BIOl1020 - Week 9.2

microevolution vs. macroevolution

• micro: small changes within a population

• macro: changes at or above species level

• same processes at different time scales

• result of genetic divergence

what is required for speciation to occur and how does it begin

• reproductive isolation is required

• steps:

  • start with one pop.

  • individuals interbreed freely

  • gene flow keeps them similar

  • pop. become separated → gene flow reduced or stops

what happens after populations are separated

• genetic differences accumulate:

  • mutations occur

  • natural selection differs

  • genetic drift acts randomly

how does speciation complete

• differences build over time

• reproductive isolation evolves

  • cannot produce viable or fertile offspring

• populations become distinct → new species

what role does gene flow play in speciation

• gene flow prevents speciation

• no gene flow → divergence → speciation

main modes of speciation

• allopatric speciation: geographic separation

• sympatric speciation: no geographic separation

types of allopatric speciation

• vicariance: population split by barrier

• dispersal: individuals move and become more isolated

• peripatric: small group forms new population

island populations and their importance

• any isolated population

• separated by unsuitable habitat

adaptive radiation

rapid evolution of many species from one ancestor

co-speciation

two groups speciate together due to close interaaction

how does sympatric speciation occur

• without geographic isolation

• often driven by disruptive selection

instantaneous (sympatric) speciation

• occurs in one generation

• caused by chromosome changes (polyploidy)

• offspring cannot breed with parent population

• leads to reproductive isolation

polyploidy and how does it cause speciation

• organisms have more than 2 sets of chromosomes

• causes immediate reproductive isolation → instant speciation

secondary contact and a hybrid zone

• previously separated populations meet again

• hybrid zone: area where hybrids are produced

• outcomes depend on hybrid fitness

possible outcomes of hybridization

• low fitness

• equal fitness

• high fitness

what happens when reproductive barriers break down

• prezygotic and postzygotic barriers are gone

• hybrids are viable and fertile

• gene flow increases

• species boundaries collapse