Help
Options
Didn't know it?
click below
click below
Knew it?
click below
click below
Don't Know
Remaining cards (0)
Know
retry
shuffle
restart
0:00
Embed Code - If you would like this activity on your web page, copy the script below and paste it into your web page.
Normal Size Small Size show me how
Normal Size Small Size show me how
Evolutionary Biology
Speciation 2: Isolating barriers and modes of speciation
| Question | Answer |
|---|---|
| What are isolating barriers? | = Any intrinsic process or phenomenon that forms a barrier to gene flow between populations Excludes (large-scale) physical separation per se |
| What are isolating barriers? | Barriers may vary in whether they are products of selection or by-products of other differences between populations |
| Premating barriers | By-products or products of selection for avoidance of hybridisation (reinforcement), if populations have hybridised in the past and hybrids have reduced fitness |
| Postmating barriers | By-products, since, for example, cannot be selection for greater degree of sterility in hybrids |
| Premating, prezygotic isolating barriers | = Features that impede transfer of gametes to members of other populations |
| 1) ecological isolation | Temporal: e.g. Bishop Pine v. Monterey Pine in California: former sheds pollen late in year, latter early in year can be made to hybridise Habitat isolation: e.g. Dixie Iris (Iris hexagona) v. Copper Iris (Iris fulva) in SE USA |
| 2) potential mates meet but don't mate | - Behavioural (sexual or ethological) isolation in animals - Pollinator isolation in plants |
| Mating or gamete transfer occurs, but zygotes are not formed | - Mechanical mismatch/copulatory failure - Gametic incompatibility |
| Hybrid zygotes are formed but have reduced fitness | - Hybrid inviability - Hybrid sterility |
| F2 hybrid breakdown | e.g. cytotypes of grasshopper Caledia captiva in Australia: in hybrid zone, F1 hybrids are common but F2 hybrids are rare (because die during embryonic development) |
| Which barriers cause isolation? 1) multiple barriers (apple maggot fly) | Prezygotic isolation: host plant fidelity, allochronic isolation Postzygotic isolation: no intrinsic inviability/sterility of hybrids (expect ecological isolation from host specific adaptation |
| Primary hybrid zone | Population genetic differentiation correlated with environmental discontinuities (with hybrid zones along these discontinuities) |
| Secondary hybrid zone | Two formerly separate populations expand, meet and interbreed, but do not necessarily merge |
| ie. Yellow-bellied toad | Hybrids have lower rates of survival (= postzygotic isolating barrier) |
| Characterisation of Bombina hybrid zone | Coincidence of clines in genetic (allozyme) and morphological features |
| What maintains stability of hybrid zones? | = Balance of dispersal and degree of hybridisation - bringing about gene flow - and selection against hybrids - preventing gene flow |
| e.g. two populations fixed at a locus for alleles A and a (hence populations are AA and aa) | Dispersal of AA individual into aa range, followed by mating, creates Aa hybrid; mating of this F1 hybrid with aa individual introduces A into aa population (introgression) |
| e.g. two populations fixed at a locus for alleles A and a (hence populations are AA and aa) | But if Aa hybrids are less fit (postzygotic isolating mechanism), then introgression does not occur or occurs rarely Hence steepness of cline determined by balance of amount of gene flow and strength of selection against hybrids |
| Summary | Isolating barriers are processes that keep species separate by preventing or reducing gene flow They may be products of selection, by-products of other differences, or a mixture of these |
| Summary | Isolating barriers may be prezygotic (pre- or postmating) or postzygotic Hybrid zones demonstrate the possibility of stable species boundaries (genetical and physical) despite hybridisation occurring |
| Summary | Stability of hybrid zones depends on relative strengths of gene flow and fitness of hybrids |
| Allopatric speciation | (geographically separated populations become reproductively isolated) |
| 1) With vicariance | (an existing population is subdivided into one or more smaller populations) |
| 2) Peripatric speciation | (a larger parental population gives rise to a smaller population at its periphery, by subdivision or by dispersal) |
| Parapatric speciation | (geographically neighbouring populations become reproductively isolated) |
| Sympatric speciation | (geographically overlapping populations become reproductively isolated) |
| Criteria to meet for sympatric speciation | 1) The species must be largely or completely sympatric 2) The species must have substantial reproductive isolation, preferably based on genetic differences |
| Criteria to meet for sympatric speciation | 3) The sympatric taxa must be sister groups. The genetic similarity used to establish this fact must not result from hybridisation 4) The biogeographic and evolutionary history of the groups must make the existence of an allopatric phase very unlikely |
| Summary - modes of speciation | Speciation can occur in several modes (based on presence, absence or nature of geographical barriers to gene flow) |
| Summary - modes of speciation | Evolutionary causes of speciation can, in principle, operate via any of these modes Evidence for sympatric speciation in the wild is rare and frequently proved incorrect later on |
| See powerpoint for diagrams |
Created by:
reub8n
Popular Biology sets