Survey
* Your assessment is very important for improving the workof artificial intelligence, which forms the content of this project
* Your assessment is very important for improving the workof artificial intelligence, which forms the content of this project
Speciation and Extinction Definitions • Speciation – The origination of new species • Extinction – The death of all organisms in a species • Adaptations – Specific morphological changes as a result of environmental pressures • Taxonomy – Discipline that assigns names to organisms and classifies biological diversity (Linnaeus binomials) What is a species? What is a species? Revisiting a few species concepts Morphological Biological Phylogenetic Morphological Species Concept Species can be distinguished from each other by morphological traits “Classical species concept” Morphological SC limitations High plasticity Morphological SC limitations Undetected speciation African bush elephant African forest elephant Biological Species Concept Groups of interbreeding natural populations that are reproductively isolated from other such groups Biological SC limitations Asexual reproduction Biological SC limitations Spatially disjunct populations/species: -Reproductively isolated due to spatial segregation, but may be reproductively compatible -Lab tests Biological SC limitations Hybridization: different species often hybridize naturally (very common in plants) Coulter pine Jeffrey pine Biological SC limitations Fossils: impossible to know for sure if similar looking species interbreed Phylogenetic Species Concept “the smallest diagnosable cluster of individual organisms within which there is a parental pattern of ancestry and descent” (Cracraft 1983) Clusters must be: 1) Monophyletic (ancestor + all descendants) Phylogenetic Species Concept “the smallest diagnosable cluster of individual organisms within which there is a parental pattern of ancestry and descent” (Cracraft 1983) Clusters must be: 1) Monophyletic (ancestor + all descendants) 2) derived through an evolutionary process of descent from an ancestral lineage 3) diagnosable through examination of derived characters Phylogenetic Phylogenetic tree Phylogenetic SC limitations Can inflate the number of species if species designations are based on a few characters The genetics of a species may not always fully explain the differences in species. Variations below the species level: Subspecies - morphologically and genetically different populations – Geographically isolated – Trinomial Latin name Lodgpole pine (Pinus contorta) Distribution of subspecies Contorta Latifolia Murrayana Variations below the species level: Ecotype – genetic subpopulations adapted to specific physical environmental factors (interfertile with other ecotypes of same species) – Differences are genetically based – Differences may be morphological, physiological or phenological – Ecotypes occur in distinct habitats – Genetic differences are adaptive (enhance survival/reproduction) Example: Achillea experiment (Clausen et al. 1948) How does a new species “emerge”? • New species evolve from preexistent species • Speciation processes – Cladogenetic speciation – Reticulate speciation – Phyletic speciation new species originate from a splitting event in which each branch is a species Time Speciation Processes Cladogenetic speciation new species results from interbreeding of two existing species Time Reticulate speciation ancestral species transforms into a single descendant species Time Phyletic speciation From White 2008 Time Time Why could it be difficult to distinguish past cladogenetic and phyletic speciation? When can we say speciation happens? • When two populations of one species become sufficiently different so they are no longer the same species (this will partly depend on the species concept that you choose) • The divergence of an ancestral species into 2 or more daughter species requires genetic change among populations Brief genetics review • Diploid organisms have 2 homologous copies of each chromosome (2n) • Allele – one of two or more alternative forms of a gene located at a single point (locus) on a chromosome -Homozygote: alleles same -Heterozygote: alleles different • Populations contain individuals with different alleles Mechanisms of genetic differentiation • Mutation • Genetic drift • Natural selection • Gene flow Mutation – Random changes in DNA – Source of all new alleles – If beneficial, allele frequency increases – Negative & neutral mutations also occur Genetic Drift – Change in allele frequencies that occurs entirely from chance – Largest effects in small, isolated populations Jaguars Example Genetic drift 1. Genetic drift acts faster and has more drastic results in smaller populations 2. Can reduce genetic variation in populations 3. Genetic drift can contribute to speciation Natural Selection Genetic traits that enhance the survival and reproduction (fitness) of carriers relative to other individuals in the population will increase in frequency over time Peppered moth, Manchester, UK www.ibri.org/Books/ Pun_Evolution/Chapter3/3.2.htm Gene Flow Movement of alleles within a population or between populations caused by the dispersal of gametes or offspring How does gene flow affect the progression of natural selection? Adaptation and gene flow • Niche of species not fixed • Expect adaptations at periphery of distribution • So, why don’t the peripheral populations of all species adapt to local conditions resulting in continual expansion of range? Deer Mice, Geographic Variation Geographic modes of speciation Allopatric Sympatric Parapatric Allopatric speciation Geographic isolation cuts off gene flow between populations and generates reproductive barriers Allopatric speciation modes: – Vicariance – Jump dispersal Allopatric speciation: Vicariance Dispersal barrier forms isolating populations of the species e.g. rising sea level, plate tectonics, mountain building, glaciation, climate change From White 2008 Allopatric speciation: Vicariance Gondwanaland breakup and Ranidae family Madagascar India Allopatric speciation: Jump dispersal A few individuals cross a preexisting barrier and start a new isolated population 1 2 3 From White 2008 Allopatric speciation: Jump dispersal 1 2 3 From White 2008 Allopatric speciation: Jump dispersal Galapagos tortoises Video Dispersal or Vicariance? • Vicariance usually results in multiple species diverging at the same time • Dispersal only one (or a few) species diverging at the same time Allopatric speciation What if populations get together again? Fail to interbreed or produce fertile descendants Interbreeding produces fertile hybrids and backcross to parental populations Interbreed but hybrids are less fit so natural selection acts against the hybrids (reinforcement) Sympatric speciation Populations of ancestral species overlap extensively throughout population differentiation From White 2008 Sympatric speciation Cichlids in Lake Malawi, eastern Africa Parapatric speciation Overlap occurs only partially & in zone of marginal fitness (i.e. isolating forces operating) Sympatric and parapatric speciation How to overcome gene flow in a population? Fitness 1. Disruptive selection: selection that favors the extreme traits in a population Trait Apple flies Apple Hawthorns Sympatric and parapatric speciation How to overcome gene flow? 2. Chromosomal changes (e.g. polyploidy) Gen 1 2n fuse 3n Gen 2 n 3n fuse 4n n Allopatric vs. Sympatric speciation • Allopatric speciation likely most predominant • Sympatric/parapatric speciation likely occurs more commonly than suspected but hard to detect Speciation is successful…what now? Adaptive Radiation – divergence of a taxon into a number of different forms and adaptive zones E.g. silversword plant family Hawaiian Honeycreepers Adaptive radiation, trophic specialization Extinction! “The extinction of species and of whole groups of species, which has played so conspicuous a part in the history of the organic world, almost inevitably follows on the principle of natural selection; for old forms will be supplanted by new and improved forms.” - Darwin Extinction • All species eventually go extinct • Over 99.9% of all species that have existed are now extinct • Counterbalances speciation Animals Driven to Extinction Passenger pigeon Great auk Dodo Dusky seaside sparrow Aepyornis (Madagascar) Old magazine illustration of hunters shooting Passenger Pigeons (Ectopistes migratorius). Note the density of the flight. (From copy in Schorger, 1955.) Historical range of passenger pigeon. The most similar species in existence today is probably the mourning dove (Zenaida macroura) Two kinds of extinction • Background extinction – Continuously operating – Low rate of extinction • Mass extinction – High rate of extinction (~75%) – Short period of time (100,000s-millions years) – Affects many lineages Mass Extinction Events K-T Background Extinction Foote 2003 Few possible causes of mass extinctions • Ice Ages – Rapid change in climate – Large areas uninhabitable • Asteroids – Catastrophic damage – Debris/climate • Volcanoes (massive volcanoes) – Change in climate via particulates & SO2 Five or six mass extinctions? Dramatic changes in fossil record Eg – Cretaceous-Tertiary event ("K-T extinction") Until 1980s - climate P-Tr O-S Late D Tr-J K-T K-T extinction 1980 – Asteroid hypothesis 1. Rock strata enriched with Iridium 2. Selective extinctions 3. Crater 6th mass extinction Rate of extinctions • Magtoday < Magprior • Ratetoday = Rateprior Causes of 6th Mass Extinction • • • • • • • Habitat destruction Habitat fragmentation Pollution Overharvesting Invasive species Climate change Human overpopulation