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CHAPTER 3 Natural Selection, Speciation and Extinctions Prepared by Peter Stiling University of South Florida 4/30/2017 Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. 1 CHAPTER 3 Natural Selection, Speciation & Extinctions Outline 3.1 3.2 3.3 3.4 Patterns of natural selection Patterns of speciation The history of life Continental drift, range collapse and geographic distribution of species 3.5 Current patterns of extinction 4/30/2017 Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. 2 Natural Selection, Speciation and Extinctions Peter Ryan and colleagues (2007) • Two islands Inaccessible and Nightingale in the Tristan de Cunha archipelago in the South Atlantic ocean contained separate species of Neospiza inches, whose ancestors had blown there from S. America, 3000km distance. • One small-billed seed generalist and one large-billed seed specialist had evolved independently on each island. Generalists eat a variety of seeds of different plant species. Specialists feed upon seeds of just one species of plant. • This work showed how the formation of species does not always occur via the simplest route. – One large billed species evolved on one island, one small billed species on another island and both species dispersed onto the other island, forming different species. 4/30/2017 Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. 3 We will be covering: • How natural selection can follow four different patterns: directional, stabilizing, balancing and disruptive. • We will examine the species concept and the two main mechanisms of speciation, allopatric and sympatric speciation. • We will briefly examine the history of life on Earth and patterns of origination and extinction of species. • We will discuss the current extinction crisis and current factors endangering life on Earth. 4/30/2017 Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. 4 Natural Selection, Speciation and Extinctions 3.1 Patterns of Natural Selection Natural selection can follow different patterns • Natural selection tends to lead to an increase in the frequency of the allele that confers the highest fitness in a given environment. • Selection will act to maintain a number of alleles in a population if the relative fitness of different alleles changes on a spatial or temporal scale. 4/30/2017 Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. 5 Patterns of Natural Selection Natural selection can follow four different patterns: 1. Directional 2. Stabilizing 3. Balancing 4. Disruptive (given time disruptive selection can lead to speciation) 4/30/2017 Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. 6 Patterns of Natural Selection 3.1.1 Directional Selection Favors phenotypes at one extreme • Directional selection favors individuals at one extreme of a phenotypic distribution that have greater reproductive success in a particular environment. • Directional selection may arise when a new allele is introduced into a population by mutation, and the new allele may confer a higher fitness in individuals that carry it (e.g., the melanic forms of the peppered moth). 4/30/2017 Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. 7 Patterns of Natural Selection Directional Selection Figure 3.1 Graphical representation of directional selection. This pattern of selection selects for a darker phenotype that confers higher fitness, for example in a polluted environment as with the peppered moth, Biston betularia. 4/30/2017 Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. 8 Patterns of Natural Selection Directional Selection Figure 3.2 The study organism and study site of the Grants’s work on natural selection. (a) The Grants focused much of their work on one of the Galapagos Islands known as Daphne Major. 4/30/2017 Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. 9 Patterns of Natural Selection Directional Selection Figure 3.2 The study organism and study site of the Grants’s work on natural selection. (b) The small island of Daphne Major (0.3km2) has a resident population of the medium ground finch, Geospiza fortis. 4/30/2017 Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. 10 Patterns of Natural Selection Directional Selection • The Grants quantified beak size of Galapagos finches by measuring beak depth (a measure of the beak from top to bottom) • They compared beak sizes of parents and offspring by examining broods over many years. • They discovered the depth of the beak was inherited by offspring from parents regardless of environmental conditions. • Beak depth is a heritable trait. 4/30/2017 Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. 11 Patterns of Natural Selection Directional Selection In 1978, a drought reduced seed abundance on Daphne Major, leaving only large seeds. Birds with larger beaks were more likely to survive because they were better at breaking open the large seeds. In the year after the drought the average beak depth of birds in the population increased almost 10%. 4/30/2017 Figure 3.3 Variation in the beak size of the medium ground finch, G. fortis on Daphne Major in 1976 and 1978. Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. 12 Patterns of Natural Selection 3.1.3 Stabilizing Selection favors intermediate phenotypes • Stabilizing selection favors the survival of individuals with intermediate phenotypes. • The extreme values of a trait are selected against • An example of stabilizing selection involves clutch size, the number of eggs laid, in animals. • Too many eggs dooms the chicks to starvation. • Too few eggs results in fewer genes passed on. 4/30/2017 Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. 13 Patterns of Natural Selection Stabilizing Selection Figure 3.4 Graphical representation of stabilizing selection. Here the extremes of a phenotypic distribution are selected against while individuals with intermediate traits are favored. These graphs illustrate stabilizing selection in clutch size of birds. 4/30/2017 Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. 14 Patterns of Natural Selection 3.1.3 Balancing Selection promotes genetic diversity • Balancing selection is a type of natural selection that maintains genetic diversity in a population. • In balancing selection two or more alleles are kept in balance and are maintained in a population over the course of may generations. • Balancing selection does not favor one particular allele in the population. 4/30/2017 Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. 15 Patterns of Natural Selection Balancing Selection • Population geneticists have identified two common pathways along which balancing selection can occur. 1. Heterozygote advantage 2. Frequency-dependent selection 4/30/2017 Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. 16 Patterns of Natural Selection Balancing Selection Heterozygote advantage • • • • 4/30/2017 Classic example is the sickle cell allele of the human β-globin gene. A homozygous individual with two copies of this allele has sickle-cell disease, a hereditary disease which damages blood cells. The sickle cell homozygote has a lower fitness than a homozygote with two copies of the normal and more common β-globin allele. However, in areas where malaria is prevalent, the heterozygote has the highest level of fitness and the sickle cell allele is maintained. Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. 17 Patterns of Natural Selection Balancing Selection Frequency-dependent selection • Positive frequency-dependent selection: common phenotypes have an advantage. Example: Prey are warningly colored, to advertise bad taste or toxicity, the prey gain most benefit where many individuals have warning coloration and the prey is deterred. • Negative frequency-dependent selection: rare phenotypes are favored over common ones. Example: visually searching predators form a search image for the more common color forms of prey. 4/30/2017 Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. 18 Feature Investigation John Losey and colleagues (1997) showed how the existence of green and red color morphs of the pea aphid, Acyrthosiphon pisum, are maintained by natural enemies. • Red morphs are more likely to be eaten by ladybird predators than green morphs. • Green morphs are more likely to be parasitized than red morphs. • The frequency of color morphs in a population therefore changes according to the frequency of parasites or ladybird predators in the area. 4/30/2017 Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. 19 Feature Investigation Figure 3.6 4/30/2017 Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. 20 Patterns of Natural Selection 3.1.4 Disruptive Selection favors the survival of two phenotypes • Disruptive selection favors the survival of individuals at both extremes of a range rather than the intermediate • Disruptive selection is similar but not identical to balancing selection where individuals of average trait values are favored against those of extreme trait values. 4/30/2017 Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. 21 Patterns of Natural Selection Disruptive Selection Figure 3.5 Graphical representation of disruptive selection. In this example, the normal wild-type colonial bentgrass, Agrostis tenuis, is intolerant to metals in the soil. A mutation creates a metal tolerant variety, which can grow in soils contaminated with metals from mining operations. 4/30/2017 Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. 22 Natural Selection, Speciation and Extinctions 3.2 Speciation Occurs where genetically distinct groups separate into species • Over a long time span, disruptive selection can result in speciation. • Two alternative mechanisms by which speciation occurs: – Allopatric speciation – Sympatric speciation 4/30/2017 Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. 23 Speciation Table 3.1 Four different species concepts There are over 20 species concepts, each with its own advantages and disadvantages. Here are four of the more widely accepted species concepts, biological, phylogenetic, evolutionary and ecological. 4/30/2017 Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. 24 Speciation 3.2.1 There are many definitions of what constitutes a species • The biological species concept defines species in terms of interbreeding. • The biological species concept has been used to distinguish morphologically similar yet reproductively isolated species. 4/30/2017 Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. 25 Speciation There are many definitions of species “ Groups of populations that can actually or potentially exchange genes with one another and that are reproductively isolated from other such groups. ” ~Ernst Mayr (1942) 4/30/2017 Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. 26 Speciation There are many definitions of species Figure 3.7 The biological species concept The northern leopard frog, Rana pipiens (left), and the southern leopard frog, R. utricularia (right), appear very similar but are reproductively isolated from each other. 4/30/2017 Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. 27 Speciation There are many definitions of species The biological species concept suffers from at least three disadvantages: 1. For many species with widely separate ranges, we have no idea if the reproductive isolation is by distance only or whether there is some species isolating mechanism. 2. In plants, hybrids often form when parents from two different species are crossed with each other, and sometimes these hybrids can be fertile. This greatly blurs species distinctions. Example: Oaks from different species interbreed and their offspring are themselves viable, capable of reproducing with other oaks. 3. The biological species concept cannot be applied to asexually reproducing species such as bacteria and some plants and fungi, or to extinct species. 4/30/2017 Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. 28 Speciation 3.2.2 Phylogenetic Species Concept advocates species are identified by a unique combination of characters. • This definition incorporates the classic taxonomic view of a species based on their morphological characters and molecular features such as DNA sequences. 4/30/2017 Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. 29 Speciation Phylogenetic Species Concept The disadvantage of the phylogenetic species concept is: • Determining how much difference between populations is enough to call them species. – Using this definition, many currently recognized subspecies or distinct populations would be elevated to species status. – Example: The black racer, Coluber constrictor. – Example: Cotton mouse, Peromyscus gossypinus 4/30/2017 Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. 30 Speciation Phylogenetic Species Concept Figure 3.8 Difficulties with the phylogenetic species concept 4/30/2017 The subspecies of the racer, Coluber constrictor, appear different yet are members of the same species. Modified from Conant, 1975. Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. 31 Speciation Phylogenetic Species Concept Figure 3.9 Difficulties with the phylogenetic species concept The cotton mouse, Peromyscus gossypinus, exists as 6 subspecies. Each subspecies possesses a slightly different coat color which resembles the color of the local habitat and sometimes different lengths of tail and hindfeet which are related climbing ability infordifferent Copyright © The McGraw-Hill to Companies, Inc. Permission required reproduction or habitats display. 4/30/2017 32 Speciation 3.2.3 Evolutionary Species Concept Whereby a species is distinct from other lineages if it has its own evolutionary tendencies and historical fate 4/30/2017 Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. 33 Speciation 3.2.4 Ecological Species Concept Each species occupies a distinct ecological niche, a unique set of habitat requirements (Leigh Van Valen, 1976). • Competition between species is likely to result in each individual species occupying a unique niche. 4/30/2017 Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. 34 Hybridization and Extinction Introduced species can bring about a form of extinction via hybridization. Figure 3.10 (a) Mallard duck, Anas platyrhynchos, (b) New Zealand gray duck, A. superciliosa, and (c) the hybrid between them. 4/30/2017 Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. 35 Speciation 3.2.5 The Main Mechanisms of Speciation Two mechanisms: Allopatric and Sympatric speciation • Allopatric Speciation involves spatial separation of populations by a geographical barrier. • Sympatric Speciation is when members of a species that initially occupied the same habitat within the same range diverge into two or more different species. 4/30/2017 Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. 36 Speciation The Main Mechanisms of Speciation Example of Allopatric Speciation: • Non-swimming populations separated by a river may gradually diverge because there is no gene flow between them. • Aquatic species separated by the emergence of land may undergo allopatric speciation. • Upthrusting of mountains can also divide populations among which speciation occurs. 4/30/2017 Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. 37 Speciation The Main Mechanisms of Speciation Figure 3.11 Allopatric speciation in porkfish. About 3.5 million years ago the Isthmus of Panama arose, separating porkfish into two separate populations with no opportunity for mixing. Since then, genetic changes in each population have led to the formationCopyright of two species, one inInc.the Caribbean and one in the Atlantic. © The McGraw-Hill Companies, Permission required for reproduction or display. 4/30/2017 38 Speciation The Main Mechanisms of Speciation Example of Sympatric Speciation: • Metal tolerant plants in Wales, Agrostis tenuis are starting to show a change in their flowering season. – Over time this population may evolve into a new species that cannot interbreed with the original metal-sensitive species. • Individual herbivorous insects are restricted to individual host plant species. As plants speciate each develops its own species of herbivores. 4/30/2017 Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. 39 Speciation The Main Mechanisms of Speciation • Sympatric speciation is more common in plants than animals because: – Plants commonly exhibit polyploidy (contain three or more sets of chromosomes). – Such changes can result in sympatric speciation. • 4/30/2017 Some insects and about 30 species of reptiles and amphibians are also polyploids. Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. 40