Lecture 4: Geographic distribution Flashcards
hybrid zones:
where genetically distinct populations meet, mate and reproduce
-challenge our view of ‘species’
why are hybrid zones good to study?
- involved in nearly all speciation events
- range of genotypes show genetic differences and selection pressures that separate the taxa
hybrid zones: covers the CLINE between
the two populations of alleles
ability to study hybrid zones has increased over the years as
massive advances in DNA studies have occurred
hybrid zones show historical patterns example:
Hewitt (1999) European hedgehog -2 sub species -- Erinaceus europaeus -- Erinaceus concolor -used mitochondrial DNA you can see recolonisation pattern after ice age
Post glacial recolonisation is known as:
Refugia;
facilitated the persistence of components of biodiversity over millennia and under changing conditions
–>movement of hedgehogs, grasshoppers and bear (& oaks) (all took refugia in south of europe)
consequences of hybridisation:
- can remain indefinitely (pop never rlly mix with each other)
- merge (become 1 again)
- reproductive isolation (hybrids rlly unfit)
- third species
Cline =
(graph) a change in the allele frequency over a geographical transect
we can use cline characteristics to tell us about the mixing of populations
- shape
- co-occurance
- movement
things which influence cline shape:
width of cline from one population to another:
dependant on dispersal (how many genes are flowing between the two populations) and selection (how much selection is acting against those alleles in the two population.)
things which influence cline shape: dispersal
- no dispersal = rigid line (at right angle)
- more dispersal = more gene flow and cline gets wider and flatter
- ABO blood groups in humans
things which influence cline shape: selection
stronger selection = steeper cline
-steeper S
high selection against hybrids known as
tension zone
-heterozygotes DISadvantage
heterozygote DISadvantage
- lower intrinsic fitness than either parental individual
- normally keeps deleterious heterozygotes from appearing
clines appearance in tension zones
v steep
primary hybrid zone
-rock pocket mice
change in the environment where the populations simply change with the environments (mice)
-natural selection alters alleles in a continuers population
-neutral alleles stay the same for both pops
secondary hybrid zone
- -hedgehogs
- formally allopatric species expand to meet
- pops isolated and then meet again
- neutral alleles vary
weird cline behaviour: movement hybrids
-the can move –> if hybrids are less fit, selection will move cline to where density is lower so fewer hybrids produces
weird cline behaviour: asymmetry
Clines move in response to strength of dispersal (may not even swap populations)
-neutral genes can pass through without key ones
– hybrid X homozygouse
(A1A2B1B2 XA1A1B1B1) =introgression
dispersal (gene flow) influences
the shape and location of the cline
introgression=
movement of genes from one species or population into a mother by hybridisation and backcrossing
consequences of hybridisation: indefinite
- selection maintains steep clines at some loci
- could be a tension zone
- only if character differences are favoured by different environments
- could move - area of low density
consequences of hybridisation: Merge:
- fitness of hybrids not lower than the original populations
- introgression and post zygotic reproductive barrier broken down
- variation and distinction between 2 populations lost
consequences of hybridisation: Reproductive isolation
- strengthening of barriers to gene exchange
- large areas of genome protected from introgression
- mechanism controversial –> reinforcement?
consequences of hybridisation: third species
hybrids becomes reproductively isolated from original populations
-new species
