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Immagini e concetti della biologia Sylvia S. Mader Sylvia S. Mader, Concepts of Biology © Zanichelli editore, 2018

A8 - Principles of evolution Sylvia S. Mader, Concepts of Biology © Zanichelli editore, 2018

Darwin’s journey In 1831 Charles Darwin made a journey around the world. He observed that species change from place to place and through time. Charles Darwin (1809-1882) Sylvia S. Mader, Concepts of Biology © Zanichelli editore, 2018

Darwin’s journey Sylvia S. Mader, Concepts of Biology © Zanichelli editore, 2018

Darwin’s observations Rheas living in South America had the same adaptations to desert as african ostriches. Darwin's Rhea Marine fossils in the rocks of the mountains. Earth’s strata contain fossils Sylvia S. Mader, Concepts of Biology © Zanichelli editore, 2018

Darwin’s observations Similar organisms living in the Patagonia desert and in tropical forests showed different characteristics. Patagonia desert Tropical rain forest Sylvia S. Mader, Concepts of Biology © Zanichelli editore, 2018

Darwin’s observations Galapagos iguanas had claws and muzzle adapted to the environment. Marine Iguana New species of finches on the Galàpagos had different beaks in relation to their feeding habits. Woodpecker finch Sylvia S. Mader, Concepts of Biology © Zanichelli editore, 2018

Darwin’s knowledges supporting evolution Charles Lyell (geologist) Earth is old enough for evolution to have resulted in the species seen today. Georges Cuvier (paleontologist) Catastrophes caused evolution to occur. Jean-Baptiste de Lamarck (zoologist) Inheritance of acquired traits is a mechanism of evolution. Sylvia S. Mader, Concepts of Biology © Zanichelli editore, 2018

Artificial selection mimics natural selection wild mustard Kohlrabi Brussels sprout Chinese cabbage In agriculture humans select and maintain specific traits in certain populations. Sylvia S. Mader, Concepts of Biology © Zanichelli editore, 2018

The natural selection Members of a population have inheritable variations Stronger individuals are more likely to reproduce A population is able to produce more offsprings than the environment can support There is always a struggle for survival Sylvia S. Mader, Concepts of Biology © Zanichelli editore, 2018

The natural selection Darwin observed natural selection in Galàpagos finches. Sylvia S. Mader, Concepts of Biology © Zanichelli editore, 2018

Studying the natural selection today Scientists are observing variations in the beak of Medium Ground Finch (Geospiza fortis) in relation with climate changes at Galàpagos islands. Sylvia S. Mader, Concepts of Biology © Zanichelli editore, 2018

Evolution “A series of changes in a population over time, due to an accumulation of inherited differences”. Sylvia S. Mader, Concepts of Biology © Zanichelli editore, 2018

Fossils provide records for the past. Evidences for evolution Fossils provide records for the past. Sylvia S. Mader, Concepts of Biology © Zanichelli editore, 2018

Evidences for evolution The fossil records indicate that life has progressed from the simplest organisms to more complex ones. Sylvia S. Mader, Concepts of Biology © Zanichelli editore, 2018

Evidences for evolution Some fossils (as Archaeopteryx lithographica) have intermediate characteristics between two different groups (i.e. birds and reptiles). Sylvia S. Mader, Concepts of Biology © Zanichelli editore, 2018

Evidences for evolution COMPARATIVE ANATOMY Homologous structures are anatomically similar among certain organisms and suggest that organisms have a common descendent. Some organisms have vestigial structures, memories of functional structures in ancestors. Sylvia S. Mader, Concepts of Biology © Zanichelli editore, 2018

Evidences for evolution BIOGEOGRAPHY Some plants and animals evolved in particular locations. Therefore similar environments, widely separated, contain different organisms with similar adaptations. Sylvia S. Mader, Concepts of Biology © Zanichelli editore, 2018

Evidences for evolution MOLECULAR EVIDENCES The degree of similarity of DNA (or amino acid) base sequences shows a pattern of relations between organisms. Sylvia S. Mader, Concepts of Biology © Zanichelli editore, 2018

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, 2018

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, 2018

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, 2018

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, 2018

Hardy-Weinberg principle Sylvia S. Mader, Concepts of Biology © Zanichelli editore, 2018

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, 2018

Natural selection In stabilizing selection extreme phenotypes are negatively selected, intermediate phenotypes are favored. Sylvia S. Mader, Concepts of Biology © Zanichelli editore, 2018

Natural selection In directional selection an extreme phenotype is favored. Sylvia S. Mader, Concepts of Biology © Zanichelli editore, 2018

Natural selection In disruptive selection two or more extreme phenotypes are favored over the intermediate one. Sylvia S. Mader, Concepts of Biology © Zanichelli editore, 2018

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, 2018

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, 2018

New species and biodiversity Speciation occurs when: one species splits into two or more species; one species becomes 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, 2018

Evolutionary species concept Every species has its own evolutionary history and a species can be recognized by diagnostic traits. Sylvia S. Mader, Concepts of Biology © Zanichelli editore, 2018

Biological species 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, 2018

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, 2018

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, 2018

Gradual or rapid speciation Gradualistic model: speciation occurs due to gradually changing environmental conditions. Sylvia S. Mader, Concepts of Biology © Zanichelli editore, 2018

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, 2018

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. 39 Sylvia S. Mader, Concepts of Biology © Zanichelli editore, 2018

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, 2018

Reproductive barrier Sylvia S. Mader, Concepts of Biology © Zanichelli editore, 2018

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, 2018