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Evolutionary Biology > Speciation and Phylogenies > Flashcards

Speciation and Phylogenies Flashcards

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1
Q

What are hybrid zones?

A
  • 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
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2
Q

What happens when diverging populations meet? They could..

A

Remain completely distinct (reinforcement)

Merge together (fusion)

Form a stable hybrid zone (stability)

Form a new hybrid species

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3
Q

Why are hybrid zones are good to study ?

A
  • 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
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4
Q

Hedgehog example of how hybrid zones reveal historical patterns

A

Different Mitochondrial DNA genotypes

Relationships between haplotypes to see how similar they are

Refugia

post glacial recolonisation

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5
Q

Cline theory

A
  • 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
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6
Q

Wide cline example

A

ABO blood groups in humans (high dispersal)

Stronger selection = steeper cline

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7
Q

what is Extrinsic Fitness ?

A

Selection purely due to the environment

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8
Q

what is Heterozygote Disadvantage?

A

lower intrinsic fitness than either parental individual

Heterozygotes have disadvantageous intermediate phenotypes

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9
Q

What causes a tension zone?

A

When hybrids are really unfit

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10
Q

2 types of hybrid zones

A

primary or secondary

(common after ice age)

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11
Q

Tension zone example (Heliconius butterflies)

A

Predators learn to recognise different warning colours

Hybrids with mixed colour patterns will have lower fitness

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12
Q

Primary hybrid zones

A

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

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13
Q

Primary hybrid zone example (mountains)

A

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

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14
Q

Founder effect

A

A random change in allele frequencies from the parent population that occurs when a small founding group establishes a new population

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15
Q

Introgression

A

movement of genes from one species or population into another by hybridisation and backcrossing.

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16
Q

Rock pocket mice hybridisation

A

Different colour morphs controlled by 1 gene

Maintained by strong selection pressures

Primary hybridisation

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17
Q

House mice hybridisation

A

Even the neutral genes show some selection (not strong)

Secondary hybridisation

steep cline

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18
Q

Why can it be hard to distinguish between primary or secondary hybrid zones?

A

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)

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19
Q

Consequences of Hybridisation: Indefinite

A

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

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20
Q

Consequences of Hybridisation: Merge

A
  • Fitness of hybrids not lower than the original populations
  • introgression
  • Variation and distinction between 2 populations lost
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21
Q

Consequences of Hybridisation: Reproductive isolation

A

Strengthening of barriers to gene exchange

Large areas of genome protected from introgression

Mechanisms controversial - reinforcement?

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22
Q

Consequences of Hybridisation: Third species

A

Hybrids become reproductively isolated from original populations

new species

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23
Q

Hybrid zones can be asymmetric

A

They tend to congregate in areas of low density and can be barriers to gene exchange

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24
Q

Prezygotic isolation

A

Barriers before the egg and sperm meet

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25
Q

Prezygotic isolation examples

A
  • Occupying different parts of a habitat
  • Different mate choice - sexual selection
  • Different reproductive anatomy
  • Temporal - different reproductive seasons
  • Gametic - gametes don’t fuse
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26
Q

Postzygotic isolation

A

Barriers that occur once the gametes have fused

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27
Q

Postzygotic isolation examples

A

Hybrid breakdown - hybrids are less fit

Hybrid inviability - hybrids don’t survive

Hybrid infertility - hybrids survive but are infertile (e.g. mules)

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28
Q

what does the Dozhansky-muller incompatability show?

A
  • 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
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29
Q

Magic trait speciation

A

Speciation occurring from divergence in an ecological trait that results in reproductive isolation

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30
Q

Dobzhansky-Muller incompatibilities

A

Alleles that differ between species and are incompatible when found together in hybrids, causing postzygotic barriers

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31
Q

Haldane’s rule

A

When a cross produces inviable or sterile offspring, the heterogametic sex is more strongly affected.

Infertility/inviability tends to affect the heterogametic sex first

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32
Q

The biological species concept

A

“Species are a group of actually or potentially interbreeding populations,
which are reproductively isolated from other such groups”

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33
Q

Peripatric speciation

A

Founder effect

speciation without gene flow

34
Q

Genetic drift

A

Any change in allele frequencies due to chance events

(Has a largest effect on smaller populations)

35
Q

Mutation

A

Any change in the hereditary material (DNA) of an organism

36
Q

Allopatric speciation by divergence

A

Species diverge due to genetic drift, natural selection, mutations or the founder effect

Range extension

37
Q

Allopatric speciation due to vicariance

A
  • 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
38
Q

Parapatric speciation

A
  • 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
39
Q

Sympatric speciation

A
  • Genetic differences result in reproductive isolation
  • Non-geographical barrier
  • speciation with gene flow
40
Q

How allopatric speciation arise?

A

Vicariance

Dispersal

41
Q

Ecological speciation

A

The evolution of reproductive isolation between populations as a result of ecologically-based divergent natural selection

42
Q

Sexual selection

A

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)

43
Q

Causes of speciation

A

Ecological speciation

Sexual selection

Reinforcement of reproductive isolation

Polyploidy

Hybrid speciation

Random genetic drift

44
Q

Causes of speciation: Reinforcement

A

Evolution of enhanced reproductive isolation between populations due to natural selection for greater isolation

45
Q

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

A

Two populations will hybridise and eventually the two populations will collapse into one again

46
Q

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) ? ?

A

Reinforcement & speciation.
Hybrids don’t mate

47
Q

Not all types of isolating mechanisms can evolve via reinforcement

A

Alleles that strengthen prezygotic isolation gain an advantage because individuals with them have higher fitness than those who hybridise.

48
Q

Not all types of isolating mechanisms can evolve via reinforcement

A

Stronger postzygotic isolation usually cannot evolve by reinforcement because an allele that lowers hybrid fitness cannot increase in frequency.

49
Q

Which mechanisms can evolve by reinforcement

A
  • Prezygotic isolation mechanisms can evolve by reinforcement
  • Postzygotic isolation mechanisms generally cannot
50
Q

Under what conditions is reinforcement likely?

A

When populations are well differentiated

Populations that already exhibit high pre and post zygotic isolation

51
Q

What percentage of Drosophila taxa are impacted by reinforcement?

A

60-83% of all sympatric Drosophila taxa

Reinforcement enhances prezygotic isolation bu 18-26%

52
Q

What is Reproductive character displacement ?

A

The accentuation of differences between sympatric populations of two species as a result of reproductive interactions between them

53
Q

Reproductive character displacement: Flycatcher example

A

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

54
Q

What is polyploidy?

A

Occurs when chromosomes fail to segregate during meiosis

Known to be polyploid If they have more than two paired sets of chromosomes

55
Q

What is duplication of chromosomes of the same species called?

A

Autopolyploidy

56
Q

Allopolyploidy

A
  • An allopolyploid is an individual having two or more complete sets of chromosomes derived from different species.
57
Q

Different types of ploidy

A
  • Haploid - 1 copy of each chromosome
  • Diploid = 2 copies
  • Triploid = 3
  • Tetraploid = 4
  • Hexaploid = 5
58
Q

How are Tetraploids and Triploids reproductively effected?

A
  • Tetraploids are reproductively isolated from their ancestors
  • Triploids are largely sterile because their gametes are unbalanced (aneuploidy)
59
Q

How does polyploidy effect animal and plant species?

A

Rare in animal species but relatively common in plants

Est. 15% of angiosperm and 31% of fern speciation events involve a ploidy increase.

60
Q

How do new polyploid individuals build up a population?

A

By reducing hybridisation with parental diploids

  • Self fertilisation
  • Vegetative propagation
  • Habitat segrigation
61
Q

What is hybrid speciation?

A
  • Hybridisation between two or more distinct lineages that contributes to the origin of a new species.
  • When two becomes three
62
Q

what is Homoploid Hybrid Speciation ?

A

Speciation via hybridisation without a change in chromosome (ploidy) number

When the hybrid population becomes distinct from parental species (when still in contact)

63
Q

Combinatorial mechanism of speciation and adaptive radiation

A

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

64
Q

Combinatorial mechanism of speciation and adaptive radiation: Radiation of cichlid fish in lake victoria

A
  • 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
65
Q

What is peripatric or founder effect speciation ?

A

Speciation by evolution of reproductive isolation in small peripheral populations as a consequence of genetic drift and natural selection.

66
Q

Two suggested models for founder effect

A
  1. Peak shift model
  2. Adaptive ridge model
67
Q

What is the Peak shift model?

A
  • New adaptive populaiton shifts from ancestral fitness peak to new peak
  • Unlikely because seletion wouldn’t drive away from adaptive peak
68
Q

What is the adaptive ridge model?

A

Speciation occurs when a populaiton moves along a fitness ridge

69
Q

Why does species richness vary?

A
  1. Chance alone
  2. Variation in clade age
  3. Variation in net diversification rate
70
Q

Phylogenetic tree imbalance

A

A difference in the number of descendent tips (usually species) either side of a focal node

71
Q

Equal-rates Markov model

A
  • 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.
72
Q

What is the raitio of unbalanced to balanced trees when randomly simulating phylogenies ?

A
  • 2:1 ratio unbalanced to balanced trees
  • Unequal species richness
73
Q

How are phylogenies balanced in nature?

A
  • Unbalanced trees are more common that expected.
  • The imbalance is not caused by chance alone
  • More than predicted by the null model
74
Q

Is there a relationship between species richness and clade age?

A
  • 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
75
Q

What is net diversification rate ?

A
  • Net rate of species diversification
  • Net diversification rate is a strong predictor of species richness in higher taxa across the tree of life
76
Q

Net diversification rate equation (R)

A

R = b - d

Speciation rate - extinction rate
lineage birth rate (b) - lineage death rate (d)

(expressed per lineage per million years)

77
Q

Measuring net diversification rate using the Yule (Pure Birth) model (R)

A

R = ln(N) / t

ln (calculator function for log)
N = number of species?
/ t = divided by time

78
Q

An example using the net diversification rate using the Yule (Pure Birth) model

A

Hawaiian silverswords have diversified into 25 species over 5 million years

R = ln (25) / 5
= 0.64 species per lineage per My
Fast diversification

79
Q

Lateral gene transfer

A
  • 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

80
Q

what is C4 and C3 photosynthesis?

A
  • 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)
81
Q

Why are C4 photosynthesising plants a source of much research?

A

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)

82
Q

Lateral gene transfer conclusions

A
  • Widespread LGT among grasses (rate varies)
  • Large DNA fragments spread functional genes
  • LGT can accelerate adaptive evolution
  • The mechanism may be reproductive contamination

Evolutionary Biology (3 decks)

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