1. Chapter 15 How Organisms Evolve
15.1 How are populations, genes, and evolution related?
15.2 What causes evolution?
15.3 How does natural selection work?

2. Figure: 15-CO
Title:
How Organisms Evolve
Caption:
We think of hospitals as places in which to seek protection from disease, but they also foster the evolution of drug-resistant supergerms.

3. 15.1 How Are Populations, Genes, and Evolution Related?
Genes and the environment interact to determine traits
The gene pool is the sum of the genes in a population

4. The Gene Pool For example, coat color in hamsters:
A population of 25 hamsters contains 50 alleles of the coat color gene (hamsters are diploid)
If 20 of those 50 alleles code for black coats, then the frequency of the black allele is 0.40 or 40% [20/50 = 0.40]

5. FIGURE 15-2 A gene pool
In diploid organisms, each individual in a population contributes two alleles of each gene to the gene pool.

6. 15.1 How Are Populations, Genes, and Evolution Related?
Evolution is the change over time of allele frequencies within a population
The equilibrium population is a hypothetical population that does not evolve

7. The Hardy-Weinberg Principle
A mathematical model (1908) proposed independently by
Godfrey H. Hardy (English mathematician)
Wilhelm Weinberg (German physician)

8. Hardy-Weinberg Principle (no evolving)
There must be no mutation
There must be no gene flow
The population must be large
All mating must be random
There must be no natural selection

9. Section 15.2 Outline 15.2 What Causes Evolution?
Mutations Are the Source of Genetic Variability
Gene Flow Between Populations Changes Allele Frequencies
Allele Frequencies May Drift in Small Populations
Mating Within a Population Is Almost Never Random
All Genotypes Are Not Equally Beneficial

10. Causes of Evolution Five factors contribute to evolutionary change:
Mutation
Gene flow
Small population size
Nonrandom mating
Natural selection

11. Source of Genetic Variability
Mutations are rare changes in the base sequence of DNA in a gene
Usually have little or no immediate effect
Are the source of new alleles
Can be passed to offspring only if they occur in cells that give rise to gametes
Can be beneficial, harmful, or neutral
Arise spontaneously, not as a result of, or in anticipation of, environmental necessity

12. FIGURE 15-3 Mutations occur spontaneously
This experiment demonstrates that mutations occur spontaneously and not in response to environmental pressures. When bacterial colonies that have never been exposed to antibiotics are exposed to the antibiotic streptomycin, only a few colonies grow. The observation that these surviving colonies grow in exactly the same positions in all dishes shows that the mutations for resistance to streptomycin were present in the original dish before exposure to the environmental pressure, streptomycin.

13. 15.2 What Causes Evolution?
Gene flow between populations changes allele frequencies
Immigration adds alleles to a population
Emigration removes alleles from a population

14. FIGURE 15-4 Pollen can be an agent of gene flow
Pollen, drifting on the wind, can carry alleles from one population to another.

15. 15.2 What Causes Evolution?
Genetic drift is the random change in allele frequencies over time, brought about by chance alone
Has minor impact in very large populations
Occurs more rapidly and has bigger effect on small populations

16. FIGURE 15-5 Genetic drift
If chance events prevent some members of population from reproducing, allele frequencies can change randomly.

17. Population Size Matters
Population size affects genetic drift

18. FIGURE 15-6 The effect of population size on genetic drift
Each colored line represents one computer simulation of the change over time in the frequency of allele A in a (a) large or (b) small population in which two alleles, A and a, were initially present in equal proportions, and in which randomly chosen individuals reproduced.

19. Causes of Genetic Drift
There are two causes of genetic drift
Population bottleneck
Founder effect

20. Population Bottleneck
A population bottleneck is a drastic reduction in population size brought about by a natural catastrophe or over-hunting
A population bottleneck can change allele frequencies and reduce genetic variation

21. FIGURE 15-7a Population bottlenecks reduce variation
(a) A population bottleneck may drastically reduce genetic and phenotypic variation because the few organisms that survive may carry similar sets of alleles.

22. Population Bottleneck
Northern elephant seal
Hunted almost to extinction in the 1800s
By 1890s, only 20 individuals remained
Hunting ban allowed population to increase to 30,000
Biochemical analysis shows that present-day northern elephant seals are almost genetically identical

23. FIGURE 15-7b Population bottlenecks reduce variation
(b) The northern elephant seal passed through a population bottleneck in the recent past, resulting in an almost total loss of genetic diversity.

24. Founder Effect
The founder effect occurs when a small number of individuals leave a large population and establish a new isolated population
By chance, allele frequencies of founders may differ from those of original population
Over time, new population may exhibit allele frequencies that differ from original population

25. 10-18a
Title:
The effects of genetic drift.
Caption:
(a) Its founders were not representative of the original population.

26. 10-18b
Title:
The effects of genetic drift.
Caption:
(b) A short-lived drop in population size caused a change in allele frequency in one of the populations.

27. 10-18c
Title:
The effects of genetic drift.
Caption:
(c) One population is so small that low-frequency alleles are lost by chance.

28. Mating Is Almost Never Random
Nonrandom mating can change the distribution of genotypes in a population
Organisms within a population rarely mate randomly

29. Mating Is Almost Never Random
Most animals are likely to mate with nearby members of their species
Certain animals, such as snow geese, exhibit assortative mating, where there is a preference for mates that are similar in appearance

30. 15.2 What Causes Evolution?
Mating Within a Population Is Almost Never Random
Nonrandom mating among snow geese
Competition between males favors evolution of structures for ritual combat
Peahens are attracted to the peacock’s showy tail

31. FIGURE 15-9 Nonrandom mating among snow geese
Snow geese, which have either white plumage or blue-gray plumage, are most likely to mate with other birds of the same color.

32. Figure: 15-7
Title:
Peahens are attracted to the peacock's showy tail
Caption:
The ancestors of today's peahens were apparently picky when deciding on males with which to mate, favoring males with longer and more colorful tails.

33. Endangered Habitat Destruction
Figure: E15-1
Title:
Endangered by habitat destruction
Caption:
The bighorn sheep is among the species designated “endangered” by the U.S. government.
Endangered Habitat Destruction

34. 15.2 What Causes Evolution? All Genotypes Are Not Equally Beneficial
Antibiotic resistance evolves by natural selection
Case study of chapter on penicillin resistance illustrates key points about evolution

35. All Genotypes Are Not Equal
Evolution is change in allele frequencies of a population, owing to unequal reproductive success among organisms bearing different alleles
Penicillin-resistant bacteria had greater fitness (reproductive success) than non-resistant bacteria

36. Penicillin Resistance Illustrates Key Points about Evolution
Natural selection does not cause genetic changes to individuals
Natural selection acts on individuals, but it is populations that are changed
Evolutionary changes are not “good” or “progressive” in any absolute sense

37. A compromise between opposing pressures
Figure: 15-8
Title:
A compromise between opposing pressures
Caption:
(a) A male giraffe with a long neck is at a definite advantage in combat to establish dominance. (b) But a giraffe's long neck forces it to assume an extremely awkward and vulnerable position when drinking. Thus, drinking and male–male contests place opposing evolutionary pressures on neck length.

38. Table 15-1 Causes of Evolution

39. Section 15.3 Outline 15.3 How Does Natural Selection Work?
Natural Selection Stems from Unequal Reproduction
Natural Selection Acts on Phenotypes
Some Phenotypes Reproduce More Successfully Than Others
Selection Can Influence Populations in Three Ways

40. Natural Selection
Natural selection is often associated with the phrase “survival of the fittest”
The fittest individuals are those that not only survive, but are able to leave the most offspring behind

41. Natural Selection
The selection on phenotypes affects genotypes present in the population
If environmental conditions favored tall pea plants, then tall pea plants would leave more offspring
These offspring would carry tall alleles

42. Success of Phenotypes
Successful phenotypes are those that have the best adaptations to their present environment
Adaptations are characteristics that help an individual survive and reproduce

43. Success of Phenotypes
Adaptations arise from the interactions of organisms with both the nonliving and living parts of their environments

44. The Environment Nonliving (abiotic) components include:
Climate
Availability of water
Concentration of minerals in the soil
Living (biotic) components include:
Other organisms

45. The Environment Interactions with other organisms include: Competition
Coevolution
Sexual selection

46. Some Phenotypes Reproduce More Successfully Than Others
Competition acts as an agent of selection
Both predator and prey act as agents of selection
Coevolution
Predation
Sexual selection favors traits that help an organism mate

47. Agents of Selection
Coevolution is the evolution of adaptations in two species due to their extensive interaction
e.g. predator-prey relationships

48. Examples of Coevolution

49. Some Phenotypes Reproduce More Successfully Than Others
Sexual selection favors traits that help an organism mate
Female preference for males with a certain phenotype
Conspicuous features (bright colors, long feathers or fins, elaborate antlers)
Bizarre courtship behaviors
Loud, complex courting songs

50. Figure: 15-6
Title:
Competition between males favors evolution of structures for ritual combat
Caption:
Two male bighorn sheep spar during the fall mating season. In many species, the losers of such contests are unlikely to mate, while winners enjoy tremendous reproductive success. Question Imagine that you studied a population of bighorn sheep and were able to identify the father and mother of each lamb born. Would the difference in number of offspring between the most reproductively successful adult and the least successful one be greater for males or for females?

51. Selection Influences Populations
Natural selection and sexual selection can affect populations in three ways:
Directional selection
Stabilizing selection
Disruptive selection

52. FIGURE 15-13 Three ways that selection affects a population over time
A graphical illustration of three ways natural and/or sexual selection, acting on a normal distribution of phenotypes, can affect a population over time. In all graphs, the beige areas represent individuals that are selected against•that is, they do not reproduce as successfully as do the individuals in the purple range.

53. Selection Can Influence Populations in Three Ways
Directional selection shifts character traits in a specific direction
Favors extreme values of traits
Stabilizing selection acts against Individuals who deviate too far from the average
Favors average values of traits
Disruptive selection adapts individuals within a population to different habitats
Favors both extreme values of traits

54. A balanced polymorphismHH Hh hh
dies of malaria
lives and
reproduces
dies of
sickle-cell anemia
A balanced polymorphism HH Hh hh
Figure: 15-10
Title:
A balanced polymorphism
Caption:
Natural selection may preserve two or more phenotypes in a population. In some African human populations, selection for malaria-resistant heterozygotes ensures that the alleles for both normal (H) and sickle-cell (h) hemoglobin persist in the population. Question What will happen to the h allele in malaria-free environments?
dies of malaria
lives and
reproduces
dies of
sickle-cell anemia HH Hh hh