Macroevolution I: Origin of Species

microevolution

change in the frequencies of genotypes in a population —> leads to the formation of species

species

a group of actually or potentially interbreeding natural populations that are reproductively isolated from other such groups

• this is the biological species concept

problems with the biological species concept

• does not apply to asexual organisms

• can not be applied to fossils

• “potentially interbreeding” is ill-defined, particularly in the case of hybrids

reproductive barriers between species (prezygotic vs. postzygotic)

• prezygotic barriers: barriers that prevent fertilization

• postzygotic barriers: barriers that operate after fertilization occurs

prezygotic barriers

• barriers that prevent fertilization

  • habitat isolation

  • temporal isolation

  • behavioral isolation

  • mechanical isolation

  • gametic isolation

postzygotic barriers

• barriers that operate after fertilization occurs

  • reduced hybrid survivorship

  • reduced hybrid fertility

allopatric speciation

• geographic barriers divides population

• gene flow is cut off

• microevolution takes over

• populations differentiate

  • most prevalent form of speciation

sympatric speciation

• species arise from a connected population

• common in plants, occasional in animals

• generally two types: polyploidy, and behavioral change

sympatric speciation by polyploidy (plants)

• mistake during cell division

• new individual is viable but genetically isolated

• in plants, individual with new ploidy-level can often self-fertilize

sympatric speciation by behavioral change (animals)

• parents separate into new subpopulations in same region

• young return to new place to mate

• no gene flow with original population

autopolyploidy

chromosome duplication in a single species

allopolyploidy

combining chromosomes of two different species

the dobzhansky-muller model

• explains how species become reproductively isolated

  • a subdivided population evolves independently

  • in each lineage, new alleles become fixed at different loci, but they are incompatible

adaptive radiation

relatively rapid evolution of many new, diverse species from a common ancestor under new environmental conditions

coevolution

mutual evolutionary influence between two species occurring in the same environment

• red queen hypothesis: hypothesis that species must constantly adapt, evolve, and proliferate in order to survive while pitted against ever-evolving opposing species.