lec21: species, speciation, and hybridization Flashcards
what are the 4 MICROevolution sources that drive evolutionary change?
natural selection, mutation, genetic drift, migration
what is the “species problem”?
the problem of how best to define a species
what is a species? (hint: traditionally defined by…)
phenotypic similarity
sympatric
When two species live in the same geographical area
allopatric
Two organism that occur in different geographical areas
based on the traditionally definition of species, it fairly easy to identify within a region (___), but problems arise from gradual differences across regions (____)
sympatric and allopatric
other than the phenotypic similarity of species, _____ similarity also used to identify and define species which is a ____ species concept
genetic and phylogenetic (might say they are same or different species)
how different is different enough and how similar is similar enough to be called different or same species… now there’s 2 main species concepts; what are they?
taxonomic (or morphological) and biological
taxonomic (or morphological)
based primarily on distinct measurable differences
biological
based on inter-fertility among individuals
what does this example tell us about universal species concept: bacteria don’t interbreed so we can’t use the biological species concept to define what is a bacterial species
there’s not universal concept that applies to everything. so CONCEPTS VARY AMONG GROUPS OF ORGANISMS: NO universal species concept
for Darwin; species defined as groups of organisms that are sufficiently similar/different in _____
similar in phenotype
whats the biological species concept (BSC) stated by Mayr
defines species as groups of interbreeding individuals that remain reproductively isolated from other such groups
he was defining species not by similarity but by reproductive isolation from other such groups
(1) so if 2 individuals can interbreed with each other, we call them the same species under this definition
(2) if they’ve become reproductively isolated from each other, we call them distinct species
(3) if they’re not able to interbreed from each other, genetic variation can’t be shared
so with reproductive isolation, these populations can continue to diverge without collapsing back into one
BSC helps frame “the species problem” as a tractable research question; it is the most useful species concept we have which leads to the best research on the speciation process. BSC points to highlight (explain each except the last point):
- focuses on the _____
- geographical isolation alone is or is not sufficient?
- isolation does or does not have to be absolute?
- must be possibly interbreeding in the ___
- does not apply well for __, __, highly ____ species, or ___
- focuses on the PROCESS
- geographical isolation alone is NOT sufficient
eg. 2 populations of the same species are far apart and so therefore they’re not interbreeding with each other, so we’re going to call them distinct species - THAT IS NOT the messaging because we want to understand the process of speciation.
maybe there could be changes in climate or gene flow or migration that brings them back together again and if they can still interbreed then they are still the same species so 2 isolated species do not make a species by themselves
- isolation does NOT have to be absolute
you can get some interbreeding between populations at a low level that still keeps the species distinct. talked about Neanderthals interbreeding with humans, we still consider Neanderthals and modern humans as separate species even though there’s some low level of reproductive isolation. so it’s about where we cut things off. what levels of reproductive isolation is large enough that we call them distinct species
- must be possibly interbreeding in the WILD
when defining species as potential to interbreed it has to be in the wild
- does not apply well for bacteria, asexuals, highly self-fertilizing species, or fossils
in terms of “WHERE” does speciation occur, draw a a model for allopatric vs sympatric.
based on the models, allopatric OR sympatric speciation is more common and easier to evolve due to evolution with minimal gene flow
allopatric speciation is more common and easier to evolve due to evolution with minimal gene flow
Allopatric speciation (1) occurs when a species separates into two separate groups which are isolated from one another. A physical barrier, such as a mountain range or a waterway, makes it impossible for them to breed with one another. but other than this case, allopatric speciation can interbreed.
what are the stages when reproductive isolation (RI) can occur?
finding a compatible mate and mating
↓
fertilization
↓
development and growth of zygote (F1)
↓
adult survival and reproduction (F1)
↓
growth, survival, reproduction of offspring (F2)
in the stages of reproductive isolation (RI), where do pre-zygotic RI barriers and post-zygotic RI barriers lie?
finding a compatible mate and mating
↓
fertilization
↓
development and growth of zygote (F1)
↓
adult survival and reproduction (F1)
↓
growth, survival, reproduction of offspring (F2)
(pre-zygotic RI barriers - before the growth of the zygote)
finding a compatible mate and mating
↓
fertilization
↓
———————————————————
(post-zygotic RI barriers - there’s differential survival of the zygote after fertilization)
↓
development and growth of zygote (F1)
↓
adult survival and reproduction (F1)
↓
———————————————————-
↓
growth, survival, reproduction of offspring (F2)
anything that could go wrong at any of these stages after populations that have been isolated from each other can be a cause of speciation or reproductive isolation.
what causes reproductive isolation (RI); what are the reproductive isolating barriers? (2)
pre-zygotic RI barriers - before the growth of the zygote
post-zygotic RI barriers - there’s differential survival of the zygote after fertilization
pre-zygotic barriers
prevent mating or fertilization so no zygote gets formed
what are 4 pre-zygotic barriers?
- geographical, ecological
- populations might differ in their ecology - temporal, behavioural (mate recognition)
- might differ in their timing of their mating or behavioural traits - mechanical (genital structure compatibility)
- there might be mechanical problems, so there can be evolutionary divergent between species that mean that just mating doesn’t work - cellular (sperm-egg compatibility)
- or cellular problems that are preventing proper fertilization where the sperm and egg are incompatible
what’s are 2 examples of pre-zygotic isolation? explain
look at slide for both
- apple maggot flies: habitat and temporal isolation
- in Abalone
post-zygotic barriers + caused by what?
prevent proper functioning of zygotes once they are formed
caused by combinations of genes with low fitness in the hybrid
what are 2 post-zygotic barriers
intrinsic and extrinsic
intrinsic
Postzygotic barrier that has nothing to do with the environment
when hybrids exhibit lower fitness than either parent, regardless of the environment, due to:
- inviability, sterility, or abnormal development of hybrids
extrinsic
ecological mismatch of hybrid phenotype due to the environment
intrinsic problems with an offspring can never be favoured by natural selection. so how do we get the evolution of a post-zygotic barrier?
easiest to think in allopatry; we have these different populations that have been separated for some time. they’re accumulating changes separately. now if they come back together and interbreed, there might be combinations of those genes that have never been seen before by natural selection; evolution happened over here in one direction, over here in another direction, and now we’re shuffling up those combinations and either by whatever evolutionary force that drove those differences, now that they’re back together and interbreeding, something about those combinations doesn’t work anymore and causes reduced fitness. that reduced fitness could be intrinsic so that the organism is just inviable, it has nothing to do with the environment, it might be sterile or have an abnormal development of hybrids. or it could be extrinsic, there could be an ecological mismatch in the hybrid where this combination that was never seen before is not adaptive.
Does natural selection favour pre or post-zygotic isolation?
Since natural selection acts on one’s survival and ability to pass genes along, it makes sense that pre-zygotic isolating mechanisms would be more prevalent in nature.
because since pre-zygotic barrier prevent the formation of a zygote, it prevents organisms from expending expensive energy on non-viable offspring
can post-zygotic barrier be directly favoured by natural selection? + why
it cannot be directly favoured by natural selection
- arise as an indirect byproduct of evolution acting separately in different populations
natural selection can indirectly influence the evolution of post-zygotic barriers by acting on traits that affect hybrid fitness.so by indirect, it means that natural selection acts on the individual traits of organisms rather than directly on the post-zygotic barriers themselves
so the idea is that reproductive isolation post-zygotically is an incidental byproduct of evolution. it’s not directly under natural selection or even under genetic drift, but whatever evolutionary changes happened in these different populations. because they happen independently, now that they’re back together, it’s incidentally caused reproductive isolation
draw a diagram/model of an intrinsic post-zygotic isolation inherent genetic problems with hybrids
have species A and B interbreeding somewhere along the way either in the F1 hybrid offspring or in the next generations, there’s some reduced ability to survive, there’s lethality and or sterility
what is the mule an example of? + explain
intrinsic post-zygotic isolation
focusing on “how much reproductive isolation happens over time”, explain the relation between genetic distance and post-zygotic isolation in fruit flies.
the more that fly pairs are genetically differentiated, the more likely they are to be reproductively isolated
what is an example of extrinsic post-zygotic isolation: poorly adapted hybrids + explain
the butterflies and their wing coloured pattern
look at slide
what does the butterfly example tell us to answer this question: is adaptive evolution required for speciation? + what is it termed as?
local adaptation by different populations CAN lead to reproductive isolation and speciation
- distinct evolutionary responses to different selective pressure (populations diverging by natural selection will diverge faster than random genetic drift)
- LA not absolutely necessary, but accelerates population divergence and evolution of RI
such as the butterflies that adapted differently from its hybrids due to selective pressure
termed ECOLOGICAL SPECIATION
considering this question: what genes might be important in causing to become reproductively isolated; explain the example of the fish adaptation to freshwater from ocean.
on slide + Eda gene
recall from lec15: what is adaptive radiation
the evolution of ecological and phenotypic diversity within a rapidly multiplying lineage as a result of speciation
- the species differ in traits allowing exploitation of a range of habitats and resources
eg. adaptive radiation of beak morphology in hawaiian honey creepers
adaptive radiation originates from what
a single common ancestor
adaptive radiation process results in what
an array of many species
what are the 4 features that commonly identify an adaptive radiation
- recent common ancestry from a single species
- phenotype-environment correlation
- trait utility
- rapid speciation
what causes adaptive radiation? (3)
- ecological opportunity
- origin of a key innovation
- high rates of speciation characterize the clade
(look at slide for more info)
Hybridization
When two organism from different species make an offspring
def: the exchange of genes between species as a result of occasional inter-species mating
what does neanderthal say about hybridization speciation process
sometimes hybridization can reverse speciation process to merge 2 groups into 1
what can hybridization result in? and especially by what?
can result in complex patterns of variation
- can be evolutionarily significant for speciation, specially by POLYPLOIDY
polyploidy
an organism, tissue, or cell with more than 2 complete sets of homologous chromosomes
the heritable condition of possessing more than two complete sets of chromosomes
what are the 2 types of polyploidy?
- allopolyploidy
- autopolyploidy
allopolyploidy: eg + arises from what
eg: AA x AA → AA AA
arises from duplicated karyotype following hybridization between species
occurs when the individual has more than two copies but from different species.
autopolyploidy: eg + arises from what
eg. AA → AA AA
arises from duplicated karyotype within a species (eg. non-disjunction)
both from the same parental species
which one is the commonest type of polyploidy; autopolyploidy or allopolyploidy
allopolyploidy
draw the polyploid complex including both types as a phylogenic tree
slides
Allopolyploids can generally be distinguished from autopolyploids because they produce a more diverse set of gametes
what are the benefits of hybrid polyploid offspring? (2)
they exhibit novel phenotypes so they can exploit new habitats
they show hybrid vigour (improved traits) due to heterozygosity, mainly seen in allopolyploid organisms
are polyploids reproductively isolated from themselves (each other) or from their diploid parents? … hence a form of sympatric or allopatric speciation?
polyploids are reproductively isolated (the inability of a species to interbreed with related species due to barriers) from their diploid parents because meiosis doesn’t work well if they try to interbreed with their diploid parents
- hence a form of sympatric speciation
polypoids often shows quite high fitness, so even though it’s sort of a massive, drastic mutational event, sometimes it can be favored and reproductively isoalted from their parents
Speciation continuum
A continuous non unidirectional process of changes within species
How do isolating barriers evolve?
isolating barriers evolve from ecological speciation which is a form of speciation arising from reproductive isolation that occurs due to an ecological factor that reduces or eliminates gene flow between two populations of a species.
what are the cons of hybrid polyploid offspring?
offspring are reproductively isolated from their diploid parents
The reproductive isolation between polyploids and their diploid parents is not necessarily a con or a disadvantage; rather, it is a natural consequence of genetic differences. Polyploids have a different number of sets of chromosomes compared to their diploid parents. This difference in chromosome number can lead to difficulties in successful reproduction between the two because the chromosomes need to pair up correctly during processes like meiosis for successful gamete formation.
