W10L2 speciation 1: genetic mechanism Flashcards
What is speciation
-the splitting of one species into two. Occupies a unique place in the theory of evolution
- connect small-scale (micro-evolutionary process) population genetic differences to large-scale (macro evolutionary )
The biological species concept
Species are groups of actually or potentially interbreeding natural populations, which are reproductively isolated from other such groups
Basic speciation process
- One or more barriers to gene flow develops
* Geographic barriers result in allopatric speciation
* Ecological barriers result in sympatric speciation
* Parapatric and peripatric speciation are also possible - Accumulation of genetic differences through mutation, drift and natural selection
- Selection/genetic divergence leads to reproductive isolation in hybrids
* Pre-zygotic isolation mechanisms
* Post-zygotic isolation mechanisms
Some pre-zygotes barriers
Potential mates (despite being sympatric) do not meet
* Temporal isolation (eg. mate at different times of the year)
* Ecological isolation (eg. utilise different resources/ habitats)
Behavioural isolation
* Potential mates meet but do not mate (eg. different courtship songs, pheromones)
Mechanical isolation
* Copulation occurs, but gametes are not transferred (eg. changes in genital morphology)
Gametic isolation
* Gametes are transferred, but eggs are not fertilized (eg. corals)
Post-zygotes barriers
Hybrid inviability
* Zygote lethality/ developmental errors = no offspring
* Partial F1 hybrid inviability (lower number of hybrid offspring) F1 infertility
* Hybrid viable, but is sterile or has reduced fertility F2 or backcross has reduced viability or fertility
Simple hyphothesis for genetic incompatibility
- having a single speciation gene
-but how can a new alleles become fixated if a hybrid is unfit?
Dobzhansky-Muller Incompatibility (DMI)
This incompatibility model assumes that hybrid incompatibility arises from epistatic interactions across two or more genes
-in a same species in two different population, there is a fixation of new mutation which cause problem for hybrid individual
DMI model example in Platyfish and Swordtail fish
-viable F1 hybrid but partial inavaiabillity in backcross
Two loci:
1. Tumour locus (Tu)
2. Suppressor locus (R)
Tu/Tu; R/R genotype produces black spots—tumours that have suppressed
In hybrid backcrosses to swordtails, loss of the R allelein 25% of F2 hybrids leads to severe melanomas
-Tu contains a novel duplicated gene, Xmrk-2, that induces tumours in a dominant manner
-R contains an allele of CDKN2AB with promoter mutations that make it much more transcriptionally active
DMI model example in Drosophila fruit flies
- female melaogaster mating with male stimulants mating does not produce sons
-Ancestral species have Hmr;Lhr
-But divergence in each species cause deleterous interaction in hybrid
-Mutation of either Lhr (lethal hybrid rescue) in D. simulans or Hmr (hybrid male rescue) in D. melanogaster rescues hybrid male lethality
Haldane rule
When in the F1 offspring of two different animal races one sex is absent, rare, or sterile, that sex is the heterozygous [heterogametic] sex
What explain Haldane rule
Hypothesis 1: Species carry incompatible alleles on different sex chromosomes
-this is not well supported by data
-some species of with XX and X0 also show Haldane rule
-in drosophila Y have little or no effect on hybrid fitness
Hypothesis 2: Dominance model
Dominance rule indept
- If interactions involved in DMIs are sex linked and there is dominance, the homogametic sex (XX or ZZ) will be shielded from the effects of deleterious recessive interacting alleles because it has two copies of the sex chromosome
- Heterogametic (XY or ZW) or XO/ZO sex is not shielded, thus has reduced fitness in hybrids
Dominance model in Drosophila DMI example
The Hmrmel allele must be at least partially recessive, as one copy in the hybrid does not cause lethality in hybrid females
-Hybrid females have only one copy of the Hmrmel allele in conjunction with the negatively interacting Lhrsim allele (dominant)
Fish DMI example not an example of Haldane’s rule
Tu is on the X chromosome, but there is no sex bias in the incidence of lethal tumours—why is this?
This is because Tu is dominant!
Tu/Y ; -/- males and Tu/- ; -/- females both have a copy of Tu and no copy of R
If Tu was recessive, only Tu/Y ; -/- males (12.5% of F2 backcross offspring) would die of melanoma
Key points about genetic incompatibilities
Dobzhansky-Muller incompatibilities (DMIs) underpin many examples of hybrid incompatibility
DMIs arise from epistatic interactions across two or more genes
