Extinction Extinction was common if not inevitable because species could not always adapt to large or rapid environmental changes. The impact of extinction can conveniently be considered at three levels. Species extinction eliminates an entire species, by an environmental event (flood etc.,) or by biological event (disease or non availability of food resource half or more). Mass extinction eliminates half or more species in a region or ecosystem, as might occur following a volcanic eruption.
Five major mass extinction that occurred since the Cambrian period. This mass extinction is often referred to as K-T extinction. Table . represents the K-T extinction.
frogs, beetles, even if fertilization takes place between two species, due to genetic incompatibility they do not leave any surviving offspring. ii. Hybrid sterility – In this type, hybrids are formed due to inter specific crosses but they are sterile due to the failure of the chromosomes to segregate normally during meiosis, example Mule (inter specific cross between a horse and a donkey). iii.
Hybrid breakdown – F1 Hybrids are viable and fertile, but F2 hybrids may be inviable or sterile.