Reproductive isolation It is a mode of speciation through which new species form from a single ancestral species while both species continue to inhabit the same geographical region. Two or more species are involved. New species formed due to genetic modification in the ancestor that is naturally selected can no longer breed with the parent population. Sexual isolation is strongest.
Phenotypic plasticity has emerged as potentially important first step in speciation initiated within an isolated population. A well studied example is the adaptation of Apple maggots that feed on apples in North America. When the apple trees were imported to North America, Apple maggot flies ( Rhagoletis pomonella ) a parasitic insect that normally laid its eggs in the fruit of wild hawthorns until one subset of population began to lay its eggs in the fruit of domesticated apple trees ( Malus XII Std Zoology Chapter XII Std Zoology Chapter Evolution Table . Details of the five major mass extinctions since the Cambrian period Extinction Period Date (Mya) Estimated % of Marine Animal Extinction Genera Species End of the Ordovician End of the Devonian End of the Permian End of the Triassic End of the Cretaceous K-T Extinction refers to the German word Cretaceous and Tertiary periods.
Global extinction eliminates most of the species on a large scale or larger taxonomic groups in the continent or the Earth. Snow ball Earth and extinction following elevation in CO levels are example. Extinction events opens up new habitats and so can facilitate the radiation of organisms that survived the mass extinction.