“That mystery of mysteries…”
Darwin never actually tackled how new species arose…
So…what is a species?
•	Biological species concept
•	defined by Ernst Mayr
•	population whose members can interbreed & produce viable, fertile offspring
•	reproductively compatible
How and why do new species originate?
•	Species are created by a series of evolutionary processes
•	populations become isolated
•	geographically isolated 
•	reproductively isolated
•	isolated populations 
evolve independently
•	Isolation
•	allopatric
•	geographic separation
•	sympatric
•	still live in same area
Pre-zygotic barriers
•	Obstacle to mating or to fertilization if mating occurs
Geographic isolation
•	Species occur in different areas
•	physical barrier
•	allopatric speciation
•	“other country”
Ecological isolation
•	Species occur in same area, but occupy different habitats so rarely encounter each other
Temporal isolation
•	Species that breed during different times of day, different seasons, or different years cannot mix gametes
•	sympatric speciation
•	“same country”
Behavioral isolation
•	Unique behavioral patterns & rituals isolate species
•	identifies members of species 
•	attract mates of same species 
•	courtship rituals, mating calls
Recognizing your 
own species
Mechanical isolation
•	Morphological differences can prevent  successful mating
Mechanical isolation
•	For many insects, male & 
female sex organs of 
closely related species do 
not fit together, preventing 
sperm transfer
•	lack of “fit” between sexual organs: 
hard to imagine for us… but a big issue for insects with different shaped genitals!

Gametic isolation
•	Sperm of one species may not be able to fertilize eggs of another species
•	mechanisms
•	biochemical barrier so sperm cannot penetrate egg
•	receptor recognition: lock & key between egg & sperm 
•	chemical incompatibility 
•	sperm cannot survive in female reproductive tract
Post-zygotic barriers
•	Prevent hybrid offspring from developing into a viable, fertile adult
•	reduced hybrid viability
•	reduced hybrid fertility
•	hybrid breakdown
Reduced hybrid viability
•	Genes of different parent species may interact & impair the hybrid’s development
Reduced hybrid fertility
•	Even if hybrids are vigorous 
they may be sterile
•	chromosomes of parents may differ in number or structure & meiosis in hybrids may fail to produce normal gametes
Hybrid breakdown
•	Hybrids may be fertile & viable in first generation, but when they mate offspring are feeble or sterile
A love story……
Rate of Speciation
•	Current debate:
	Does speciation happen gradually or rapidly 
•	Gradualism
•	Charles Darwin
•	Charles Lyell
•	Punctuated equilibrium
•	Stephen Jay Gould
•	Niles Eldredge
Gradualism 
•	Gradual divergence over long spans of time
•	assume that big changes occur as the accumulation of many small ones
Punctuated Equilibrium
•	Rate of speciation is not constant
•	rapid bursts of change 
•	long periods of little or no change
•	species undergo rapid change when they 1^st  bud from parent population
Evolution is not goal-oriented
An evolutionary trend does not mean that evolution is goal-oriented. 

Surviving species 
do not represent 
the peak of 
perfection. There 
is compromise & 
random chance 
involved as well

Remember that for 
humans as well!
Any Questions??