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Species concepts
Darwin’s other stumbling block
 How do new species arise?
 i.e., speciation
The nature of species
 The concept of species must account for two phenomena:
 The distinctiveness of species that occur together at a single locality
 The connection that exists among different populations belonging to the
same species
Speciation
 The process by which new species arise, either by
 transformation of one species into another
 by the splitting of one ancestral species into two descendant species
Sympatric speciation
 The differentiation of populations within a common geographic area into
species
 Species that occur together:
 Are distinctive entities
 Are phenotypically different
 Utilize different parts of the habitat
 Behave separately
Subspecies
 Within a single species, individuals in populations that occur in different areas
may be distinct from one another
Overlap of subspecies
The Biological Species Concept
 Ernst Mayr’s biological species concept defines species as…
 “…groups of actually or potentially interbreeding natural populations which
are reproductively isolated from other such groups.”
 In short, members of a population mate with each other and produce fertile
offspring
 Reproductive isolation is therefore key to the BSC
Gene pools of biological species are isolated
 Any factor that impedes two species from producing viable, fertile offspring
contributes to reproductive isolation
 Various barriers classified by whether they function before or after zygote
formation:
 Pre-zygotic barriers
 Post-zygotic barriers
Geographic isolation
Deer mice
Ecological isolation
 Two species living in different habitats may not encounter each other:
 Two species of garter snake (Thamnophis) occur in the same area but one
species lives in water and the other is terrestrial
Lions and tigers are ecologically isolated
In captivity lions and tigers can mate and reproduce offspring that survive
Behavioral isolation
 Species-specific signals and elaborate behavior to attract mates
 Sympatric species avoid mating with members of the wrong species in a
variety of ways, including differences in:
 Visual signals
 Sound production
 Chemical signals: pheromones
 Electrical signals: electroreception
Behavioral isolation
 Many animals recognize mates by sensing pheromones:
 Female gypsy moths emit a volatile compound to which olfactory organs of
male gypsy moths are specifically tuned
Behavioral isolation
 Eastern and western meadowlarks only recognize songs of the same species
 Specific courtship rituals
Temporal isolation
 Temporal isolation:
 Brown trout breed in the autumn whereas rainbow trout living in the same
streams breed in the spring
Mechanical isolation
 Clasping appendages in dragonflies
 Floral anatomy corresponding to specific pollinator
Genital arch in Drosophila
Gametic isolation
 Sperm of one species may not survive internal environment of female
reproductive tract in another species
Conspecific sperm precedence
 Conspecific sperm wins in multiple matings
Post-zygotic isolating mechanisms
 Reduced hybrid viability:
 Genetic incompatibility may abort development at embryonic stage
 Reduced hybrid fertility:
 Species mate and hybrid is viable but sterile
Sterile hybrids
Post-zygotic isolating mechanisms
 Hybrid breakdown:
 First generation hybrids are fertile but subsequent generations are defective
Shortcomings of Biological Species Concept
 50% of California plant species, in one study, were not well defined by genetic
isolation
Shortcomings of Biological Species Concept
 Hybridization is not uncommon in animals
 10% of bird species have hybridized in nature
 Hybrid offspring of Galápagos finches appeared to be at no
disadvantage for survival or reproduction
 Reproductive isolation may not be the only force for maintaining the
integrity of species
Ecological Species Concept
 Theories & criticisms
 Hybridization has little effect because alleles introduced into one species’ gene
pool from other species are quickly eliminated by natural selection
 Difficult to apply biological species concept to populations that are
geographically separated in nature
 Many organisms are asexual and do not mate
Other species concepts