Evolution and its processes

1. Evolution and its processes Chapter 11

2. Discovering How Populations Change:Observations of Nature Changed our Thinking • Aristotle • Described nature as a continuum of organization • Lifeless matter through complex plants and animals • Influenced later European thinkers • Modified the continuum based on their own beliefs • By the 1400th century they believed in a “great chain of being” • Lowest form (snakes)  humans  spiritual beings • Each link in the chain = a species • Each was forged at the same time, same place, in a perfect state • Complete and continuous • Because everything that needed to exist already did, there was no room for change

3. Discovering How Populations Change:Observations of Nature Changed our Thinking • European naturalists • Globe-spanning expeditions • Asia, Africa, North and South America • Catalogued each new species found in the chain of being • 1800s • The naturalists found patterns that raised questions that could not be answered within the framework of the chain of being • Where species live • Similarities in body plans • Prompted questions about the natural forces that shape life

4. Discovering How Populations Change:Observations of Nature Changed our Thinking • European naturalists • Biogeography • Study of patterns in the geographic distribution of species • Naturalists found plants and animals living in extremely isolated places that looked suspiciously similar to species living across oceans or over mountains • “links”? • “perfect state”?

5. Discovering How Populations Change:Observations of Nature Changed our Thinking • European naturalists • Biogeography • Study of patterns in the geographic distribution of species • Flightless birds • Look alike (and are closely related), on different continents • If related, how could they have ended up on different continents? • How would they be placed in the “great chain of being”?

8. Discovering How Populations Change:Observations of Nature Changed our Thinking • European naturalists • Biogeography • Study of patterns in the geographic distribution of species • Cactus • Look alike (but are not closely related), on different continents • Cannot be as closely related as their outward appearance might suggest • How would they be placed in the “great chain of being”? American spiny cactus African spiny spurge

10. Discovering How Populations Change:Observations of Nature Changed our Thinking • European naturalists • Fossils • Physical evidence of an organism that lived in the ancient past • Fossils unlike anything living were being unearthed and lack of fossils for some living organisms • “forged” time/place/perfect? • Geologists found simple fossils in deeper layers and more complex in more recent layers • “forged” time/place/perfect? 58 million years old 64.5 million years old

11. Discovering How Populations Change:Observations of Nature Changed our Thinking • All the observations • did not fit with the prevailing view • suggested that change had occurred in species • Several naturalists around 1800 attempted to describe and explain how life on Earth had changed over time

12. Change over Time • Georges Cuvier (1769-1832) • Proposed that many species that had once existed were now extinct • Catastrophic geological events "Why has not anyone seen that fossils alone gave birth to a theory about the formation of the earth, that without them, no one would have ever dreamed that there were successive epochs in the formation of the globe." Georges Cuvier, Discourse on the Revolutions of the Surface of the Globe http://www.ucmp.berkeley.edu/history/cuvier.html

13. Change over Time • Jean-Baptiste Lamark (1744-1829) • Proposed an idea about what processes might drive change • Environmental pressures cause an internal “need” for change • The resulting change is seen in offspring • Inherent drive toward perfection, up the chain of being

14. Change over Time • Charles Darwin (1809-1882) • Influenced by • 5-year expedition on the Beagle as the naturalist • Charles Lyell’s Principles of Geology • Challenged the prevailing belief about the age of the Earth • Thomas Malthus • Correlated human population sizes with famine, disease, and war • Humans have the capacity to produce more individuals than their environment can support = competition to survive • Selective breeding of pigeons, dogs, and horses

15. Change over Time • Alfred Wallace (1823-1913) • Influenced by • Existing studies indicating “change” in populations • Studying wildlife in the Amazon basin and the Malay Archipelago

16. Natural Selection • Charles Darwin and Alfred Wallace both proposed that natural selection could be a mechanism by which evolution (change) occurs • Many naturalists/scientists had accepted the idea of descent with modification (evolution) • Darwin and Wallace suggested a mechanism

17. Natural Selection • Natural Selection • Observations about populations • All organisms have a far greater reproductive potential than is ever realized • As a population expands, resources that are used by its individuals are limited • Limited resources results in competition • Observations about genetics • Species share traits • Inherited variations exist (alleles) • Inferences • A certain form of a shared trait may make its bearer better able to survive • Those that survive tend to leave more offspring • Thus, an allele associated with an adaptive trait tends to become more common in a population over time

18. Natural Selection • Natural selection example in sea stars • Can produce 2,500,000 eggs/year! • A given population of sea stars has various traits (alleles) • Body wall thickness: T = thick t = thin (Simplified) • Crashing waves and predators make life a constant struggle and many sea stars die • A thick body wall might help protect from crashing waves and predators • Those with adaptive traits (TT and Tt) are more likely to survive and reproduce • Thus a larger frequency/percentage of the offspring will inherit the T allele than was present in the parent generation

19. Natural Selection • Vocabulary • Change/Evolution/Modification • Change in the line of descent • Adaptations • An adaptation is a structure or a process that increases an animal’s potential to survive and reproduce in specific environmental conditions • Can be manifested as behavioral, physiological, or morphological traits • Fitness • Degree of adaptation to an environment

20. Evolution • Organic evolution or microevolution is the change in allele frequency of a population over time • A population is a group of the same species • Populations share morphological and physiological traits • However, within the shared traits there are allelic variations • These variations are created by • Mutation and changes in chromosome number or structure • Crossing over at meiosis I • Independent assortment at meiosis I • Fertilization

21. Evolution • Organic evolution is not…

23. Evolution • Allele frequencies • All of the alleles of all the genes of a population are referred to as the gene pool • The abundance of any particular allele in the gene pool of a population is the allele frequency

24. Evolution • Allele frequencies • Population level phenotype frequency • Total # of phenotypes = 20 • 15 blue/20 total = 0.75 • 5 white/20 total = 0.25

25. Evolution • Allele frequencies • Population level genotype frequency • Total # of genotypes = 20 • Homozygous dominant • 5/20 = 0.25 • Heterozygous • 10/20 = 0.5 • Homozygous recessive • 5/20 = 0.25

26. Evolution • Allele frequencies • Population level allele frequency • Total # of alleles = 20 x 2 = 40 • 20 A (blue)/40 = 0.5 • 20 a (white)/40 = 0.5

27. Hardy-Weinberg and Population Genetics • To follow the allele frequency in a population we have to change our perspective from individuals to populations • Previously: crossed 1 purple flowering plant with 1 white flowering plant • In a population there might be 75 purple flowering plants and 25 white flowering plants • Any of them could end up mating with any other plant • Must know the frequency of each possible allele to determine the possible offspring genotype and phenotype ratios

28. Hardy-Weinberg and Population Genetics • Two parents • 1:2:1 genotype • 3:1 phenotype • Population • AA = .56 = 56% • Aa = .19 + .19 = .38 (38%) • Aa = .06 = 6%

29. Hardy-Weinberg and Population Genetics • Hardy and Weinberg derived an equation to determine allele and genotype frequencies of a population • Based on the results of a Punnett square for a population that has a given gene with two alleles (A and a) • p = one allele frequency (A) • q = the other allele frequency (a)

30. Hardy-Weinberg and Population Genetics • Hardy and Weinberg derived an equation … • p2 = homozygous dominant genotype frequency (AA) • 2pq = heterozygous genotype frequency (Aa) • q2 = homozygous recessive genotype frequency (aa)

31. Hardy-Weinberg and Population Genetics • Hardy and Weinberg derived an equation … • The frequencies of homozygous dominant (AA), heterozygous (Aa) and homozygous recesive (aa) should add up to 1 (100% of the population) • AA + Aa + aa = 100%  p2 + 2pq + q2 = 1

32. Hardy-Weinberg and Population Genetics • Hardy and Weinberg derived an equation … • Because there are only two alleles the following equation is also true • p + q = 1

33. Hardy-Weinberg • Example • Daisies with a gene for flower color • 490 dark blue flowers (BB) • 420 light blue flowers (incomplete dominance) (Bb) • 90 white flowers (bb) • 1000 total • How many copies of flower color alleles are there? • How many of these copies are B? • How many copies are b? • What are the allele frequencies (divide the number of B and b alleles by the total number of alleles) ?

34. Hardy-Weinberg • Example • Daisies with a gene for flower color • How many copies of flower color alleles are there? • 2000 (2 x 1000) • Two alleles per individual • How many of these copies are B? • (490 x 2) + (420) = 1400 • How many copies are b? • (420) + (90 x 2) = 600 • What are the allele frequencies? • B 1400/2000= 0.70 • b 600/2000= 0.30

35. Hardy-Weinberg • Example • Daisies with a gene for flower color • 490 dark blue flowers (BB) • 420 light blue flowers (Bb) • 90 white flowers (bb) • 1000 total • What are the genotype frequencies? • (divide the number of each genotype by the total number of flowers)

36. Hardy-Weinberg • Example • Daisies with a gene for flower color • Genotype frequencies • Homozygous dominant (BB) 490/1000 = 0.49 • Heterozygous (Bb) 420/1000 = 0.42 • Homozygous recessive (bb) 90/1000 = 0.09

37. Hardy-Weinberg and Population Genetics • The Hardy-Weinberg equation can theoretically predict the genotype proportions of the next generation if the population meets five specific conditions • No mutations • No new alleles introduced to the population • Large population • Ensures change doesn’t occur by chance alone • No migration • Prevents alleles from entering or exiting the population • Random mating • Each individual has an equal chance of mating • No selection • All individuals survive and produce the same number of offspring

38. Hardy-Weinberg and Population Genetics • If a population meets all five specific conditions it is said to be at genetic equilibrium • Equilibrium occurs when the allele frequency of a population does not change (not evolving) • This is a theoretical reference point for a population 1st generation 2nd generation 3rd generation

39. Hardy-Weinberg and Population Genetics • If a population meets all five specific conditions it is said to be at genetic equilibrium • These conditions never occur all at once in nature • When the equilibrium is disturbed then the frequency of alleles changes • Populations evolve when the frequency of alleles changes

40. Evolutionary Mechanisms • How do allele frequencies change? • Effects of evolutionary mechanisms • When a population does not meet one of the five criteria for genetic equilibrium, the allele frequency can change

41. Evolutionary Mechanisms • Mutation • Changes in the genetic information results in different proteins creating different traits • Mutations are random and may result in beneficial, neutral, or detrimental changes • Not all mutations matter to evolution • Can add new alleles to a population

42. Evolutionary Mechanisms • Genetic Drift • Chance events that change the allele frequency • More likely to occur in small populations • Similar to flipping a coin • Should get equal numbers of heads and tails • In a small population (10 flips) you might get 7 heads and 3 tails • In a large population (1000 flips) you will get closer to 50% heads and 50% tails

44. Evolutionary Mechanisms • Genetic Drift • Founder Effect • A few individuals (founders) from a population colonize a new habitat • Founders may bring a different frequency of alleles

45. Evolutionary Mechanisms • Genetic Drift • Bottleneck effect • Occurs when the number of individuals in a population is drastically reduced • Disease, starvation, over-hunting, etc. • Population is left with only a remnant of the original gene pool

48. Evolutionary Mechanisms • Gene Flow • Also called migration • Any movement of genes from one population to another • Changes the relative allele frequency • Adds new alleles or remove alleles

49. Evolutionary Mechanisms • Natural Selection • Occurs when some phenotypes are more successful at leaving offspring than others

50. Evolutionary Mechanisms • Natural Selection • Modes of selection • Directional selection occurs when one extreme phenotype is at a disadvantage