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

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

4. 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 ( )
Sylvia S. Mader, Concepts of Biology © Zanichelli editore, 2018

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

6. 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

7. 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

8. 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

9. 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

10. 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

11. 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

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

13. 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

14. 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

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

16. 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

17. 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

18. 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

19. 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

20. 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

21. 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

22. 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 = ( )/200 = 120/200 = 0.6 L
l frequency = ( )/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

23. 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

24. 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

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

26. 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

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

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

29. 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

30. 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

31. 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

32. 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

33. 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

34. 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

35. 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

36. 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

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

38. 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

39. 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

40. 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

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

42. 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