Mockford Flashcards
What are the 4 main evolutionary forces (drivers of evolution)?
- Mutation
- Genetic drift
- Natural selection
- Migration (gene flow)
Mutation is … and introduces new … into a population
random, variation
Genetic drift is random changes in … allele frequency. It occurs more in … populations, tends to lower … and can cause isolated populations to …
unselected, smaller, heterozygosity, diverge
Migration counteracts…
divergence due to drift
Natural selection is … and … focussed
fitness, adaptation
Mutation can be … or in the …
somatic (body cells - not passed onto next generation), germline
Germline mutations can be … mutations or … mutations.
point, block
Block mutations affect vast areas of a …, which may be…
chromosome
- Deleted
- Inserted
- Translocated
- Inverted
- Duplicated
Point mutations can be…
- Substitutions
- Insertions
- Deletions
- inversions
Point mutations may also be silent (…) or …-…
synonymous, non-synonymous
Non-synonymous point mutations can be…
- Missense (codes for different amino acid)
- Nonsense (codes for stop codon)
- Frame shift
- all mutations add variation
Neurofibromatosis affects 1 in 3000-4000 people in the UK. It is caused by the … gene, which has one of the highest … rates of any human disorder. The gene’s role is to…. So far 255 different mutations have been reported. Around 50% of cases are inherited and the other 50% are new mutations
NF1, mutation, suppress tumours along nerve cells
Genetic variation within a population is dynamic and changes between generations. Allele frequency can change by …
chance
Population size matters. Endangered populations have gone through … … and have a limited gene pool. Satellite populations can take a different evolutionary pathway, called the … …
genetic bottlenecks, founder effect
Genetic drift depends on the … population size (Ne), which is the number of … … individuals. This rarely equals the census (Nc)
effective, sexually reproducing
Gene flow is when … move between populations, generally via …, although other ways include … and … … …
genes, migration, travellers, lateral gene transfer
Natural selection is…
the differential survival and/or reproduction of classes of entities that differ in one or more characteristics.
Fitness is the….
probability of survival x the average number of offspring for a class of individuals
An adaptation is…
a characteristic that enhances the survival or reproduction of organisms that bear it, relative to alternative character states
Characteristics are not all adaptations. They may just be a consequence of …, for example red blood cells are red because they contain iron and haemoglobin, and genetic drift can spread … traits. They may be … (now used for something they weren’t originally), or may be a consequence of gene …
physics/chemistry, neutral, exaptations, linkage
Linkage disequilibrium is…
alleles appearing together more often than you would expect by chance
What can cause linkage disequilibrium? give e.g.s
- Weakly beneficial alleles grouping together due to stronger cumulative effect (not much effect when on own)
- Structural changes e.g. inversion, which blocks recombination
- Hitchhiking alleles - non-selected alleles around selected allele passed onto next gen (in “selective sweep”) - has been found when selecting for body size in mice and in humans for the lactose digesting allele
How do we show natural selection exists?
- Correlations between trait and environment
- Responses to experimental change in the environment
- Show correlations between the trait and fitness component
Guppies in Trinidad. 1 predator eats only …, 1 predator eats …. In areas of low predation, guppies have a different colour to those in areas of high predation. This is due to a trade-off between … … and not getting … before reproduction. … and … spots were reduced in high predation environments. In both treatments, spot size … background grain size, but not when there were no predators.
Juveniles, all guppies, attracting females, eaten, blue, iridescent, matched
When there were no predators, the spots were doing the opposite to the background (grain size) - so were trying to stand out. Why is this the case, especially when pigments such as melanin and carotenoids are costly to produce?
- Correlation between trait and fitness component - more carotenoids = faster and more viable sperm
- 2013 study found some effect of orange BUT more of an effect on female selection if the male is rare (negative frequency dependent selection)
What are the types of selection?
Directional selection - shifts the overall population by favouring the extreme
Stabilising selection - acts against extreme phenotypes
Divergent selection - favours variants of opposite extremes (normally in 2 different environments, between populations. When in the same population it is disruptive selection)
The spadefoot toad (Spea multiplicata) has two different morphs:…
- omnivores - rounded bodies, long intestines, not many jaw muscles, many teeth - feed at bottom of pond
- carnivores - longer bodies, shorter intestines, stronger jaw muscles, fewer teeth - feed in water column on fairy shrimp
The two spadefoot toad morphs are the same species so can interbreed, although…
the intermediates are less likely to survive to metamorphosis - they have a lower feeding efficiency and smaller body size
A mark-recapture study showed that the intermediate form…
was being selected against - divergent selection occuring
However, carnivores were actually…
favoured over omnivores by directional selection as well
If the intermediate form is so unfit, why hasn’t it been lost?
When co-occuring with Spea bombifrons (another species of spadefoot toad), the intermediate form is favoured. Spea bombifrons is dominant and occurs in the same habitats, so the intermediate form avoids the competition.
- this means that all 3 types of selection occur simultaneously
… is necessary for selection, despite selection reducing it
variation
Sockeye salmon…
Quinn et al., 2007
migrate earlier to avoid the peak fishing season in canada - time of migration is genetically controlled
- e.g. of directional selection
Dog whelks: positive correlation between…
Berry & Crothers, 1968
stabilising selection of whelks and intensity of wave action experienced by the whelks.
Variance of more exposed whelks reduced by 90%
How is genetic diversity maintained?
- Sex
- Ploidy
- Gene flow
- Mutation
- Balancing selection (heterozygote advantage + frequency dependent selection)
How does sexual reproduction introduce variation into a population?
- Independent assortment
- Random fertilisation
- Crossing over
Ploidy - genetic variation can be hidden in….
recessive alleles
Balancing selection acts to…
maintain the proportion of alleles in the population
Give an e.g. of heterozygote advantage
Sickle cell anaemia - if heterozygote then more protected against malaria (both recessive - serious anaemia, no recessive allele - vulnerable to malaria) - maintained in population in areas where malaria is prevalent
Connexin 26:
- 2 copies = deafness
- 1 copy = increased cell repair
- heterozygote advantage
What are the 4 critiques as to why heterozygote advantage might not be common?
- Surely a good adaptation should be fixed at 100% in the population as soon as possible? - NO - when a mutation arises in a population it only has to be fit in the heterozygote form (1 copy) and usually is - chance of mutation in both alleles miniscule
- What appears to be HA may not actually be HA - linkage disequilibrium - beneficial alleles for one gene may be linked with fitness-reducing alleles of another gene, which can make the heterozygote for one or both of the genes look advantageous - called ASSOCIATIVE OVERDOMINANCE
- HA imposes a ‘load’ on the population (adds genotypes that are detrimental) - decreases mean fitness of the population
- HA is unstable because of duplication - normally duplication only leads to redundant genes. But with HA a duplication mutation event would lead to only needing one copy instead of two for the detrimental effect - would be selected against very quickly
What is the selection coefficient (s)?
The difference between the mean relative fitness of individuals of a given genotype and that of a reference genotype
fitness is represented by the letter ‘…’
w
The genetic load is…
the difference between the maximum fitness and the mean fitness
Positive frequency-dependent selection is when..
the survival and reproduction of any one morph increases if that phenotypic form becomes more common in the environment
Negative frequency dependent selection is when…
the survival and reproduction of any one morph decreases if that phenotypic form becomes more common in the environment
- advantage of being rare
What is an example of positive frequency-dependent selection?
- Mullerian mimicry
What is an example of negative frequency-dependent selection?
- Batesian mimicry
Positive FD selection … help with maintaining polymorphism, whereas negative FD …
doesn’t, does
Lake Tanganyika cichlids (Perissodus microlepis)…
display NFDS. They have mouths that point left or right as they consume pieces of their larger prey fish from the side. If more (>50%) attacks come from one side the prey fish learns to be more defensive of this side. This means that left and right mouthed individuals is maintained at around 50%, with fluctuations around this over time
Left handedness is … and associated with costs such as lower height and reduced longevity. Raymond et al. (1996) suggested negative frequency-dependent hypothesis for why it remained in the population…
When fighting, left handed may be beneficial, as, due to it being rarer in the population, opponents would not expect left handed punches and would not have as much practice defending against them, therefore making them more effective in the fight.
- there is evidence that left-handers are more successful in baseball, cricket and homicide
How could the evolution of correlated traits occur?
- Linkage disequilibrium
- Pleiotropy (genes effecting more than one character
G. fortis…
- beak size increased after drought on G. islands in 1977 - as bigger seeded plants with harder seed cases survived more. However bill length was actually reduced as bill depth increased in the population (as closely linked on genome), which actually hindered the adaptive path of the species
The direction of evolution depends on genetic …
correlation
Species with a low genetic variance are unlike to have…
a great physiological tolerance
Species with greater physiological tolerance to climatic variables will be able to extend their distribution to…
higher latitudes
The problem with tropical species is that their environment is very …, so they don’t have much … variation which would give them a greater physiological tolerance range
stable, background
Tropical drosophila species…
Kellerman et al., 2009
have lower standardised cold resistance and desiccation resistance than those drosophila species found globally
- More tolerant species had more genetic variation involved with cold and desiccation tolerance
One suggestion is that inbreeding in tropical regions is the cause of this lack of variation. However…
there is not the same lack of variation found in relation to genes affecting other traits, for example wing size
How can a species get around the problem of a lack of variation?
- Mutation (mosquitos became resistant to insecticide because of one base pair mutation) - rare and often more detrimental than beneficial
- Phenotypic plasticity - same genotype but different phenotypes in response to environment - useful for changing environments but may be maladaptive outside certain range + could be hindering selection (see butterflies)
- Transposable elements - potentially makes more of fitness landscape available - may not be as detrimental as once thought - prob really involved in genome evolution - could be very influential in adaptability (e.g. invasive species)
The fly Ormia ochracea is a parasite of the field cricket (Teleogryllus oceanus) which occurs on 3 Hawaiian islands (the fly only occurs on 2). It finds crickets using their song (which it uses to attract a mate. What happened?
(Zuk et al., 2006)
In 1991 30% of male crickets were parasitised. In 2001 one cricket was heard calling, and there were very few crickets around. In 2003, many crickets were found but none of them were calling. In 2004 it was found that 121/133 males were a flatwing morph (which cannot produce a call)
- hypothesised very simple mutation (probably on x chromosome) - mutation arose because of strong selection pressure
How are the crickets attracting mates now?
The flatwing males flock towards the normal morph males when they call to find mates
Mutation offers a jump to another … …
fitness peak
Mukai (1972) found that, as mutations in the genome (of drosophila) increase, … … decreases (despite variation increasing)
mean viability (likelihood of survival) - generally mutations are more detrimental than beneficial
Phenotypic plasticity is when..
the same genotype produces different phenotypes in response to environmental conditions
Phenotypic plasticity has been identified as the major mechanism in response to the … and …-… environmental change
climate, human-induced
Why may phenotypic plasticity not be a good thing?
- Mate preference (if male displays phenotypic plasticity that female isn’t likely to choose as an attractive trait - however in some cases both can be plastic, or females can select for plasticity)
The mosquitofish - spring-born fish have significantly different metabolism to summer-born fish. However, slight variations in development can lead to them being offset with their environment and therefore not very well adapted.
Yupp
Blue tits in south of France - habitat (evergreen vs deciduous) influences how different populations deal with cues. Evergreen populations…
only brought their laying dates earlier to match increasing temperatures and therefore food sources - same cues are used differently
In spadefoot toads (Spea multiplicata), all tadpoles begin as … and only some have the phenotypic plasticity to become … based on their early diet. Carnivores reach … much more quickly, but omnivores come out of it more strongly and are favoured in more stable environments. Kelly et al. (2019) found that male call rate is related to how … their offspring is, and females choose males with … call rates. Therefore females are sexually selecting for …
omnivores, carnivores, metamorphosis, plastic, higher, plasticity
How does phenotypic plasticity evolve in short lifespans?
as it maximises fitness in the long term, but selection will probably favour non-plastic phenotypes in the short term
Plasticity can theoretically evolve if adaptive rate is slow (weak selection)
The squinting bush brown butterfly has two morphs depending on the temperature it develops in. Predation is lower in the wet season so it has more …, and predation is higher in the dry season so the morph is much more …. The study (wijngaarden et al., 2002) tried to put the butterflies under strong … …, but found that the reaction norm was not …. This was because the variation wasn’t in the …, it was phenotypic plasticity. This may actually be … if temperatures change beyond these norms.
eyespots, camouflaged, artificial selection, changed, genes, maladaptive
Transposable elements are parts of a genome that … … and cause impactful mutations and huge adaptive changes
move around
Transposable elements may actually have a very significant role in …. Evidence for this has come from research into … species which go through a very extreme … …
adaptation, invasive, genetic bottleneck
One theory into the success of invasive species is that they have a lot of … …. It suggests that genes are expressed differently when animals are put under …, which can increase the … effect of transposable elements, increasing the chance of a significant … mutation
transposable elements, stress, epigenetic, beneficial
The human genome is around … transposable elements
half
Dachshunds have short legs (chondrodysplasia) because of a … …
transposable element
What is the biological species concept?
If two individuals can produce fertile offspring they are of the same species
Hybrid zones are areas where…
genetically distinct populations meet, mate and reproduce
Hybrid zones are involved in nearly all speciation events, except … speciation
polyploid
The change in proportion alleles across the two distinct populations is called the …
cline
There are two species of hedgehog across Europe, Erinaceus europaeus and Erinaceus concolor, that can produce hybrids. Based on mitochondrial DNA (entirely from mother), there is more distinction between different hedgehog populations than this. There is a pattern that suggests they have moved up from the south of Europe (common in european species due to previous ice in northern regions).
Just read it.
What can happen at hybrid zones?
- Indefinite HZ - hybrids produced at low number and kept in one particular area
- Merging - two entirely compatible species that move into each others populations and produce viable hybrids
- Reproductive isolation - hybrids are so unfit that selection acts against hybrids being produced in the first place
- Third species - hybrids fitter than ancestral genotypes and create an entirely new species
A cline is a…
change in the allele frequency over a geographical transect
We can use cline characteristics to tell us about the … of populations. If we produce a simple graph of the clines and alleles present within them we can analyse their …, … and …
mixing, shape, co-occurrence (with other clines) and movement
Cline shape (width) changes with…
dispersal and selection (greater dispersal, e.g. more gene flow = wider cline)
In human blood groups, A and B are ….
codominant (e.g. AA would result in type A, AB would result in type AB, BO would result in type B, OO would result in type O)
Across Europe, the frequency of the B blood type allele…
declines in an East to West gradient (reflecting invasions into Europe from Mongolia after the collapse of the Roman Empire)
- This is a wide cline
Stronger selection =
steeper cline
Give an example of selection keeping a cline narrow
Rock-pocket mice (Chaetodipus intermedius) in mexico and southern USA - Light and dark morphs, light (sandy) and dark (lava) substrate. - so can be cryptic or conspicuous (which leads to very strong selection pressure from predators) - gene flow smooths out the change in phenotype frequency but predation keeps cline steep
… … can also keep clines steep.
Heterozygote disadvantage - deleterious heterozygotes kept out of population by selection by separating into populations with homozygous dominant and recessive genotype - prevents gene flow
- This type of hybrid zone is called a TENSION ZONE
A primary hybrid zone is one where…
natural selection alters alleles in a continuous population
- tends to be one population diverging
- the environment tends to affect different loci in different geographical locations
- neutral alleles tend to remain relatively constant
- e.g. rock-pocket mice
A secondary hybrid zone is one where…
formerly allopatric species expand to meet
- clines are usually in the same geographical location
- clines can be any shape
- neutral alleles also tend to change
- e.g. hedgehogs
Why are x-chromosomes under strong selection?
Males only have one copy so if contains deleterious alleles then they are more likely to manifest and be selected against
If the neutral alleles match the cline of those under strong selection, the hybrid zone is likely …
secondary
Cline asymmetry - dispersal (gene flow) influences the … AND … of the cline. Different alleles can follow different patterns. Neutral alleles may become … from the selected alleles, and introgression of these alleles may occur.
shape, location, uncoupled
When a gene moves from one population into the other by hybridisation or backcrossing it is called…
introgression
Where hybrids are unfit, … … are common
tension zones
What are the two types of reproductive isolation?
Prezygotic and postzygotic
What are examples of prezygotic reproductive isolation (before egg and sperm meet)?
Habitat isolation, morphological isolation (non-behavioural traits that don’t attract mate e.g. colour), mechanical isolation, behavioural isolation (e.g. some birds display to attract mates, some build, some call), temporal isolation (time of day or year), gametic isolation
What are examples of postzygotic reproductive isolation?
Hybrid inviability (don’t survive), hybrid infertility (survive but can’t reproduce), hybrid breakdown (hybrid less fit than ancestral populations)
What is the genetic basis of a barrier?
Usually comes down to Dobzhansky-Muller incompatibility
- Population temporarily split up
- background mutations always occuring (good, bad and neutral)
- one population has one mutation, which is fixed into the population, the other population has another, which is also fixed
- When the two populations meet again and mate, there are two alleles which have never been in combination with each other
- there is potential here for interaction and incompatibility (not always incompatible, but potentially)
- works without opposing selection as alleles are just background mutations
- over time, the potential for these incompatibilities increases
How is mating preference linked to diverging traits?
You have to have assortative mating to complete RI
Direct system - the same gene that is adaptive also affects mating preference (e.g. Darwin’s finches - females in certain island populations prefer male finches with more suitable beak types to the food sources on the island - prefer males with the same beak type as them) - called MULTIPLE-EFFECT TRAITS
Indirect system - the gene that is adaptive is linked to one which affects mating preference - selection acts in favour of females who choose mates with adaptation due to the linkage disequilibrium of the two traits
+ REINFORCEMENT produces REPRODUCTIVE CHARACTER DISPLACEMENT - selection for prezygotic isolation due to selection against hybrids - theoretically completes speciation - can only happen in sympatry
Reinforcement is…
the selection for prezygotic isolation dye to selection against hybrids
- prezygotic isolation introduced to support post-zygotic isolation (which produces character displacement)
reinforcement can only occur in …
sympatry
(reproductive) character displacement is…
when differences among similar species whose distributions overlap geographically are accentuated in regions where the species co-occur, but are minimised or lost where the species’ distributions do not overlap
Give an example of RCD
Pied flycatcher and collared flycatcher look similar - males are pied (black and white), females are brown. In allopatry both species have pied males, whereas in sympatry, male collared flycatchers remain pied whereas male pied flycatchers become brown due to selection against hybrids (postzygotic isolation) and selection for females choosing browner males (prezygotic isolation). Hybrids are more common when the species look similar. This selection is not present in allopatry as no hybrids are being produced and no postzygotic isolation is occuring.
+ the gene that is adaptive (brown plumage) is linked to one which affects mating preference (gene for females choosing brown males)
How quickly do barriers evolve?
Price and Bouvier (2002) collected data from literature about bird matings. They found that within species, 62% of matings produced fully fertile offspring, and within genera 8% of matings produced fully fertile offspring. This percentage decreased the higher the taxonomic group was. By looking at cytochrome b clock (very steady divergence rate) in the birds, they found that species that produce infertile hybrids last shared a common ancestor on average about 7MYA (suggests an average speciation rate). Passerines had a much faster rate than non-passerines, which may be a large contributor in their diversity as a group - strong support of Haldane’s rule shown
What is Haldane’s rule?
When a cross produces inviable or sterile offspring, the heterogametic sex is more strongly affected (as only have one copy of many alleles)
- males in drosophila
- females in birds
In drosophila…
Coyne and Orr, 1997
speciation has been very gradual and very variable (shown by looking at isolation compared to genetic distance). They also found that most of the first half of post-zygotic isolation was due to male sterility or inviability (Haldane’s rule). They found that prezygotic isolation evolved faster when the species were in sympatric pairs (from similar areas) - good evidence for reinforcement
How much does the genome need to differ to cause isolation?
Hypothesis:
Stage 1: small adaptive divergence
Stage 2: divergent areas grow via linkage
Stage 3: genomes diverge so much that interbreeding very reduced
Stage 4: isolation
What are speciation genes?
Genes which cause reproductive isolation on their own - probably not very common but there is evidence e.g. in Drosophila one allele that is normal in one species (mauritiana) can cause male sterility in another species (simulans) - may be due to duplication event
- duplicated genes that are in the process of evolving into new functions at the time of species separation are likely to contribute to species differentiation - divergence probably down to selection
Duplicated genes are good candidates to look for as … genes as they are searching for a new role
speciation
Howea belmoreana and Howea forsteriana…
are two palm species endemic to Lord Howe island near Australia that are thought to have diverged from a single species. There is almost no chance that they have diverged in allopatry as the island is so small, and they differ genetically in only 4 loci. They differ in their flowering times by 6 weeks, which is thought to have lead to their exclusively sympatric speciation.
What are the 4 main contributors in speciation?
- chance (very important in speciation - what everything else is built upon
- ecology
- reinforcement
- sexual selection
How can chance contribute to speciation?
- random mutations
- genetic drift
- freak natural events
- long-term drift can lead to Dobzhanksy-Muller incompatibilities
- Mutation order speciation
- Founder effect speciation
- Polyploid speciation
Mutation order speciation is when…
different and incompatible mutations (alleles) fix in separate populations that are experiencing similar selective regimes
In Ecological speciation (divergent selection) different populations find different genetic solutions to … problems, whereas in mutation order speciation, different populations find different solutions to…
the same selective problem - ecology is involved but doesn’t favour divergence, divergence is just random, but not drift (as selection is involved)
Founder effect speciation is when…
a few individuals get isolated from the main population and begin a satellite population on their own. The allele proportion of this population relies on the allele population of the “founders”, which is very unlikely to be the same as that in the original population
- e.g. paradise kingfishers (Tanysiptera) in New Guinea - different species on each small surrounding island, thought to all have originated from small groups from those species found on the main islands (some may have been birds blown over by wind - complete chance - or perhaps some selection was involved i.e. stronger flying birds travelled to islands)
Polyploidy is when an individual has more copies of a genome than …. It is really important in … speciation, not in … reproducing taxa. It is often associated with … isolation and … differentiation. It can … produce a new species and is usually formed by a failure of division in …
2, plant, sexually, morphological, instantly, meiosis
How can ecology contribute to speciation?
Divergent selection between environments causes barriers to gene flow
- Environmental differences (Habitat, climate, resources)
- Ecological interactions (Disease, Competition, Behavioural interference)
- Sexual selection
Give an example of environmental differences contributing to speciation
T. cristinae is a species of stick insect with two ecomorphs - one lives on plants with needle leaves, the other lives on plants with broad, flat green leaves. Their hybrids are unfit. This is partly because of habitat isolation (never met individuals from other plant), immigrant inviability (much lower chance of survival to reproduction on other plant), sexual isolation (female didn’t choose other morph) and cryptic isolation (female choice after copulation).
Give an example of ecological interactions contributing to speciation (normally in sympatry)
ECOLOGICAL CHARACTER DISPLACEMENT in sticklebacks - brook sticklebacks show characteristics of more benthic living when in sympatry with ninespine sticklebacks, which are the more dominant species - those brooks who had been living in sympatry showed more growth in the presence of ninespines than those who had been living in allopatry (separate to ninespines)
- this is due to interspecific competition
intraspecific competition (ecological interaction) can also lead to speciation. Give an example
anolis lizards (Anolis sagrei) - perch when they display their dewlaps - bigger lizards have bigger perches, and smaller ones have smaller perches - there has been divergence between large and small lizards based on perch size - small ones have got smaller to avoid competition and fit on small perches that the large ones cannot fit on. They have evolved a difference in limb size
Reinforcement is often thought of as a mechanism to … the speciation process. However, some argue that it could be a … … as well, called … reinforcement, which could potentially result in … different populations
finish, starting point, cascade, four
Pfennig (2003) found that between the two spadefoot toad species (Spea multiplicata and bombifrons) the number of F1 hybrids significantly decreased between the early 80s to the early 2000s. Alternative hypotheses to reinforcement included:
- Opportunities for hybridisation declined - but least abundant S. bombifrons increased in population over the time period XXX
- Hybridisation is initially common when one species is rare - but no correlation of F1 hybrids and S. b. abundance XXX
- Habitat changes reduce hybridisation - but no change in rainfall and no correlation between rainfall and F1 hybrid number XXX
Females of both species normally prefer a fast male call rate, but in sympatry S. multiplicata females prefer a lower call rate ( may be prezygotic isolation selected for)
S. m. females suffer much more decrease in fitness from hybridisation than S. b. females (hence why THEY changed) - this is the driver - reinforcement
In 2014 found divergence was also happening within the species (cascade reinforcement) - between those living alongside S. b. and those on their own (reduced gene flow between)
Yes
Reinforcement acts to prevent … …, despite needing … … initially - but not too much!
gene flow, gene flow
Sexual selection is…
the result of the differential reproductive success that arises from competition for mates and access to fertilisations
Sexual selection can involve natural selection, for example…
- when there is a direct benefit to females
- good genes hypothesis
- sensory bias
However some sexual selection doesn’t involve natural selection, for example…
- fisherian runaway selection
- chaseaway selection
Why do we need to distinguish between natural and sexual selection?
- SS favours mate acquisition and fertilisation whereas NS favours survival and fecundity
- They can act in opposition of a trait
- differences in rate, strength, direction (SS can be stronger and faster)
- SS may build linkage disequilibrium more effectively (via non-random mating)
- one type might contribute to divergence, one type may inhibit it
Sexual selection can generate … isolation, but … divergence is needed first for coexistence
prezygotic, ecological
How can food resources effect sexual selection?
(Snowberg and Benkman, 2009)
(Ballentine, 2006)
Crossbills:
- 10 ecotypes with overlapping ranges
- mate assortatively by call type in the wild
- Even without call prompts females choose males with more suitable beak type to food given
- here sexual selection is related to feeding efficacy (a direct system)
- NS and SS working in same direction
- Beak morphology may be multiple effect trait
North American Swamp Sparrow
- Females prefer higher performance song with range of frequencies and high trill rate (requires small beak)
- Recent colonisation of coastal areas (requiring larger beaks for feeding)
- Sexual selection is working against natural selection
+ although mate preference appears to be gradually changing to prefer better adapted males
Give an example of how resources and environment can influence mating systems
Cape penduline tit
- lives in harsh environment w/ limited resources
- Monogomous and shows biparental care
Eurasian penduline tit
- live in rich environment
- shows sexual conflict over investment (one parent always leaves to produce more offspring with other individuals)
How can predation effect sexual selection?
Guppies
- Crenicichla (eat all guppies) and rivulus (eat juveniles)
- Low predation - females choose males with more black spots
- High predation - females choose males who are more conspicuous (fewer black spots)
- no speciation despite rapid co-adaptation of male and female reproductive strategies (too much gene flow)
How can parasitism play a role in sexual selection?
In general, strong honest signals in males show that they are in good condition, whereas weaker signals suggest that they aren’t
- e.g. melanin often is used as a SS signal and costs a lot to produce - if have high parasitic load energy will go to defending against parasites more than sexual signalling
- e.g. common yellowthroat males have black mask to attract mates - mask size positively correlated with alleles involved in immune defence (MHC class II alleles)
How can sensory environment influence SS?
- visual/acoustic/olfactory signals have to navigate an environment
sound, for example, transmits differently through different environments (absorption, reflection, diffraction (due to temperature)) - dense foliage often leads to bird songs with lower frequencies, a purer tone, slower syllable repetition rates and different frequencies in succession (largely due to echoes)
- open grassland often leads to higher frequencies, more broadband songs, and any rate of syllable repetition and any frequency in succession
– Madagascan paradise flycatchers adapt their song depending on the environment of their territories (lower and longer songs in denser foliage)
Significantly more clades show … in diversity in the direction predicted by … … than expected … …
asymmetry, sexual selection, by chance
However, a paper (morrow et al., 2003) found no evidence that higher …, dichromatism or … size were correlated with higher species …
dimorphism, testes, richness
- depends on taxa
- problem of only looking at extant species
- not particularly strong evidence that SS leads to higher speciation rates - think yes but evidence is equivocal
What is Time For Speciation (TFS)?
Time for reproductive isolation to evolve once the process has begun
What is the Biological Speciation Interval (BSI)
Average time between origin of a new species and when that species branches again
Lake victoria is presumed to be … years old. Phylogenetic studies predicted (>500) cichlids to be … years old. However, lake victoria dried out around … years ago. Estimated that when the lake dried up…
750,000, 200,000, 15,000, the fish used rivers and other lakes as refugia (which will have introduced different selection pressures) and then reintroduced themselves into the lake when it reformed
Speciation rates are higher on larger …
islands. The larger the area a species occupies the more likely it is to speciate. (ferns didn’t bc of polyploidy being a very large contributor to their speciation (31%)
Groups with higher gene flow (further dispersal) required a larger…
minimum area for speciation
Rates of speciation are highly … and hard to …
variable, estimate
Adaptation –> divergence –> … –> … –> …
hybridisation, isolation, divergence
