Speciation Flashcards
What does Speciation involve
Involves the breakdown of mating systems between populations
Importance of speciation
Shows where biodiversity comes from –> Speciation process generates diversity
Important to understanding patterns of variation in nature
- All variation = driven by speciation
Affects of being different Species
Species = puts things on independent evolutionary paths
What is a species
Still up for debate – Some even argue if spcies are even real
Argument about species
- Some argue if species are real – argue if there is something biologically meaningful in being different species
- Species seems like an important level of biologic organization but is it really meaningful
- Argue what a species is
Is species meanigful
Species seems like an important level of biological organization BUT is it really meaningful – our other levels of linnean classification are rather artubtrary –> we know they all have to be monophyletic groups but the level at which we place socies is not objective
Species in hierachy + meaningfulness of species
We know species is part of heirachy
+
We know Linean nomelcature has levels that are reflective of physiological relationships – each level shows a monophyletic cleft
Question: Is there anything special about where we designate levels acorss phylogeny – is it objective or subjective?
In mammalia where do we decide to delineate orders or family is?
Many levels of varaition to put monophyletic group in – question is - is it subjective what level of organization to put animals in to?
Dividing Order
Order is divided into families –> Had split between monophyletic groups
Why do we label things? – some of the way things are divided is obvious = put in the same monophyletic clave BUT it is also somwhat arbutrary
- Someone decided what to name things
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Arbitrary in deciding species
There is a level or artbirainess in species –> Leads to the question is there anything special in species level variation (Does the discontunity we see in nature correspond to anything meaningful?)
Does dicontunity we see in nature correspond to anything biologically meaningful (is species meaningful)
Most biologists agree that discontunity is realo and corresponds to species level class = teh differences are unique to species –> MEANS species are meaningful level of organization that requires some degree of specilization
Nature of evolution
Evolution –> Allele frequencey chnage –> IT IS CONTINOUS
Question – does this just lead to continous allelic variation across teh living world OR is discontunity a function of evolution
IF evolution is continous –> is all varaition continous and humans are just imposing that teh vraaition is discontinous???
Our answer for are species biologically meaningful?
ANSWER: YES –> Discontinuity among organisms indicates that species is biologically meaningful level of organization
NOW – if we know that species are real –> what do they reflect?
Step after knowing species are real
Can we come up with an objective concept that describes discontinuity that epxlains what a sepcies is
We know nature is full of discontinous varieties – can we come up with an objective concept of what a species is?
Species concepts vs. Species diagnostics
Related BUT different things
Species concept = idea of what species means
Class perspective on species
Focus on species of sexually reproducing organisms
WHY – explaining discontinuity in asexual lineages is rather straightforwards BUT sex and recombination homogenizes genomes
- Focus on sexual because of framework we have been using + where discontinuity ends up is harder to understand
Effect of sex
Sex and recombination homogenize genomes
- Sex and recombination breakdown and homogenize differences among groups
Mating minimizing discontinuity = harder to explain discontinuity in sexual reproducing
Explaining asexual speciation
Explaining discontinuity is rather straightfoward – because can’t come back into contact = explaining discontinuity is not hard
Species concepts
- Morphospecies concept
- Phylogenetic species concept
- Ecological Species concept
- Biologic species concept
- Genotypic clustering species concept
Morphospecies concept
Overall: Species are groups that are morphologically distinguishable
- Of see some difference in shape or size or traits = different species
Issues in Morphospecies
No one studying speciation uses this – still doesn’t mean it is not useful in some circumstances
Used to describe speciation BUT only if it is the only option
Issue = people argue over how many species there are
Example – argue over horns (if different horns means they are different species) – using morphology is difficult
Use of morphospecies
Only potentially useful for long extict organisms – used for drawing distictions when we don’t have any other information other than the size and shape of organism
Group that needs morphospecies = people dealing with extict organisms – if just have variation in fossil bodies to know if they are different species
Example – Paleotontology –> Shows issue in concepts + People argue how many species there are
Limitations of morphospecies
- Polyphenism – have the same genotypes but different morphologies
- They shouldn’t be different species but might be placed as such because look different
- Might have different body shapes – Means they are not different species just different morphs + Sexual dymorphism + developmental differences
- People argue over how many there are + using mrpholguy is hard
Phylogenetic species concept
Overall: Species are the smallest recognizable monophyletic groups in phynology
- Make phylogentic tree –> Smaller monophyletic = species level diversity
- See monophyletic group and see groups of species
***There are multiple versions of this
Example phylogenetic tree
See 7 groups of varaition = 7 species
Seems objective –> Take in genetic varaition – if evolution is about genetics then this seems useful
Problem = NOT USED OFTEN
Issue in phylogenetic species concept
Gives an Incomplete lineage story –> Part of reason why varaition in genes doesn’t always match varaition in species
- Variation in ancestrak Might get one allele in one popultion and another alleles in other popultion BUT don’t know if it will end up that way
- Ancestral vraiation is evoloving – which alelles goes to fixatino changes
Get different answer for phylogeny based on gene used = NOT objective
Example Incomplete lineage
Start – 3 types of flies
- 3 groups of drosphila with different morphs – we suspect they are different species
The phylogenetic relationship is different based on 2 different genes –> the two genes gives two answers regarding the flies relationships to each other
- Might get one answer if lookat one one gene or another – NOT reliable way to deleniate if something is the same species
Use of phylogenetics
Useful – If species do not interact with each other
Example – looking to see if they vary genetically + if they are different phylogenies from different clades)
- Look across genome regions = name 2 species in group that was thought to be one
Good if have seperation between organisms – can conclude that they are not exchanging genes BUT we do not know if they were in the same habitat if differences would break down
***Phylogeny might be the best in some cases even if not the best
Ecological species concept
Overall: Species are co-occuring organisms that occupy different niches (Adaptive zones)
- Popultions that are adapted to particular combination of resource bases, predators/parasites, and envirnmental factores in a particular place
- Organisms in the same habitat but different nuche –> Have different requirements (Ex. Different predators)
- Different set of ecological charachteristics = seperate ecological popultion = difefrent species
IDEA = comes from niche exclusion
- If niche is the same then one will go to extiction
- If niches are different = niche that dirves discontuniuty = drives species level diversity
Niche Exlusion
States that two popultions that completley overlap in niches cannot coexist in the long term
- If overlap = can’t co-occur through time
Problems in ecological species concept
- It is impossible to define a niche extrinsilcally of the popultion occupying it
- Can’t define niche without organisms that occupy –> only can define for organism that exists in that space
- Niche are not like parking spaces
- Species do drive each other to extiction due to intense competition BUT does tat mean they weren’t species to begin with
***Not used over the years
Use of Ecological species concepts
Application in asexual organisms
Used in things like prokaryotes
- Useful in some cases – shows how to miantain discontuniuty + drives discontunity in asexual organisms
Biologic species concept
Overall: Species are groups of interbreeding populations that are reproductively isolated from other such groups
- Discontiunity has to do with connective gamete pools of reproductive systems
IF they are exchnaging alleles THEN this is no longer the case –> no longer reproductivley indeoendent = discontinuity is harder to maintain
- If popultions are not reproducing they become evolutionary independent
Variation is discontinous because reproduction is discontinous
Ecologic + morpho + Phylogenetic species concepts
Not very useful for understanding process of speciation –> Not used by evolutionary biologists for understadning how species arise BUT they are useful when in limited situations
What does species drive
Species are meaningful levels of biologic organization that drives discontinuity in biology
Biologic + genologic species concpets
Much more used + get to the crux of what speciation is about
Biologic species Concept (Overall)
Articulated from after the modern synthesis – formed the basis of a lot of speciation reserach
Important reveleation in understanding what species are + gets to idea of how species form
Start to couple pattern + process
Why is variation discontinous according to biologic concept
Variation is discontinous because reproduction is not continous
Different species remain different because they are not exchanging genes through reproduction
Evolution in biologic species concept
The popultions are no reprodcuing with each other + no migration connecting them –> means the evolutionary forces acting ion popultions or within species are independent of each other
- Drift + migration are indepent = species are in independent evolutionary trajectories
- Forces in one popultion does not affect the other
***If popultions are not reproducing with each other they become evolutionarily independent
Closest thing we have to unified species concept
Evolutionary indepence –> closest thing we have to a unfied species concept
When we talk about discontinuity – see different groups that are evolutionarily independent – different trajectories through evolutionary time
Drawbacks to Biologic species concept
- Requires absolute complete reproductive isolation
- Species designations are alwats relative – interbreeidng popultions that are isolated from other groups
- If we are basing this on the potential for interbreeding its hard to say anything about popultions that aren’t actually in contact
Drawback #1 in BSC - Absolute reproductive isolation
In intial incarnation – people held closley that it requires absolute reproductive isolation in order for them to be considered different species and anything less is not speciation
-This is a high hurdle to set for discontinuity if it requires absolute barreiers to gene flow
- This does NOT match with what we see in nature
reproductive Isolation = Different species are incapable of prodcing fertile offspring with each other in the wild
Issue in saying BSC requires absolute reproductive isolation
Doesn’t match what we see in nature –> Hybridization is common in nature between groups that otherwise stay seperate – NOT blending together
- Real species can have hybrids + gene flow
What do we mean by evolutionary independence
Means that the evolutionary forces acting on a popultion do not have any baering on the evolutionary outcome of another popultions
Issue #2 in BSC – always relative
Species designations are always relative –> you are reproductivley isolated from ANOTHER group (not just looking at one group – looking at two groups relative to each other)
- Looking at how they are relative to each other NOT intrinsic about a group itself
Makes BSC less than satisfactory
Issue #3 in BSC – If things would be incontact with each other
One of the biggest when thinking about diagnostic –> the BSC functions poorly because it is hard to test whether things are caopable of reproducing with each other if there were in contact
- If we are looking at groups that are not interacting this is hard to determine
EVEN if do an experiment in lab –> they might not breed in the lab or they might BUT even if they do breed in the lab they might never in nature because of the nature of where they live
- Means that applying BSC in a lot of situations where people think about two things being diverged gets hard in practice
Hybridization (overall)
Pointed out in 1990s – documentaing rayes of hybridiztion in wild for organisms that we otherwise consider species (match strong discontinuity and functional evolutionary independence)
Example Hybrid
- Polar baers + grizzly baers
Example Hybrid
- Polar baers + grizzly baers
- They are very different (easy to tell apart) –> We theerfore call them different species and they mainatin discontinuity (speciation) BUT they can hybridize and their hybrids are fertile
- Seen in zoos (mate readily)
- NOT just done when in caoptivity –> happens in the wild – occuring at increased rates as climate chnage is the blending the linkes of habitats – they are pushed together into the same range and ar hybridzing at significant rates
BUT we are not seeing them collapse as one baer – still have discontinuity even though they are exchanging genes in nature
Shows: Things we think of as good species that match discontinuity and evolutionary independence can still form hybrids and do so in anture + shows that the species can be phylogenetically distant and still mate
- Ducks + geese – can find hybrids easily
- 76% of duck and geese in britain form hybrids with other spcies in nature (form fertile hybrids)
- Chicks + guinea Foul – shwos get hyrbids from things further in ohylogentics distances
- Produce hybrid offspring even though they seperated 35 million years ago
- Example = Sturgeon + American paddle fish – mate –> get hybrid offspring (don’t know if it is fertile) BUT the species have been seperated for at least 200 Million years
What do hybrids show
Shows that discontinuity can be maintained even if have some amount of gene flow
Shows: Things we think of as good species that match discontinuity and evolutionary independence can still form hybrids and do so in anture + shows that the species can be phylogenetically distant and still mate
Affect of seeing hybrids/fertile hybrids in nature
If hybrids/fertile hybrids are seen in nature –> MEANS we need to soften our view on BSC
Means we do not need absolute barriers to gene flow because we can have some amount of gene flow and discontinuity can persist
Result: Led people to make different version of BSC that softens view point
Soften view of BSC
Says that species are groups that are mostly reproductivley isolated
- Less satisfying than an objective idea of what species are
Potential outcome of hybrids
Hybrids can become own species – often occurs in plants
Biggest Issue in BSC
First idea of complete reproductive isolation = most problamatic
Complete repdouctive isolation often does not match up with the discontinuity seen in nature –> hybrids are fairly common and many are fertile YET discontinuity exists
Solution – often soften BSC to mostly reproductively isolated
Genotypic clustering species concept
Overall: Species are genetically distinguishable groups of individuals with few or no intermediates when in contact with other such groups
- Came because of rapid nature of hybrids in nature
- Thinking about organisms occupying diffrent genetic spaces
Gets close to idea of evolutionary independence
Issue in Genotypic Clustering concept
- Still relative – have to compare to another group
- Somehwat wishy/washy – “few or no”
Why are species genetically different according to GCSC
Genetic varaition that no longer overlaps between species because evolutionary forces in one popultion do not act on evolutionary forces of the other
BUT STILL the differences are mainatined in the face of some gene flow
GSCS = gets closer to idea of evolutionary independence
Migration + GCSC
Effect = to homogenize allele frequencies between populations
High migration = we are not able to maintain genetic differences –> cam’t maintain genotypic clusters unless there are other forces acting STRONGLY against migration
What does it mean if there is migration but still have discontiniuity
If there is some migration and they are not collapsing (they still have discontinuity) –> tells us that migration is not meaningful
- If popultions are able to maintain discrete genotypic space even if some reproduction is still going on migration is no longer consequential THEREFORE the popultions are evolutionarily independent
Means the two popultions are still evolving independently without needing absolutely no gene flow
- One popultion does not effect the other depsite some potential continuity
How can genotypic clusters stay intact if reproductive isolation is not absolute
Example:
Start – have two groups in two environments (in two niches within the same envirnmnet)
Start = only red + blue (no intermediates)
IF exchange some genes –> NOW have some purple
- Migration acts to hemogenize allele frequencies SO if we have exchange of genes why doesn’t migration do that?
Occurding to BSC – The purple (result of gene flow) would be sterile = they can’t make offspring = get some hybrids BUT the hybrids can’t go anywhere
BUT what if we have fertile hybrids –> They do not have to be sterile they just ahve to be lower in fitmess
- Lower fitness could be due to reproductive abvility (decrease in fecundity) –> People focused on things that deal with repdoruction because thinking about reproductive isolation as sole driving force of isolation BUT there are ways of affecting reproduction that doesn’t have to do with repdouction directley
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Instead – indiviuals could just be ecologically unfit –> could be infit in envirnment
- Example – The purple might not camoflouge as well = stands out = gets eatten
- OR Hybrids could have lower fitness because hybrid genomes are not as well adpted as parental genomes would have been
Selection of intermediates = important way we view process of speciation rather than just looking at reproduction system directley
Overall: Hybrid just needs to have low fitness –> if pruple does poorly alleles aren’t liley to spread between blue and red = popultions can stay distict
What can cause lower fitness
Lower fitness could be due to reproductive abvility (decrease in fecundity) –> People focused on things that deal with repdoruction because thinking about reproductive isolation as sole driving force of isolation BUT there are ways of affecting reproduction that doesn’t have to do with repdouction directley
Instead – indiviuals could just be ecologically unfit –> could be infit in envirnment
- Example – The purple might not camoflouge as well = stands out = gets eatten
OR Hybrids could have lower fitness because hybrid genomes are not as well adpted as parental genomes would have been
Selection on intermediates
Selection of intermediates = important way we view process of speciation rather than just looking at reproduction system directley
What do we need for two popultions to be different species if have gene flow
Hybrid just needs to have low fitness –> if pruple does poorly alleles aren’t liley to spread between blue and red = popultions can stay distict
Genotypic clustering vs. BSC in practice
In practice genotypic clustering is not very different from soften view of BSC –> two sides of the same coin BUT considering them sepertlet allows us to look at pattern vs. Process
Pattern = what are species
Process = how do species evolove
Unfied concept (BSC + Genotypic)
Species are genetically distuiguishable groups of individulas maintained in contact with other such groups –> this pattern is driven by the evolution of reproductive isolation
- Discontinuity = driven by diffrent forms of reproductive isolation to keep them apart BUT not absolute barriers to complete genomes
Reproductive isolation is…
Complicated
Universality of species concepts
No one species concepts is perfectley applicable across all of the varaition we see
Issue in species concepts
Have issue of dealing with purley asexual organisms _ the range of sexuality in other organsims
- Main limits of either end that many species concepts run into
Concpets = only work whenb there is the right amount of sexual reproduction going on
IN BSC – issue of sexuality = if have too much sex or not enough sex
Issue in BSC
Issue when have too much sex (rapid hybirization) OR not enough sex
- Issue = some groups do not conform to biologic continuity that we epxect on species levels
Examples that are probabmatic for any of our species conepts:
- Have many exmaples in plants because they are self pollinating (Asexual linages) + Hybridize between unrealted forms = complex varaition
1. Hawthorns – 180 species OR 2000 spoecies in the world –> because hard to know what is reproductivley isolated when have a huge range of varaition from asexual to sexual on a regular basis
2. Oaks – have asexual and rapid hybridization – makes drawing boarders between species hard
Speciation (Paradox)
Speciation poses a bit of a paradix for evolution by NS
For a whgile was a paradox for evolutionary biologists –> Hard to know how this happens
- Issue = breakdown of reproductive compatability that used to be able to interbreed – why should this happen
Paradox: Shouldn’t NS favor inivividulas caopable of breeding with everyone + How do popultions go through a phase of limiting reproductive options
Issue in breaking down reproductive compatability
Breakdown in reproductive compatability = causes decrease in reproductive sucess = reduces fitness –> Why evolove to be not compatable if ancestors were – breaks down an important part of fitness
Figuring out speciation
Thought about how to go through phase from interbreeding to stage of being incompatable
Do we need string ecological divergence or absolute reproductive isolation – how do you breakdown the compatability
Led people to start thinkning about geopgraohic context of speciation
Geographic view of species
For a long time biologists were hung up on geographic context of speciation
- Emphisis on this has fallen out of favor (partly because reality is more nuanced) BUT it is still important to discuss
Looks at how the ranges of popultions relate to eachother throughout the process of speciation
Importance of geograohic context
Has string consequences for ease ordifficulty of speciation
Issue in geograophic view of speciation
Issue = two species ranges now might not reflect the ranges when they were diverging
- What we see now might not be the context that organisms diverged in BUT we want to know what is the context of divergence
- The current distrubution of completley reproductivley isolated taxa doesn’t necessarily tell us about the nature of their divergence
Geographic modes of speciatation
- Allopatry (Vicariance + Peripatric)
Allopatry
Complete geograohuc seperation of popultions during divergences – seperated range
NO migration at the start (m = 0) – totally different regions during divergence
Two forms:
1. Vicarance
2. Peripatric
Vicariance
Seperation of a large popultion into two parts –> one large popultion that gets cleaved into two
- Seperation of an otherwise connected population
- Seperates what was one larger interbreeding popultions into two seperate popultions
Could happen if a barreier appears OR if organisms shoft range around a geographic barrier
Example – if near a mountain range –> might go over and split across the mountain range – NOW they evolove indepentley because seperated and by the time they come back into contact they may no longer be compatible
Peripatric divergence
Allopatry caused by colonization of a distant habitat from a larger popultions
- Started as a big popultion and founded a small satelite popultion
- Founder events will play a role
Example – On a mainland –> have an island off of coast and maybe have one pregnant female goes to island and starts a new popultion in island (now seperated)
What forces are important in Perpatric divergence
Drift – have strong effects at the start of the process
- Strong founder effects causing rare alleles in the samll popultions might be important
- Only issue in periparty
If found a new satlite popultion with few indivdiuals = involves a lot of drift from the start
Example Peripatric divergence
If have flies on mountain and have a satelite mountain range (there is a big seperation between the two) and happen to get one individual on the other mountain that founds a new popultion
Strong founder effects causing rare alleles in the samll popultions might be important
Commonality between peripatric + Allopatric speciation
BOTH – have a geographic barreier from gene flow form the start – no genes exchanged because the two popultions are physically seperated
- BOTH – results in absolute barrier to gene flow between popultions
BUT it is worth having them as seperated because only peripatric has drift
Examples of Allopatry
- Panama – Vicarinace – seperates from pacific from atlantic between NA and SA –> before had one mass contecting the two (befoe the waters were connected) – THEN have a land mass that sperates them – now different species on sides of land mass because reproductivly isolated when land mass rose
- Example – snapping shrimp seperated
- Bambies + Chimps – diverged due to vicarinace –> Seperated form each other after popultions were seperated by the congo ruver
- They ae bad swimmers = popultions seperated
- Now evolove independently
- Example peripatry –> Hawaii
- Because we know the timing well – we know about how peripatric divergence occurs
- Spiders are different species on the different islands
- Drosphilla in Marchyus Vs. seychells – two different species
- The mainland insect goes to island and now is three species NOT one
- Peripatry = includes many model systems for geentoc basis of reproductive isolation
What causes most Peripatry
Many have to do with island colonization
Difference between Peripatry + Vicariance
DRIFT – plays an important in peripatric divergence
Example of vicariance
Snapping shrimp seperated
Parapatry
Geographic seperation BUT not complete isolation – often occuring in adjoining habitats
- Alongside each other (Adjoining habitats)
- NOT seperated ranges – have adjacent ranges –> ranges touch + could overlap to some gree in the middle
HERE – m>0 – Divergence still occurs even in the face of some inital gene flow
Migrayion in Parapatry
m>0 – > not complete geographic isolation
- Overlaping individuals can mate
Divergence still occurs even in the face of some inital gene flow
Migrayion in Parapatry
m>0 – > not complete geographic isolation
- Overlaping individuals can mate
Divergence still occurs even in the face of some inital gene flow
Example Parapatry
Might have flies adapted to different habitats that vary by elevations
- Might have high and low eleveation ones – they ate geographically isolated because they stau in different habitats but the habitats touch each other – range where the habitats are next to each other
Seen in Andes with buterflies thta are low and high land that are likley in some contact
Sympatry
No geographic seperation among diverging popultions
- Diverging occurs in complete overlap of popultion ranges
- More restictive for speciation to occur
Migration in Sympatry
m > 0 –> Over 0 for all individuals in both popultions
Have some emergence of discontunity while still conneted
Divergence in Sympatry
Divergence begins as polyphenism that eventually leads tp evolution of some reproductive or genetic incompatability that leavds to discontinuity in a single habitat
- Have some emergence of discontunity while still connected
Geographic variation in Sympatry
Modes of geograohic varaition = range of potential migration and gene flow in the process
Speciation + Migration
Easy to explain speciation at m= 0 BUT as increase potential migration = gets more restrictive to explain how speciation occurs in the face of more gene flow
How does reproductive incompatability or traits that maintain divergence evolove – what are ways effective gene flow can be limited between genotypic clusters of organisms
Geographic modes
Ways of classifying context of speciation BUT also classifying the traits involoved in the process
- Look at ways phenotypes can affect evolutionary independence of popultions
- Looks at ways the gene flow can be restricted in popultions
More nuanced view – covers a range of migration levels
What do we need in order to look at ways phenotypes can affect evolutionary independence
Need to think about when in the life cycle gene flow can be restricted and when gene flow can be restricted in formation of zygotes
- One of the ways to classify isolating mechanisms is by the timing of their action along the life cycle
Points in lifecycle/zygote formation gene flow is restructed
- Premating – no mating
- Isolation occurs before gametes are exchnaged
- After mating but before zygote formation – mating but no zygote
- Isolation due to unsucessful fertalization
- Post zygotic – have zygote but have an issue in zygote that prevents effective gene flow across popultions
- Isolation due to problems with the offspring’s genotype
Pre-mating isolation
- Behavioral Isolation – Might be because of difference in mating behaviors that make two organisms unattracted to each other
- Organisms fail to attarch each other as mates
- Mechanicam isolation prevents reproductions – geneticals do not fit toegther
- Reproductive structures are incompataible for mating
Example – Bird with corkscrew fallace
- Reproductive structures are incompataible for mating
- Ecological traits – Divergent adaptation leads to decreased mating events across popultions
Example — Peackock dancing spiders –> do a dance
- Two species might not exchnage genes because do not find the dancing attarctive
- MIght not be impressed by dance
Affect of ecological traits on Pre-mating
Ecological traits could play a role in premating isolation
- COuld have an ecological adaptation that prevents gene flow
- Example – Different polinators – flowering plants polinated by different insects (one might be polinated by an insect and one might be bat polinated) –> pollen won’t flow from one to the other
- Could have different mating areas – one might dance in open and one might dance in isolated area
- Could have different reproductive seasonality (different mating seasons)
- If one mates in jine and one mates in augast = have barrier that keeps them apart
Post mating Presyzgotic
Most occur when gametes themselves are incompatible (sperm and egg can’t make zygote)
- Often gametic isolation –> Gametes are transfered byt fail to form a zygote
Might be because of incompatible cell types – not reacting right to get gene in egg cells (Fail to react properly) or the sperm might not be able to survive the female reproductive track
Example – Gemete incompatability for broadcast
- Sessile organisms that don’t move – cast ganetes out into the ocean and hope they mix and form zygote –> might spew spew out but when sperm and egg comes together nothing happens (seen in Sea urchins)
Post zygotic isolation
Have some amount of gene flow – when mate they make zygotes but some isolation makes zygote not effective gene flow between popultions
- Hybrids might be bad at reproducing or bad at surviving or both
Can have:
1. Sterelity
2. Inviablity – fitness is low
Catagories of Post repdouctive isolation
- Intrinsic
- Extrinsic
Postrepductive Intrinsic
Context independent – doesn’t matter what environment
- Independent of the environment
They are bad no matter what
Post reproductive Extrinsic isolation
Decrease in fitness = depends on the envirnment
- the lower fitness is due to envinrment
In a lab they might be fine
Example Post reproduvctive isolation
- Grizzly Baer + polar baer–> They are fertile – might be extrinsic post zygotic that keeps them apart
Intermediate is not good at being a polar baer + not godd at being a grizzly baer –> they are only bad in the envirnment (not in the zoo) = extrinsic
- In the zoo it is fine It is only bad in envirnment = exitrinsic
- Hybrids that are steile – MUle –> intrisic becaise doesn’t matter where the Mule lives they are always sterile
What mechanisms keep genotypic cliusters apart
Not often one mechansims that keeps genotypic clusters apart – as speciation proceeds often have more than one mechanism acting in different parts of the life cycle – act in a sequential pattern
- Multiple mechanisms can work together in a sequential fashion
Example – have 80% isolation due to premating = only 20% gene flow after THEN can reduce that even more to create repdouctive isolation
Example – multiple mechanisms keeping genotypic clusters apart
Overall: Have reduction from full compatability at each stage
Example – have 80% isolation due to premating = only 20% gene flow after THEN can reduce that even more to create repdouctive isolation
Can measure the strength of reproductive isolation for different mechanisms in system in organsms and measure them indepentley
- Can go into the lab and say that you have some traits that causes 65% reduction in potebtial matings and of the matings that occur 55% reduction in ability to form zygotes and after mating the offspring that form from those zygotes have 90% reduction in fitness
- Shows different measures of mechanisms + how their act in a sequential fashion
Overall: Have 65% further reduced by 55% which which is further reduced by 90% – cumalative reduction to 84.25% at post mating and 98% reduction by the end
SHOWS – any one factpr by itself might not be very good but the different traits acting toegtehr limit to less than 2% because of the way that they interact with each other
Explaining how does speciation occur in the absence of gene flow
In the absence of any gene flow – incompatibilities are easy to eexplain – easy to explain that given time these incompatabilities are almost ineviatble
- If they are seperated geographically then they evolove indpenetley then when they come back together there has been enough time that they become incompatable = speciation is complete
How do we get gradual breakdown of reproductive isolation
Use Dozbhnzke Muller model –> key is to explain how reproductive incompatability evoloves in the absense of gene flow
Considers incompatiblility evoloving at multiple genes
Dozbhnzke Muller model (overall)
Explains how incompatabilities form –> key is to explain how reproduction incompatability evoloves in the absence of gene flow
Consideres incompatibility evolving at multiple genes
- Considers interaction between more than one locus (epistasis)
Overall: Given amount of polygenic traits + Amount of epistasis in nature – if two popultions evolove independently = end with epistatsic differences in different contexrs
Dozbhnzke Muller model
Explains Inevitable model of reproductive isolation is driven by inevitable buildup of epistatic incompatibilities between organisms
Example – Have a post mating Pre zygotic isolation mechanism – have sperm + egg issue
- IF you have locus A that is a cell surface protein on the sperm and locus B for receptor protein on egg surface
Start = A and B are compatible with each other in intial population
THEN you separate the two populations physically = evolve independently – if evolve independently = mutations in one population will not occur in the other population – the timing of mutations + what mutations occur might be different
In one population might get a varient in A –> now have individuals with a not A – and a needs to be compatible with B gene
IF have a change in A and both A and a are compatible with B (this needs to be the case otherwise a would not rise in population - a works with B = increase in a
Then the second population is fixed for A (they never have the mutation) BUT in the second population you get B –> b – b needs to be compatible with A – eventually get fixation for p
NOW
Pop 1 = foxed for a
Pop 2 = Fixed for b
WHEN population 1 and 2 come back together = have an issue because a is not compatible with b (a is compatible with B) = breakdown in compatibility in fitness
Overall: Given amount of polygenic traits + Amount of epistasis in nature – if two populations evolve independently = end with epistatic differences in different contexts
Overall Idea in Dozbhnzke Muller model
Overall: Given amount of polygenic traits + Amount of epistasis in nature – if two popultions evolove independently = end with epistatsic differences in different context
Different popultions eveoloving indepentley sets the stage for downstream evolution = get some incompatabilities that effect many things (effects all stages) –> Indendent evolution cuases genomes to be incompatible because of inevitable buildup of epistasis
Where is speciation easy to see
Easy to see in Allopatry because mutations don’t occur in context of each other
Example – Have a diploid organism with 6 chromosomes THEN have seperation barrier (because allopatry) –> mutation in habitat 2 in locus a AND in other popultion have D –> d – NOW have subsequent changes in bases
The The mutation occured in indiviuald popultions and went to fixation that leads to genome wide seperations
- Mutation in one popultions might be favored in both habitats but only occurs in one and stays in one because they are seperated
IF you take away barrier – if exchnage genes might get initial chnage in one popultion in one habitat thats a works well with d BUT NOT D –> Ability to get estabiloshed is reduced
Mutations on the same chromosomes + Speciation
If lock A and a on the same chromsome = always inherited together but still hvae gene flow – HAVING SEX = have recombination = put incpomatible getes = hard for sepeciation to get started
Required of allopatry
People thought that speciation requires allopatry (requires geographic seperation) – because if have any gene flow = harder to explain genetic popultions for spceiation = easy tp say that it will only occir if have barriers to gene flow driven by geography
Change in thought about speciation
Stopped thinking about other geographic modes (not considering modes of speciation) BUT this is now changing
Where is DM model easy to see
DM type isolating mechanisms evolve easily when no gene flow BUT they are harder to evolve in situations with more migration
They are almost inevitable in pure allopatry given enough time, but the conditions for reproductive isolation to evolve in sympatry are much narrower
Genotypic clustering figure
Figure: Shows genotypic clustering – Groups of ants that form distict clusters in genomic space
Overall – looks at genomic varaition in seed dispersing ants in eastern NA
SHOWS – variation in genomic space across 284 colonies of ants thought to be one of two species
- Showed that the species designation based on morphospecies is inaccurate with genomic data
In Genomic clustering model – have 3 species and a whole other set of less distict clusters that represent some earlier phase of speciation process before string reproductive isolation and evolutionary independence is completed
What do we expect in genomic clustering figure
Distcit discrete clustering – what we expect to occpuy spcies level divergence – get few or no intermediates between individuals in genomic space vs. other groups of colonies that have a lot of intermediate indivoduals indicative of ongoing hybridzation
D/M model (again)
If geographically seperate popultions (have allopatry) = not exchanging genes with the other AND if we allow enough time –> THEN the independent evolution occuring in isolated popultions will inevitably lead to epistatic incompatabilities that result in breakdown of reproductive compatability between popultions
What does DM model work for
For epistatic incompatabilities – requires epistasis
What does DM require
requires epistaisis –> changes in a couple of genes that interact with each other that allow breakdown of reproductive compatabiulity
What does DM answer
Answers how should a breakdown in reprodycrtive sucess evolove if a fundemental poart of NS is the ability to repriduce with memebers of popultion
Answer in DM = if they are seperate popultions and evolove indepentley (mutations in one popultion occur in one and not the other or occur in different orders) = sets the stage for subsequent evolution = get inevitable buildup of epistatic incompatabilities
Where does DM function best
Functinons best in complete absence of gene flow or migration = best in pure allopatry
Sucess of DM model
Was such a sucessful model that is became the null for understanding how new species arise for decades
Many thought that because DM incompatabilities occur so readily in isolated popultions that perhaps this explains ALL of biodiversity (that geographic happenstance in seperated popultion either through viracariance or allowparty) might explain all species diversity because process happens automatically given seperation and time
Came out of Modern synthesis
Issue in DM model
Might not match biodeiveristy in all contexts –> is diveristy at species level reaoly just because of rivers and mounatins splitting popultions – doesn’t match with understanding of how biodiveristy is structyred + importance of ecological interactions in maintaining biodiveristy
Second model of speciation
Explains how reproductive isolation can evolve in the absence of hard geographic barriers
HERE – speciation can occur when ecology is driving force between seperation of popultions
Apple Maggot Fly example
Overall: Shows how ecological adaptation can result in reproductive isolation = have speciation if no geographic component separating populations
Background:
Fruit flies –> lay eggs in ripe fruit – larvea developes feeding in fruit –> – THEN triggers the fruit to drop and larevae crawls out and gies to soil –> When there is hawthron in the fall the next year = the flie leaves the round and starts again
- Life cycle is synced with avalibility of ripe fruit – intamiate relationship between insect and host plant
- Before = all adapted to infesting hawthorn fruit THEN colonists introduced apples to NA – didn’t affect flies for a while BUT in 1850s in NY some of the flies shifted from attacking Hawthrons to attacking apples
Entomologosts documenting this shift were reading Dawrins work and started talking to Dawrin about this being a potential case of formation of a new species in action – Dawrin agreed BUT the entomologost was hit by a train and everyone forgot about the story until some reseracghers picke dit up and started looking to see if this was a case of formation of a new species
NOW – this is well studied and is the best supported case of ecological speciation in nature
2 phenotypes in shift from Hawthorn to apples
Shift = required adapatation in 2 Phenotypes
- Chemosensory response to fruit volitle – find host by smelling them
- Know the apples prefer the complex voltile blend of apple odor and hawthrons prefer hawthron odor
- As they look for host plants where they will find mates and lay eggs apples to to apple trees and hawthrons go to hawthorn tree
- Life history Time
- Apples fruit 3-4 weeks earlier than hawthorns –> Apple flies come out of ground 3-4 weeks eariler than hawthorn = shirfts the mating sead of the two insects apart through time
- flies only have a single life cylce per year = need to be tightly synced around windows of avalble host resoruces
- Even if on the same host plant they are out at different times – as hawthron hatch the apples are dying
Isolation in apple fly
The two traits (life history + smell) = forms the basis of pre-mating isolation
- Difference in chemo repsone = causes them to go to difefrent trees + have different phenology (seasonal timing) –> driving mating season apart = creates strong but incomplete reproductive siolation
Isolation in apple fly is…
Incomplete –> premating isolation limits portential gene flow to 4-6% per year –> Means the strength of reprductive isolation = 94-96%
4-6% migration = BIG reudction from random breeding but migration is a string evolutionary force that is able to homogenize allele frequnecies between popultiomns exchanging genes
MEANS – even in the face of 4-6% migration rate –> THE difefrence still persistnce because also have post zygotic dilution on system
- Post and pre allows differences to cause consistent allele frequencey difefrences to build up between the popultions when in sympatry
- differences are genome wide
- Know there is no geographical compoenent because we know where the shift occured
Maintaining differences in face of migartion (Apple)
Even in the face of 4-6% migration rate –> THE difefrence still persistnce because also have post zygotic dilution on system
- Post and pre allows differences to cause consistent allele frequencey difefrences to build up between the popultions when in sympatry
- differences are genome wide
- Know there is no geographical compoenent because we know where the shift occured
Pre zygotic isolation (Apple)
Looked at chemo sensing – looked at pre mating and post mating level
Figure – Looked at strong phenotypic differences in chemo behavior
Have apples + hawthron in wind tunnel that has a blend of volitle that makes apple odor
- Apple respond positivley to apple – Hatwhorn have limited response to apple
Have Apples + HAwthron in wind tunnel that makes Hawthorn odor
- Hawthorn respond positivley to hawthorn
Y axis – looked at phenotypic response of upwind flight
SHOWS pre mating component of phenotype – if apple preferntially go to apple and hawthrons go to hawthorns = seperating where flies find theor mate and lay eggs
Pre zygotic isolation (Apple)
Looked at chemo sensing – looked at pre mating and post mating level
Figure – Looked at strong phenotypic differences in chemo behavior
Have apples + hawthron in wind tunnel that has a blend of volitle that makes apple odor
- Apple respond positivley to apple – Hatwhorn have limited response to apple
Have Apples + HAwthron in wind tunnel that makes Hawthorn odor
- Hawthorn respond positivley to hawthorn
Y axis – looked at phenotypic response of upwind flight
SHOWS pre mating component of phenotype – if apple preferntially go to apple and hawthrons go to hawthorns = seperating where flies find theor mate and lay eggs
Options for Hybrids of Apple and Hawthron
- Dominance – Heterozygotes between apple and hawthron just prefer Hawthron (hawthron effect masks the apple effect) OR apple is dominant to hawthorn phenotype
- Here = get initial reprodtive isolation from pre mating BUT when have hybridization event have substantial gene flow back into one popultion = if this was teh case (if apple is dominant) = woulod have a lot of hawthron genes going into apple popultion in hybrid that is behavirally apple
- Co-dominance – lead to hybrid flies likeing both –> repsonds positivley to both
- POOR barrier to gene flow
- If all hyrbrids were just able to move between hawthron and apple mating popultions = increase gene flow between them – if indiviuals can respond to both why shoudl they adapt divergentley why wouldn’t they all be generalists between hawthron and apple if that opens more ecological oppertunities for them
Response of hyrbids to voltile of plants
Took F1 from 3 populations and compared them to initial apple and hawthorn blends + have some mixes of different groups of volatiles
Result: Found that there was no response of hybrids to apple or hawthorn blends – ONLY small percent response to some mixes of blends
- Go form 100% response of upwind flight in pure bred to complete lack of behaviral response = hybrids seems to be inert
MEANS – hyrbids prefer neither –> if can’t respond behaviorally to either of teh fruit smells in feild in nature = can’t find matyes = can’t ocaposit = very low fitness = reason why this is strong post zygotic isolation
Fitness of Hybrids/ post zygotic isolation
Hyrbids prefer neither –> if can’t respond behaviorally to either of teh fruit smells in feild in nature = can’t find matyes = can’t ocaposit = very low fitness = reason why this is strong post zygotic isolation
- Hybrids = intert = no response = can’t find mates
Ecological sterelity of hybrids
If put hyrbids in cage with males and females = can get them to mate in the lab – means that they are not intrinsically sterile they are ecologically sterile – in context of ecological envirnment they will never be able to find mates
Why do Hybrids have complete breakdown of behavioral phenotype
Seen in feild experiment looking at differential trapping of apple and hawthron based on odor blend in nature
Experiment – Have two bated sets of lores – has shiny red christmas ornaments in sticky material to trap flies
- Some of parafin lure that releases the volitle (apple or hawthorn odor) + have a control trap with blank paraffin
Looked at Apple in Apple orchord and hawthron foron in stands of hawthron trees and visa versa
- Seeing how hawthorn flies react to apple in their envrinment and visa versa
Results: See percent of flies caught on sticky odor rather than blank control
- Expect 50/50 – if catching by chance
- In apple orchard = caught more than expected by chnace in trap with apple odor
- Hawthhrn odor in hawthron trees = catch more hawthron than expected by chnace compared to control
- Hawthron in apple area AND apple in hawthorn area –> get far fewer flies on baited lure than by chnace – catch more on blank
SHOWS – not just a matter of phenotype about prefermaces not that hawthrons like the smells of hawthron and apple like the smeel of apple –> SHOWS that they avoid the smell of alternative host fruit (nit only about preferance but about avoidance behaviuors too)
Avoidance of chemo sense
Not just a matter of phenotype about prefermaces not that hawthrons like the smells of hawthron and apple like the smeel of apple –> SHOWS that they avoid the smell of alternative host fruit (nit only about preferance but about avoidance behaviuors too)
NOT unexpected based on the kind of chemo sensory ecology of insects in general –> specialized insects need to navigate a chemcially noisy world – many plants making many voltiles that might overlap = need to make the right decision on roght plant smell to follow for getting a good host plant
Example of picking good host plant
Example = crab apples –> very acidic = the flies can’t survive on them = if the flies makes a mistake and follows the smell of crab apples rather than hawthorn or apples = they will ovaposit into the crab apple = that will give low fitness = insects are wired to prefer a good habitat BUT also wired to avoid bad habitats
Genetics of chemo sensing
Found based on genetic cross
Likely have 4-5 genes involoved –> have a mixture of pro apple loci or pro hawthorn loci AND anti apple or anti hawthron loci
- Apple flies have genotypes to prefer apples BUT also activley avoid hawthorn blend
- hawthrons have pro hawthorn and anti apple genotypes
***Know the nueorophysilogy of the phenotyope well for how the brain is wired
Hybirds low fitness
When Apple and hawthorn come together in hybrids NOT the prefernace for specific fruit that matters it is the avoidance part where the hybrids end up avoiding both –> Avoids hawthron and avoids apple = casues teh flies to ebcome ecologically sterile – flies with genotype for anti apple and angti ahwthron end up ebing anti to both
What drives the post zygotic isolation
Driving the strong post zygotic isolation is NOT just the effect of single loci it is the compbination of preferance and avoidance alleles = what overall phenotyope there is and therefore what fitness you have is not just based on alleleic state of one locus – it depends on the allelic state of multiple loci = epistasis
Epistasis in ecological barriers
Kinds of ecological barreiers in order for them to be effective = they are also epistatic too
Have epistasis even in cases where have ecology driving reproductive isolation and speciation
- Have epistasis in the same way have epistasis when have allopatry in DM model
MEANS the most effective isolatinmg mechanism often evolve epistasis
Ecological speciation on Adpative topograohy
Since have epistasis – on AT = see two different potential high fitness outcomes (one specializes in apple and one in hawthron) with some fitness valley between the two
Means have one popultion specialized at one peak and the one specialzied on the other peak – generating few or no intermediates because have such low fitness
Case of recent ongoing speciation
Have morphologically residential flies in genus that infest a wide numver of host plants where fly poputions are divergnetly specialized on the same kinds of traits involoved in chemo sense + life cycle timing in a way that have different species of flies attcking different host plants
Apple flies = have being doing this for 160 years BUT others have been opccur the past 10,000 or 100,000 years
Have range of ongoing levels of divergenes that indicating of adapting radiation
More recentley – some of flies where introduces via apples in 1970s and have shifted to hawthorn populations there - see not just evolution of speciation in past century but in the past few decades
Adaptive radiation
have divergent ecolgical specialization and speciation not just in teh case of a single new species being generated but a whole diversification of a group of organisms based on tehse kinds of ecological speciation process
Commonality of ecologically driven speciation
When ecologic oppertunities post fitness tradeoffs that generate epistatic incompatabilities in ecological phenotypes the process of ecologically driven speciation and adpative radaition might be pretty common in nature
Speciation
Speciation is process of taking varaition in popultion and splitting it into varaition at species level – into different evolutionary independent lines
Speciation as interface
Can think of speciation as the interface of micro and macro evolution
Change in perspective in sepciation
Thinking from allele freq change within population to type of larger scale evolutionary change between populations or between species
Speciation is connection between two levels of thinkning about evolutionary change
Change in perspective in sepciation
Thinking from allele freq change within population to type of larger scale evolutionary change between populations or between species
Speciation is connection between two levels of thinkning about evolutionary change
Limitation of adaotive radaition
Idea of adaptive radiation is not limited to the fruit flies – a lot of major studies understanding evolution process driving diversification have takem place to understadn adaptive radiotion in general
Example – Figure –> study of Adapative radiation of lizards in Greater antilles → difefrent ecoligcall oppertunities on islands lads to different diversification of lizards into different ecological groups through time
Adaptive Radiation
Not one speciation event BUT have multiple speciation events based on adaptations to many ecological environments
Example – Lizards –> specialized based on ecological opportunity to increase biodiversity
- Have different islands + difefrent ecologies = different behaviors + phenotypes –> Causes to diversify in many ways
Affect of Adaptive radiation
Adaptive radiation puts speciation at the interface between micro and macro evolution (variation within species to between species variation to evolve to macro evolution processes)
