Since we re not going to have review this week either I am posting these slides to help with reviewing the material that we didn t cover during discussion sessions these past two weeks. Of course, take your notes from Dr. Herrera s lectures as your main study material. As I always say, take these slides as a sample of what I consider the most complex concepts that most students might benefit from reviewing. These slides are not intended as a substitute to Dr. Herrera s lectures, nor are they an exhaustive review of all the material that Dr. Herrera covered in the past two weeks. Use your discretion and look up in the book any material that may not be completely clear. I will hold office hours on Thursday MORNING this week. I will pretty much be available from 8:30 until 2pm, so you can come in any time. However, it will be best if you email me to set up a time so I make sure I m not in a meeting or anything like that. Good luck to everyone in the exam! Silvia
Sources of genetic variation in sexually reproducing populations New genes and new alleles originate only by mutations Phenotypic variations result from recombinational shuffling of existing alleles. Crossing over Independent assortment Random fertilization
Concept check 23.2 1. Of all the mutations that occur, why do only a small fraction become widespread in a gene pool? 2. How does sexual recombination produce variation?
Of the five conditions for H-W equilibrium, which ones change allele fequency, and which ones change phenotypic frequency? Mutation (negligible in most pop) Natural selection Migration Genetic drift Alelle freq Non-random mating Phenot freq
Preservation of genetic variability Diploidy A recessive allele, even if lethal, will be maintained in the population in heterozygosis Balanced polymorphism When balancing selection maintains stable frequencies of two or more phenotypic forms Heterozygote advantage Frequency dependent selection Environmental heterogeneity
Balanced polymorphism Heterozygote advantage A heterozygous individual has a greater fitness than a homozygous individual E.g. sickle cell anemia (four independent origins of this allele) Frequency dependent selection relative fitness of a phenotype declines if the frequency increases E.g. host-parasite dynamics Environmental heterogeneity Fluctuating environmental conditions may give normally selected-against organism some form of advantage E.g. pepper moth with and without snow
Balanced polymorphism Heterozygote advantage A heterozygous individual has a greater fitness than a homozygous individual E.g. sickle cell anemia (four independent origins of this allele) Frequency dependent selection relative fitness of a phenotype declines if the frequency increases E.g. host-parasite dynamics Environmental heterogeneity Fluctuating environmental conditions may give normally selected-against organism some form of advantage E.g. pepper moth with and without snow
Frequency dependent selection.60.58.56.54.52.50 Fluctuations in frequency of left-sided Perissodus microlepis Value FREQ.48.46.44.42 1981.00 1983.00 1985.00 1987.00 1989.00 1982.00 1984.00 1986.00 1988.00 1990.00 YEAR
Balanced polymorphism Heterozygote advantage A heterozygous individual has a greater fitness than a homozygous individual E.g. sickle cell anemia (four independent origins of this allele) Frequency dependent selection relative fitness of a phenotype declines if the frequency increases E.g. host-parasite dynamics Environmental heterogeneity Fluctuating environmental conditions may give normally selected-against organism some form of advantage E.g. pepper moth with and without snow
Population genetics problems 1. For an experiment, you started out a population of a species of wild flowers using 150 true breeding tall plants (TT) and 150 true breeding dwarf plants (tt). These alleles show incomplete dominance, therefore the hybrids (Tt) show an intermediate height. Five years later you want to know whether this population is at Hardy-Weinberg equilibrium. What do you expect to find if the population reached equilibrium? 2. A population of a butterfly species has two alleles for a gene determining wing color. Individuals homozygous for allele B look brown (BB), those homozygous for allele W look white (WW) and heterozygous individuals (BW) are mottled brown and white. The population is composed of 2 brown, 32 white, and 16 mottled butterflies. Is this population at H-W equilibrium?
Macroevolution
Anagenesis vs. Cladogenesis
Both processes in the same tree Anagenesis: the evolution of species involving a change in gene frequency in an entire population. The ancestral population can be considered extinct. Cladogenesis: an evolutionary mechanism and a process of adaptive evolution that leads to the development of a greater variety of sister organisms
Evolutionary change may lead to speciation Allopatric speciation: occurs when populations physically isolated by a barrier evolve reproductive isolation. If the barrier breaks down, individuals of the two populations can no longer interbreed.
Evolutionary change may lead to Sympatric speciation: species undergoing sympatric speciation are not geographically isolated by, for example, a mountain or a river. The speciating populations generally share the same territory. speciation
Sympatric speciation by autopolyploidy and allopolyploidy
Sympatric speciation in animals: same process, different mechanism 1. Specialization in different resources Example: fig wasps
Sympatric speciation in animals: same process, different mechanism 2. Balanced polymorphism and assortative mating - Example: Snow geese white and blue morphs
Adaptive radiation Defined as the rapid speciation of a single or a few species to fill many ecological niches. It is an evolutionary process driven by mutation (heritable/genetic variation) and natural selection.
Adaptive radiation Caused by Opportunity: colonization of uninhabited islands: e.g. Darwin s finches, anoles in Caribbean islands, monotremes and marsupials in Australia. Extinction: Mass disappearance of dinosaurs allowed for the diversification of mammals by the end of the Mesozoic.
Punctuated equilibrium vs. Punctuated equilibrium: (evolution by jerks) species will show little to no evolutionary change throughout their history. When evolution does occur, it happens sporadically (by splitting) and occurs relatively quickly compared to the species full duration on earth. Phyletic gradualism: ("evolution by creeps"), most evolution occurs uniformly and by the steady and gradual transformation of whole lineages (anagenesis). gradualism
Vocabulary Extant vs. extinct Adaptation vs. exaptation Cladogenesis