Transcript Speciation
Species and Speciation BIOLOGY Biological Species Concept • A group of organisms that can interbreed and produce viable, fertile offspring • Photo Credit of Galápagos Marine Iguanas: Putneymark, 2008, Wikimedia Commons Biological Species Concept • Assumes – Common characteristics – Genetically compatible – Interbreed under natural conditions – Sexual Reproduction • Photo Credit of Iguana iguana: Ianare, 2007, Wikimedia Commons Drawbacks of Biological Species Concept • Cannot be used with exclusively asexual organisms – Prokaryotes – Amoeba & some other protists – Some animals, plants & fungi • Photo Credit of Amoeba proteus: Angel, BIOL 1407 student, fall 2008 Drawbacks: Isolated Populations • How do you test for interbreeding under natural conditions? • Can a deer in Florida interbreed with one in Wisconsin? • • Photo Credit of Key Deer: Scott Bauer, 2006, USDA Photo Credit of Deer Running: Paul Frank, 2006, USFWS Drawbacks • Can a Great Dane interbreed with a Chihuahua? • Photo Credit: David Shankbone, 2006, Wikimedia Commons Drawbacks • Some species look and behave differently • Can interbreed and have viable, fertile offspring • Example: coyotes, wolves, dogs • Picture Credits: Courtesy of Smithsonian National Zoo @ nationalzoo.si.edu (coyote and wolf); Pam Wolfe (dog) REPRODUCTIVE ISOLATING MECHANISMS in order for one population to become very different from another, they must be reproductively isolated, this means that there will no longer be a free exchange of alleles between the two populations there are Prezygotic mechanisms that prevent mating or fertilization and Postzygotic mechanisms that prevent development of a zygote 1. PREZYGOTIC MECHANISMS a. Ecological Isolation two populations do not exchange alleles with each other because they are in different geographic places or at different places within the same ecosystem eg. cheetah and tiger do not meet because one is in Africa and one in Asia cheetah range tiger range eg. Asiatic lion and Bengal tiger are both in northern India, but lion is on savannah and tiger in forest b. Temporal Isolation two populations do not exchange alleles because they are only available to exchange alleles at different times of year or even of the day eg. morning glory opens its flower at sunrise; cactus opens its flowers at sunset eg. purple finch mates in June so babies have access to lots of berry seeds goldfinch mate in August so that babies have access to lots of thistle seeds c. Behavioural isolation two populations do not exchange alleles because they do not respond to each others mating rituals eg. male grey crickets rub legs at 25 times a second male black crickets at 45 times a second the females of each species only responds to the sound made by the male of that same species Prezygotic Mechanisms: Behavioral Isolation • Different courtship rituals • Blue-Footed Booby Video: http://www.youtube.com/watch?v=4 MPfTzXEZdY • Photo Credit: Richard001, 2007, Wikimedia Commons Behavioral Isolation Eastern and Western Meadowlarks • Isolated by Songs • Listen to both species (Links to songs are below the picture): • http://www.lewis-clark.org/content/content-article.asp?ArticleID=401 d. Mechanical Isolation two populations do not exchange alleles because of some physical barrier that prevents this eg. many insects have modifications on their exoskeletons such that the male and female parts are a perfect 'lock-and-key' fit Prezygotic Mechanisms: Mechanical Isolation • Structural differences prevent mating • Example: Genital openings (arrows) not aligned No Mating eg. orchids are shaped so that only certain beetles can reach the nectar and therefore pick up the pollen, that beetle will the go to another orchid of the same type to deposit the pollen e. Gametic isolation two populations exchange sperm and eggs but chemical markers prevent the eggs from being fertilized by the 'wrong' sperm and so no alleles are exchanged eg. wind blows the pollen of corn onto the flowers of milkweed, but the pollen can not grow down through the stigma because it does not possess the correct enzyme eg. clams and fish both shed eggs and sperm into the same water, but the clam sperm can not penetrate the fish eggs and vice versa, due to enzymes not being able to eat through zona pellucida 2. POST-ZYGOTIC ISOLATING MECHANISMS a. Zygotic mortality - even though the zygote is created, it fails to develop to maturity b. Hybrid inviability- even though the hybrid is born, it does not live long or is not as healthy c. Hybrid infertility - even though the hybrid is healthy and vigourous, it is not able to reproduce eg. donkey X horse mule eg. lion X tiger liger healthy and strong but mules are sterile healthy & strong; but ligers are sterile 3. SPECIATION when two populations become completely isolated and no longer exchange alleles, they are said to have formed separate species a. Allopatric speciation - this is a situation in which the two populations are geographically isolated prior to them becoming separate species the population is isolated and then the changes occur due to new environments Allopatric Speciation Geographic separation Genetic exchange Populations become genetically different Reproductive isolation b. Sympatric speciation - this is a situation where the two populations remain in physical contact with each other but still stop exchanging alleles and become separate species although not separated, the population undergoes different selection pressures and changes occur over time in allele frequencies Sympatric Speciation • Populations not geographically separated • Can be due to rapid genetic changes: – Alter morphology – Alter behavior – Alter habitat preferences Figure 24.5 (a) Allopatric speciation. A population forms a new species while geographically isolated from its parent population. (b) Sympatric speciation. A subset of a population forms a new species without geographic separation. Figure 24.11-4 Species A 2n = 6 Normal gamete n=3 Species B 2n = 4 Meiotic error; chromosome number not reduced from 2n to n Unreduced gamete with 4 chromosomes Hybrid with 7 chromosomes Normal gamete n=3 Unreduced gamete with 7 chromosomes New species: viable fertile hybrid (allopolyploid) 2n = 10 Figure 24.UN03 Ancestral species: Triticum monococcum (2n = 14) Test Your Understanding, question 10 Wild Triticum (2n = 14) Product: T. aestivum (bread wheat) (2n = 42) Wild T. tauschii (2n = 14)