The Origin of Species Galápagos Islands, landforms newly emerged from the sea, despite their geologic youth, are filled with plants and animals known no-where else in the world, Speciation: The origin of new species in evolution. Microevolution: Evolutionary change below the species level; change in the genetic makeup of a population (gene pool) from generation to generation. Evolutionary theory must also explain how new species originate and develop through the subdivision and subsequent divergence of gene pools. Macroevolution: Evolutionary change above the species level, including the appearance of major evolutionary developments, such as flight, that we use to define higher taxa. Two basic patterns of evolutionary change can be distinguished: anagenesis and cladogenesis (Search for figures). Anagenesis, also called phyletic evolution, is the accumulation of changes that gradually transform a given species into a species with different characteristics. Cladogenesis, also called branching evolution, is the splitting of a gene pool into two or more separate pools, which each give rise to one or more new species. Only cladogenesis can promote biological diversity by increasing the number of species. Objectives: - Exploring the mechanisms by which species arise. - Examining the possible origins of some novel features that define higher taxonomic groups. - Species concept(s); what we really mean when we refer to a species. 1- The biological species concept emphasizes reproductive isolation - Are organisms truly divided into the discrete units we call species, or is this classification an arbitrary attempt to impose order on the natural world? - The results from comparing morphology, physiology, biochemistry, and DNA sequences of different groups of organisms generally confirm that morphologically distinct species are indeed discrete groups, with many differences in addition to morphological ones. The Biological Species Concept - 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 1
offspring, but are not able to produce viable, fertile offspring with members of other populations. - i.e. The biological species concept is based on the potential to interbreed rather than on physical similarity. - Humans and chimpanzees remain distinct biological species because many factors keep them from interbreeding and producing fertile offspring. Reproductive Isolation: - Reproductive isolation is the existence of biological factors (barriers) that impede members of two species from producing viable, fertile hybrids. - The reproductive barriers between more closely related species are not so obvious. - These barriers can be classified according to whether they contribute to reproductive isolation before or after fertilization, prezygotic barriers ( before the zygote ) and postzygotic barriers ( after the zygote ). --- Prezygotic barrier: (prezygotic barrier is a reproductive barrier that impedes mating between species or hinders fertilization of ova if interspecific mating is attempted) - Habitat isolation - Temporal isolation - Behavioral isolation - Mechanical isolation (morphological differences in plant species) --- Postzygotic barrier: - Gametic isolation - Reduced hybrid viability - Reduced hybrid fertility - Hybrid breakdown 2
Limitations of the Biological Species Concept - The number of species to which this concept can be usefully applied is limited. - For example, there is no way to evaluate the reproductive isolation of fossils or asexual organisms such as prokaryotes. (Many prokaryotes do transfer genes by conjugation and other processes, but this transfer is different from sexual recombination. Furthermore, genes are often transferred between distantly related prokaryotes). - It is also difficult to apply the biological species concept to the many sexual organisms about which little is known regarding their ability to mate with different kinds of organisms. For such reasons, alternative species concepts are useful in certain situations. Other Definitions of Species: - Morphological species concept: Defining species by measurable anatomical criteria. - Paleontological species concept: Definition of species based on morphological differences known only from the fossil record. Ecological species concept: Defining species in terms of ecological roles (niches). Phylogeny: The evolutionary history of a species or group of related species. 2- Speciation can take place with or without geographic separation - Speciation can occur in two main ways (depending on how gene flow between the populations is interrupted): --1- allopatric speciation --2- sympatric speciation - (search for figures and examples) Allopatric speciation: A mode of speciation induced when an ancestral population becomes segregated by a geographic barrier or is itself divided into two or more geographically isolated subpopulations. Allopatric ( Other Country ) Speciation In allopatric speciation, gene flow is interrupted when a population is divided into geographically isolated subpopulations. 3
- A river may change course and split a population of animals that cannot cross it. - when individuals colonize a remote area, and their descendants become geographically isolated from the parent population. - the speciation that occurred on the Galápagos Islands following colonization by mainland organisms. - - How formidable (difficult) must a geographic barrier be to keep allopatric populations apart? The answer depends on the ability of the organisms to move about. - Example, allopatric speciation of antelope squirrels on opposite rims of the Grand Canyon. - In contrast, birds and other organisms that can disperse easily across the canyon have not diverged into different species on opposite rims. - Once geographic separation has occurred, the separated gene pools diverge through any or all of the mechanisms described in Chapter 23: different mutations arise, sexual selection takes a different course in the respective populations, other selective pressures act differently on the separated organisms, and genetic drift alters allele frequencies. - In less than 2 million years, the few animals and plants from the South American mainland that colonized the Galápagos Islands gave rise to all the new species now found there. - To confirm a case of allopatric speciation, it is necessary to determine whether the allopatric populations have changed enough that they no longer have the potential to interbreed and produce fertile offspring. - In some cases, researchers evaluate whether speciation has occurred by bringing together members of separated populations in a laboratory setting (look at figures). -The geographic isolation, though obviously preventing interbreeding between allopatric populations, is not in itself a biological isolating mechanism. - Isolating mechanisms which are intrinsic to the organisms themselves prevent interbreeding even in the absence of geographic isolation. - Can divergence of allopatric fruit fly populations lead to reproductive isolation? Sympatric ( Same Country ) Speciation (mechanisms that can produce a new species without geographic isolation from the parent population) In sympatric speciation, speciation takes place in geographically overlapping populations. How can reproductive barriers between sympatric populations evolve when 4
the members remain in contact with each other? Mechanisms of sympatric speciation include chromosomal changes and nonrandom mating that reduces gene flow. 1- Polyploidy Some plant species have their origins in accidents during cell division that result in extra sets of chromosomes, a mutational change that results in the condition called polyploidy. Polyploidy: A chromosomal alteration in which the organism possesses more than two complete chromosome sets. - An autopolyploid is an individual that has more than two chromosome sets, all derived from a single species. For example, a failure of cell division can double a cell s chromosome number from the diploid number (2n ) to a tetraploid number (4n ) (search figures). Autopolyploid: An individual that has more than two chromosome sets, all derived from a single species. - This mutation prevents a tetraploid from successfully interbreeding with diploid plants of the original population the triploid (3n) offspring of such unions are sterile because their unpaired chromosomes result in abnormal meiosis. - However, the tetraploid plants can still produce fertile tetraploid offspring by self pollinating or mating with other tetraploids. Thus, in just one generation, autopolyploidy can generate reproductive isolation without any geographic separation. Allopolyploid: A common type of polyploid species resulting from two different species interbreeding and combining their chromosomes. The allopolyploids are fertile with each other but cannot interbreed with either parental species thus they represent a new biological species. One mechanism for allopolyploid speciation in plants (search for figures or models):. The new species resulted from this mechanism has a diploid chromosome number equal to the sum of the diploid chromosome numbers of the two parent species A hybrid of two different species is usually sterile because its chromosomes are not homologous and cannot pair during meiosis. However, such a hybrid may be able to reproduce asexually. In subsequent generations, through various mechanisms, the sterile hybrid may change into a fertile polyploid (as new species) known as an allopolyploid. - Scientists have documented several speciations of plant species through the origin of new polyploid. 5
- Many important agricultural crops such as oats, cotton, potatoes, tobacco, and wheat are polyploids. The wheat used for bread, Triticum aestivum, is an allohexaploid (six sets of chromosomes, two sets from each of three different species). The first of the polyploidy events that eventually led to modern wheat probably occurred about 8,000 years ago in the Middle East as a spontaneous hybrid of an early cultivated wheat and a wild grass. 2- Habitat Differentiation and Sexual Selection Allopatric and Sympatric Speciation: A Summary Adaptive Radiation 6