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Biological Sciences
BIOSC 0160

Exam 3 Notes BIO 0160 EVOLUTIONARY GENETICS I – NATURAL SELECTION I. By what mechanism does adaptive evolution occur? a. Lamarck – “felt needs” b. Darwin – natural selection II. Darwin’s reasoning about natural selection a. Ernst Mayr (1980) i. Observation 1: individuals have incredible reproductive potential; populations have the potential for exponential growth ii. Observation 2: most populations are stable in size iii. Observation 3: natural resources needed for survival and reproduction are limited iv. Inference 1: the environment can support only so many individuals; only a few of produced offspring survive and reproduce because there is a “struggle for existence” v. This inference is supported by direct observations of mortality III. Darwin’s reasoning about mechanism – Part 2 a. Ernst Mayr i. Observation 4: organisms in populations vary greatly; no two individuals are exactly alike ii. Observation 5: much of this variation is heritable iii. Inference 2: survival in the struggle for existence is not random; individuals whose inherited characteristics fit them better to their environment are likely to leave more offspring than less fit individuals IV. Short definition of natural selection a. Natural selection is a process in which individuals differ in survival/reproduction, that difference resulting from the possession of alternate, heritable forms of a trait V. Formal requirements a. John Endler (1984) i. Trait variation ii. Trait heritability iii. Consistent relationship between trait form and survival/reproduction iv. When all here of the above occur, natural selection is certain to operate v. If any one of the three requirements do not occur, natural selection will not operate VI. Subtle aspects of natural selection a. Populations, not individuals, evolve b. The selection process is decidedly non-random c. Fitness is not defined merely as those who reproduce; it is defined independently of representation in subsequent generation, and with respect to the environment d. Mutation and recombination, not natural selection, generate variation within populations e. Outcomes of selection depend heavily on the specifics of the environment context VII. Observed examples of evolution by natural selection a. Peppered moth i. Light and dark moth, industrial revolution made smog, dark colored moth reproduced more and avoid predation b. Medium ground finch i. Researches went to volcano island and marked finches and took data on them ii. Drought came about and compared distribution of birds before drought to birds after drought iii. Birds with higher beak depth were able to crack open harder seeds to find food and survived Exam 3 Notes BIO 0160 c. Many, many other examples EVOLUTIONARY GENETICS II – MICROEVOLUTION I. Species and populations a. Species: a “nature kind” b. Species: group of populations whose individuals can interbreed c. Population: group of conspecifics (member of the same species) living in a particular location II. Concept of a gene pool a. The complete set of genes in a population; the total aggregate of genes b. Basic terms: i. Locus – location of a gene on a chromosome/simple way to say gene ii. Allele – form of a gene at a given locus iii. Diploid – two alleles at a given locus iv. Genotype – genetic makeup of an individual at a locus v. Phenotype – physical manifestation of the genotype vi. Homozygous – two of the same allele at a locus vii. Heterozygous – two different alleles at a locus viii. Microevolution – a change in a populations gene pool over time III. Non-changing populations – “Hardy-Weinberg Theorem” a. What happens when only sexual shuffling occurs? b. Frequency of alleles and genotypes in a population will remain constant i. No mutation ii. No natural selection iii. No gene flow between populations iv. No genetic drift v. No non-random mating (mating is random) c. Test of theorem by example i. Population size = 500 individuals ii. Total # of genes = 1000 iii. Observe population of: 1. 480 red-flowered plants a. 320/480 homozygous AA b. 160/480 heterozygous Aa 2. 20 white-flowered plants a. 20 homozygous aa iv. Gene pool makeup v. Frequency A = p = vi. Frequency a = q = vii. p + q = 1 viii. p=0.8 and q=0.2 ix. IV. Evolving populations a. Microevolution – generation-to-generation change in a population’s frequency of alleles or genotypes Exam 3 Notes BIO 0160 i. Non-adaptive mechanisms 1. Genetic drift a. Chance change in a gene pool b. Any non infinite population size will exhibit genetic drift c. Coin toss example – lower # of flips = greater genetic drift d. Real life examples i. Northern elephant seals 1. Low population so high genetic drift ii. Cheetahs e. Smaller the population, the greater the genetic drift f. Drift yields non-adaptive evolution g. Related phenomena: i. Bottleneck effect - Occurs when population experiences a “crash” (Fig 25.8) ii. Founder effect - Occurs when small # of individuals from source population start a new population away from source population iii. Both yield small populations greatly affected by drift 2. Gene flow a. Movement of genes between populations b. Occurs by dispersal of fertile individuals (Fig 25.9 pg 447) c. Causes two different populations to become more similar 3. Mutation a. Will change allele frequencies…but very, very little 4. Non-random mating a. Inbreeding (Fig 25.12 pg 451) i. Between closely related partners b. Positive assortative mating (like mates with like individuals) i. Self-pollination 1. Heterozygotes mate with heterozygotes, they produce homozygotes half the time 2. Over many generations, heterozygotes diminish and approaches 0 as homozygotes increase in number c. Negative assortative mating (like mates with dislike) i. “Outbreeding” ii. Results in increase in the frequency of heterozygotes d. All of these alter genotype, not allele frequencies (Fig 25.12) i. Alleles are being sorted into different genotypes (move toward homozygotes) ii. Still same allele pool, allelic frequencies doesn’t change e. Probably non-adaptive ii. Evolution by natural selection 1. Sources of variation a. Mutation – rar-4 rare-8 beneficial i. Rate: 10 to 10 per locus per generation 1. 1 in 1,000,000 mutation/locus/generation b. Recombination i. Recombines existing alleles in new combinations c. Maintenance of variation i. Diploidy: masks recessives Exam 3 Notes BIO 0160 1. Deleterious traits becomes more rarely expressed due to dominant allele masking recessive 2. As allele drops in frequency, it becomes harder and harder for natural selection to remove the allele 3. If deleterious allele becomes advantageous, over many generations the new advantageous allele will become more favorable and population will evolve to increase fitness ii. Balanced polymorphism 1. Heterozygote advantage a. E.g., humans and malaria i. Heterozygotes are malaria resistant and don’t have sickle cell ii. Homozygote dominant don’t have sickle cell but can get malaria iii. Homozygote recessive die from sickle cell 2. Patchy environments a. E.g., finches (Fig 25.5) i. Two types of seeds for finches to eat: small soft shell and large hard shell ii. Large beak finches can crack large seeds iii. Small beak finches can crack small seeds iv. Creates disruptive selection 3. Frequency-dependent selection a. E.g., African swallowtails i. Batesian Mimickry ii. 3 colors in the females iii. 3 morphs have evolved to mimic three toxic species iv. Any predator that eats the mimic will stay away from the tasty butterfly forever v. If any of the 3 mimics frequencies gets too high, it will decrease until the three are about equal 2. Modes of selection a. Directional Selection i. Individuals at one end of the variation spectrum have higher fitness ii. A change in mean trait value occurs iii. Variation is reduced iv. Evolutionary change occurs (Fig 25.3 pg 441) b. Stabilizing Selection i. Individuals at and around the mean trait value have higher fitness ii. No change in mean trait value occurs iii. Variation is reduced iv. No evolutionary change occurs (fig 25.4 pg 442) v. Graph becomes skinnier but mean doesn’t change at all vi. Shows natural selection and evolution are not synonymous c. Disruptive Selection Exam 3 Notes BIO 0160 i. Individuals at and around mean trait value have lower fitness ii. No change in mean trait value occurs** iii. Variation enhanced when expressed statistically** iv. Is adaptive (e.g. finches) 1. Some patches have small and others have large seeds 2. Bill beak size has selective advantage for fitness due to fight for food 3. Small and large bills are able to get food easier than medium size since they are all fighting for the same source due to their bill size v. Hence, EVOLUTION OCCURS (Fig 25.5 pg 443) vi. Center of the graph lowers fitness and extreme ends fitness increases V. Sexual Selection a. Selection that enhances ability to obtain mates i. Via increasing within-sex competitive ability (male-male competition) 1. E.g., elephant seals (Fig 25.16 pg 454) 2. Males fight other males to obtain all the females and reproduce ii. Via increasing between-sex attractiveness (sexual selection by female choice) b. Can result in a decrease in fitness within other contexts (e.g., predation) VI. Origin of evolutionary novelties a. Basic principles i. New structures are modifications of existing structures or tissues ii. Novelties arise as modifications of development iii. Function is retained during modification b. Some important Mechanisms i. Allometry 1. Difference in relative growth rate of parts 2. Evolutionary modifications of Allometry give rise to new shapes 3. New shapes of high relative fitness compared to other shapes 4. Homeobox genes affect rates of cellular proliferation ii. Heterochrony 1. Evolutionary change in timing of developmental events 2. Also known as onset and offset 3. E.g., human brain development a. Brain growth stops later in humans than gorillas b. Offset occurs later in time c. Event shifted from relative early to late in later generations d. Steeper growth curve occurs changing brain size due to allometry as well 4. Change in developmental processes a. E.g., suppression of apoptosis in formation of webbed foot i. Ducks suppress apoptosis so they can swim more efficiently b. E.g., activation of arthrogenesis (developmental production of joints) in formation of mammalian dentary-squamosal jaw joint iii. Homeosis 1. Alteration of basic body plan a. Arthropod has mutation in UBX gene (insects lack gene so no legs in abdomen region Exam 3 Notes BIO 0160 SPECIATION I. What is a species? a. Biological species concept i. Pioneered by Ernst Mayr ii. Groups of conspecifics (groups within the same species) who have the potential to interbreed and cannot interbreed with other such groups b. Other concepts i. Morphological 1. Uses anatomical differences to establish species boundaries 2. Most frequently used concept in practice ii. Cohesion 1.
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