species
a population that can breed and produce offspring that can also breed, but it is not able to breed successfully with other populations
speciation
- process by which an existing population gives rise to a new population that no longer has the potential to recombine with it as a single gene pool.
- the key to this is reproductive isolation.
- As long as individuals can share genes freely in a gene pool, that gene pool may change over time, but we would not consider it to be a new species unless it had changed enough that present members of the population would be unable to breed with past members.
reproductive isolation (what is it)
Reproductive isolation can occur as a result many different changes in genetic compatibility that may lower fitness of hybrids or changes in phenotype that may provide a barrier to successful mating. can happen through post or pre zygotic methods.
Post-zygotic isolation
occurs when hybrids are formed but have reduced fitness.
a) - hybrid viability: offspring dies young
b) - hybrid sterility: when you can’t reproduce with another member of the population is not hybrid
c) - Hybrid Fitness: fitness is reduced because it is not going to survive. it is a mix between the best of the both parents but because it is a mix, it is eaten. if poisonous mates with camouflage, it will be half camouflaged and half poisonous. it doesn’t do either well so it gets eaten
prezygotic isolation
prevents the formation of hybrids in the first place
gametic isolation (prezygotic isolation)
sperm and egg can’t form zygote. they just don’t fertilize
mechanical isolation (prezygotic isolation)
parts don’t fit
termporal isolation (prezygotic isolation)
fertile periods don’t match. ex. one animal is only fertile in spring, others only in fall
behavioural isolation (prezygotic isolation)
behaviours don’t match. the mating activity they do is different from the mating that the organism is attracted to so they don’t recognize each other as mates
habitat isolation (prezygotic isolation)
they live so far away that they will never encounter each other. ex. tigers in asia vs lions in africa
allopatric speciation
- Speciation requires reproductive isolation. It usually also involves divergence as the two populations become different phenotypically.
- The standard model of speciation assumes that a gene pool MUST BE physically split before the two sub-populations can diverge enough to speciate.
geographic isolation
causes all gee flow to stop between two subpopulations:
- natural disasters, colonization, fragmentation, physical barrier, continental drift
microevolution
causes two gene pools to diverge from one another.
- genetic drift, natural selection, differential mutation, sexual selection
steps in model
1) geographic
2) microevolution
3) reproductive isolation
types of prezygotic isolation
gametic, temporal, behavioural, mechanical and habitat
sympatric speciation
sympatric (sym = same) speciation, in which reproductive isolation occurs between two sub-groups of a single population before divergence.
divergence
when two populations start to become different (they are still same species). If they diverge enough, they will speciate
