L4 Geographic distribution Flashcards
What are hybrid zones?
Where genetically distinct populations meet, mate and reproduce
What is polyploidy speciation?
When a new individual has a different number of chromosomes than its parents, so cannot mate with parent group
Why are hybrids good to study when understanding speciation?
Involved in nearly all speciation events and have a range of genotypes that the different selection pressures that separate the taxa
What is a cline?
Change in the frequency of an allele from one geographical point to another
How do European hedgehogs exhibit hybridisation?
Two main species, originated during the ice age when populations were isolated, then moved out of refugee after ice retreated, populations were isolated for a significant amount of time, and hybrid zones occur where the groups cross over
What are the different consequences for hybrid zones?
indefinite
merge
become reproductively isolated
Third species evolves
How may hybrid zones be indefinite?
Two different populations remain, some interaction occurring in the hybrid zone, all remain the same
Why may hybrid zones merge?
Populations may join up across a hybrid zone if there is no detriment to the fitness of either population and interbreeding is high
When does reproductive isolation occur?
If hybrids are unfit with interbreeding then selection will act to keep the two populations apart.
What shape is the cline when populations are not interacting?
Get a steep, or stepped cline
What does a cline with lots of gene flow look like?
Shallow
What cline characteristics can we use to tell us about the mixing of populations?
Shape
co-occurence
movement
What controls the shape of the cline?
Dispersal and how much selection is acting on the alleles
What happens as dispersal increases?
Cline gets shallower
What is an example of a wide cline?
Blood groups in humans - AB/B has the B allele, which occurs at a low frequency. Was brought up through Europe with the Roman Empire, covers an entire continent so has a very shallow slope
What happens if selection is acting on alleles?
One allele is more likely to survive in each environment, and selection will act to keep it that way. Strength of selection depends own the strength of the allele
What does the cline look like under strong selection?
Steep
What is heterozygote disadvantage?
Disadvantage to be heterozygote, have a reduced fitness
What is a tension zone?
High selection against hybrids where there are two homozygous populations, and heterozygotes are less fit. Hybrid numbers will be kept down
What does the cline in a tension zone look like?
Steep
Give an example of a tension zone
Rock pocket mice Two different morphs, black and beige, living on two different sand types. Are under high predation pressure Mutation caused in 4 different amino acids . Selection acts to keep the morphs in the correct environments, would struggle to survive in opposite coloured sand
What is a primary hybrid zone?
Natural selection alters alleles in a continuous population Covers a vast amount of area and changes alleles seen depending on the environment. Different clines are seen across the transect
What is a secondary hybrid zone?
Species that spent a significant amount of time being allopatric move out of refugee and meet again - all clines happen at that one point.
What did the study into hybridisation in the horse mouse show?
Looked at 105 sites in 1800 mice, at allonym sites, that they assumed were neutral, and at x linked site, which were assumed to be under selection. Found a strong barrier to gene flow, with a steep cline. Even neutral alleles are influenced by selection in the esteem
