Chapter 24 Notes
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This 8 page Class Notes was uploaded by Ozerk Turan on Friday February 12, 2016. The Class Notes belongs to BIL 160 at University of Miami taught by Dr. Paul Groff in Spring 2016. Since its upload, it has received 28 views. For similar materials see Evolution and Biodiversity in Biology at University of Miami.
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Date Created: 02/12/16
Biology Chapter 24 Notes Microevolution vs. Macroevolution o Microevolution – changes over time in allele frequencies in a population Evolution within the species level Three main mechanisms cause allele frequency change Natural selection Genetic drift Gene blow Only natural selection consistently causes adaptive evolution o Macroevolution – the broad pattern of evolution above the species level The origin of new groups of organisms, such as mammals or flowering plants through a series of speciation events Speciation – the process by which one species splits into two or more species Concept 24.1: The biological species concept emphasizes reproductive isolation o Biologists compare morphology, physiology, biochemistry, and DNA sequences when grouping organisms o Biological species concept – according to the concept, a species is a group of populations whose members have the potential to interbreed in nature and produce viable, fertile offspring, but do not produce viable, fertile offspring with members of other such groups Thus, the members of a biological species are united by being reproductively compatible, at least potentially Gene flow between populations (of the same species) holds a species together genetically o Reproductive isolation – the existence of biological factors (barriers) that impede members of two species from interbreeding and producing viable, fertile, offspring Prezyogtic barriers – block fertilization from occurring Impede different species from attempting to mate Preventing the successful completion of mating Hindering fertilization if mating is successful Different prezygotic barriers: Habitat isolation – two species counter each other rarely, or not at all, because they occupy different habitats, even though not isolated by physical barriers Temporal isolation – species that breed at different times of the day, different seasons, or different years cannot mix their gametes Behavioral isolation – courtship rituals and other behaviors unique to a species are effective barriers to mating Mechanical isolation – morphological differences can prevent successful completion of mating Gametic isolation – sperm of one species may not be able to fertilize eggs of another species Postzygotic barriers – barriers that prevent the hybrid zygote from developing into a viable, fertile adult Reduced hybrid viability – genes of the different parent species may interact and impair the hybrid’s development or survival in its environment Reduced hybrid fertility – even if the hybrids are vigorous, they may be sterile Hybrid breakdown – some first-generation hybrids are fertile, but when they mate with each other or with either parent species, offspring of the next generation are feeble or sterile Such barriers block gene flow between the species and limit the formation of hybrids – offspring that result from an interspecific mating o Limitations of biological species concept: The biological species concept cannot be applied to fossils or asexual organisms (including all prokaryotes) The biological species concept emphasizes absence of gene flow However, gene flow can occur between distinct species Grizzly bears and polar bears can mate to produce “grolar bears” o Morphological species concept – concept that distinguishes species by body shape and other structural features The concept can be applied to asexual and sexual organisms, and it can be useful even without information on the extent of gene flow o Ecological species concept – defines a species in terms of its ecological niche, the sum of how members of the species interact with the nonliving and living parts of their environment Example: two species of oak trees might differ in their size or in their ability to tolerate dry condition, yet still occasionally interbreed Because they occupy different ecological niches, these oaks would be considered separate species even though they are connected by some gene flow Emphasizes the role of disruptive selection o Phylogenetic species concept – defines a species as the smallest group of individuals that share a common ancestor, forming one branch on the tree of life Concept 24.2: Speciation can take place with or without geographic separation o Allopatric speciation – gene flow is interrupted when a population is divided into geographically isolated subpopulations A population forms a new species while geographically isolated form its parent population The process of allopatric speciation: The definition of barrier depends on the ability of a population to disperse o For example, a canyon may create a barrier for small rodents, but not birds, coyotes, or pollen Separate populations may evolve independently through mutation, natural selection, and genetic drift Reproductive isolation may arise as a by-product of genetic divergence o For example, isolated populations of mosquitofish have evolved reproductive isolation as a result of selection under different levels of predation Evidence of allopatric speciation Reproductive barriers can develop when laboratory populations are experimentally isolated and subjected to different environmental conditions Allopatric speciation can also occur in nature o Fifteen pairs of sister species of snapping shrimp (Alpheus) are separated by the Isthmus of Panama o These species originated 9 to 13 millions years ago, when the Isthmus of Panama formed and separated the Atlantic and Pacific waters Regions with many geographic barriers typically have more species than do regions with fewer barriers Reproductive isolation between populations generally increases as the distance between them increases Reproductive barriers are intrinsic to the organisms themselves, physical separation alone is not a biological barrier o Sympatric speciation – a subset of a population forms a new species without geographic separation (speciation taking place is geographically overlapping populations) Sympatric speciation can occur if gene flow is reduced by factors including: Polyploidy Sexual selection Habitat differentiation Polyploidy – an accident during cell division that results in extra sets of chromosomes (can produce new biological species in sympatry within a single generation) Autopolyploid – an individual that has more than two chromosome sets that are all derived form a single species Allopolyploid – a species with multiple sets of chromosomes derived from different species Sexual selection can also drive sympatric speciation Sexual selection for mates of different colors has likely contributed to speciation in cichlid fish in Lake Victoria Habitat differentiation Sympatric speciation can also result from the appearance of new ecological niches For example, the North American maggot fly can live on native hawthorn trees as well as more recently introduced apple trees Concept 24.3: Hybrid zones reveal factors that cause reproductive isolation o A hybrid zone is a region in which members of different species mate and produce hybrids o Hybrids are the result of mating between species with incomplete reproductive barriers o Pattern within hybrid zones: A hybrid zone can occur in a single band where adjacent species meet For example, two species of toad in the genus Bombina interbreed in a long and narrow hybrid zone Hybrids often have reduced fitness compared with parent species The distribution of hybrid zones can be more complex if parent species are found in patches within the same region o Hybrid zones over time: When closely related species meet in a hybrid zone, there are three possible outcomes Reinforcement (strengthening reproductive barriers) Fusion (weakening reproductive barriers) Stability (continued formation of hybrid individuals) When hybrids are less fit than parent species, reinforcement of reproductive barriers may occur through strong selection for prezygotic barriers Over time, the rate of hybridization decreases Where reinforcement occurs, reproductive barriers should be stronger for sympatric than allopatric species o For example, in populations of flycatchers, males are more similar in allopatric populations than sympatric populations If hybrids are as fit as parents, there can be substantial gene flow between species if gene flow is great enough, reproductive barriers weaken and the parent species can fuse into a single species For example, pollution in Lake Victoria has reduced the ability of female cichlids to distinguish males of different species Extensive gene flow from outside the hybrid zone can overwhelm selection for increased reproductive isolation inside the hybrid zone For example, parent species of Bombina routinely migrate into the narrow hybrid zone resulting in ongoing hybridization Concept 24.4: Speciation can occur rapidly or slowly and can result from changes in few or many genes o Many questions remain concerning how long it takes for new species to form, or how many genes need to differ between species o Patterns in the fossil record The fossil record includes examples of species that appear suddenly, persist essentially unchanged for some time, and then apparently disappear Niles Eldredge and Stephen Jay Gould coined the term punctuated equilibria to describe periods of apparent stasis punctuated by sudden change § The punctuated equilibrium model contrasts with a model of gradual change in a species over time Punctuated Equilibria – in the fossil record, long periods of apparent stasis, in which a species undergoes little or no morphological change, interrupted by relatively brief periods of sudden change o Speciation rates The punctuated pattern in the fossil record and evidence form lab studies suggest that speciation can be rapid For example, the sunflower Helianthus anomalus originated from the hybridization of two other sunflower species o Studying the genetics of speciation A fundamental question of evolutionary biology: how many genes change when a new species forms? Depending on the species in question, speciation might require change in a single gene or many genes For example, in Japanese Euhadra snails, the direction of shell spiral affects mating and is controlled by a single gene In monkey flowers (Mimulus), two loci affect flower floor, which influences pollinator preference o Pollination that is dominated by either hummingbirds or bees can lead to reproductive isolation of the flowers In other organisms, speciation can be influenced by larger numbers of genes and gene interactions o From speciation to macroevolution Macroevolution is the cumulative effect of many speciation and extinction events
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