Lecture 4: Macroevolution & Phylogenetics Intro - PDF

Lecture 4 Macroevolution & Phylogenetics Intro Chapter 24 pages 536-546, 550-553 Chapter 25 pages 555, 572 - 580.  Origin of Species Microevolution: changes in allele frequencies in a population. Speciation: the process by which one species splits into 2 or more species, i.e....

Lecture 4 Macroevolution & Phylogenetics Intro Chapter 24 pages 536-546, 550-553 Chapter 25 pages 555, 572 - 580.  Origin of Species Microevolution: changes in allele frequencies in a population. Speciation: the process by which one species splits into 2 or more species, i.e., new species. Macroevolution: Evolutionary change above the species level. Includes the origin of a new group of organisms or the cumulative evolutionary change over a long period of time. Evolutionary theory: Must explain how new species originate in addition to how populations evolve.  What Is a Species? Traditionally, MORPHOLOGICAL (body form) differences have been used to distinguish species The BIOLOGICAL SPECIES CONCEPT emphasizes reproductive isolation – The biological species concept states that a species is a population or group of populations of whose members have the potential to interbreed in nature and produce viable, fertile offspring; they do not breed successfully with other populations  Biological Species Concept emphasizes reproductive isolation According to the biological species concept, in order for organisms to be of the same species, they must be able to reproduce with each other. What causes members of a common species to cease being able to reproduce with each other? Reproductive Barriers!  Reproductive Barriers 2 types of Reproductive Isolating mechanisms: 1. Prezygotic barriers (5 types) Impede mating between species (even when ranges overlap) or block the fertilization (union of sperm & ova) if members of different species attempt to mate (zygote never produced) 2. Postzygotic barriers (3 types) Often prevent the hybrid zygote from developing into a viable, fertile adult  5 Prezygotic barriers impede mating or hinder fertilization if mating does occur Habitat isolation: Two species that occupy different habitats within the same area may encounter each other rarely, if at all. Temporal isolation: Species that breed during different times of the day, or seasons, or years cannot mix their gametes. Female will only mate with a male Behavioral isolation: with a certain courtship ritual Courtship rituals that attract mates Breed at different and other behaviours unique to a times of the year species as they enable mate Aquatic vs recognition. terrestrial  5 Prezygotic barriers impede mating or hinder fertilization if mating does occur Mechanical isolation: Mating attempted, but morphological differences prevent its successful completion. Gametic isolation: Sperm of one species may not be able to fertilize the eggs of another -Shells spiral in species. For example, sperm may different directions not be able to survive in the -Genital openings Sperm will not fertilize egg of another species (i.e. unable reproductive tract of females of not aligned to survive in reproductive tract of female of other another species. species, biochemical mechanism …) Reproductive Barriers and Speciation Prezygotic Barrier: Habitat Isolation The group in green is separated from the group in red in space: Therefore they can’t reproduce with each other. They will be exposed to different environments -> Natural selection can select for different adaptive genotypes. Genetic drift will also increase the genotypic differences between the two populations Different mutations will accumulate. This will cause the two population to accumulate genetic differences -> eventually leading to speciation. Prezygotic Barrier: Temporal Isolation Behavioral Isolation Females in the group in green The group in green reproduce select males differently reproduce at a different time than females in the group in red: than the group in red: Therefore, both groups are not Therefore, they can’t reproducing with each other due reproduce with each other. to “mate-choice”. Genetic drift will increase Sexual selection will cause the the genotypic differences accumulation of different traits between the two between the two populations. populations Genetic drift will also increase the Different mutations will genotypic differences between the accumulate. two populations Different mutations will accumulate. This will cause the two population to accumulate genetic differences -> eventually leading to speciation.  Reproductive Barriers Prezygotic Barrier: Mechanistic Isolation Gametic Isolation Due to differences in The sperm of males in the green morphology, individuals in the group can only survive in the females green group cannot physically from the green group (and vice- reproduce with individuals in versa). red. Therefore, both groups are The morphological not reproducing with each differences should increase other. with time. Genetic drift will increase the Genetic drift will also increase the genotypic differences between genotypic differences between the the two populations two populations Different mutations will Different mutations will accumulate. accumulate. This will cause the two population to accumulate genetic differences -> eventually leading to speciation.  3 Postzygotic barriers prevent a hybrid from developing into a viable, fertile adult Reduced hybrid viability: The genes of different parent species may interact in ways that impair the hybrid’s development or survival in its environment Reduced hybrid fertility: Even if hybrids are healthy and Hybrid is frail vigorous, they may be sterile. F2 generation feeble or sterile Hybrid breakdown Some first-generation hybrids are viable and fertile, but when they mate with one another or with either parent species, offspring of the next Mule is robust generation are feeble or sterile. but Sterile Postzygotic Barriers: Reduced Hybrid Viability Reduced Hybrid Fertility Hybrid Breakdown Fertilization occurs Fertilization occurs between Fertilization occurs between individuals from individuals from green and between individuals from green and red group, but red group, but hybrid is green and red group, but hybrids do not develop. sterile. hybrids die after 2 generations. Therefore, Therefore, “technically” “technically” the only the only way these Therefore, “technically” way the individuals individuals can produce the only way these can produce offspring offspring that are not individuals can produce is with members of biological dead ends is offspring that are not their own group. with members of their biological dead ends is own group. with members of their own group. Genetic drift will increase the genotypic differences between the two populations Different mutations will accumulate. This will cause the two population to accumulate genetic differences - > eventually leading to speciation.  The biological species concept has limitations The biological species concept cannot be applied to: – Asexual organisms – Organisms about which little is known regarding their reproduction (ex. Fossils) Other Definitions of Species: 1. The morphological species concept Characterizes a species in terms of its body shape, size, and other structural features (applies to asexual & sexual organisms) 2. The ecological species concept Views a species in terms of its ecological niche 3. The phylogenetic species concept trace phylogenetic history of organisms (comparing DNA sequences)  Mechanisms of Speciation speciation can occur with or without geographic separation Speciation can occur in two main ways: 1) Allopatric speciation, geographic separation of populations restricts gene flow. 2) Sympatric speciation: occurs in geographically overlapping populations when biological factors, such as chromosomal changes and nonrandom mating reduce gene flow.  Allopatric Speciation (Greek: allos = other, patra = homeland) Geographic Separation (ex: mountain ranges, glaciers, land bridges, splintering of lakes, colonization of islands, ie. Galápagos). The significance of the barrier depends on the ability of organisms to move about Once separated, the gene pools may begin to diverge through a number of mechanisms: Mechanism: 1. Different selective pressures in differing environments act on the two populations. 2. Mutations arise. 3. Genetic drift alters allele frequencies.  Evidence of Allopatric Speciation Different species of snapping Before isthmus shrimp - separated by Isthmus of formed, gene flow Panama occurred between populations These species originated 9 to 3 million years ago, when Isthmus of Panama formed and separated Atlantic and Pacific waters Some species live on Atlantic side, others on Pacific side Since the isthmus formed the two populations have been geographically isolated and have evolved into distinct species via natural selection and other evolutionary mechanisms (i.e. genetic drift, mutations arise)  Sympatric (“Same Country”) Speciation In sympatric speciation – Speciation takes place in geographically overlapping populations. Requires the emergence of a reproductive barrier that isolates a subset of a population from the remainder of the population in the same area. Can occur if: – There is a change in *ploidy (We are NOT going to talk about this! more common in plants) – A change in ecology (more common in animals) – habitat isolation – Sexual selection Requires the emergence of a reproductive barrier  Sympatric Speciation: changes in ecology Habitat Differentiation Example: North American maggot fly (Rhagoletis pomonella). The fly’s original habitat was native hawthorn trees. About 200 years ago, some populations colonized newly introduced apple trees. Because apples mature more quickly than hawthorn fruit, the apple- feeding flies have been selected for more rapid development and now show temporal isolation from the hawthorn-feeding maggot flies (= prezygotic restriction) Gene flow restricted →Speciation is underway.  Sympatric Speciation: Changes in ecology Sexual selection Lake Victoria, Africa is home to 600 species of cichlid fishes. Why so may species? Nonrandom mating: Sexual selection – Reproduction is dependent on secondary sexual characteristics. → Eventually, new species may evolve from sexual selection Females will select for male colour – by only selecting males of a particular colour, the females of one species will never mate with the males of the other species and vice- versa –Keeps gene pools of 2 species separate  Macroevolution evolution above the species level. Macroevolution: phenotypic differences are so great that the new species are assigned to different taxonomic categories Evolutionary Novelties can arise from: 1. Gradual Refinement of existing structures ( cumulative change over millions of speciation episodes) 2. Major changes in body can result from changes / mutations in: a) genes that control development b) genes that control the spatial organization of body parts  Macroevolution: Gradual refinement of existing structures for new functions. It is important to recognize that natural selection can only improve a structure in the context of its current utility, not in anticipation of the future. Ex: The fossil record indicates that light bones predated flight. o Therefore, they must have had some function on the ground, perhaps as a light frame for agile, bipedal dinosaurs. o Feathers originally were used for courtship, thermoregulation, or camouflage. Once flight became an advantage, natural selection would have gradually changed the structures to be adapted for flight.  Macroevolution: mutation in genes that control the placement and spatial organization of body parts. The products of one class of genes, the Hox genes, provide positional information in an animal embryo. This information prompts cells to develop into structures appropriate for a particular location. One major transition in the evolution of vertebrates is the development of the walking legs of tetrapods from the fins of fishes. Investigating the Tree of Life A phylogenetic tree is a diagram that represents evolutionary relationships among organisms InvestigatingReproductive the Tree ofBarriers Life Evolutionary Patterns Darwin recognized that the species he observed were the modified descendants of earlier ones. Distinct populations of an ancestral species separate and diverge through time, again, and again. Just as a tree sprouts new branches on old ones and adds rings to a thickening trunk, the pattern of nested similarities among species strongly indicates a process of descent with modification and the accumulating of change. Investigating the Tree of Life Evolutionary Patterns Evolution produces two distinct but related patterns: 1. Patterns of similarities found among species on present day earth. 2. The historical pattern of evolution recorded by fossils Investigating the Tree of Life Evolutionary Patterns Today, an estimated 10 million species inhabit the planet. How do scientists categorize these “patterns” ? Similarities or Differences in: 1. MORPHOLOGICAL characteristics. 2. MOLECULAR characteristics (DNA) These characteristics are used to create a PHYLOGENY: A hypothesis for the evolutionary history of species or group of species. Phylogenies are represented as evolutionary trees/phylogenic trees They are “hypotheses” because they represent the best model, or explanation of the relatedness of organisms on the basis of all the existing data.  Building a Phylogenetic Tree based on morphological characters Phylogenetic trees are inferred by comparison of characters shared among different groups of organisms Characters are the anatomical or physiological features that make up organisms. To be useful a character must must vary among species but not within species. All species contain some character that are shared with other members of their group, some that are shared with members of other groups, and some that are unique.  Phylogenetic tree based on morphological characters... Sometimes taxonomists have placed a species within a genus (or other group) to which it is not most closely related. Why?  Building a Phylogenetic Tree based on molecular characters Molecular phylogeny involves the comparative analysis of the nucleotide sequences of DNA from which evolutionary histories and relationships can be inferred. The process of using molecular data is conceptually similar to the process described earlier for morphological data. These first step after sequencing the DNA is to align comparable sequences. If the species are closely related, they will differ at only one or a few regions. In contrast, distantly related species usually have different bases at many regions and may even differ in length.  Phylogenetic Trees show evolutionary relationships Sister Taxa Phylogenetic trees are groups of organisms that represent hypotheses share a common of how different ancestor that organisms relate to is not shared by any other one another group diagram showing relationships between A lineage organisms that evolved early and remains Starts with most ancestral unbranched is a basal form (1 in diagram) taxon. Each branch point ancestor time *Present day species represents a divergence event or Polytomy: A node on a phylogeny where more splitting apart of a single than two lineages descend from a single ancestral lineage. A polytomy may indicate either group into two descendant that we don't know how the descendent lineages groups. are related or that we think that the descendent lineages speciated simultaneously.  How to read an evolutionary tree? The vertical lines, called branches, represent a lineage, and nodes are where they diverge, representing a speciation event from a common ancestor. Time is also represented, proceeding from the oldest at the bottom (or left) to time the most recent at the top (or right). A group of taxa that includes a common ancestor and all of its descendants is called a monophyletic group. (aka clade) A group that excludes one or time more descendants is a paraphyletic group Test your understanding 3 2 1  How to read an evolutionary tree? It’s about how the branches connect, not where the tips are placed! Another critical point about these trees is that if you rotate the structures, using one of the branch points as a pivot, you don’t change the relationships. These are examples of the same tree  How to read an evolutionary tree? Are mammals more closely related to amphibians or to birds? a) Amphibians b) Birds Mammals are more closely related to birds than they are to amphibians. From the tree we can see that mammals and birds share a common ancestor that evolved after amphibians. Review: Review: Allopatric and Sympatric Speciation: in allopatric speciation, geographic isolation restricts gene flow between populations Reproductive isolation may then arise by natural selection, genetic drift, or sexual selection in the isolated populations Even if contact is restored between populations, interbreeding is prevented In sympatric speciation, a reproductive barrier isolates a subset of a population without geographic separation from the parent species Sympatric speciation can result from polyploidy, natural selection, or sexual selection Macroevolution: evolutionary novelties (modified versions of older structures, evolution in genes that control development, changes in genes that control the placement)  Videos Evolution of the Eye: https://www.youtube.com/watch?v=4SXHMm5I-68 https://www.youtube.com/watch?v=qrKZBh8BL_U Dinosaurs / Birds https://www.youtube.com/watch?v=3_RLz0whDv4