation
Harris’ antelope squirrel
White-tailed antelope squirrel
Sympatric speciation
• Under the sympatric model
– More restrictive conditions for speciation
– Individuals of the two diverging groups may
come into contact and reproduce
– When they do, the genetic differences
developing between the groups would be
eliminated
Intrinsic reproductive isolating mechanisms are
always required for speciation
Harris’ antelope squirrel
White-tailed antelope squirrel
Intrinsic mechanisms invol ve changes to organisms
that prevent interbreeding
In allopatric s peciation, intrinsic mechanisms come into pl ay once
populations are physically separated
In sympatric s peciati on, intrinsic mechanisms are the onl y ones invol ved
Many intrinsic reproductive isolating
mechanisms drive speciation
Prezygotic isolation
Leopard Frog Rana sphenocephala
Wood Frog Rana sylvatica
Many intrinsic reproductive isolating
mechanisms drive speciation
Many intrinsic reproductive isolating
mechanisms drive speciation
Euhadra
Many intrinsic reproductive isolating
mechanisms drive speciation
Reproductive
barriers keep
species separate
Speciation occurs at widely differing rates
A slow rate of s peciati on evi denced by a li vi ng horseshoe crab (13 extant
species) and a 300 million year-ol d fossil species
A rapi d rate of s peciati on evi denced by Gal apagos finches which have
di versified into 13 species within the last 100,000 years
Species Come and Go
Best estimates from the fossil
record indicate that greater
than 99% of species that have
existed are now extinct
A typical "lifetime" for a
species is about 1 million years
Mass Extinctions are a fact of life
Mass extinction
• Several major mass extinctions
– Exceeded the background extinction rate
– Most recent  the Cretaceous–Paleogene
extinction event  ~ 65.5 million years ago
– In the past 540 My  five major events
when over 50% of animal species died
Are We Now Causing a Mass Extinction?