sp3 - Speciation Models Flashcards
speciation
the process by which new species are formed, often described as the origin and spread of reproductive isolating barriers
incipient speciation
the early stages of a species diverging into two or more distinct populations
allopatric speciation
**Speciation without gene flow
geographically separate limiting gene flow accumulating differences
sympatric speciation
same geographic environment however separated by other isolating barriers thus still accumulating differences over time
current emphasis in speciation
forces driving evolution and accumulation of isolating barriers
how does geographicaly isolation arise
- dispersal
- vicariance
Dispersal
geographical isolation in which a few individuals disperse to a separate geographical region which already existed and reproductive barriers evolve as evolutionary processes act on two popualtions independantly
**FOUNDER EVENT
Vicariance
geographic barrier forms (was not there before) thus separating populations so evolutionary process act on them independently
key predictor to indicate geographical isolation occured
speciation associated with characteristics of geographic isolation
Process/recipe of allopatric spec.
- populations become geographically separated and gene flow no longer occurs
- mut/drift/selection/gene flow occur independently in pops
- as time passes, if populations came back into contact may no longer be able to freely interbreed
example of vicariance - isthmus of panama
North and south America formerly connected
formation of panama isthmus separates Pacific and Atlantic Ocean
isolated populations of shrimp in either oceans (+also ALL SPECIES in waters), thus would expect to see same patterns in sister species
Ex ONE common ancestor, diverge into TWO populations which are then separated by isthmus formation and diverge into two more populations each
Therefore pop on one side sister to other, ANOTHER species one side sister to other side
**sister species occur on either side of isthmus
example of dispersal - Hawaiian Drosophila
island chain 10MY old, originating from volcanic activity therefore all life on Island there bc of dispersal
islands in NW older than those in the SE
Many groups of species diversified after diversification event
Diff drosophila species phylogeny corresponds to age of islands, older species on older islands
how to identify dispersal versus vicariance
dispersal applies to a FEW species
vicariance would apply to ALL species in area
relationship btwn length of time since divergence and prezygotic isolation in allopatric taxa
positive relationship
ICLICKER QUESTION FROM CLASS
same pattern if looking at pre-zygotic and post-zygotic barriers, positive relationship btwn time sinze divergence and isolation!
YES - geographic barrier permanent therefore no extra cost if meet
Pops are physically isolated!!!!!
secondary contact
previously separated populations come back together
evidence for allopatric
speciation events within clades coinciding with rare dispersal event
rep. isolation accumulate as func of time since divergence
do multiple unrelated groups w/ sister species on either side of geo barrier
current distributions not overlapping - WEAK evidence, geo barriers can change
possible results of secondary contact
- NOTHING
- merge into a single species
- invasion of one species, detected if some individuals some part of genome indicating separate species
- form a hybrid zone in area of contact
- introgression - exchange of a few genes incorporated into common genome of each species
- form a new hybrid species - rep. isolated hybrid species, and 2 parents species still exist
Hybrid Zones
very common
stable because parent species always interbreeding BUT hybrids not diverging enough to be distinct species
Reinforcement
if prod poor quality offspring, fitness of parents reduced
selection favours individuals that can avoid hybridization
selection acts to strengthen pre-zygotic barriers to avoid formation of unfit hybrids
process of reinforcement
a: divergence in allopatry
b. secondary contact, with hybs. that prods low fitness offspring
c. reinforcement of pre-zygotic barriers by natural selection for those that can avoid interbreeding
requirements of sympatric speciation
strong selection to overcome gene flow
ex. ecological selection by shift in habitat preference/resource use
a link between ecology and mating preferences, traits subject to divergent selection also contribute to non-random mating
magic traits
traits subject to divergent selection also contribute to non-random mating
speed up process of divergence
extra on symnpatric spec in howea palms
researchers find juvenile hybrids, and very few adult hybrids
juvenile hybrids but very rare they survive to adult hood
Ex. of extrinsic hybrid viability
ex. speciation in Helianthus petiolaris
widespread, normally in sandy soil sometimes in dunes
species comparison in dunes vs sand sheets: very different seeds between dune and nondune (5X heavier seed in dune environment, dune plants flower later, > nitrogen use efficiency)
spec. diverged 12,000 years ago
ASYM gene flow, more from dune to non-dune than non-dune to dune
estimated hybrid and parental fitness
strength of various reproductive barriers
sequenced genome, dune specific traits map to large supergenes which define dune ecotype AND reproductive barriers map onto supergenes!!!
Speciation WITH gene flow
resources/hab favour diff phenotypes
individuals/subpops differ in traits that affect fitness - diff alleles of same gene or diff genes selected
selection may act to reduce gene flow/recombination among loci that affect
two forms of hybrid speciation
homoploid hybrid speciation: new hybrid spec has same ploidy as parents
polyploid hybrid speciation: new hybrid spec increase ploidy in relation to parents
homoploid hybrid characteristics
F1 hybrid, cont generations, frequent cross overs
- ecological separation of hybids
ability to populate area parents cannot - rapid and strong isolation from parents
ex. hybrid sparrows cannot mate with parental species only each other
Do hybrid zones = hybrid species?
NO!!!!!
