Speciation Flashcards
What is speciation?
the process by which new species arise
What is a species?
According to (a slightly modified version of) the Biological Species Concept:
a species is a group of individuals that have the potential to interbreed with one another but are reproductively isolated from other such groups
Why is it so tough to define a species?
- a wide range of variation can exist in multiple traits within species or between species = it’s challenging to select which traits we would use to decide what defines a species
- how do we know whether species are fundamentally different from other groupings of organisms (eg., populations, races/subspecies, genera, strains, families)?
- some species speciate/radiate at different temporal scales than others
How are Heliconous butterflies a good example for how difficult defining a species can be?
looking at the image from the slides, there’s two rows of 5 butterflies
the two butterflies in each of the 5 columns have the same wing colouration and patterns, but this is not what makes up the different species
there are only 2 different species = each row
these two species are geographically isolated but have convergently evolved similar wing patterns to warn predators of toxicity
T or F: some lineages diversify into multiple species in a short time period
true
How and why is it possible for some lineages to diversify into multiple species relatively quickly?
Depends on their conditions
ex. if they were dramatically separated by a reproductive barrier such as a geographic barrier - the gene flow is rapidly reduced and they will only be able to pass on the present genes = variation between the two populations will increase
Explain how African cichlids are a good example of rapid speciation
in African Great Lakes, cichlids have experienced huge radiation (speciation)
250 species in Lake Tanganyika
700 species in Lake Victoria
800+ species in Lake Malawi
all of this radiation occurred in ~10 million years (very short time period for such a large amount of species to arise)
How are Tropheus cichlids in Lake Tanganyika a good example of speciation? How might this have occurred?
within Lake Tanganyika, there are 8 different species of Tropheus cichlids
they are very closely related but have become reproductively isolated and distributed at different regions of the lake
they have a lot of variation in their colouration and patterns
Is there a single, universal definition for a species?
Probably not, there’s always exceptions to everything in biology
What is the biological species concept?
states that a species is a group of individuals that can interbreed with each other, but not with other individuals from other such groups (ie., reproductively isolated)
What is reproductive isolation?
a good measure for determining what a species is
it occurs when there is no gene flow (or rare gene flow) between distinct species and little hybridization
Does reproductive isolation make hybridization and introgression impossible?
no, these can still occur and be important for increasing diversity and adaptations
ex. EPAS1 allele in Tibetans from an introgression with Denisovans
Why can the biological species concept be problematic?
demonstrating reproductive isolation can be a challenge for many organisms
how do we define species for organisms that don’t reproduce sexually? what about fossils?
How can we overcome the limitations of the biological species concept in defining species?
we can make inferrences from patterns of gene flow and DNA sequences
T or F: speciation is complex and dynamic and acts like a continuum
true
Why is speciation a dynamic continuum?
it’s a very slow process and there can be a lot of back and forth
How can speciation being a dynamic process make it challenging to study?
how do you know what period of time to study? how do you know at what stage of the speciation process you’re looking at?
What are the 4 basic steps of speciation?
- one species with a polymorphism, no reproductive isolation (lots of gene flow)
- reduced (not fully) gene flow (eg., maybe there’s a new geographical barrier or they’ve colonized an island, could be changes to behaviour) = developing into a new species?
- complete (or near complete) reproductive isolation
- many differences (eg., genetic, reproductive, ecological) and isolating barriers that are independent from the original isolation (the differences don’t need to be the differences that initially caused the separation) = now there are two closely related groups but will not reproduce together
What stages of speciation are most dynamic?
- when the gene flow is not completely reduced, there is still transfer of genes between the two groups and if a species range or ecology changes, the process can be reversed and they can re-combine as one species
- when there is complete (or almost) reproductive isolation, there can still be a secondary contact - if the groups are not isolated anymore - one species could be lost, both could be retained?
What happens if there’s contact after speciation is ‘complete’? ie., they’re not isolated anymore
one species could be lost
both species could be retained
it depends
What are the two types of speciation?
allopatric
sympatric
Define allopatric speciation
speciation that occurs when there is complete geographic isolation and no gene flow
reproductive isolation caused by geographic isolation
Define sympatric speciation
aka ecological isolation
speciation that occurs when there is no geographic isolation and gene flow still exists
eventually caused by ecological isolation
Which type of speciation is most commonly observed? why is the other more difficult to demonstrate?
allopatric is most common
sympatric is tough to demonstrate because there is still gene flow occurring
and, if there’s two closely related species, you’d expect one to ecologically outcompete the other or that because they’re still trading genes, eventually there’d be no difference between them
How do differences between species arise and persist if there’s still gene flow? ie., how does sympatric speciation occur?
this is tougher to demonstrate, but there are multiple forms of reproductive isolation barriers that are not just geographic
ex. premating, postmating and prezygotic, postzygotic
What is a challenge for determining whether speciation is allopatric or sympatric?
just because two species are geographically isolated now, doesn’t mean they have always been = species ranges can change - was there sympatric speciation before allopatric?
it’s tough to know whether or when there was geographic isolation or not
What are three major types of reproductive isolation barriers?
premating
postmating, prezygotic
postzygotic
Describe premating isolating barriers. Give examples of types
these are reproductive isolation barriers that prevent mating between individuals of two closely related species
they are ecological and behavioural barriers
ex. geographical barriers, migrational behavioural changes, food specializations, different mating and reproduction times
What type of speciation have Southern capuchino seedeaters been experiencing? what is driving this?
sympatric driven by premating barriers
Explain how Southern capuchino seedeaters are an example of sympatric speciation
a case study of adaptive radiation where 10 species arose within 1 million years (very fast)
the 10 species are closely related, they have overlapping ranges, they are ecologically similar and breed in the same places at the same time - so how did they speciate?
Explain the study by Turbek et al. on the Ibera and Tawny-bellied seedeaters
this study looked at the sympatric speciation between Ibera and Tawny-bellied seedeaters which have males with very different plumage (females nearly identical), different song calls, but overlapping ranges and breeding times and found evidence for speciation driven by pre-mating isolation
the males of both species reacted aggressively to models representing another male from their own species and producing their own songs = song call and plumage are important traits in species recognition
Was there any genetic differentiation between the Ibera and tawny-bellied seedeaters? What does this tell us about their reproductive isolation?
they found FST outlier values only in 12 gene regions = very genetically similar
3 of these related to pigmentation of plumage
the only real differences genetically were related to plumage colouration, which when connected to the behavioural data, shows that the main thing separating these species is premating barriers = different colours and songs
HAs there been evidence of hybridization between the Ibera and tawny-bellied seedeaters?
no, even though closely related so these are distinct species
What did the authors do to test the behavioural responses of the Ibera and tawny-bellied seedeaters?
they used painted models of the males and played their songs (and used a control, a male from a third closely related species) to determine how males from each species reacted / whether they recognized their own species markers
What did the authors find from the behavioural experiments of the Ibera and tawny-bellied seedeaters?
males reacted most aggressively to models displaying plumage and playing songs from their own species = they are able to distinguish
conclusion: premating barriers (behavioural and genetic) are preventing these two closely related species from mating
Describe postmating, prezygotic isolation barriers and give an example
reproductive isolation barriers that allow mating but prevent fertilization (no potential hybrids produced)
ex. Japanese ground beetles
Explain the study by Sota et al., about 2 closely related species of Japanese ground beetles
the authors looked at the differences between genitalia and reproductive tracts of two closely related, almost identical looking species of Ground beetles
they conducted different matings between the two species to measure the outcomes
What did Sota et al., find in their study about Japanese ground beetles?
the different species were able to mate but were not compatible and could not produce offspring and there were consequences to the individuals in the mating pair (death or damage to reproductive organs)
in the crosses between individuals of different species, females were way more likely to die and/or have damaged reproductive tracts than matings between individuals of the same species
males were also more likely to have damaged copulatory organs between interspecific mating crosses
conclude: these are different species caused by reproductive isolation driven by postmating, prezygotic barriers
Describe postzygotic reproductive isolation barriers and give examples
reproductive isolation barriers that allows mating and production of a hybrid offspring but it is unfit (sterile, nonviable, or both)
unfit because:
- ecological inviability = they are not well suited to their environment
- hybrid inviability = if they die before adulthood, they cannot reproduce
- hybrid sterility = they may survive, but cannot reproduce their own offspring - no gene flow
ex. mules = donkeys x horses
Describe how Heliconius melpomene and Heliconius cydno are examples of postzygotic reproductive barriers
these two species can mate and reproduce offspring, but the hybrid is ecologically unfit
the hybrid has a wing pattern which is not recognized by predators, so it is much more likely to be attacked and not survive to adulthood
this prevents gene flow and speciation
T or F: only one barrier can contribute to isolation
false
Provide and explain an example of multiple barriers contributing to isolation
Drosophila sechellia on the Seychelles islands with closest relatives living on African mainland and other islands in the Indian Ocean
premating isolation:
- geographic isolation (on different islands)
- D. sechellia specializes on toxic morinda fruit which the other species do not consume = this is where they reproduce, they would not overlap with the others
postzygotic isolation:
- if they did mate, the male offspring are sterile
Why is it difficult to study the genetic basis of hybrids?
they’re often dead or sterile
How do hybrid incompatibilities arise and evolve? what is the genetic basis?
the best organizing principle we have to explain this is based on the Bateson-Dobzhansky-Muller model of Hybrid Incompatibility
What does the Bateson-Dobzhansky-Muller model of Hybrid Incompatibility state?
over time, incompatibilities accumulate and later ones may not have anything to do with the initial processes for the speciation
- There’s one species with AABB
- there’s some isolation, splitting the species into two groups and individual mutations arise = AaBB in one group, AABb in the other
- in the first group, a becomes fixed and replaces A = aaBB. In the second group, b becomes fixed and replaces B = AAbb
- if individuals from the two groups mate and produce a hybrid offspring with AaBb, this combination may not be compatible
What did the study by Bikard et al., look at?
they studied variation within a single species (A. thaliana) by looking at strains to determine whether the differences could give rise to a new species
What did the study by Bikard et al., find?
they found that individual evolution of the same gene in different strains of a single species (A. thaliana) caused incompatibilities in hybrids
one caused death in hybrid offspring
they found that this was because the genes were duplicates
Which combination of which 2 genes did Bikard et al., find were the cause of incompatibility in A. thaliana?
when plants were AABB = dead
homozygous for At1g71920 (AA) and homozygous for At5g10330 (BB) there were aborted seeds
Why did the AABB combination in hybrid A. thaliana offspring cause incompatibilities?
At1g71920 and At5g10330 are duplicated genes of HPA1 and HPA2, which code for histidinol-phosphate amino-transferase = essential in histidine biosynthesis
AT1 has a premature stop codon in one strain, but is normal in the other
AT5 has a large deletion in one strain, but is normal in the other
so if both the mutated strains are inherited in one offspring (AABB), histidine biosynthesis is effected and the offspring dies
What could Bikard et al. conclude from their findings about genetic incompatibilities within a single species and potential speciation?
the incompatibilities between the two duplicated genes could cause speciation by introducing a postzygotic reproductive barrier
