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
How can you distinguish between a primary and secondary hybrid zone?
Primary hybrid zone occurs when environmental change causes an allele increase, happens across an area. Secondary occurs when separated, and even neutral allies are affected
What are allozymes?
Different forms of enzymes that do the same job
What is an example of a primary hybrid zone?
Mussels in Long Island sound and surrounding area, Sea is salty and pulls water out of the cells. Mussels have to adapt around it - Remove an amino acid, which helps retain salt and waters. As salinity increases across the sound, more individuals with this allele occur
What is cline asymmetry?
When cline doesn’t move in different directions at the same rate
What is backcrossing?
Mating of a hybrid with one of its parents or an organism with the same genetic characteristics as one of the parents.
What is introgression?
The movement of genes from one species or population to another, by hybridisation and backcrossing - alleles move into another population
What are the consequences of indefinite hybridisation?
Selection can maintain steep clones at some loci, could be a tension zone. Tends to remain only if character differences are favoured by different environments. May be subject to change, will exist in the areas of low density
What are the consequences of merging hybrid zones?
Occurs when fitness of hybrid isn’t lower that original populations, so introgression leads to breakdown of post zygotic reproductive barriers. Variation and distinction between the two populations is lost.
What are the consequences of reproductive isolation in hybrid zones?
Strengthens barriers to gene exchange, so large areas of the genome are protected from introgression,
