Speciation
When speciation occurs, the new ‘daughter’ species share characteristics
because they are descended from a common ancestor. Species are therefore related to one another to differing degrees, like individuals
in a family tree
Prezygotic barriers
Before mating:
• habitat isolation
• temporal isolation
• behavioural/pollinator isolation
After mating:
• mechanical isolation
• gametic isolation (including sperm competition & sperm preference)
Postzygotic barriers
1) Genetic incompatibilities
– genes of the two parent species interact in ways that reduce
the viability (i.e., survival) and/or fertility (i.e., sterility) of F1
or later generation hybrids
– independent of the ecological context of the hybrid
– can arise due to any mechanism of speciation (due to any
microevolutionary process)
2) Ecologically-dependent postzygotic isolation
– a reduction in hybrid fitness due to a mismatch between their phenotype and their environment
(i.e., hybrids ‘fall between niches’)
– can only be produced by divergent natural selection
Prezygotic
These prevent mating or fertilization from happening in the first place.
Postzygotic
These reduce the survival or reproduction of hybrid offspring after fertilization has occurred
Allopatric? Sympatric?
Allopatric – occurring in separate, non-overlapping geographic areas
Sympatric – occurring in the same, overlapping geographic area
Allopatric speciation
– Gene flow is interrupted by some extrinsic barrier
– Populations evolve independently, eventually accumulating reproductive barriers
– If secondary contact occurs, populations are reproductively isolated
Sympatric speciation
– Speciation in the absence of any extrinsic or geographic barrier
(i.e., between populations that inhabit the same region)
– Difficult to overcome the initially unrestricted gene flow
– Can occur via polyploidization or via strongly disruptive selection alongside
some mechanism for positive assortative mating (i.e., so mating tends to
occur within, and not between, each incipient species)
– Restrictive conditions, and biogeographic evidence of
allopatric speciation, suggest sympatric speciation is rare
Ecological speciation
• reproductive isolation arises as a by-product of adaptation to
different environments or niches. i.e., natural selection differs in each population due to the unique biotic
and/or abiotic environments they inhabit (termed divergent natural selection), and the phenotypic differences that evolve cause reproductive isolation as a side-effect
• can occur under any geographic context
Polyploidizaton
• increase in the # of the entire set of chromosomes (i.e., 2n, 3n, 4n, etc.)
• postzygotic can be instantaneous (caused by the production of gametes with unbalanced #’s of chromosomes)
• occurs in sympatry
• common in ferns & flowering plants (estimated 2-4% of speciation events in flowering plants & 7% in ferns involved polyploidization)
2 Type of Polyploidization
1) autopolyploid - an individual with more than two
chromosome sets derived from a single species
2) allopolyploid - an individual with more than two
chromosome sets derived from different species
Reinforcement
• The strengthening of prezygotic barrier by natural selection in response to reduced hybrid fitness (i.e., in response to existing postzygotic isolation)
• Can complete a speciation event initiated by any mechanism
• Expected outcome is a pattern of stronger prezygotic isolation in sympatry than allopatry
• Requires secondary contact (i.e., at least partial sympatry)
Mechanisms of speciation
1) Ecological
2) Polyploidization
3) Reinforcement
