Speciation Flashcards
What is a species according to the biological definition?
A population of reproducing organisms that is isolated from other populations
This definition is attributed to Mayr and Dobzhansky.
What are the two main contexts in which species can appear?
- In time along a lineage (e.g. humans/chimps and our common ancestors)
- In space due to geographic isolation (e.g. on an island)
What is allopatric speciation?
A mode of speciation that occurs when populations are geographically isolated
Conditions are slightly different in each geographical location
What is an example of allopatric speciation?
Two different species of antelope squirrels on either side of the Grand Canyon
What is genetic drift?
Random fluctuations in allele frequencies that can cause major changes in small populations, lowering the genetic variation
What are founder effects and bottlenecks?
Events that can lead to a loss of genetic variation in a population
What is sympatric speciation?
Speciation that occurs without geographic isolation, often debated in animals but observable in plants
What is assortative mating?
A mating pattern where individuals mate with others that are phenotypically similar to themselves
What role does temporal isolation play in speciation?
It can lead to reproductive isolation based on differences in breeding times
What is polyploid speciation?
A type of speciation that occurs in plants due to genome duplication events
How long can speciation take?
Up to 5,000,000 years for a single eukaryotic population to become two reproductively isolated populations
Why is estimating the number of species difficult?
Due to massive unknowns, especially in the sea, and variability in plants, fungi, and bacteria
What is mating isolation in the context of the biological species concept?
Lack of interbreeding is key to defining species
Often called isolating mechanisms - selection may be involved directly or indirectly
What are prezygotic barriers?
Factors that prevent mating or fertilization between species
What are postzygotic barriers?
Factors that reduce the viability or reproductive capacity of hybrid offspring
At this stage, natural selection can become involved by reinforcing species differences
Sometimes can produce fertile hybrids but reproductive failure at F2 or afterwards (hybrid breakdown)
What can hybrid zones lead to?
- Fusion of groups (when lots of gene flow)
- Reinforcement of isolation (when some gene flow)
- Stability of hybrid populations (limited gene flow with negligible fitness loss)
What is the role of natural selection in speciation?
Natural selection can reinforce species differences after populations have diverged
What are the different types of selection?
- Directional selection - selects advantageous allele (curve shifts to one side)
- Disruptive selection - selects both extremes (curve dips in the middle)
- Stabilising selection - extremes disappear (curve peaks in the middle)
How do genetic factors play a role in speciation?
- sampling and statistical effects such as drift and bottlenecks, combined with long periods of isolation can lead to speciation
- these random effects do not undermine the power of natural selection
- speciation is not an adaptation, it is a consequence of particular conditions which can involve the byproduct of subsequent selection
What are the types of premating prezygotic barriers?
- Habitat isolation
- Temporal isolation
- Behavioural isolation (mating behaviours e.g. dance, calls)
What are the types of post-mating prezygotic barriers?
- Mechanical isolation = sexual organs are incompatible
- Gametic isolation = sperm and egg don’t ‘recognise’ each other
What are the types of postzygotic barriers?
- Reduced hybrid viability
- Reduced hybrid fertility
- Hybrid breakdown
