Unit 4, Week 1: Speciation and Biodiversity
Unit 4, Week 1: Speciation and Biodiversity BIOL 1108
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This 4 page Class Notes was uploaded by Skyler Tuholski on Saturday November 21, 2015. The Class Notes belongs to BIOL 1108 at University of Georgia taught by Dr. Farmer in Fall 2015. Since its upload, it has received 336 views.
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Date Created: 11/21/15
11/4, 6, and 9/2015 Speciation and Biodiversity Wednesday, November 4, 12:20 PM Learning Objectives: • Understand the difference between anagenesis and cladogenesis • Understand the 4 modes of speciation in terms of geographic origin • Understand the 4 modes of speciation as classifed by genetic causation • For sympatric speciation, be able to describe the isolating mechanisms responsible for pre-zygotic and post- zygotic reproductive isolation • Clicker Question: What is the most important factor that holds a gene pool of a species together and prevents speciation? ○ Gene flow • Macroevolution- origin of new taxonomic groups (species, genera, families, etc.) • Speciation is the bridge between microevolution and macroevolution ○ Origin of new species ○ Can be gradual or rapid but is too rapid to be well-documented in the fossil record • Patterns of speciation ○ Anagenesis (aka phyletic evolution): accumulation of microevolutionary changes that result in the earlier population is considered a separate species from the later population ○ Cladogenesis (aka branching evolution): Over time, you have several species resulting from a parent species. The formation of a new branch is a macroevolutionary effect. Describes origin of biodiversity • Reproductive isolation ○ Through reproductive isolation, organisms within one group will be more closely related to one another than to organisms outside that group ○ Speciation is the outcome of reproductive isolation over time • Clicker Question: What evolutionary factors are responsible for genetic divergence among populations? ○ Mutation, genetic drift, and selection • Modes of Speciation are ○ classified by geographic origin of reproductive barriers Allopatric speciation: Reproductive barrier resulting from a geographic barrier. This happens in 2 ways: □ Vicariance: divergence of 2 large populations (ex: volcano) □ Peripatric speciation: divergence of small population from the larger ancestral population (ex: colonizing of an island) Parapatric speciation: neighboring populations, between which have a small amount of gene flow, diverge and become reproductively isolated Sympatric speciation: evolution of reproductive barriers within a single, initially random mating pop. ○ Classified by genetic and causal basis: Genetic divergence (allele substitution) Biology Page 1 Genetic divergence (allele substitution) □ Genetic drift □ Natural selection Cytoplasmic incompatibility Cytological divergence □ Polyploidy □ Chromosomal rearrangement Recombinational speciation • Allopatric speciation: reproductive barriers that result from geographic isolation ○ Ex: continental drift, isthmus of Panama, mountain range formations, canyons, rivers ○ By Vicariance: Grand canyon split a population of ancestral squirrels into 2 species b/c they were separated by canyon ○ By Peripatry: Some of the population separates from the majority of the parent population and then diverges into a separate species Leads to Adaptive Radiation: organisms diversify to fill different niches □ Ex: Darwin's finches. Key innovation is some variable trait that can allow access to a niche (beak shape) • Parapatric Speciation: ○ Ex: ancestral species of salamander was in WA, but some migrated. As they migrated southward, habitats varied. Adjacent populations could interbreed, but more distant populations could not: RING SPECIES • Sympatric speciation: genetic isolation and divergence occurs despite lack of geographic barrier ○ Ex: white mice are less visible out in the field than in the low lit forest. So mice that live in forest become darker in fur color ○ Ex: Cichlid fish in different African lakes ○ Ex: feed same species of fruit flies and feed them different food sources. When they reunite, the flies who lived off the same food source were more likely to mate: POSITIVE ASSORTATIVE MATING- like mates with like Test for positive assortative mating: character displacement. Describes characters that are more divergent in populations that live sympatrically than in allopatric populations. ○ Sympatric Reproductive Isolation: Prezygotic isolation: potential mates never meet, possibly due to □ behavior differences: mating signals, color differences □ temporal isolation: nocturnal/diurnal, seasonal cycles □ mechanical isolation: cannot physically reproduce b/c the sexual organs are incompatible □ Gametic mortality: gametes fail if not implanted into the proper species (ex: pollen) Postzygotic isolation □ Zygote mortality: forms but dies soon after fertilization dies □ Hybrid inviability: hybrid dies during development □ Hybrid sterility: hybrid survives but can't reproduce (Ex: F horse + M donkey = sterile mule) □ Low hybrid fitness: hybrid survives but is not very successful • Cryptic species: different species with nearly identical morphology that usually live in the same habitat. They are reproductively isolated based on different behaviors • Clicker Question: What is the best explanation for the presence of 12 subspecies of the giant tortoise endemic to the Galapagos islands? One colonization from a small tortoise from South America gave rise to the 12 giants of Galapagos Biology Page 2 ○ One colonization from a small tortoise from South America gave rise to the 12 giants of Galapagos • Sexual Selection: Female mate choice is important cause of speciation ○ To test if sexual selection influences speciation, compare species diversity of sister groups to see if sexual selection is related to increased species diversity • Modes of Abrupt Speciation ○ Polyploidy: failure of cell division that fails to separate chromosomes into 2 cells. Lead to accidental formation of something other than a diploid organism. Organism is more likely to succeed if it has an even # ploidy. Most common in plants Ex: Wheat is doubly allopolyploid (6N) □ 3 different parental species ○ Hybridization: progeny of hybridization has characteristics that allow it to take advantage of a different niche and may become isolated from the parental forms ○ Chromosomal rearrangement: inversion that reproductively isolates these individuals Often in small populations subject to severe stress. Two breaks in a single chromosome cause inversion, deletion, or ring structure. AKA saltation diversification ○ Recombinational speciation: F1 hybrids produce a great variety of recombinant offspring, some of which may be fertile but reproductively isolated from the 2 parent species. Very common in plants, not common in animals. (Ex: sunflowers) • Tempo of evolution ○ Coelcanth is the link between fish and 4 legged animal. Related to lungfishes and tetrapods. Thought to have been extinct for 65 million years ○ However the species was found off the coast of South Africa in 1938: Lazarus taxon (thought to have been dead but we found it!) ○ View 1: Evolution is gradual and slow ○ View 2: Punctuated equilibrium: something that very suddenly gives rise to new species ○ Both views are right! • Clicker Question: Which of the following statements best describes when evolution has occurred? ○ A population has a different allele frequency in this generation compared to the last generation • Species Concept ○ What are the essential attributes of a species that define it? ○ 2 meanings of species: "Family" used to classify plants "Relatedness" describes evolutionary relatedness ○ Goals in classifying species 1) to enable us to classify organisms systematically 2) that a species corresponds to discrete groups of similar, closely related organisms 3) to help us understand how these discrete clusters of organisms arise in nature 4) to represent products of evolutionary history and/or 5) to apply to the largest possible variety of organisms ○ Species is the smallest independent unit of evolution ○ All modern species fall under a lineage concept ○ At least 23 different species concepts ○ The essence of speciation is the lack of gene flow. Species are distinct b/c they don't have gene flow with each other Biology Page 3 • Species Concepts Examples ○ Typological: a species is a group whose members share certain characteristics that distinguish them from other species Conformed members of a species to a "type" Essentialist notion: there is an ideal form that is imperfectly imitated by other earthly forms The type specimen defines the species Doesn’t work with evolutionary change Who gets to decide what the ideal type is? Clicker Question: What is the fundamental problem with the typological species concept? □ The type specimen may not represent the whole species □ Morphological variation exists within a species □ Speciation is an ongoing process ○ Darwin states that species is an artificial categorization for our convenience Qualitatively there is no difference between varieties and species, it is just a continuum He states that varieties are connected present day, and species were connected in the past ○ Morphological: species is a group of organisms with similar anatomical characteristics Preferred for plants and asexually reproducing organisms Problems: □ sometimes morphological criteria are arbitrary □ What about sympatric species? They live in the same location and are very similar but are different lineages? □ Cryptic species have different genes but are phenotypically very similar □ What about sexual dimorphism/ morphology changes at different stages of life? ○ Biological: species is defined as naturally occurring groups of individuals that actually or can potentially interbreed and produce viable offspring Involves development of reproductive barriers and isolation First concept to emphasize the process by which species arise Can only be used on contemporary populations. Inferences cannot be drawn over a time span Difficult to apply this concept to hybrid species (Ex: Lonicera fly, Pomarine skua, Mariana mallard) Only applies to sexually reproducing organisms. What about asexual? Parthenogenic? How can you apply this to bacteria, which can change their genome? Despite the problems, this concept is applicable to many species and is the most used concept Biology Page 4