Species and speciation
What is a species?
 Species = Latin for “kind” or “appearance”
 Linnaeus described species in terms of their morphology
 Modern taxonomists also consider genetic makeup and functional and
behavioral differences when describing species
The biological species concept (BSC) emphasizes reproductive isolation
 Ernst Mayr 1942
 A biological species is defined as a population or group of populations whose
members have the potential to interbreed and produce viable, fertile offspring
but cannot do so with members of other species
– The species is the largest unit of population in which gene flow is possible
– It is defined by reproductive isolation from other species in natural
environments (hybrids may be possible in the lab or in zoos)
Gene pools of biological species are isolated by pre- and post-zygotic barriers
 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
Pre-zygotic barriers: habitat 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
Pre-zygotic isolating mechanisms: temporal isolation
 Temporal isolation:
– Brown trout breed in the autumn whereas rainbow trout living in the same
streams breed in the spring
Pre-zygotic barriers: behavioral isolation
 Species-specific signals and elaborate behavior to attract mates
 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
Other behavioral mechanisms
 Eastern and western meadowlarks only recognize songs of the same species
 Specific courtship rituals
Pre-zygotic isolating mechanisms: Mechanical isolation
 Clasping appendages in dragonflies
 Floral anatomy corresponding to specific pollinator
Genital arch in Drosophila
Pre-zygotic isolating mechanisms: gametic isolation
 Sperm of one species may not survive internal environment of female
reproductive tract in another species
Pre-zygotic isolating mechanisms: 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
Other species concepts
How species are diagnosed
 Morphology
 Phenotypic characters
 Molecular characters
Gradual divergence among species in allozymes
Differences among species
 Genetic difference (D) can be used as a molecular clock to estimate relative
times of divergence
 Used by Coyne and Orr (1989)
Strength of prezygotic and postzygotic isolation increases gradually
Conclusions of Coyne and Orr
 Time required for full isolation to evolve is variable – averages 1.3 to 1.5
million years, but many happen under 1 million years
 Postzygotic isolation evolves more rapidly in males than females
Premating isolation is a stronger barrier to gene exchange than postzygotic
isolation in sympatric taxa
Genes cause reproductive isolation
 Genes have been identified that have an effect on sperm motility in hybrids
Epistatic interactions cause sterility
Chromosome differences
Hybridization
 Hybrids may represent intermediate stages in the process of speciation
 A character that changes along a hybrid zone exhibits a cline
 Primary hybrid zones
– Natural selection alters allele frequencies in continuously distributed
populations
 Secondary hybrid zones
– Secondary contact of two formerly separated species
Primary hybrid zone
Secondary hybrid zone
Speciation
Mechanisms of speciation
– Allopatric speciation
 Vicariance
 Peripatric speciation or founder effect speciation
– Sympatric speciation
– Parapatric speciation
Geographic isolation
Allopatric speciation
 Populations are first geographically isolated, then become reproductively
isolated
 Species separated by geographic barriers will diverge genetically
 If divergence is great enough it will prevent breeding even if the barrier later
disappears
 Effectiveness of barrier varies with species
Isolation is correlated with genetic distance
Historic isolation
Reproductive isolation may result from adaptive divergence of two populations
Apple maggot fly
Rhagoletis pomonella
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Native to Midwest and northeastern US
Relied on native hawthorne fruit trees
Adults mate on the fruit, females lay eggs in the fruit
Flies began using apple trees sometime after they were introduced from
Europe ~ 300 years ago
Apple maggot fly
 Expt: Give flies choice of haw vs apple. Flies from apple choose apple, flies
from haw choose haw
 Mating occurs on the plant, so host preference  in strong nonrandom mating
 Flies associated with different hosts are genetically divergent!
Suggests: Natural selection based on host preference for different food
created different races/species of flies.
 Even though they are sympatric!
 It appears that rather than being isolated by dispersal or vicariance, the flies
are isolated on different host species
Rhegoletis pompella
 Selection has favored distinct habitat preferences in apple and hawthorn flies
 There appears to be an increase in fitness from switching to apple trees:
– Escape from parasitoids: The average level of wasp parasitism is 70% less
on apple than on hawthorne
– Escape from intraspecific competition due to the large size of apple fruit
– Escape from interspecific competition
Speciation in sticklebacks
 Sympatric pairs of species of three-spine stickleback inhabit small lowelevation lakes in coastal British Columbia
 In lakes with 2 species: large-bodied benthic form & small bodied limnetic
forms are present.
 In lakes with 1 species, form is intermediate
Sticklebacks in British Columbia lakes
 Both Pre-zygotic and post-zygotic mechanisms
Two pre-zygotic barriers:
 Limnetic species & benthic species spawn in different habitats
 Limnetic females prefer red throated males
 Benthic females prefer blacker throated males
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Hybrid Sticklebacks have lower fitness in both habitats
Parallel evolution
Sexual selection
 More species in sister group with promiscuous mating systems
Peripatric or founder effect speciation
Peripatric speciation
 Genetic drift in founder population
Sympatric speciation by polyploidy
 Change in chromosome number
(3n, 4n, etc.)
– In autopolyploids, offspring have a doubling of chromosome number from
parents
– Allopolyploids are interspecific hybrids
 Offspring with altered chromosome number cannot breed with parent
population
 Common mechanism of speciation in flowering plants
Evolution of wheat