Lesson 4 Chapter 27
Lesson 4 Chapter 27 05260
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Date Created: 01/21/16
Speciation KEYCONCEPTS Speciation occurs when populations of the same species become genetically isolated by lack of gene flow and then diverge from each other due to selection, genetic drift or mutation. Populations can be recognized as distinct species if they are reproductively isolated from each other, if they have distinct morphological characteristics, or if they form independent branches on a phylogenetic tree. Populations can become genetically isolated from each other if they occupy different geographical areas, if they use different habitats or resources within the same area, or if one population is polyploid and cannot breed with the other. When populations that have diverged come back into contact, they may fuse, continue to diverge, stay partially differentiated, or have offspring that form a new species. 27.1 How are species defined and identified? A species is defined as an evolutionary independent population or group of populations. Researchers use several criteria to identify whether populations represent distinct species. The biological species concept uses reproductive isolation as a criterion to identify species. The Morphospecies concept identifies species using distinctive morphological traits. The phylogenetic species concept identifies species as the smallest monophyletic group on the tree of life. You should be able to explain whether human populations would be considered separate species under the biological species, Morphospecies and phylogenetic species concepts. 27.2 Isolation and Divergence in Allopatry Speciation is a splitting event in which one lineage gives rise to two or more independent descendant lineages. Speciation occurs when populations of the same species become genetically isolated by lack of gene flow and then diverge from each other due to selection, genetic drift and mutation. Allopatric speciation occurs when populations diverge in geographic isolation. Geographic isolation occurs through dispersal when small groups of individuals colonize a new habitat, or through vicariance, when large, continuous population become fragmented into isolated habitats. You should be able to design an experiment that would, given enough time, result in the production of two species from a single ancestral population. 27.3 Isolation and Divergence in Sympatry Sympatric speciation occurs when populations diverge genetically despite living in the same geographic area. Sympatric speciation can occur when disruptive selection favors individuals that breed in different ecological niches. Mutations that produce polyploidy can trigger rapid speciation in sympatry because they lead to reproductive isolation between diploid and tetraploid populations. You should be able to evaluate whether your experiment on speciation represents a case of dispersal, vicariance, different habitat use or polyploidy. 27.4 What happens when Isolated Populations come into contact? If gene flow occurs, populations that have diverged may fuse into a single species. If prezygotic isolation exists, populations that come back into contact will probably continue to diverge. Secondary contact can lead to reinforcement – the evolution of mechanisms that prevent hybridization. Gene flow between different species can lead to the formation of hybrid zones that move over time or are stable. In some cases, hybridization between species can create new species with unique combinations of traits. You should be able to predict how a hybrid zone will change over time when hybrid offspring have higher fitness than the parental populations. LEARNING OBJECTIVES Explain the roles that gene flow, selection, genetic drift, and mutation play in the process of speciation. Define, compare, and contract the three “species concepts”. Explain why we need three different sets of criteria to recognize species. Describe the processes of allopatric speciation, sympatric speciation, and polyploidy and give examples of each Predict what will happen when two partially divergent populations come into contact again under various circumstances. 27.1 How are species defined and identified? Speciation is a splitting event that creates two or more distinct species from a single ancestral species. When speciation is complete, a new branch has been added to the tree of life. In essence, speciation results from genetic isolation and genetic divergence. Genetic isolation results from lack of gene flow and genetic divergence occurs because selection, genetic drift and mutation proceed independently in the isolated populations. According to the biological species concept, the main criterion for identifying species is reproductive isolation. This is a logical yardstick because no gene flow occurs between populations that are reproductively isolated from one another. Specifically, if two different populations do not interbreed in nature or fail to produce viable and fertile offspring. Prezygotic Isolation: prevents species from mating with each other. 1. Temporal: breed at different times 2. Habitat: Breed in different habitats. 3. Behavioral: Different courtship displays. 4. Gametic Barrier: matings fail because the egg and sperm are incompatible. 5. Mechanical: Matings fail because the reproductive structures are incompatible. Postzygotic Isolation: The offspring of matings do not survive or reproduce. 1. Hybrid Viability: Do not develop normally and die as embryos. 2. Hybrid Sterility: The offspring mature but are sterile as adults. The criterion of reproductive isolation cannot be evaluated in fossil or species that reproduce asexually. The concept is also difficult to apply when they’re closely related but don’t overlap with each other geographically. Under the MORPHOSPECIES CONCEPT researchers identify evolutionary independent lineages by differences in size, shape and other morphological features. This is because they believe that distinguishing features are most likely to arise of populations are independent and isolated from gene flow. The Morphospecies concept is compelling simply because it is so widely applicable. It is a useful criterion when biologists have no data on the extent of gene flow and it is equally applicable to sexual, asexual and fossil species. Despite that, it has 3 major disadvantages. 1. It may lead to the naming of two or more species when there is only one polymorphic species with different phenotypes, such as the spotted and black morphs of jaguars. 2. It cannot identify cryptic species which differ in traits other than morphology. 3. The morphological features used to distinguish species are subjective, meaning the researches can disagree on what is a large enough difference. The PHYOLOGENETIC SPECIES CONCEPT identifies species based on the evolutionary history of populations. According to Darwin all species link back to a common ancestor and form monophyletic groups, which consist of an ancestral population, all of its descendants and only those descendants. There are several monophyletic groups on any given evolutionary tree. A Synapomorphy is a trait unique to a certain monophyletic group. Under the phylogenetic species concept, species are defined as the smallest monophyletic groups on the tree of life. The phylogenetic species concept has 2 distinct advantages: 1. It can be applied to any population (fossil, asexual or sexual). 1. It is logical because different species have different synapomorphies only if they are isolated from gene flow and have evolved independently. Its distinct disadvantage is that carefully estimated phylogenies are available for only a tiny subset of the population on the tree of life. Critics also say it would lead to too many “new” species being named. In practice, researches use a combination of species concepts to identify evolutionary independent populations in nature. Explain why the criteria invoked by the biological species, Morphospecies and phylogenetic species concepts allow biologists to identify evolutionarily independent groups. Describe the disadvantages of the biological species, Morphospecies and phylogenetic species concepts. Criterion for Advantages Disadvantages identifying species Biological Reproductive isolation Reproductive isolation Not applicable to (don’t breed or produce = evolutionary asexual or fossil viable offspring) independence species. Difficult to assess is populations don’t overlap geographically Morphospecies Morphologically Widely applicable Miss cryptic species distinct populations. and subjective Phylogenetic Smallest monophyletic Widely applicable and Very few phylogenies group on the based on testable currently available. phylogenetic tree. criteria 27.2 Isolation and Divergence in Allopatry Speciation begins when gene flow between populations is reduced or eliminated, causing genetic isolation. Genetic isolation happens routinely when populations become geographically separated. Populations that are geographically isolated are said to be in Allopatry. Geographic isolation can occur through vicariance or dispersal. Dispersal is the moving of individuals to a new habitat and colonizing it, separating themselves from the original group. They then begin to diverge due to mutation, genetic drift and selection. Eventually the two populations are genetically isolated from one another. Dispersal and colonization followed by genetic drift and natural selection, are thought to be responsible for the speciation in Galapagos finches and many other island groups. Vicariance is said to occur when a widespread population becomes fragmented into isolated subgroups by a chance event that physically separates them. This could be a mountain range or river, or even continental shift and climate fluctuations that have occurred again and again on earth. To summarize, a geographic isolation of populations via dispersal or vicariance produces genetic isolation due to interruption of gene flow-the first requirement of speciation. 27.3 Isolation and Divergence in Sympatry When populations or species live in the same geographic area, or at least close enough to one another to make interbreeding possible, biologists say they like in Sympatry. Sympatric speciation occurs even though populations live in the same geographical area. Two events can initiate the process of sympatric speciation: 1. External events such as disruptive selection for extreme phenotypes based on different ecological niches. 2. Internal events such as chromosomal mutations. A key realization in the study of sympatric speciation was that even though sympatric populations are not geographically isolated, they may become reproductively isolated by adapting to different ecological niches via disruptive selection. This can be seen in apple maggot flies and hawthorn maggot flies. While they live close to each other, their response to scents for their specific fruit keep them as separate groups in the same population. Prezygotic isolation is occurring as a result of natural selection for adaptations to two different niches. Polyploidy occurs when an error in meiosis or mitosis results in the doubling of the chromosome number- a massive mutation. 1. Autopolyploid: a mutation results in a doubling of chromosomes number and all come from the same species. Much less common than allopolyploid. 2. Allopolyploid: a mutation results in a doubling of chromosomes number and one chromosome set comes from a different species. Polyploid individuals are reproductively isolated from the original diploid population and thus evolutionary independent, because breeding between diploids and tetraploids generally results in sterile offspring. Compared to the gradual process of speciation by geographic isolation or by disruptive selection in sympatry, speciation by polyploidization is virtually instantaneous. It is fast, sympatric and common. Mechanisms of Sympatric Speciation Process Example Disruptive Selection Genetic divergence is caused by Apple maggot flies that are adapted natural selection for different to breed on apple verses hawthorn habitats or resources. Must be fruits mate on those fruits, so no accompanied by some mechanism interbreeding occurs. of genetic isolation. Polyploidization Genetic isolation is created by the Particularly common in plants formation of polyploid individuals because frequent hybridization that can only breed with one occurs and many mitotic divisions another. prior to meiosis. Autoploidy Gain duplicate chromosome sets A maidenhair fern individual from the same species due to became tetraploid due to an error in chromosomal doubling in meiosis meiosis. or mitosis. Alloploidy Gain duplicate chromosome sets Tragopogon species introduced to from different species due to a North America hybridized and hybridization event followed by formed offspring that became doubling tetraploid & formed new species 27.4 What happens when Isolated Populations come into contact? Three different things can happen when they come into contact again, reinforcement, hybrid zones and speciation by hybridization. Reinforcement: natural selection for traits that isolate populations by selecting differences that evolved when they were isolated from one another. If two populations have diverged extensively and are distinct genetically, it is reasonable to expect that their offspring will have lower fitness than their parents are they aren’t adapted to ANY population as hybrids. If closely related species are sympatric, individuals from the two species are seldom willing to mate with each other. If they are allopatric, then they are often times willing to mate with each other. Hybrid Zones: hybrid offspring are not always dysfunctional. In some cases, they are capable of mating and producing viable offspring that have features that are intermediate between of the two parental populations. A hybrid zone is a geographic area where interbreeding occurs and hybrid offspring are common. Depending on the fitness of hybrid offspring and the extent of breeding between parental species, hybrid zones can be narrow or wide, and long or short lived. Speciation by Hybridization: The hybrid offspring created a third, new species that had unique combinations of alleles from each parental species and therefore different characteristics. Secondary contact of two populations can produce a dynamic range of possible outcomes: fusion of the populations, reinforcement of divergence, founding of stable hybrid zones, extinction of one population or the creation of new species. Possible Outcomes of Secondary Contact between Populations Process Example Fusion of the Populations The two populations freely Occurs whenever populations of interbreed the same species come into contact Reinforcement of divergence If hybrid offspring have low Appears to be common in fruit fitness, natural selection will fly species that occupy the same lead to the prevention of geographic areas interbreeding Hybrid Zone formation An area where hybrid breeding Many stable zones can be occurs that can move and found, Townsends warblers change over time moved over time Extinction of 1 population If one population is better Townsends warblers push competitor for shared resources hermit warblers to this Creation of a new species If combination of genes allows Hybridization between the hybrid to occupy new sunflowers gave rise to new habitats or use new resources species with unique traits.
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