envs lecture 9 pt 2 Flashcards
2 classes of postzygotic barriers
extrinsic and intrinsic
extrinsic postzygotic barrier
ecological inviability, behavioral sterility
intrinsic postzygotic barriers
hybrid inviability, hybrid sterility
What is low survival of hybrids an example of
postzygotic isolation caused by an extrinsic factor (postzygotic ecological inviability)
describe intrinsic postzygotic barrier
hybrids have high mortality or are partly/entirely sterile, regardless of environment
example of behavioral sterility (postzygotic)
songs of hybrid grasshoppers are intermediates, don’t attract mates
what is hybrid variability reduced by
incompatible interactions among genes from the two populations
how else can hybrid fertility be reduced
by differences in number or structures of chromosomes
example of intrinsic post-zygotic barrier
Dobzhansky-Muller incompatibility
what is Dobzhansky-Muller incompatibility
alleles A1 and B2 and A2 and B2 have never been in the same population, so no selection for them to function well together; may be incompatible
another definition of Dobzhansky-Muller incompatibility
when population is split into 2, A evolves into a in one population, B evolves into b in another population; mutually incompatible
what is Dobzhansky-muller a model of
hybrid incompatibility
what is another definition of Dobzhanksy-Muller
new alleles at diff loci not in same population; haven’t been selected to function well together
another rule associated w/ intrinsic post-zygotic
Haldane’s Rule
Haldane’s rule
hybrid inviability or sterility often limited to heterogametic sex
explain Haldane’s rule
in mammals, males (XY) are heterogametic, male hybrids are often sterile, in birds (ZW) and butterflies females are heterogametic; females often sterile
describe Haldane’s rule in males
in mammals, males (XY) are heterogametic, and male hybrids are sterile
describe Haldane’s rule in females
in birds and butterflies females (ZW) are heterogametic, females are sterile
who is heterogametic in mammals
males
so who is sterile in mammals
male hybrids
who is heterogametic in birds and butterflies
females
so who is sterile in birds and butterflies
females
what is cytonuclear incompatibility
low fitness of hybrids b/w populations w/ most mismatch of nuclear and mitochondrial genes
what is low fitness of hybrids b/w populations caused by (in cytonuclear incompatibility)
genetic mismatch b/w mitochondrial and nuclear genes
why do paternal backcross (cytonuclear incompatibility) have low fitness
most of nuclear genes come from population B, are mismatched to mitochondrial genes from population A
describe offspring of maternal backcross (cytonuclear incompatibility)
most nuclear genes come from same population as mitochondria –> normal, high survival
what is cytonuclear incompatibility
low fitness of hybrids b/w populations w/ most mismatch of nuclear and mitochondrial genes
what do common shrew in siberia have
two chromosomal races that form a narrow hybrid zone
what do chromosomal races in common shrew form
narrow hybrid zone
what do chromosomal races differ by
fusion of single-armed chromosomes (o and g) into double armed chromosomes
what happens in meiosis of hybrids
multiple rearrangements cause a chain of 9 chromosomes to form
what does this chain of 9 chromosomes do
irregular segregation causes unbalanced gametes and low fertility
what does a transect in common shrew show
cline in frequency of Novosibirsk chromosome arrangement less than 9 km wide
what happens to chromosome configuration of either race
cannot increase within populations of the other race,
why can’t chromosome configuration of either race not increase within populations of the other race
b/c meiosis in F1 hybrids produces gametes that lack some chromosomal regions
what kind of isolation evolves faster in flies and fishes
prezygotic isolation evolves faster than post-zygotic
what does the strength of prezygotic and post-zygotic reproductive isolation b/w pairs of populations and species of Drosophila increase w/
amount of time since their lineages split
what is time estimated by
genetic distance b/w each pair
how is strength of pre-zygotic isolation measured
by observing mating between flies in the lab
how is strength of post-zygotic isolation measured
by survival and fertility of hybrid individuals
when does strong pre-zygotic isolation evolve
shortly after isolation (at small genetic distances)
when does strong post-zygotic isolation evolve
only after strong pre-zygotic isolation
at what distance does pre-zygotic isolation appear
relatively small genetic distances
what does value of 0 indicate
pairs are no more isolated than conspecific individuals (same species)
what does value of 1 indicate
complete reproductive isolation
what does strength of prezygotic isolation increase more rapidly b/w
sympatric species
does strength of pre-zygotic isolation increase more rapidly b/w allopatric or sympatric species
sympatric species
what is genetic distance (an index of)
time since divergence
what is seen at small genetic distances
sympatric pairs show strong isolation
do sympatric pairs show strong isolation at small genetic distances
yes
do allopatric pairs show strong isolation at small genetic distances
no
how long do sympatric sister species of Drosophila take to evolve
200,000 years
how long do allopatric species of Drosophila take to evolve
1.1-2.7 million years
do prezygotic barriers always evolve to completion
no; may not evolve to completion
can pre-zygotic barriers become weaker
yes
how can pre-zygotic barriers become weaker
if habitats change
what does increasing turbidity in lake victoria cause
harder for females to see differences in male coloration that is basis for sexual isolation
what is differences in male coloration in fish the basis of
sexual isolation
what happens to fish species that were previously isolated in lake
now hybridizing
is strong postzygotic isolation irreversible
probably
what is strong post-zygotic isolation
complete hybrid sterility
what are prezygotic barriers for these fish
attraction of females to brightly colored males
how does this rapid response reverse
since these are pre-zygotic barriers
what does speciation proceed quickly with
with pre-zygotic barriers
what takes much longer
formation of post-zygotic barriers
what is another way speciation can occur
polyploidy
where is speciation by polyploidy common in
plants (15%)
where is speciation by polyploidy rare in
animals
what does speciation by polyploidy change
changes amount of gene products, phenotypes, environmental tolerances
what does hybridization between diploids produce (in goatsbeards/tragopogon)
tetraploids
describe the tetraploid offspring compared to parents
reproductively isolated
allotetraploids
tetraploids formed by hybridization; a type of polyploidy in which an organism or cell possesses four sets of chromosomes derived from two species
what can differences between a newly formed polyploid and its ancestor in Yarrow confer
ecological differences that could reduce the opportunity for crossing between them
describe flowering time of a newly originated hexaploid (neo-6n yarrow) planted in a dry dune
intermediate b/w those of its tetraploid parent (4n) and existing hexaploid (6n) species
if you think you have one species in a sample but you have two, what might you see in a hardy weinberg test?
no heterozygotes were observed, so no hybridization was occurring, evidence of reproductive isolation. morphological differences.
does speciation occur at same or varying rates
varying rates; sometimes instantaneous, sometimes over millions of years
what mechanisms are involved in speciation
various reproductive isolating mechanisms
can multiple reproductive isolating mechanisms involved in speciation operate simultaneously
yup
describe isolating mechanisms as the basis of speciation
some isolating mechanisms may be the basis of speciation, others may have evolved later
which kind of isolating mechanism evolves faster
pre-zygotic isolating mechanisms
is hybridization between species normal or rare
not unusual
what can hybridization b/w species lead to
formation of stable hybrid zones, introgression, merging of species
what must populations have in order to be considered different species
some level of genetic differentiation
