The Origin of Species The Origin of Species Chapter 24 Chapter 24 The Biological Species Concept Defines a species as a population or group of populations whose members have the potential to interbreed in nature and produce viable fertile offspring Cannot be applied to Asexual organisms Organisms about which little is known regarding their reproduction Speciation Dependent on Reproductive isolation Genetic divergence From Figure 24 14 Reproductive Isolation The existence of biological factors that impede members of two species from producing viable fertile offspring Prezygotic barriers before fertilization sperm egg never unite Postzygotic barriers develops normally but can t reproduce itself Prezygotic Barriers Impede mating between species or prevent fertilization if members of different species attempt to mate Figure 24 3 Prezygotic mechanisms Habitat isolation species rarely if ever encounter each other Temporal isolation species mate at different times of the year Behavioral isolation courtship rituals are unique to a certain species Prezygotic mechanisms Mechanical morphological changes prevent successful mating Gametic isolation sperm of one species cannot fertilize egg of another species Postzygotic Barriers Often prevents the hybrid zygote from developing into a viable fertile adult Figure 24 3 Postzygotic barriers Reduced hybrid viability Genes of the different parent species may interact and impair the hybrid s development Reduced hybrid fertility Even if hybrids are vigorous they may be sterile Hybrid breakdown Some first generation hybrids are fertile but when they mate with another species or with either parent species offspring of the next generation are feeble or sterile Two species of pine are found in the same habitat but release pollen at different times during the year This is an example of isolation 1 geographical 2 ecological temporal 3 4 behavioral 0 of 5 Concept 24 2 Speciation can take place with or without geographic separation Speciation can occur in two ways Allopatric speciation Sympatric speciation Figure 24 5 Allopatric Speciation Other Country Gene flow is interrupted or reduced when a population is divided into two or more geographically isolated subpopulations Single species homogeneo us habitat Geographic barrier isolates populations Genetic drift mutation natural selection cause genetic divergence Barrier removed populations mix but don t interbreed Sympatric Same Country Speciation Single species homogeneous habitat Speciation that takes place in geographicall y overlapping populations Environmental change two habitats populations isolated by habitat Environmental pressure to adapt genetic divergence Sufficient divergence reproductive isolation In cichlid fish Figure 24 12 EXPERIMENT Sympatric speciation has resulted from nonrandom mating due to sexual selection Researchers from the University of Leiden placed males and females of Pundamilia pundamilia and P nyererei together in two aquarium tanks one with natural light and one with a monochromatic orange lamp Under normal light the two species are noticeably different in coloration under monochromatic orange light the two species appear identical in color The researchers then observed the mating choices of the fish in each tank Monochromatic Normal light orange light P pundamilia P nyererei RESULTS CONCLUSION Under normal light females of each species mated only with males of their own species But under orange light females of each species mated indiscriminately with males of both species The resulting hybrids were viable and fertile The researchers concluded that mate choice by females based on coloration is the main reproductive barrier that normally keeps the gene pools of these two species separate Since the species can still interbreed when this prezygotic behavioral barrier is breached in the laboratory the genetic divergence between the species is likely to be small This suggests that speciation in nature has occurred relatively recently Polyploidy Presence of extra sets of chromosomes in cells due to accidents during meiosis Has caused sympatric speciation in many plant species Autopolyploidy individual has more than two chromosome sets all derived from a single species Allopolyploidy species with multiple sets of chromosomes derived from different species Page 495 One mechanism for allopolyploidy Figure 24 11
View Full Document
Unlocking...