21- Speciation P2 Flashcards
Causes Of Speciation
1) Ecological Speciation
2) Sexual Selection
3) Reinforcement of reproductive isolation
4) Polyploidy
5) Hybrid Speciation
6) Genetic Drift
1) Ecological speciation
reproductive isolation between populations -
- due to ecologically-based divergent selection
Prezygotic isolation= mating incompatible
Postzygotic isolation Hybrid has reduced fitness
Example= Monkeyflower (Mimulus)
M. lewisii: High elevation - adapted for bee pollination
M- cardinalis: Low elevation - adapted for hummingbird pollination
Hybrid zone highlights important ecological traits:
- Elevation differences alone reduced gene exchange by 60%
- Pollination differences meant 98% reduction in gene flow = almost complete barrier
Example= Drosophila experiment in the lab
raised populations with different food sources for 20 generations : starch v maltose
- when populations were mixed…
- significant sexual isolation between the 2 groups compared to within population mating -> highlights speciation due to an ecological factor.
Pleitropy
Speciation is a pleiotropic outcome of these adaptations to the conditions.
(speciation not being selected for itself)
2) Sexual selection
Differences in reproduction
- due to varying abilities to attain mates
- extravagant sexual traits in a lineage leads to higher species richness e.g. birds of paradise.
= engine driving speciation
Phenotypically distinct sexual traits –> accentuates preference for them -> increases strength of selection
This divergent selection can drive rapid prezygotic isolation (if this preference stops them mating with other individuals.)
Example: Treehoppers
Signal via acoustic pulses
- Variation in preferences of signal frequency
Crickets (Hawaii)
Preferences in sexual signal have led to rapid evolution of the lineages with multiple species emerging.
Why does sexual selection
vary across populations?
* To improve conspecific recognition (i.e. reinforcement)
* To provide optimal direct benefits to mates
* As a result of pleiotropic effects (e.g. different perceptual biases)
* Because of variation in “good genes” mechanisms (e.g. novel indicator
traits)
* Because of runaway mechanisms when trait and preference alleles
become genetically linked
3) Reinforcement
Selection for further isolation enhances reproductive isolation between populations
- to reduce likelihood of forming maladaptive hybrids.
Second contact after allopatry…
1) Hybridisation & collapse (due to no postzygotic isolation, fitness of hybrids is good & gene flow gets reintroduced)
2) Reinforcement- post-zygotic isolation now also present (reduced fitness of hybrids.)
Alleles that help identify members of their own population are selected for -> helping increase prezygotic isolation.
Until Selection is complete
Other Isolating Mechanisms
Postzygotic mechanisms can’t evolve by reinforcement because hybrids have decreased fitness so none of their alleles increase in fitness.
Conditions favouring Reinforcement
- Low fitness: 0.3 or below
- High levels of population difference (measure of prezygotic isolation).
Hybrids with fitness over 0.3 often leads to maintenance of the hybrid.
V low prezygotic isolation/population differences= extinction of the hybrid (collapse).
Drosophila Reinforcement
common process in sympatric taxa
- hybrids in sympatry have higher levels of prezygotic isolation in relatively short times since divergence than hybrids formed from allopatry.
- reinforcement impacts 70% of all sympatric dresophila taxa
- enhances premating isolation by 25%
Reproductive Character Displacement
reproductive interactions cause a trait difference to further accentuate when 2 populations come into contact
Example Flycatchers:
- females in sympatry prefer lighter colouration compared to the 2 populations in allopatry where they prefer shades of darker ancestral coats.
- Coat colour diverges further in this area of secondary contact showing adapative benefit of the lighter coat being reinforced.
4) Polyploidy
Individuals with more than 2 sets of paired chromosomes (2N)
- occurs when chromosomes fail to segregate during meiosis…
so gametes both pass on a full set of chromosomes: 2N + 2N -> 4N
Autoploidy= these unreduced gametes come from the same species
Alloploidy= gametes come from different species (hybrid).
Polyploid Speciation
Speciation occurs in a single step via a change in chromosome numbers whereby tetraploids (4N) have complete reproductive isolation from their diploid ancestors.
Triploid offspring are sterile as the gametes are unbalanced =aneuploidy
(2N + 4N -> 2x 3N)
Polyploid speciation
Rare in animals
Common in plants
- 30% in Ferns
- Polyploid ancestor in the evolutionary tree of every plant.
Goatsbeard example= allopolyploids hybrid species formed very recently in Europe
How polyploids build up population
Initial rareness due to the chance mutation
1) is to reduce hybridisation with the parental diploids (as offspring is infertile) via:
- Self fertilisation
- segregate into new habitats
- Vegetative propagation: plants use biomass of dead plants to grow.
2) Capitalise on polyploid advantage
- chance to access new fitness optima by accessing a new adaptive landscape.
(- extreme phenotypic traits & heterozygote advantage)
5) Hybrid Speciation
2 (or more) distinct lineages hybridise to form the origin of a new species
- has to stabilise and remain distinct from original 2 lineages
Different to normal hybridisation as actually leads to a new species instead of remaining as an intermediate between the 2.
Hybrid Speciation Examples
1) Alloploidy
2) homoploid= hybrid speciation without chromosome number changing
- more common than first believed
Helianthus Sunflowers= Homoploid Example
3 hybrid species formed:
Hybrids live in more environmental extremes compared to parents - - - Recombinant genome allows colonization of this new habitat -> which is out of the parental range.
Combinatorial mechanism
bringing together a novel set of genes (due to recombination of the 2 parent genomes) has the potential to form multiple hybrid species, adaptive to new environments.
- Potential to fuel speciation and evolution
Cichlid fish, Lake Victoria: adaptive radiation via combinatorial mechanism
Fastest rates of speciation known in vertebrates.
- ancient hybridisation event between 2 distantly related species -> provided genetic variation which was recombined multiple times to form many new species.
700 species in under 150,000 years.
6) Random Genetic Drift
Random changes in allele frequencies
- overtime can result in reproductive isolation
Dobs-Muller incompatibility theory:
- new mutation is fixated in a population
- another mutation is fixated in other individuals
2 individuals mate but hybrid has reduced fitness due to negative interactions/incompatibilities between the alleles.
- Creates post-zygotic isolation between the populations.
Peripatric speciation involves genetic drift.
The small founder population is heavily impacted by genetic drift
- as genetic variation is severely bottlenecked by the sudden reduction in population size.
- Rare alleles can easily be fixed -> creates incompatibilities at secondary contact.
Example= Paradise Kingfishers
Founder effect speciation (via genetic drift) is still controversial
Peak Shift model= fixation of new allele leads to a new fitness peak forming
- hybrids which form sit in a valley between the 2 fitness peaks.
Adaptive Ridge Model:
- Allele fitness moves along the ridge instead of travelling through a valley to get across to the new fitness optima.
- dropping into the valley should be heavily selected against so adaptive ridge model was proposed
– Hybrids still sit below the peaks with reduced fitness.
Genetic drift vs Selection
Speciation by genetic drift in the absence of selection would take a long time and is often quite unlikely in nature
- presence of any strong selection is much more influential.
Drosophila: founder effect experiment
1000 flies were bottlenecked by taking just 1 male & 1 female from the population -> this was repeated for 30 generations.
(extreme bottleneck)
- only 0,3% of replicates evolved significant reproductive isolation
- 90% of flies went extinct.
