Leslie Samuel
Issues in Origins and Speciatioin
Freeman/Herron 15
Mechanisms of Speciation
15.1 Species Concepts
- A species is the smallest evolutionarily independent unit
- The essence of speciation is lack of gene flow
The Biological Species Concept (Ernst Mayr)
- Criterion for identifying species is reproductive isolation
- Shortcoming:
o Cannot be tested if nearby populations do not actually overlap
o Can never be used in studying fossil forms
o Irrelevant to asexual populations
o Difficult to apply in many plant groups
The Phylogenetic Species Concept/genealogical species concepts
- Criterion for identifying species called monophyly.
- Estimates the phylogeny of closely related populations and finding the smallest
monophyletic group
- Based on unique derived characters
- Shortcomings:
o Cost a lot of time, money, and careful analysis.
o Could easily double the number of named species
 Not as big an issue
The Morphospecies Concept
- Based on morphological differences among fossil1s
Applying Species Concepts: two case histories
Each species concept has advantages and disadvantages
- Diversification in Marine Copepods
o Biological species concept was first used
o Phylogenetic species concept, based on gene sequencing, showed that
species diversity in copepods is likely to be far greater than previously
thought.
- How Many Species of Elephant Live in Africa?
o Phylogenetic analysis showed that forest and savanna elephants qualify as
distinct phylogenetic species
o This discovery was essential for preserving biodiversity
15.2 Mechanisms of Genetic Isolation
How do Species form?
- Speciation has been hypothesized to be a 3-stage process
o Initial step isolates populations
o Second step results in divergence in traits such as mating system or habitat
use
o Final step produces reproductive isolation (secondary contact
 In significant number of speciation events, this never occurs.
Physical Isolation as a Barrier to Gene Flow
- Gene flow tends to homogenize gene frequencies
- Allopatric model holds that small populations become physically isolated
o Can occur rapidly when selection for divergence is strong and gene flow is
low
Geographic Isolation Through Dispersal and Colonization
- Populations can become geographically isolated when individuals colonize a new
habitat (Founder hypothesis)
o Two Predictions
 Closely related species should almost always be found on adjacent
islands
 At least some sequences of branching events should correspond to
the sequence in which islands were formed
Geographic Isolation through Vicariance
- Vicariance events split a species’ distribution into two or more isolated ranges and
discourage or prevent gene flow between them
A Role for Mutation: Polyploidy and Other Chromosome Changes as a Barrier to Gene
Flow
- Changes in chromosome number isolate populations genetically (polyploidy)
- Speciation triggered by changes in chromosome number has been especially
important in plants
- Until more causative links are established, we have to be cautious in interpreting
the importance of small-scale karyotype differences in speciation
15.3 Mechanisms of Divergence
- Genetic Drift
o Sampling error
o Can produce genetic divergence in small, isolated populations
o Leads to a random loss of alleles and the random fixation of existing and
new alleles.
o E.g. bottlenecking
- Natural Selection
o Can lead to divergence if one of the populations occupies a novel
environment or uses a novel resource.
 E.g. Apple and hawthorn maggot flies. In a single generation,
researchers succeeded in replicating the selection events that have
produced divergence between the apple race and the hawthorn race
in nature over the past 150 years.
- Sexual Selection
o Acts on characters involved in mate choice.
o Changes in sexual selection may isolate populations and cause rapid
divergence.
o In Drosofila, male-male combat and female choice determined the
differentiation between populations leading to different head shapes and
fighting strategies.
15.4 Secondary Contact
Hybridization occurs when recently diverged populations interbreed
- Reinforcement
o A type of selection that leads to assortative mating and the prezygotic
isolation of populations
o Prezygotic isolation evolves much faster in sympatric species pairs than it
does in allopatric species pairs
- Hybridization
o Creation of New Species through Hybridization
 Secondary contact and gene flow between recently diverged
species can result in the formation of a new, third, species.
o Hybrid Zones
 Region where interbreeding between diverged populations occurs
and hybrid offspring are frequent
 Three possibilities
 When Hybrid and parental forms are equally fit, the hybrid
zone is wide. Allele frequency change is dominated by drift
 When hybrids are less fit than purebred individuals, the fate
of the hybrid zone depends on the strength of selection
against them
 When hybrids are more fit, the fate of the hybrid zone
depends on the extent of environments in which hybrids
have an advantage.
The Genetics of Speciation
- Large-scale changes in the genome are not only unlikely, but unnecessary for
divergence and speciation to occur
- Pea Aphids and Three-Spined Sticklybacks