Speciation and Phylogenies Flashcards
What are hybrid zones?
- Where genetically distinct populations meet, mate and reproduce
- Forming between populations that are not yet ‘true species’
- Distinct populations overlap and there is mixing of genetic material
What happens when diverging populations meet? They could..
Remain completely distinct (reinforcement)
Merge together (fusion)
Form a stable hybrid zone (stability)
Form a new hybrid species
Why are hybrid zones are good to study ?
- Tell us about the process of speciation
- Can tell us about historical patterns e.g. where populations were in the past compared to where they are now
- Range of genotypes show genetic differences and selection pressures that separate the taxa
Hedgehog example of how hybrid zones reveal historical patterns
Different Mitochondrial DNA genotypes
Relationships between haplotypes to see how similar they are
Refugia
post glacial recolonisation
Cline theory
- A cline → change in the allele frequency over a geographical transect
- Graph of differences in allele frequency over a geographical transect
- Width of cline - how long it takes to get from one allele being completely fixed to another
- more dispersal = wider cline
Wide cline example
ABO blood groups in humans (high dispersal)
Stronger selection = steeper cline
what is Extrinsic Fitness ?
Selection purely due to the environment
what is Heterozygote Disadvantage?
lower intrinsic fitness than either parental individual
Heterozygotes have disadvantageous intermediate phenotypes
What causes a tension zone?
When hybrids are really unfit
2 types of hybrid zones
primary or secondary
(common after ice age)
Tension zone example (Heliconius butterflies)
Predators learn to recognise different warning colours
Hybrids with mixed colour patterns will have lower fitness
Primary hybrid zones
Populations have never separated from one another
Natural selection alters alleles in a continuous population
populations start to diverge
Environment affects different loci in different places
Primary hybrid zone example (mountains)
Population that lives on a mountain diverge with altitude
population higher up the mountain is better adapted to cold conditions
population at the bottom is better adapted for warmer conditions
middle of the mountain = hybrid population
Founder effect
A random change in allele frequencies from the parent population that occurs when a small founding group establishes a new population
Introgression
movement of genes from one species or population into another by hybridisation and backcrossing.
Rock pocket mice hybridisation
Different colour morphs controlled by 1 gene
Maintained by strong selection pressures
Primary hybridisation
House mice hybridisation
Even the neutral genes show some selection (not strong)
Secondary hybridisation
steep cline
Why can it be hard to distinguish between primary or secondary hybrid zones?
Secondary hybrid zones can look like primary if they are old enough
Primary hybrid zones can look like secondary if multiple clines have become trapped in a density trough
If enough of the genome is under selection (non-selected regions diverge due to hitchhiking)
Consequences of Hybridisation: Indefinite
Selection maintains steep clines at some loci
could be a tension zone
only if character differences are favoured by different environments
could move: - area of low density or differences in migration rates
Consequences of Hybridisation: Merge
- Fitness of hybrids not lower than the original populations
- introgression
- Variation and distinction between 2 populations lost
Consequences of Hybridisation: Reproductive isolation
Strengthening of barriers to gene exchange
Large areas of genome protected from introgression
Mechanisms controversial - reinforcement?
Consequences of Hybridisation: Third species
Hybrids become reproductively isolated from original populations
new species
Hybrid zones can be asymmetric
They tend to congregate in areas of low density and can be barriers to gene exchange
Prezygotic isolation
Barriers before the egg and sperm meet
Prezygotic isolation examples
- Occupying different parts of a habitat
- Different mate choice - sexual selection
- Different reproductive anatomy
- Temporal - different reproductive seasons
- Gametic - gametes don’t fuse
Postzygotic isolation
Barriers that occur once the gametes have fused
Postzygotic isolation examples
Hybrid breakdown - hybrids are less fit
Hybrid inviability - hybrids don’t survive
Hybrid infertility - hybrids survive but are infertile (e.g. mules)
what does the Dozhansky-muller incompatability show?
- Mutations arise in two populations and become fixed
- When hybrid populations are formed the fixed mutations mix
- This causes lower fitness in the hybrids and is a barrier to reproduction
Magic trait speciation
Speciation occurring from divergence in an ecological trait that results in reproductive isolation
Dobzhansky-Muller incompatibilities
Alleles that differ between species and are incompatible when found together in hybrids, causing postzygotic barriers
Haldane’s rule
When a cross produces inviable or sterile offspring, the heterogametic sex is more strongly affected.
Infertility/inviability tends to affect the heterogametic sex first
The biological species concept
“Species are a group of actually or potentially interbreeding populations,
which are reproductively isolated from other such groups”
Peripatric speciation
Founder effect
speciation without gene flow
Genetic drift
Any change in allele frequencies due to chance events
(Has a largest effect on smaller populations)
Mutation
Any change in the hereditary material (DNA) of an organism
Allopatric speciation by divergence
Species diverge due to genetic drift, natural selection, mutations or the founder effect
Range extension
Allopatric speciation due to vicariance
- Populations seperated whilst staying in the same place
- e.g. when the Panama isthmus was formed it caused a physical barrier
- Range splitting
- Mutation and natural selection causes differences to evolve
- When panama canal was formed the two new species can back in to contact but did not interbreed
- speciation without gene flow
Parapatric speciation
- Range expansion leads to sympatry
- (non-geographical barrier)
- speciation with gene flow
- expected to be more common than sympatric speciation (less gene flow)
- results in cline
Sympatric speciation
- Genetic differences result in reproductive isolation
- Non-geographical barrier
- speciation with gene flow
How allopatric speciation arise?
Vicariance
Dispersal
Ecological speciation
The evolution of reproductive isolation between populations as a result of ecologically-based divergent natural selection
Sexual selection
Differential reproduction as a result of variation in the ability to obtain mates
E.g. Female mate choice, male plumage colour, sexual ornaments (antlers etc)
Causes of speciation
Ecological speciation
Sexual selection
Reinforcement of reproductive isolation
Polyploidy
Hybrid speciation
Random genetic drift
Causes of speciation: Reinforcement
Evolution of enhanced reproductive isolation between populations due to natural selection for greater isolation
What will happen when two divergent populations comes into secondary contact and the hybrids have the same fitness as parentals (i.e. there is no postzygotic isolation only prezygotic) ? ?
No hybrid unfitness
Two populations will hybridise and eventually the two populations will collapse into one again
What will happen when two divergent populations come into secondary contact and the hybrids have reduced fitness (i.e. there is some postzygotic isolation & prezygotic) ? ?
Reinforcement & speciation.
Hybrids don’t mate
Not all types of isolating mechanisms can evolve via reinforcement
Alleles that strengthen prezygotic isolation gain an advantage because individuals with them have higher fitness than those who hybridise.
Not all types of isolating mechanisms can evolve via reinforcement
Stronger postzygotic isolation usually cannot evolve by reinforcement because an allele that lowers hybrid fitness cannot increase in frequency.
Which mechanisms can evolve by reinforcement
- Prezygotic isolation mechanisms can evolve by reinforcement
- Postzygotic isolation mechanisms generally cannot
Under what conditions is reinforcement likely?
When populations are well differentiated
Populations that already exhibit high pre and post zygotic isolation
What percentage of Drosophila taxa are impacted by reinforcement?
60-83% of all sympatric Drosophila taxa
Reinforcement enhances prezygotic isolation bu 18-26%
What is Reproductive character displacement ?
The accentuation of differences between sympatric populations of two species as a result of reproductive interactions between them
Reproductive character displacement: Flycatcher example
Collard and Pied flycatchers living in allopatry (apart) have similar colour morphs.
When living in symptraty Pied flycatcher females prefer a brown male rather than the darker morph that looks similar to the collard Fly catcher
When living in allopatry females prefer the darker morph
What is polyploidy?
Occurs when chromosomes fail to segregate during meiosis
Known to be polyploid If they have more than two paired sets of chromosomes
What is duplication of chromosomes of the same species called?
Autopolyploidy
Allopolyploidy
- An allopolyploid is an individual having two or more complete sets of chromosomes derived from different species.
Different types of ploidy
- Haploid - 1 copy of each chromosome
- Diploid = 2 copies
- Triploid = 3
- Tetraploid = 4
- Hexaploid = 5
How are Tetraploids and Triploids reproductively effected?
- Tetraploids are reproductively isolated from their ancestors
- Triploids are largely sterile because their gametes are unbalanced (aneuploidy)
How does polyploidy effect animal and plant species?
Rare in animal species but relatively common in plants
Est. 15% of angiosperm and 31% of fern speciation events involve a ploidy increase.
How do new polyploid individuals build up a population?
By reducing hybridisation with parental diploids
- Self fertilisation
- Vegetative propagation
- Habitat segrigation
What is hybrid speciation?
- Hybridisation between two or more distinct lineages that contributes to the origin of a new species.
- When two becomes three
what is Homoploid Hybrid Speciation ?
Speciation via hybridisation without a change in chromosome (ploidy) number
When the hybrid population becomes distinct from parental species (when still in contact)
Combinatorial mechanism of speciation and adaptive radiation
When hybridisation events occur between diverged lineages, the new combinations of old genetic varience that have been brought together in hybrid populations can be the fuel for speciation & evolution
Combinatorial mechanism of speciation and adaptive radiation: Radiation of cichlid fish in lake victoria
- Ancient hybridisation event may have fueled radiation in lake victoria cichlids.
- Provided genetic variaiton that was recombined and sorted into many new species
- Plus ecological opportunity, resulted in 700 new species in 150,000 years
What is peripatric or founder effect speciation ?
Speciation by evolution of reproductive isolation in small peripheral populations as a consequence of genetic drift and natural selection.
Two suggested models for founder effect
- Peak shift model
- Adaptive ridge model
What is the Peak shift model?
- New adaptive populaiton shifts from ancestral fitness peak to new peak
- Unlikely because seletion wouldn’t drive away from adaptive peak
What is the adaptive ridge model?
Speciation occurs when a populaiton moves along a fitness ridge
Why does species richness vary?
- Chance alone
- Variation in clade age
- Variation in net diversification rate
Phylogenetic tree imbalance
A difference in the number of descendent tips (usually species) either side of a focal node
Equal-rates Markov model
- Simple null model
- Constant rate birth-death markov process
- Any model where birth and death rates are equal across all lineages in a phylogenetic tree at a particular time.
What is the raitio of unbalanced to balanced trees when randomly simulating phylogenies ?
- 2:1 ratio unbalanced to balanced trees
- Unequal species richness
How are phylogenies balanced in nature?
- Unbalanced trees are more common that expected.
- The imbalance is not caused by chance alone
- More than predicted by the null model
Is there a relationship between species richness and clade age?
- Clade age and species richness are uncorrelated across 1,397 clades of multicullular eukayotes
- Clade age is a poor predictor of how many species a clade contains
What is net diversification rate ?
- Net rate of species diversification
- Net diversification rate is a strong predictor of species richness in higher taxa across the tree of life
Net diversification rate equation (R)
R = b - d
Speciation rate - extinction rate
lineage birth rate (b) - lineage death rate (d)
(expressed per lineage per million years)
Measuring net diversification rate using the Yule (Pure Birth) model (R)
R = ln(N) / t
ln (calculator function for log)
N = number of species?
/ t = divided by time
An example using the net diversification rate using the Yule (Pure Birth) model
Hawaiian silverswords have diversified into 25 species over 5 million years
R = ln (25) / 5
= 0.64 species per lineage per My
Fast diversification
Lateral gene transfer
- The aquisition of genetic material from another organism without sexual reproduciton
- Lateral gene transfer (LGT) = Horizontal gene transfer (HGT)
- LGT can accelerate adaptive evolution
Plants that have rhizomes have more laterally aquired genes
what is C4 and C3 photosynthesis?
- C3 - photosynethesis at lower temperatures
- All plants photosynthesis using C3
- C4 = Photosynthesis at higher temperatures
- Some have adapted to C4 (split the cycle between C4 & C3)
Why are C4 photosynthesising plants a source of much research?
If you could enginer C4 into C3 plants (such as rice) you could grow at higher temps using less water (important for famine and climate issues)
Lateral gene transfer conclusions
- Widespread LGT among grasses (rate varies)
- Large DNA fragments spread functional genes
- LGT can accelerate adaptive evolution
- The mechanism may be reproductive contamination
