mirrored file at http://SaturnianCosmology.Org/ For complete access to all the files of this collection see http://SaturnianCosmology.org/search.php ========================================================== Extinction / You are visitor number / / since 18 June 1999 / ------------------------------------------------------------------------ These are lecture notes from an experimental non-majors biology class taught in 1995 by Bruce Walsh , University of Arizona. Other lectures of potential interest: # Origins of life # Cosmic impacts and life on Earth # Genetics class lecture notes ------------------------------------------------------------------------ # Extinction * 99.9% of all species have become extinct * Without extinction, we would not be here * Mass extinctions vs. background extinction rate # Mass extinctions * many mass extinctions * Cretaceous-Tertiary (KT) extinction o dinosaurs o many marine species # The Cambrian Extinction (500 MYA) * 50% of all animal families went extinct # The Devonian Extinction (345 MYA) * 30% of animal families went extinct # The Permian Extinction (230 MYA) * 50 % of all animal families * 95% of all marine species * all trilobites * many trees # Triassic extinction (80 MYA) * 35% of all animal families # Quaternary (Pleistocene epoch) extinction 20,000 YA * massive extinction of large mammals and birds * Possible causes: o The ice-age o hunting by prehistoric man + idea due to Paul Matrin (here at Arizona) # Causes of extinction: Overexploitation * *Hawaii *: Arrival of Polynesians causes extinction of at least 39 species of endemic land birds * * New Zealand*: Colonization by Polynesians (1000 YA) followed by extinction of Moas and other large flightless birds * Introduction of humans: o 20,000 YA into North America o 30,000 YA into Australia * Recent examples: The great whales # Causes of extinction: Introduction of Pests, Predators and competitors * Rats, house cats: o decimated endemic pacific island birds which are mostly ground-nesting * Lost of mutualists o In Hawaii, many species of the Lobelia tree are endangered, due to the declining populations (extinction) of native nectar feeding birds # Causes of extinction: Habitat destruction # Risks faced by small populations * Demographic stochasticity o random changes in the number of individuals due to random births and deaths * Genetic stochasticity o Genetic drift, the loss of genetic information by chance. o inbreeding depression # Demographic stochasticity * probability of extinction depends on both the population size and fine details of the population demography o sex ratio, age structure, age-specific birth and death rates * General formulae for time to extinction o T = a * Nb # Genetic stochasticity * Genetic drift o fixation of random genes do to small population size o Deleterious genes can be fixed o inbreeding depression # Effective population size, Ne * Rate of drift proportional to 1/Ne * want to maximize Ne in captive populations o Equal sex ratio o equal contribution from all members o More members of a breeding group, the better # Ne with unequal sex ratios * Ne = 4 N*r*(1-r) o N = total population size, r = % female * extreme case Suppose only a single male used o Ne = 4 - 4/n # Short-term Genetic risks faced by small populations * A population of genetically identical individuals is at high risk of disease o Wheat rusts o Cheetahs and FLV * inbreeding depression # Long-term Genetic risks * Populations require a certain amount of genetic variation in order to respond to changes in the environment # Genetic Vs. demographic stochasicity * Once alleles are lost, they cannot be recovered * mutation to reintroduce genetic variation is extremely slow (at least tens of thousands of generation) # Why save species? * genetical resources * ecosystem stability * ethical issues