Origin of Species - Chapter 24 Flashcards
What Darwin said about speciation
“mystery of mysteries”
Darwin believed that the process of natural selection could explain the diversity of life, given enough time.
many definitions of speciation
Speciation, microevolution, macroevolution
speciation - the process of splitting one species into 2 or more
microevolution - change in allele frequencies in a population
macroevolution - broad pattern of evolution above the species level
Biological Species Concept
a definition of species that defines a species as a group that is able to produce healthy offspring with each other and can’t do so with other groups
Speciation in this definition depends on reproductive isolation
speciation has occured when populations become reproductively isolated
Problems with Biological Species Concept
asexual organisms
extinct forms (what could they breed with? can’t determine)
species that hybridize
ring species
It’s difficult to apply this concept to the real world because many species that seem very different have gene flow betwen them.
Why is ambiguity in speciation expected?
nature seldom conforms to neat categories
new species often evolve gradually
not one point in time that you can identify as the moment of speciation
Reproductive isolating mechanisms
Prezygotic barriers to reproduction
prevent gene flow between populations
- habitat isolation - different species living in different habitats, but the same area, will rarely (if ever) encounter each other
- temporal isolation (gametes can’t mix between species who breed during different seasons)
- behavioral isolation (courtship displays) - mate recognition doesn’t occur between speices with different mating rituals, so they won’t interbreed
- mechanical isolation (irish wolfhound&chinese crested dogs) - morphological differences don’t allow successful mating
- gametic incompatibility (esp. for ext. fert. need to separate plants) - sperm can’t fertilize egg of other species
Mechanisms for reproductive isolation
allopatric speciation
sympatric speciation
Prezygotic vs postzygotic barriers to genetic exchange
Prezygotic barriers stop fertilization from occuring
Postzygotic barriers contribute to reproductive isolation after a hybrid zygote is formed.
Reproductive isolating mechanisms
Postzygotic barriers to reproduction
- reduced hybrid viability - mix of hybrid genes hinder survival and development
- reduced hybrid fertility - gametes don’t align and produce sterile offspring (no gene flow)
- hybrid breakdown - hybrids are fertile, but their offspring are not.
Morpohological Species Concept
Based on appearance
morph - shape
Linnaeus
concept relied on by paleontologists
Problems with morphological species concept
cryptic (sibling) speices - virtually identical, but don’t interbreed
ecotypes - look different, but can interbreed
Allopatric Speciation
most common among animals
occurs when two populations become geographically separated (size of geographic barrier depends on the speices ability to move about)
decreased by gene flow, can be fast (20 years worms at Woods)
Natural seleciton, genetic drift, mutation increase
- populations become geographically isolated
- isolation reduces gene flow (not interbreeding)
- genetic divergence of 2 populations
- eventually results in reproductive isolation (new speices!) (even if they come into contact again, they can’t interbreed)
Which mechanisms of evolutionary change increase genetic divergence of isolated populations?
Which tend to prevent divergence?
increase: mutations, natural selection, sexual selection, genetic drift
decrease: gene flow
Sympatric Speciation
speciation without geographical separation
Process: Polyploidy, habitat differntation, sexual selection. Once one of these occurs, a new species will arise if the mechanism contintues
What factors can reduce gene flow to cause sympatric speciation?
Polyploidy, habitat differentiation, sexual selection
Polyploidy
mechanism of sympatric speciation
ploidy = number of sets of chromosomes
results from an increase in chromosome nubmer
common in plants, rare in animals
instant speciation - because chromosomes can’t line up in meiosis (not same number)
results from non-disjunction of chromosomes during meiosis
Autoploidy
unreduced gametes in meiosis (supposed to halve, but didn’t - doubles)
first documented by Hugo DeVries in primrose (several new species of primrose, bigger and larger)
Alloploidy
hybridization follwed by non-disjunction
different species breeding then 1 has autoploidy error
2 rare events, but not necessarily in the same generation
event that initates: 2 different species interbreed, then hybrid reproducing asexually
Hybrids have a low fitness reinforcement in hybrid zones - why unhealthy hybrids would cause selection for individuals that only mated with members of their own population?
unhealthy hybrids are going to cause selection for individuals that only mate with their own population because these offspring will likely survive, whereas the unhealthy hybrids will not survive long enough to produce more offspring or the offspring will also be unhealthy, so theis will reinforce reproductive barriers
“mystery of mysteries”
Darwin believed that the process of natural selection could explain the diversity of life, given enough time.
many definitions of speciation
What Darwin said about speciation
speciation - the process of splitting one species into 2 or more
microevolution - change in allele frequencies in a population
macroevolution - broad pattern of evolution above the species level
Speciation, microevolution, macroevolution
a definition of species that defines a species as a group that is able to produce healthy offspring with each other and can’t do so with other groups
Speciation in this definition depends on reproductive isolation
speciation has occured when populations become reproductively isolated
Biological Species Concept
asexual organisms
extinct forms (what could they breed with? can’t determine)
species that hybridize
ring species
It’s difficult to apply this concept to the real world because many species that seem very different have gene flow betwen them.
Problems with Biological Species Concept
nature seldom conforms to neat categories
new species often evolve gradually
not one point in time that you can identify as the moment of speciation
Why is ambiguity in speciation expected?
prevent gene flow between populations
- habitat isolation - different species living in different habitats, but the same area, will rarely (if ever) encounter each other
- temporal isolation (gametes can’t mix between species who breed during different seasons)
- behavioral isolation (courtship displays) - mate recognition doesn’t occur between speices with different mating rituals, so they won’t interbreed
- mechanical isolation (irish wolfhound&chinese crested dogs) - morphological differences don’t allow successful mating
- gametic incompatibility (esp. for ext. fert. need to separate plants) - sperm can’t fertilize egg of other species
Reproductive isolating mechanisms
Prezygotic barriers to reproduction
allopatric speciation
sympatric speciation
Mechanisms for reproductive isolation
Prezygotic barriers stop fertilization from occuring
Postzygotic barriers contribute to reproductive isolation after a hybrid zygote is formed.
Prezygotic vs postzygotic barriers to genetic exchange
- reduced hybrid viability - mix of hybrid genes hinder survival and development
- reduced hybrid fertility - gametes don’t align and produce sterile offspring (no gene flow)
- hybrid breakdown - hybrids are fertile, but their offspring are not.
Reproductive isolating mechanisms
Postzygotic barriers to reproduction
Based on appearance
morph - shape
Linnaeus
concept relied on by paleontologists
Morpohological Species Concept
cryptic (sibling) speices - virtually identical, but don’t interbreed
ecotypes - look different, but can interbreed
Problems with morphological species concept
most common among animals
occurs when two populations become geographically separated (size of geographic barrier depends on the speices ability to move about)
decreased by gene flow, can be fast (20 years worms at Woods)
Natural seleciton, genetic drift, mutation increase
- populations become geographically isolated
- isolation reduces gene flow (not interbreeding)
- genetic divergence of 2 populations
- eventually results in reproductive isolation (new speices!) (even if they come into contact again, they can’t interbreed)
Allopatric Speciation
increase: mutations, natural selection, sexual selection, genetic drift
decrease: gene flow
Which mechanisms of evolutionary change increase genetic divergence of isolated populations?
Which tend to prevent divergence?
speciation without geographical separation
Process: Polyploidy, habitat differntation, sexual selection. Once one of these occurs, a new species will arise if the mechanism contintues
Sympatric Speciation
Polyploidy, habitat differentiation, sexual selection
What factors can reduce gene flow to cause sympatric speciation?
ploidy = number of sets of chromosomes
results from an increase in chromosome nubmer
common in plants, rare in animals
instant speciation - because chromosomes can’t line up in meiosis (not same number)
results from non-disjunction of chromosomes during meiosis
Polyploidy
mechanism of sympatric speciation
unreduced gametes in meiosis (supposed to halve, but didn’t - doubles)
first documented by Hugo DeVries in primrose (several new species of primrose, bigger and larger)
Autoploidy
hybridization follwed by non-disjunction
different species breeding then 1 has autoploidy error
2 rare events, but not necessarily in the same generation
event that initates: 2 different species interbreed, then hybrid reproducing asexually
Alloploidy
unhealthy hybrids are going to cause selection for individuals that only mate with their own population because these offspring will likely survive, whereas the unhealthy hybrids will not survive long enough to produce more offspring or the offspring will also be unhealthy, so theis will reinforce reproductive barriers
Hybrids have a low fitness reinforcement in hybrid zones - why unhealthy hybrids would cause selection for individuals that only mated with members of their own population?
