BIOLOGY 1B Lecture Notes - Lecture 5: Ensatina, Hybrid Zone, Hydrangea
11 Jun 2018
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Lecture 5: Speciation
9/1
• Morphological species concept (MSC): Organisms are classified in the same species if they appear
identical by morphological criteria
o Problem: polymorphism
▪ Its hard to tell in snakes if they’re the same snake morphologically because they are
different colors but if you look at genetics they’re related
▪ Butterflies that look similar are actually different species; distinguishable when you look
at larvae- different feeding, colors
▪ Non genetic variation: phenotype plasticity
• Ex. Hydrangea- different colors because of soil. Environmental variation
• Biological species concept (BSC): A species is a population whose members have the potential to
interbreed in nature and produce viable, fertile offspring
o Ernst Mayr
o Works well for birds not all groups
o The emphasis is on reproductive isolation mechanisms (barriers to gene flow)
Prezygotic mechanisms: Block breeding before a zygote forms
o Habitat isolation: spacially separated- can’t mate
o Temporal isolation: species breed at different times; don’t get to mate
o Behavioral isolation: courtship rituals differ; don’t recognize other species as mates
▪ Different songs, dances
o Mechanical isolation: breeding can be prevented because they can’t physically mate
▪ Differently coiled snails
o Gametic isolation: fertilization does not occur; molecular rejection or non recognition
o Anisogamy: sexual reproduction by fusion of dissimilar gametes
▪ Reduced hybrid viability: If zygote does form, reduced fitness, unstable, unhealthy
• Might be born but not vigorous
o Ensatina salamander
▪ Reduced hybrid fertility: Viable but not able to reproduce
• Mules! Cross a horse and donkey but mules cannot reproduce
o Hybrid breakdown: when you cross different species, the hybrids are alive but breakdown over the
generations and are no longer viable after a few generations
▪ Hybrid lethality
o Ring species:
▪ Ensatina salamander
• Occupies a ring around the central valley
o Not in the central valley because of the heat and dryness; salamanders
are moist
o Creates morphs in different areas
▪ Some cannot breed together because they evolved too much
• Population limited by environment
o Problem with BSC: hinges on reproduction and formation of gametes- not all species reproduce
that way- bacteria
• Phylogenetic species concept (PSC)
o Assess relationships of geneology based on evolutionary history
• Evolution of islands: volcanos erupting; nothing alive when lava hardens; organisms will ultimately arrive
o Spiders often first species
o Movement of species to islands; evolution on those islands
• Speciation:
• Hybrid zones: population with individuals interbreeding; gene flow; isolated population diverges
o Then original and evolved species hybridize and interbreed
▪ Possible outcomes:
• Reinforcement: hybrids are less fit than either purebred species; species diverge
until hybridization can no longer occur
• Fusion: populations come back together
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