3: Speciation Flashcards
Define Allopatric speciation
No gene flow between spatially separated populations
Define allopatric speciation by vicariance
Range splitting
- Where a once continuous range is split into 2 by a geopgraphic barrier
no gene flow
Describe Peripatric speciation
(founder effect)
long distance dispersal and colonisation event to a geographically isolated area
no gene flow
Define Parapatric speciation
Neighbouring pop.s of a continuously distributed species, diverge into diff species with gene flow!
Define sympatric speciation
A pop splits into 2 repro isolated species whilst being in geographical contact with one another at all times
What happens if reproductive isolation is incomplete? 3 options:
- Known as speciation collapse/failure/reversal (9)
- Stable hybrid zone forms, where some genetic differentiation persists (7)
- New selection to avoid hybrid maladaptation = reinforcement of reproductive isolation, driving it to completion
Why is sympatric speciation more rare than allopatric?
Because gene flow and recombination breaks down genetic combinations that might form new species
e.g linkage between ecological trait (under disruptive selection) and mating trait is difficult in sympatry because linkage is blocked by interbreeding
What can make sympatric speciation easier to arise?
The presence of a speciation trait that causes both ecolological divergence and reproductive isolation between developing species
e.g Apple maggot fly
- Ancestor lays eggs on hawthorn tree
- Later, fly started to infest apple trees in same area
- These flys mate on host plant and are now genetically divergent pops that differ in host pref and mating season
Why is parapatric speciation expected to be more common than sympatric speciation?
Because it involves less gene flow between diverging populations
What is usuallt involved in parapetric speciation?
Usually involved adaption of pop.s to an environ or ecological gradient producing a cline in allele frequencies
(Lizards at White Sands)
Describe an example of Parapatric speciation
Adaptation to environ extremes has resulted in repro isolation.
- Colour morphs have evolved in three lizard species, in at least 1 species coloration effects mating behaviour and has led to partial sexual isolation between colour morphs
Causes of speciation
Define Ecological speciation
the evolution of reproductive isolation between pops. as a result of ecologically based divergent natural selection
Causes of speciation
Define Sexual selection
Differential reproduction as a result of variation in the ability to obtain mates
- Divergent sexual selection can cause the rapid evolution of phenotypic traits and also generate really strong repro isolation between pops.
Describe the reinforcment of sexual selection
When organisms evolve traits that mate it less likely for them to make with individuals from different groups
→ happens because hybrids may be less fit
Describe how prezygotic isolating mechanisms can evolve by reinforcement
Alleles that strengthen prezygotic isolation gain an advantage because individuals with them have higher fitness than those who hybridise
Describe why postzygotic isolating mechanisms (generally) cannot evolve by reinforcement.
Stronger postzygotic isolation usually cannot evolve by reinforcement because an allele that lowers hybrid fitness cannot increase in frequency
Under what conditions is reinforcement likely?
- When pop.s are well differentiated (phenotypic traits and mating prefs …)
- Low hybrid fitness
= when pops. exhibit a high degree of pre AND postzygotic isolation
Fill in the blanks
We would expect: Populations that overlap geographically to show ——- mating preference differentiation (——— ———) than geographically separated species that have never experienced the cost of maladaptive ————.
We would expect: Populations that overlap geographically to show greater mating preference differentiation (prezygotic isolation) than geographically separated species that have never experienced the cost of maladaptive hybridisation
Define Reproductive character displacement
The accentuation of differences between sympatric pops of 2 species as a result of reproductive interactions between them
Define Autoploidy
Duplication of chromosomes of the same species
Define alloploidy
Duplication of chromosomes in hybrids between species
Describe how tetraploids can build a population (if they are unable to produce fertile offspring with diploids)
By reducing hybridisaion with diploids they gain an initial foothold, can be done by:
- Self fertilisation
- Vegetative propagation
- Habitat segregation
Define hybrid speciation
hybridisation between 2 or more distinct lineages that contributes to the origin of a new species
Define Homoploid hybrid speciation
Speciation via hybridisation without a change in chromosome (ploidy) number
- Hybrid form is stabilised and remains distinct when in contact with either parent
