Evolution and Populations

1. Evolution and Populations Chapter 17

2. Genetics and Molecular Biology • Darwin had no idea how heredity worked, and he was worried that this lack of knowledge might prove fatal to his theory. • As it happens, some of the strongest evidence supporting evolutionary theory comes from genetics. A long series of discoveries, from MendeltoWatson and Crickto genomics, helps explain how evolution works. • Also, we now understand how mutation and the reshuffling of genes during sexual reproduction produce the heritable variation on which natural selection operates.

3. Life’s Common Genetic Code • All living cells use information coded in DNA and RNA to carry information from one generation to the next and to direct protein synthesis. • This genetic code is nearly identical in almost all organisms, including bacteria, yeasts, plants, fungi, and animals.

4. A Testable Hypothesis • Darwin hypothesizedthat the Galápagos finches he observed had descended from a common ancestor. • He noted that several finch species have beaks of very different sizes and shapes. Each species uses its beak like a specialized tool to pick up and handle its food. Different types of foods are most easily handled with beaks of different sizes and shapes. • Darwin proposed that natural selection had shaped the beaks of different bird populations as they became adapted to eat different foods.

5. Genetics Joins Evolutionary Theory • In genetic terms, evolution is any change in the relative frequency of alleles in the gene pool of a population over time. • Researchers discovered that heritable traits are controlled by genes. • Changes in genes and chromosomes generate variation.

6. Genotype and Phenotype in Evolution • An organism’s genotypeis the particular combination ofallelesit carries. • An individual’s genotype, together with environmental conditions, produces its phenotype. • Phenotypeincludes all physical, physiological, and behavioral characteristics of an organism.

7. Genotype and Phenotype in Evolution • Natural selection acts directly on phenotype, not genotype. • Some individuals have phenotypes that are better suited to their environment than others. These individuals produce more offspring and pass on more copies of their genes to the next generation.

8. Populations and Gene Pools • A population is a group of individuals of the same species that mate and produce offspring. • Smallest unit in which evolution occurs • A gene pool consists of all the genes, including all the different alleles for each gene that are present in a population. • Combined genetic information of all the members of a particular population B bbbBbbbBb b bbbbBbbbb

9. Populations and Gene Pools • Researchers study gene pools by examining the relative frequency of an allele. The relative frequency of an allele is the number of times a particular allele occurs in a gene pool, compared with the number of times other alleles for the same gene occur. • Allele Frequency = # of a certain allele total # of alleles of all types in the population Red Allele - 9/36 = ¼ = 0.25 White Allele - 27/36 = ¾ = 0.75

10. Populations and Gene Pools • For example, this diagram shows the gene pool for fur color in a population of mice.

11. Allele Frequency Example • Calculate the frequency of thedominant andrecessiveallelesin the gene pool below. A a a A a A a a A a A = a = 4/10 = 0.4 6/10 = 0.6

12. Alleles Frequencies & the Gene Pool: • Phenotype Frequency = # of individuals with a particular phenotype total #of individuals in population

13. Determining Phenotype and Allele Frequencies using Japanese four o’clock flowers: • 1st Generation: RR RR RW RW RR RW RW RR • Phenotype Frequency: Allele Frequency: White - R = Pink - W = Red - 12/16 = 0.75 0 4/16 = 0.25 4/8 = 0.50 4/8 = 0.50

14. Predicting Genotypes & Phenotypes of Second Generation: • According to the laws of probability, the chance of an R gamete meeting with another R gamete is the product of the allele frequencies in the gene pool. • Red (RR) = R x R = RR = 0.75 x 0.75 = 0.5625 • White (WW) = W x W = WW = 0.25 x 0.25 = 0.0625 The frequency of all types expected in the second generation must add up to 1.0 1.0 - RR - WW = RW 1.0 - 0.5625 - 0.0625 = 0.375

15. Single-Gene Traits • A single-gene trait is a trait controlled by only one gene. Single-gene traits may have just two or three distinct phenotypes. • Dominance of an allele for a single-gene trait does not necessarily mean that the dominant phenotype will always appear with greater frequency in a given population. No widow’s peak is a recessive trait

16. Polygenic Traits • Polygenic traits are traits controlled by two or more genes. • Each gene of a polygenic trait often has two or more alleles. • A single polygenic trait often has many possible genotypes and even more different phenotypes.

17. How Natural Selection Works • Evolutionary fitness is the success in passing genes to the next generation. • Evolutionary adaptation is any genetically controlled trait that increases an individual’s ability to pass along its alleles.

18. Natural Selection on Single-Gene Traits • Natural selection for a single-gene trait can lead to changes in allele frequencies and then to evolution.  • For example, a mutation in one gene that determines body color in lizards can affect their lifespan. So if the normal color for lizards is brown, a mutation may produce red and black forms.

19. Natural Selection on Single-Gene Traits • If red lizards are more visible to predators, they might be less likely to survive and reproduce. Therefore the allele for red coloring might not become common. • Black lizards might be able to absorb sunlight. Higher body temperatures may allow the lizards to move faster, escape predators, and reproduce.

20. Natural Selection on Polygenic Traits • Polygenic traits have a range of phenotypes that often form a bell curve. • The fitness of individuals may vary from one end of the curve to the other. • Natural selection can affect the range of phenotypes and hence the shape of the bell curve. 3 Types of Selection

21. Directional Selection • Directional selection occurs when individuals at one end of the curve have higher fitness than individuals in the middle or at the other end. The range of phenotypes shifts because some individuals are more successful at surviving and reproducing than others.

22. Directional Selection: Anteaters feed by breaking open termite nests (extend their sticky tongues into the nests). New species of termites that build very deep nests. Anteaters with long tongues more effective than those with average or short tongues

23. Directional Selection • A drought on the Galápagos island of Daphne Major in 1977 reduced the number of small seeds available to finches, causing many of the small-beaked finches to die. This caused an increase in the finches’ average beak size between 1976 and 1978.

24. Stabilizing Selection • Stabilizing selection occurs when individuals near the center of the curve have higher fitness than individuals at either end. This situation keeps the center of the curve at its current position, but it narrows the overall graph.

25. Stabilizing Selection: • Most common kind of selection A – small lizards may not be able to run fast enough to escape B – large lizards may be more easily spotted, captured, & eaten by predators

26. Disruptive Selection • Disruptive selection occurs when individuals at the upper and lower ends of the curve have higher fitness than individuals near the middle. Disruptive selection acts against individuals of an intermediate type and can create two distinct phenotypes.

27. Disruptive Selection: A – white shelled limpets blend in with goose barnacles – birds can’t see them B – dark shelled limpets blend in with dark colored rocks Limpets with intermediate shell color are visible against both backgrounds

28. Genetic Drift • Genetic drift occurs in small populations when an allele becomes more or less common simply by chance. Genetic drift is a random change in allele frequency. • The smaller the population, the more susceptible it is to such random changes.

29. Evolution vs. Genetic Equilibrium • The Hardy-Weinberg principle states that allele frequencies in a population should remain constant unless one or more factors cause those frequencies to change. • The Hardy-Weinberg principle makes predictions about certain genotype frequencies. • According to the Hardy-Weinberg principle, five conditions are required to maintain genetic equilibrium: (1) The population must be very large (2) There can be no mutations (3) There must be random mating (4) There can be no movement into or out of the population (5) No natural selection

30. Evolution vs. Genetic Equilibrium • A population is in genetic equilibrium if allele frequencies in the population remain the same. If allele frequencies don’t change, the population will not evolve. • Real populations rarely exist under the rigid conditions of the Hardy-Weinberg Equilibrium.

31. The Hardy-Weinberg principle predicts that 5 conditions can disturb genetic equilibrium and cause evolution to occur: • Nonrandom mating – individuals select mates based on heritable traits • Small population size – evolutionary change due to genetic drift happens more easily in small populations • Immigration or emigration – movement of individuals into (immigration) or out of (emigration) may introduce new alleles or remove alleles from the gene pool • Mutations – introduce new alleles changing allele frequencies • Natural Selection – different genotypes have different fitness One or more of these conditions usually holds for real populations = evolutions happen most of the time

32. Nonrandom mating example – female peacocks choose mates on the basis of physical characteristics such as brightly patterned tail feathers

33. Isolating Mechanisms • When populations become reproductively isolated, they can evolve into two separate species. Reproductive isolation can develop in a variety of ways, including behavioral isolation, geographic isolation, and temporal isolation. • Speciation is the formation of a new species. A species is a population whose members can interbreed and produce fertile offspring.

34. Isolating Mechanisms • Reproductive isolation occurswhen a population splits into two groups and the two populations no longer interbreed. • When populations become reproductively isolated, they can evolve into two separate species.

35. Behavioral Isolation • Behavioral isolation occurs when two populations that are capable of interbreeding develop differences in courtship rituals or other behaviors. Blue Footed Booby Courtshipwatch to 1:30 min Blue footed boobies perform an elaborate courtship display that involves the male ‘skypointing’ (pointing the head and beak upwards and spreading the wings), alternately lifting each blue foot, with the tail held cocked, and often emitting a whistling call. Both members of the pair may then skypoint, touch beaks, lift the feet, or pick up twigs or stones and place them on the ground

36. Behavioral Isolation • Eastern and western meadowlarks are difficult to distinguish between based on size, shape, and color, however, their calls are quite distinct. • Presumably this difference serves to distinguish mates from the different species. • Behavioral isolation may also prevent different firefly populations from mating because different species have their own pattern of light pulses. Other examples include mating dances and various courtship rituals. Western Meadowlark Call Eastern Meadowlark Call

37. Geographic Isolation • Geographic isolation occurs when two populations are separated by geographic barriers such as rivers, mountains, or bodies of water. • Once completely separated, the two populations possess variations of some genes, resulting in two "species" that differ in appearance (color, size, etc.) and behavior. • For example, the Kaibab squirrel is a subspecies of the Abert’s squirrel that formed when a small population became isolated on the north rim of the Grand Canyon. Separate gene pools formed, and genetic changes in one group were not passed on to the other.

38. Geographical Isolation • The Kaibab squirrel (Sciurusabertikaibabensis, left) became geographically isolated from the common ancestor with its closest relative, the Abert squirrel (Sciurusabertiaberti, right) in the North Rim of the Grand Canyon about 10,000 years ago. • Since then, several distinguishing features, such as the black belly and forelimbs have gradually evolved.

39. Temporal Isolation • Temporal isolation happens when two or more species reproduce at different times. • For example, three species of orchid live in the same rain forest. Each species has flowers that last only one day and must be pollinated on that day to produce seeds. Because the species bloom on different days, they cannot pollinate each other.

40. Temporal Isolation • The Red-legged Frog (Rana aurora, left) breeding season lasts from January to March. The closely related Yellow-legged Frog (Ranaboylii, right) breeds from late March through May.

41. Temporal Isolation • Drosophila persimilis breeds in early morning, while closely related Drosophila pseudoobscura breeds in the afternoon

42. Founders Arrive • Many years ago, a few finches from South America—species M—arrived on one of the Galápagos islands, as shown in the figure.

43. Geographic Isolation • Because of the founder effect, the allele frequencies of this founding finch population could have differed from those in the South American population.

44. Changes in Gene Pools • Over time, populations on each island adapted to local environments.  

45. Behavioral Isolation • Natural selection could have caused two distinct populations to evolve (A and B), each characterized by a new phenotype.

46. Competition and Continued Evolution • Birds that are most different from each other have the highest fitness. More specialized birds have less competition for food. Over time, species evolve in a way that increases the differences between them, and new species may evolve (C, D, and E).

47. Speciation in Darwin’s Finches • Speciation in Galápagos finches occurred by: • founding of a new population • geographic isolation • changes in the new population’s gene pool • behavioral isolation • ecological competition

48. Gradualism • Gradualism involvesa slow,steady change in a particular line of descent. • Thefossilrecord shows that many organisms have indeed changed gradually over time.

49. The pattern of slow, steady change does not always hold. • Horseshoe crabs, for example, have changed little in structure from the time they first appeared in thefossilrecord. • This species is said to be in a state of equilibrium, which means that the crab’s structure has not changed much over a very long stretch of time.

50. Punctuated Equilibrium • Punctuated equilibrium is the term used to describe equilibrium that is interrupted by brief periods of more rapid change. • This is a proposed theory to explain the gaps in the fossil record.