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Immagini e concetti della biologia Sylvia S. Mader Sylvia S. Mader Concepts of Biology © Zanichelli editore, 2012

B4 - Microevolution and macroevolution Sylvia S. Mader Concepts of Biology © Zanichelli editore, 2012

Population genetics A gene pool is the sum of all the alleles in a population. Microevolution is evidenced by allele frequency changes within the gene pool. Hardy-Weinberg equilibrium describes microevolution in non-evolving populations. Sylvia S. Mader Concepts of Biology © Zanichelli editore, 2012

Punnett square for the II generation Population genetics Ex.: a population of 100 turtles (trait: neck length) Homozygote LL = 36%; Heterozygote Ll = 48%; Homozygote ll = 16% Allele frequencies L frequency = (36 + 36 +48)/200 = 120/200 = 0,6 L l frequency = (48 + 16 + 16)200 = 80/200 = 0,4 l Punnett square for the II generation p2 + 2pq + q2 =1 Sylvia S. Mader Concepts of Biology © Zanichelli editore, 2012

Hardy-Weinberg principle G.H. Hardy and W. Weinberg (1908): “it is possible to calculate genotype and allele frequency of a population using the formula p2 + 2pq + q2 = 1”. Microevolution does not occur (Hardy-Weinberg equilibrium) if the following conditions are satisfied: no mutations no gene flow random matings no natural selection no genetic drift Sylvia S. Mader Concepts of Biology © Zanichelli editore, 2012

Hardy-Weinberg principle Generally all frequencies do change between generations and microevolution (Hardy-Weinberg disequilibrium) does occur. Sylvia S. Mader Concepts of Biology © Zanichelli editore, 2012

Hardy-Weinberg principle Sylvia S. Mader Concepts of Biology © Zanichelli editore, 2012

Genetic variation Both mutations and sexual recombination produce genetic variation. Mutation rate is very low: 1 mutation every 105 cell divisions. Mutations are the primary source of genetic differences in prokaryotes. Random matings and gene flow (movement of alleles between populations due to migration) help microevolution. Sylvia S. Mader Concepts of Biology © Zanichelli editore, 2012

Natural selection In stabilizing selection extreme phenotypes are negatively selected, intermediate phenotypes are favored. Sylvia S. Mader Concepts of Biology © Zanichelli editore, 2012

The case of the sickle-cell disease Natural selection Stabilizing selection help maintain harmful alleles. The case of the sickle-cell disease Heterozygote advantage causes sickle-cell allele to be maintained, even though the homozygous recessive is lethal. Sylvia S. Mader Concepts of Biology © Zanichelli editore, 2012

Natural selection In directional selection an extreme phenotype is favored. Sylvia S. Mader Concepts of Biology © Zanichelli editore, 2012

Natural selection In disruptive selection two or more extreme phenotypes are favored over the intermediate one. Sylvia S. Mader Concepts of Biology © Zanichelli editore, 2012

Genetic drift Effects of the random changes in allele frequencies in a gene pool (genetic drift) are unpredictable. Sylvia S. Mader Concepts of Biology © Zanichelli editore, 2012

Genetic drift Two mechanisms are important in genetic drift: Bottleneck effect (for species close to extinction) prevents the majority of genotypes from participating in formation of the next generation. Founder effect occurs when rare alleles by the founders occur at high frequency in an isolated population. Sylvia S. Mader Concepts of Biology © Zanichelli editore, 2012

New species and biodiversity Speciation occurs when: one species splits into two or more species; one specie become a new species over time, as resulted from the changes in the allele frequencies in the genetic pool. Macroevolution depends on speciation. Sylvia S. Mader Concepts of Biology © Zanichelli editore, 2012

Evolutionary species concept Every species has its own evolutionary history and a species can be recognize by diagnostic traits. Sylvia S. Mader Concepts of Biology © Zanichelli editore, 2012

Biological specie concept Members of a same species are reproductively isolated from the members of other species. They can only reproduce with members of their own species. Similar phenotype but different species Sylvia S. Mader Concepts of Biology © Zanichelli editore, 2012

Reproductive barrier Reproductive barriers contribute to maintain genetic differences between species. Prezygotic isolating mechanisms prevent reproductive attempts. Geographic isolation: species live in different habitats. Temporal isolation: species reproduce at different periods. Behavioral isolation: different courtship mechanisms. Mechanical isolation: incompatibility due to size or morphology. Gametic incompatibility: gametes transfer but do not form zygotes. Sylvia S. Mader Concepts of Biology © Zanichelli editore, 2012

Reproductive barrier Postzygotic isolating mechanisms prevent hybrid offspring from breeding. Zygotic mortality: eggs are fertilized, but the zygote does not develop. Hybrid sterility: hybrid zygote develop, but the resulting adult is sterile. F2 sterility: although hybrids are fertile, further hybrid generations (F2) are inviable or sterile. Sylvia S. Mader Concepts of Biology © Zanichelli editore, 2012

Reproductive barrier Sylvia S. Mader Concepts of Biology © Zanichelli editore, 2012

Geographic barrier Ernst Mayr (1942) described the allopatric speciation. “Populations separated by geographic barriers will differentiate genetically and phenotypically”. Sylvia S. Mader Concepts of Biology © Zanichelli editore, 2012

Geographic barrier Adaptive radiation Hawaiian honeycreepers Geographic barrier Adaptive radiation A single ancestral species may evolve into several new species showing different phenotypic traits with which they adapt to different environments. 23 Sylvia S. Mader Concepts of Biology © Zanichelli editore, 2012

Non-geographic barrier Sympatric speciation (without geographic isolation) mostly occurs in plants, as they develop a condition called polyploidy (more than two sets of chromosomes). Polyploid individuals are reproductively isolated as they cannot reproduce with parental 2n plants. Sylvia S. Mader Concepts of Biology © Zanichelli editore, 2012

Non-geographic barrier Autoploidy Occurs when an haploid gamete fuses with a diploid gamete, resulting in a triploid plant, which is sterile. Sylvia S. Mader Concepts of Biology © Zanichelli editore, 2012

Non-geographic barrier Alloploidy Occurs when two different but related species of plants hybridize and the chromosome number doubles. Sylvia S. Mader Concepts of Biology © Zanichelli editore, 2012

Gradual or rapid speciation Gradualistic model: speciation occurs due to gradually changing environmental conditions. Sylvia S. Mader Concepts of Biology © Zanichelli editore, 2012

Gradual or rapid speciation Punctuated equilibrium model: periods of equilibrium are interrupted by rapid speciation. If the environment changes rapidly, new species suddenly arise. Sylvia S. Mader Concepts of Biology © Zanichelli editore, 2012

Speciation in the history Burgess Shale fossils represent Precambrian marine life 540 MYA. Sylvia S. Mader Concepts of Biology © Zanichelli editore, 2012

Adaptation Evolution proceeds for adaptations to the environment. Natural selection is opportunistic, not goal-oriented. Species able to adapt to the changing environment have more offsprings. Sylvia S. Mader Concepts of Biology © Zanichelli editore, 2012

Artificial selection Teosinte was the ancestor of modern corn. Thousand of years of artificial selection has changed the species. Today corn is allotetraploid which accounts for the size. Teosinte (Zea mexicana) Modern corn (Zea mays) Sylvia S. Mader Concepts of Biology © Zanichelli editore, 2012

Development genetics and speciation Eyes development, limb development and shape determination are controlled by the same genes in different headed animals. Pax6 gene is involved in the eyes development of many different animals Sylvia S. Mader Concepts of Biology © Zanichelli editore, 2012

Evolution Differential gene expression can cause dramatic changes in body shape and organs. Differential gene expression and/or new functions for old genes can explain evolution, including human evolution. Sylvia S. Mader Concepts of Biology © Zanichelli editore, 2012