LS1 Week Two Notes
LS1 Week Two Notes Life Sciences 1
Popular in Life Sciences 1 - Evolution, Ecology, and Biodiversity
Popular in Life Science
This 11 page Class Notes was uploaded by Annita Kasabyan on Monday April 11, 2016. The Class Notes belongs to Life Sciences 1 at University of California - Los Angeles taught by Kane in Fall 2015. Since its upload, it has received 19 views. For similar materials see Life Sciences 1 - Evolution, Ecology, and Biodiversity in Life Science at University of California - Los Angeles.
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Date Created: 04/11/16
Monday Lecture 1. Microevolution a. A change in the relative frequencies of alleles or genotypes in a gene pool over time 2. Mechanisms of Evolution a. Natural selection b. Gene flow c. Mutation d. Genetic drift e. Assertive mating 3. Law of independent assortment a. Gamete diversity 4. HardyWeinberg Equilibrium a. Genotype frequency i. p^2+2pq+q^2=1 1. p^2 = homozygous dominant 2. pq = heterozygous 3. q^2 = homozygous recessive b. Allele frequency i. p+q=1 c. If a population is in equilibrium, then none of the mechanisms of evolution would be occurring Wednesday Lecture 1. Antibiotics – resistance a. Antibiotics work on bacteria, not viruses b. Bacteria form resistance to antibiotics through mutation and then natural selection c. Antibioticresistant germs cause more than 2 million illnesses and at least 23,000 deaths each year in the U.S. Friday Lecture 2. What is a species? a. Species name: first part of name is the genus and the second part of the name is the species (italicized) b. BSC i. Must be reproductively compatible ii. Must produce viable, fertile offspring iii. Doesn’t work with asexual or extinct organisms c. Morphospecies concept i. Members of the same species often look alike but there are exceptions ii. Sexually dimorphic species: male and female individuals look different iii. Polymorphic species: same species, different phenotypes 1. Example: dogs, jaguar same species as black jaguar iv. Members of different species look similar (cryptic species) d. Limitation of BSC i. Ring species: sharing genetic information indirectly e. Ecological species concept i. Members of a species are characterized by ecological niche f. Evolutionary species concept i. Members of the same species share common ancestry 3. Speciation a. How do species form i. The two populations diverge genetically; speciation occurs when the two populations are no longer able to produce viable, fertile offspring ii. Allopatric speciation: geographic isolation 1. Dispersal 2. Vicariance Chapter 22 1. The biological species concept a. Definition of species has been a longstanding problem in biology i. The species must by definition be fluid and capable of changing, giving rise through evolution to new species. How can we define something that changed over time and gives rise to two species from one? b. Species are reproductively isolated from other species i. Whether or not two individuals are members of the same species is a reflection of their ability to exchange genetic material by producing fertile offspring ii. Most widely used and generally accepted definition of a species is known as the biological species concept (BSC) 1. Species are groups of actually of potentially interbreeding populations that are reproductively isolated from other such groups 2. The BSC is more useful in theory than in practice a. Can be difficult to apply i. Takes time and some individuals may not mate if not in their natural habitat b. Therefore, biologists often use a rule of thumb called the morphospecies concept i. Holds that members of the same species usually look alike ii. Today, has been extended to the molecular level iii. Members of the same species usually have similar DNA sequences that are distinct from those of other species iv. “Barcode of Life” has established a database linking DNA sequences to species v. Not always reliable; members of a species may not always look alike, but instead show different phenotypes called polymorphisms (age difference, gender difference, slight color differences) vi. Cryptic species: composed of organisms that had been traditionally considered as belonging to one species because they look similar, but turn out to belong to two species because of distinction at the DNA sequence level 3. The BSC does not apply to asexual or extinct organisms 4. Ring species and hybridization complicate the BSC a. Ring Species: species that contain populations that are reproductively isolated from each other but can exchange genetic material through other, linking populations b. Hybridization: interbreeding between two different varieties or species i. By the BSC, these different forms should be considered one large species because they are capable to reproduce and produce fertile offspring. However, because they maintain their distinct appearances, natural selection must work against the hybrid offspring 5. Ecology and evolution can extend the BSC a. A species can sometimes be characterized by its ecological niche, which is a complete description of the role the species plays in its environment – its habitat requirements, its nutritional and water needs, etc. i. Impossible for two species to coexist in the same location if their niches are too similar because competition would drive out one of the species ii. This observation gave rise to the ecological species concept (ESC), the idea that there is a onetoone correspondence between a species and its niche 1. If two lineages have very different nutritional needs, we can infer on ecological grounds that they are separate species b. Phylogenetic species concept (PSC): the idea that members of a species all share a common ancestry and a common fate i. Requires that all members of a species are descended from a single common ancestor ii. Can be useful when thinking about asexual species, but, given the arbitrariness of the decisions involved in assessing whether or not the descendants of a single ancestor warrant the term “species”, its utility is limited 2. Reproductive Isolation a. Factors that cause reproductive isolation are divided into two categories: i. Prezygotic factors act before the fertilization of the egg; prevent fertilization from taking place 1. Species are often behaviorally isolated, meaning that individuals mate only with other individuals based on specific courtship rituals, songs, or other behaviors a. For example, the prezygotic reproductive isolation of humans and chimpanzees is behavioral 2. Incompatibilities between the gametes of two different species is called gametic isolation a. For example, isolation in plants can take the form of incompatibility between the incoming pollen and the receiving flower, so fertilization fails to take place 3. Mechanical incompatibility: reproduction is prevented due to mismatch of genitalia 4. Temporal isolation: isolation due to time a. For example, members of a nocturnal species will not encounter members of a related species that are active only during the day, isolating them 5. Geographic isolation: isolation due to differences in location ii. Postzygotic factors act after the fertilization of the egg; result in the failure of the fertilized egg to develop into a fertile individual 1. Genetic incompatibility a. Example: difference in number of chromosomes b. The more closely related a pair of species, the less extreme the genetic incompatibility between their genomes 3. Speciation a. Speciation is a byproduct of the genetic divergence of separated populations i. Over a period of time, these separated species will adapt to their new environments, giving rise to new genes, therefore creating two different species that can no longer reproduce together ii. Speciation is a gradual process, so there can be species that are partially reproductively isolated, meaning that they are not yet a separate species but the genetic differences between them are extensive enough that the hybrid offspring they produce have reduced fertility compared to offspring of the same population b. Allopatric speciation is speciation that results from the geographical separation of populations i. Because genetic divergence is gradual, we find populations that are partially evolved, called subspecies c. Dispersal and vicariance can isolate populations from each other i. Dispersal: individuals colonize a distant place away from the main population 1. Peripatric speciation: a few individuals from a mainland population disperse to a new location and evolve separately, resulting in an island population – literal island or a patch of land remote from the mainland population’s habitat a. Suggests that the island population changes faster because of genetic drift since the population is a lot smaller and because the new habitat may be different from the mainland’s habitat – leading to speciation ii. Vicariance: a geographic barrier arises within a single population, isolating them from each other 1. Sea levels rise, forming new islands, and species become separated 2. This type of speciation is easier to study because we can date the time at which the population was separated if we know when the vicariance occurred iii. Adaptive radiation: unusually rapid evolutionary diversification in which natural selection accelerates the rates of both speciation and adaptation 1. Occurs when there are many ecological opportunities available for exploitation d. Sympatric populations – those not geographically separated – may undergo speciation i. For speciation to occur sympatrically, natural selection must act strongly to counteract the homogenizing effect of gene flow e. Speciation can occur instantaneously i. Instantaneous speciation, speciation that occurs in one generation, is caused by hybridization between two species in which the offspring are reproductively isolated from both parents 4. Speciation and Selection a. Speciation can occur in the presence or absence of natural selection, and natural selection does not always lead to speciation i. For example, speciation can occur entirely from genetic drift, with no role of natural selection ii. Ways in which natural selection is involved in speciation 1. Sympatric speciation required some form of disruptive selection, as when hybrid offspring are completely inferior 2. Allopatric speciation may be facilitated by natural selection a. For example, when a peripheral population is in a new environment, natural selection will act to promote its adaptation to the new conditions b. Natural selection can enhance reproductive isolation i. Natural selection contributes directly to the process of speciation when individuals better at choosing mates from their own group are selectively favored over those that frequently mat with members of the “wrong” group ii. Reinforcement of reproductive isolation is the process by which diverging populations undergo natural selection in favor of traits that enhance pre zygotic isolation, thereby preventing the production of less fit hybrid offspring iii. Speciation is caused by the accumulation of genetic differences between populations, therefore making mutation a key component of the process 1. However the mutation needs to be fixated into the population by selection if it is advantageous and by genetic drift Chapter 23 1. Evolution produces two distinct but relate patterns a. First is the nested pattern of similarities found among species on presentday earth b. Second is the historical pattern of evolution recorded by fossils c. Darwin recognized that the species he observed must be the modified descendants of earlier ones d. History of descent with branching is called phylogeny, and is much like the genealogy that records our own family histories e. Reading a phylogenic tree i. Speciation can be thought of as a process of branching ii. As species proliferate, their evolutionary relationships to one another unfold like a tree pattern, with individual species at the twig tips and their closest relatives connected to them at the nearest fork in the branch, called a node 1. Node represents the most recent common ancestor of the two descendant species f. Phylogenetic trees provide hypotheses of evolutionary relationships i. Phylogenetics, the study of evolutionary relationships among organisms, and taxonomy, the classification of organisms, are the two related disciplines within systematics 1. The aim of taxonomy is to recognize and name groups of individuals as species and to group closely related species into the more inclusive taxonomic group of the genus, and so on up through the taxonomic ranks – species, genus, family, order, class, phylum, kingdom, domain 2. Phylogenetics aims to discover the pattern of evolutionary relatedness among groups of species by comparing their anatomical or molecular features, and to depict these relationships as a phylogenetic tree a. Phylogenetic trees represent the best explanation of the relatedness of organisms on the basis of all the existing data g. The search for sister groups lies at the heart of phylogenetics i. Two species are considered to be closest relatives if they have a common ancestor not shared by any other species or group 1. Sister groups h. A monophyletic group consists of a common ancestor and all its descendants i. Monophyletic groups: all members of the group share a single common ancestor not shared with any other species 1. Examples: amphibians, tetrapods ii. Paraphyletic groups: include some but not all of the descendants of a common ancestor 1. Examples: the reptile group excludes birds even though they share a common ancestor iii. Polyphyletic groups: groupings that do not include the last common ancestor of all members 1. Examples: clustering bats and birds together as flying tetrapods even though they do not share a common ancestor i. Taxonomic classifications are information storage and retrieval systems i. Closely related species are grouped into a genus ii. Closely related genera belong to a family iii. Closely related families form an order iv. Orders form a class v. Classes form a phylum vi. Phyla form a kingdom vii. Kingdoms form a domain 1. Three domains: Eukarya, Bacteria, Archaea 2. Building a phylogenetic tree a. Homology is similarity by common descent i. Phylogenetic trees are constructed by comparison of character states shared among different groups of organisms 1. Characters are the anatomical, physiological, or molecular features that make up organisms. In general, characters have several observed conditions, called character states a. Can be as simple as whether an animal has lungs or not, or as complicated as whether a flower has a certain petal arrangement or different kind b. Character states in different species can be similar for one of two reasons: the character state was present in the common ancestor of the two groups and retained over time (common ancestry) , or the character state independently evolved in the two groups as an adaptation to similar environments (convergent evolution) b. Analogy is similarity by convergence i. Similarities due to independent adaptation by different species are said to be analogous 1. Example: wings are a character exhibited by both birds and bats. Evidence supports the view that wings in these two groups do not reflect descent from a common winged ancestor but rather evolved independently in the two groups 2. Example: unrelated fish that live in freezing water at the poles have evolved similar glycoproteins that act as molecular “antifreeze”, preventing the formation of ice in their tissues c. Shared derived characters enable biologists to reconstruct evolutionary history i. In order to build phylogenetic trees, there needs to be homologies shared by some, not all, of the members of the group under consideration 1. These shared derived characters are called synapomorphies 2. Phylogenetic reconstruction on the basis of synapomorphies is called cladistics d. The simplest tree is often favored among multiple possible trees i. Parsimony: choosing the simpler of two or more hypotheses to account for a given set of observations 1. When using parsimony in phylogenetic reconstruction, assume that evolutionary change is rare e. Molecular data complement comparative morphology in reconstructing phylogenetic history i. Tree construction relies on molecular data because a lot more detail is involved f. Phylogenetic trees can help solve practical problems i. Helps determine the origin of a species, allowing us to understand them better in order to solve problems 3. The Fossil Record a. Fossils provide unique information i. Fossils enable us to calibrate phylogenies in terms of time 1. It is one thing to infer that mammals diverged from the common ancestor of birds, crocodiles, turtles, and lizards and snakes before crocodiles and birds diverged from a common ancestor, but another matter to state that birds and crocodiles diverged from each other about 220 million years ago, whereas the group represented today by mammals branched from other vertebrates about 100 million years earlier ii. Fossils provide our only record of extinct iii. Fossils place evolutionary events in the context of Earth’s dynamic environmental history b. Fossils provide a selective record of past life i. The fossil record of marine life is more completely sampled than that for landdwelling creatures because marine habitats are more likely than those on land to be places where sediments accumulate and become rock ii. Most fossils preserve the hard parts of organism, which usually means mineralized skeletons 1. Clams and snails have excellent fossil records whereas worms and nematodes do not 2. Wood and pollen from plants enter the fossil record far more commonly than do flowers 3. Animals that lack hard parts can leave a fossil record in two ways: a. Many animals leave tracks and trails. These trace fossils preserve a record of both anatomy and behavior b. Organisms can also contribute molecular fossils to the rocks. Molecules, especially lipids like cholesterol, are more resistant to decomposition iii. Burgess Shale: a sedimentary rock formation in British Columbia, Canada, that preserves a remarkable sampling of marine life during the original diversification of animals iv. Messel Shale: a sedimentary rock formation in Germany, preserving fossils that document fish, birds, mammals, and reptiles from the beginning of the age of mammals. Formed more recently than the Burgess Shale c. Geological data indicate the age and environmental setting of fossils i. Lower the layer that the fossil is found, the older it is ii. They mapped out the geological timescale, the series of time divisions that mark Earth’s long history iii. Calibration of the timescale became possible with the discovery of radioactive decay 1. By measuring the amounts of the unstable isotope and its stable daughter inside a mineral, they can determine when the mineral formed 2. Radiometric dating: data using the decay of radioisotopes, including the decay of radioactive C to nitrogen and the decay of radioactive uranium to lead 3. Calibration of the geologic timescale is based mostly on the ages of volcanic rocks that intrude into layers of rock containing fossils d. Fossils can contain unique combinations of characters e. Rare mass extinctions have altered the course of evolution i. Repeatedly during the past 500 million years, animal diversity in the oceans dropped both rapidly and substantially, and extinctions also occurred on land 1. Known as mass extinctions, eliminated ecologically important taxa ad thereby provided evolutionary opportunities for the survivors 4. Comparing Evolution’s Two Great Patterns a. The nested similarity observed in the forms and molecular sequences of living organisms, and the direct historical archive of the fossil record b. Phylogeny and fossils complement each other i. Advantage of reconstructing evolutionary history from living organisms: we can use a full range of features such as skeletal morphology, cell structure, DNA sequence, to generate phylogenetic hypotheses ii. Disadvantage: we lack evidence of extinct species, the time dimension, and the environmental context 1. This is where the fossil record comes into play iii. Biology provides a functional and phylogenetic framework for the interpretation of fossils, and fossils provide a record of life’s history in the context of continual planetary change iv. Agreement between phylogenies and the fossil record provides strong evidence of evolution
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