Species Concept and Speciation PDF

The Origin of Species Speciation - the origin of new species from pre-existing species.  What is a species? (Latin for kind, type) Biological Species: = A set of naturally interbreeding populations that are genetically reproductively isolated from other sets of...

The Origin of Species Speciation - the origin of new species from pre-existing species.  What is a species? (Latin for kind, type) Biological Species: = A set of naturally interbreeding populations that are genetically reproductively isolated from other sets of populations.  Biospecies An array of populations which are actually or potentially interbreeding to produce viable offspring, and which are reproductively isolated from other such arrays under natural conditions So the essence of a species is…gene flow Scandinavian Y chromosome haplotype  Biospecies An array of populations which are actually or potentially interbreeding to produce viable offspring, and which are reproductively isolated from other such arrays under natural conditions  Mechanisms of reproductive isolation Geographic separation:  HYBRID ZONES  Hybridization refers to crosses between genetically differentiated forms.  Introgression refers to the movement of genes between species (or between well-differentiated populations) mediated by backcrossing.  Hybridization is common. Thousands of examples have been documented in animals, tens of thousands in plants. CLOSELY RELATED SPECIES OFTEN EXHIBIT HYBRID ZONES WHEN THEY EXPERIENCE SECONDARY CONTACT Corvus cornix HYBRID ZONE Corvus corone  Ecological differences Similar organisms use environment differently (e.g., time of day, specific locations) and so do not encounter each other http://www.rochester.edu/college/bio/professors/glor  Structural differences in reproductive anatomy http://nathistoc.bio.uci.edu/h Flowers put pollen on ymenopt/Osmia.htm bees in different places. This little bee (Osmia sp.) is able to pollinate two different species of Chinese Houses (Collinsia) because they put the pollen in slightly different places. They may have originally been the same species of Collinsia, but a mutation changed the shape of the flower, so the pollen was not delivered to the female organs in the original species.  Chromosomal differences: mules Some species with different numbers of chromosomes can mate, but produce sterile offspring - the babies can’t make “normal” gametes Zebroids: horses have 66 chromosomes; zebras have 44-64 depending on species Same phenomenon in ducks and geese Chemical differences: pheromones and other scent receptors Anise swallowtail: Tiger swallowtail: attracted to anise and attracted to cottonwoods related plants, and citrus and willows  Behavioral differences Organisms have specific mating behaviors  Species A A Species B B Evolutionary change Evolutionary change Speciation: Lineage Divergence, Lineage followed by evolutionary change. Lineage Divergence  SPECIATION  The process by which one genetically- cohesive population splits into two or more reproductively-isolated populations.  Requires the disruption of gene flow and the evolution of reproductive isolating mechanisms (RIMs).  Speciation When a population becomes reproductively isolated and persists The speciation event is NOT typically a product of natural selection, but some kind of geographic or biologic accident After the event the species may diverge by natural selection or other mechanisms Speciation involves barriers to reproduction Speciation Mechanisms 1) Allopatric Speciation 2) Sympatric Speciation  Allopatric speciation = evolutionary change occurring in different geographic ranges. Ancestral population divides; each can undergo independent evolutionary change.  Allopatric - geographic separation – A population becomes separated – No more gene flow to other populations – Over time, the new population evolves in its own direction A B Allopatric A speciation B Divergence Divergence Mtn. range A A A A Barrier Dispersal Island A A  Allopatric speciation How does the new species become morphologically different from the parent species? – Natural selection – Founder effect – NOT natural selection – Genetic drift – NOT natural selection  Allopatric speciation Natural selection - the new species may adapt differently to new environment or with a different set of mutations – E.g. Darwin’s finches - – Different food plants on different islands meant finches adapted differently  Allopatric Speciation Founder effect : The separated population will have only a piece of the total genetic variation of the species – The mutation for polydactyl cats apparently arose in Southeast England. They were carried to Maine as ship’s cats (for good luck) and were the beginning of the Maine Coon Cat. While the Maine Coon Cat illustrates founder effect, there was also strong selection for polydactyl cats (that’s what the sea captains preferred.  Here’s the Variable oystercatcher of New Zealand. Babies have all-white bellies. Adults are usually solid black, or have a bit of white feathers mixed in on their belly. http://www.tiritirimatangi.org.nz/oystercatcher Here’s the Chatham Island http://blog.doc.govt.nz/2012/02/27/chatham-island-oystercatcher/ Oystercatcher. Only about 250 of these birds live on an island off the coast of New Zealand. All adults have white bellies. Using founder effect, can you explain why these birds look different from the Variable, who they evolved from?  Allopatric speciation Genetic drift - over time the new species accumulates neutral changes in genes – E.g. accumulated changes in isolated populations of cypress trees in the Southwest – Once solid forest, broken into patches as the region’s climate became drier in the post-glacial time. – Cypresses were isolated from those in other patches. Each patch accumulated its own mutations. C-F are all considered the same species  Sympatric speciation = evolutionary divergence occurring in same (overlapping) geographic ranges. Rare in nature, but may occur by: - Initial disruptive selection (e.g., different food sources). - Local ecological niche specialization (e.g., races/ecotypes)  Sympatric speciation Occurs within a population, requires NO geographic separation Can occur from changes in any of the reproductive isolation mechanisms we already talked about: – Chromosomal, structural, ecological, chemical, behavioral Examples of sympatric speciation Chromosomal: wheat – Formed by accidental duplication of chromosomes Structural: Chinese Houses flower Chemical: anise swallowtail Ecological: palms on Lord Howe Island Behavioral: frog songs  Reproductive Isolating Mechanisms Geographic – Continental Drift – Mountain uplifting – Changes in sea level – Changes in climate – Island formation  Reproductive Isolating Mechanisms (Genetic) Polyploidy = evolution of chromosome no. that is multiple of an ancestral set. Hybridization of 2 species followed by polyploidy ----> instant speciation. Polyploid hybrid reproductively isolated from both parents.  Spartina Salt Marsh Grass, Cord Grass S. maritima X S. alterniflora (Europe & Africa, (e. North America, native species) introduced to Europe) 2n = 60 2n = 62 HYBRIDIZATION (+ loss of one chromosome) Polyploid Speciation S. X townsendii 2n = 62 : CHROMOSOME DOUBLING (+ loss of 1 chromosome pair) S. angelica NEW POLYPLOID 4n = 122 SPECIES  Reproductive Isolating Mechanisms (Genetic) PRE-ZYGOTIC (pre-mating) i) Habitat isolation - differences in habitat preference garter snakes: aquatic vs. terrestrial species ii) Temporal isolation - differences in timing of reproduction spotted skunk species: mate in different seasons  Reproductive Isolating Mechanisms (Genetic) PRE-ZYGOTIC (pre-mating) iii) Behavioral (sexual) isolation - differences in behavioral responses with respect to mating mating “dances” of birds differ among species  Reproductive Isolating Mechanisms (Genetic) PRE-ZYGOTIC (post-mating) iv) Mechanical isolation - differences in sex organs, don’t “fit” left- vs. right-handed snail species can’t mate v) Gametic isolation - sperm / egg incompatibility sperm & egg of different sea urchin species incompatible  Reproductive Isolating Mechanisms (Genetic) POST-ZYGOTIC vi) Reduced hybrid viability - embryo doesn’t live. salamander hybrids frail or don’t mature vii) Reduced hybrid fertility - hybrids develop but sterile. horse + donkey  mule: sterile  Reproductive Isolating Mechanisms (Genetic) POST-ZYGOTIC viii) Hybrid (F2) breakdown - F1 fertile, but future generations sterile or reduced fitness hybrid rice plants small, reduced fitness Time for Speciation to occur? Varies, dependent on group. E.g., Spartina angelica hybrid polyploid Ca. 20 years Hawaiian Drosophila spp. (Fruit flies) Average speciation time = 20,000 yrs Platanus spp. (Sycamores) P. orientalis & P. occidentalis separated ca. 50,000,000 years, still not genetically reproductively isolated Tempo and Pattern of Speciation Phyletic gradualism: gradual change form one species to another Punctuated equilibrium: species arise quickly, then stay the same over time  Tempo of Speciation 1) Gradualism (gradualistic speciation) = gradual, step-by-step evolutionary change Evolution of horses  Species showing very little evolutionary change: E.g.: – Coelacanth (Latimeria) - 250 myr, rediscovered 1938 – Horseshoe crab – Dawn-Redwood Tree (Metasequoia) – Maidenhair Tree (Ginkgo)  Tempo of Speciation 2) Punctuated Equilibrium = rapid evolutionary change during speciation followed by relatively long periods of stasis (no change).  Punctuated Equilibrium:  Punctuated Equilibrium:  How can rapid speciation (resulting in punctuated equilibrium) occur? 1) Founder principle or population bottleneck 2) Major environmental change, new niches open up. - both can accelerate evolutionary change  How can rapid speciation occur? 3) Major genetic change: E.g., Change in a gene that regulates development (homeotic / regulatory gene) Hox gene 6 Hox gene 7 Hox gene 8 Ubx About 400 mya Drosophila Artemia  Heterochrony = change in the rate or timing of development Neotony = type of heterochrony: decrease in rate of development  Many features of humans evolved by NEOTONY! Chimp Feature Human NEOTONY å Developmental Time ß  Heterochrony - NEOTONY Chimpanzee fetus Chimpanzee adult Human fetus Human adult Mature human adult resembles fetus of both.  Extinction “Opposite” of Speciation Over 99% of all species on earth are now extinct. E.g., – ammonites – seed ferns – dinosaurs – Irish Elk – dodo bird Extinction is a major driving force of evolution How? Opens up new niches, by removing interspecific competition.  What does each imply about evolution? Phyletic - Punctuated: – Environments change – Environments change gradually on a geologic quickly, then remain time scale, driving stable change in populations – OR geographic isolation – OR genetic drift is occurs quickly responsible for much of – OR reproductive the morphologic change isolation occurs quickly in new species and is accompanied by rapid morphologic change  So which pattern is “right”? Different kinds of organisms show different kinds of patterns Some even show a hybrid - punctuated gradualism – Rapid speciation, followed by gradual change over time – Typical of single-celled organisms that rely less on sexual reproduction