1. 15.3 Shaping Evolutionary Theory
Main idea: The theory of evolution continues to be refined as scientists learn new information.
Objectives:
Discuss patterns observed in evolution.
Describe factors that influence speciation.
Compare gradualism with punctuated equilibrium.
Review Vocabulary
Allele: alternative forms of a character trait that can be inherited.

2. 15.3 Shaping Evolutionary Theory
New Vocabulary
Hardy-Weinberg Principle
Prezygotic isolating mechanism
Genetic drift
Allopatric speciation
Founder effect
Postzygotic isolating mechanism
Bottleneck
Sympatric speciation
Stabilizing selection
Adaptive radiation
Directional selection
Gradualism
Disruptive selection
Punctuated Equilibrium
Sexual selection

3. Mechanisms of Evolution
Evolution occurs at the population level, with genes as the raw material.
Hardy-Weinberg Principle states that when allelic frequencies remain constant, a population is in genetic equilibrium. (Even though the number of owls doubled, the ratio of gray to red owls remained the same).

4. Hardy-Weinberg Principle
p2 + 2pq + q2 = 1
This equation allows us to determine the equilibrium frequency of each genotype in the population: homozygous dominant (p2), heterozygous (2pq), and homozygous recessive (q2). Note the sum of these frequencies equals one.

5. Hardy-Weinberg Principle
A population in genetic equilibrium must meet five conditions – there must be no genetic drift, no gene flow, mating must be random, mutation, and there must be no natural selection.

6. Genetic Drift
A change in the allelic frequencies in a population that is due to chance and random mating.
In smaller populations, the effects of genetic drift become more pronounced, and the chance of losing an allele becomes greater.
Examples: Founder Effect & Bottleneck

7. Genetic Drift Marble Example
Chapter 15
Evolution
15.3 Shaping Evolutionary Theory
Genetic Drift
A change in the allelic frequencies in a population that is due to chance and random mating
In smaller populations, the effects of genetic drift become more pronounced, and the chance of losing an allele becomes greater.
Marble Example
As an analogy, imagine a population of organisms represented as 20 marbles in a jar, half of them red and half blue. These two colors correspond to two different gene alleles in the population. The organisms that are reproduced in a generation are represented in another jar. Each new generation the organisms will reproduce at random. To represent this reproduction, randomly pick a marble from the original jar and deposit a new marble with the same color as its "parent" in the second jar. Repeat the process until there are 20 new marbles in the second jar. The second jar will then contain a second generation of "offspring", 20 marbles of various colors. Unless the second jar contains exactly 10 red and 10 blue marbles there will have been a purely random shift in the allele frequencies.
Repeat this process a number of times, randomly reproducing each generation of marbles to form the next. The numbers of red and blue marbles picked each generation will fluctuate: sometimes more red, sometimes more blue. That is genetic drift – random variations in which organisms manage to reproduce, leading to changes over time in the allele frequencies of a population.

8. Founder Effect
Occurs when a small sample of a population settles in a location separated from the rest of the population
Alleles that were uncommon in the original population might be common in the new population.
Examples: Amish & French Canadians

9. Chapter 15
Evolution
15.3 Shaping Evolutionary Theory
Founder Effect
The loss of genetic variation that occurs when a new population is established by a very small number of individuals from a larger population
Island 1
Island 2
Mainland
Island 3

10. Bottleneck
Occurs when a population declines to a very low number and then rebounds

11. Evolution
15.3 Shaping Evolutionary Theory
Bottleneck
a significant percentage of a population or species is killed or otherwise prevented from reproducing and can rebound later
Often caused by a natural disaster

12. Gene Flow
A population in genetic equilibrium experiences no gene flow, however, few populations are isolated
Random movement of individuals between populations, or migration increases genetic variation within a population and reduces differences between populations

13. Gene Flow Genes entering or leaving a population through migration
Emigration Genes LEAVING a population
Immigration INCOMING genes in a population

14. Nonrandom mating Rarely is mating completely random in a population.
Usually individuals mate with individuals in close proximity.
This promotes inbreeding and could lead to a change in allelic proportions favoring individuals that are homozygous for particular traits

15. Mutations A random change in genetic material.
Collective mutations violate genetic equilibrium.
Occasionally mutations provide an advantage to organisms and become more common in subsequent generations.
Basis upon which natural selection works.

16. What is a gene pool?
Gene pool – combined genetic info of all members of a population
Contains two of more alleles (genes) for the same trait
Allele frequency – number of times an allele occurs in a gene pool compared to the number of times another allele occurs (expressed in percents)

17. Relative Frequencies of Alleles
Section 16-1
allele for brown fur
allele for black fur
Sample Population
48% heterozygous black
Frequency of Alleles
16% homozygous black
36% homozygous brown

18. Natural Selection
Acts to select the individuals that are best adapted for survival and reproduction
Stabilizing selection operates to eliminate extreme expressions of a trait when the average expression leads to higher fitness. (Birth Weights)
Directional selection makes an organism more fit. (Peppered Moths)
Disruptive selection is a process that splits a population into two groups. (African Finches)

19. Chapter 15
Evolution
15.3 Shaping Evolutionary Theory
Stabilizing selection operates to eliminate extreme expressions of a trait when the average expression leads to higher fitness.
Ex. Siberian Husky
Stabilizing selection favors the norm, the common, average traits in a population. Look at the Siberian Husky, a dog bred for working in the snow. The Siberian Husky is a medium dog, males weighing 16-27kg (35-60lbs). These dogs have strong pectoral and leg muscles, allowing it to move through dense snow. The Siberian Husky is well designed for working in the snow. If the Siberian Husky had heavier muscles, it would sink deeper into the snow, so they would move slower or would sink and get stuck in the snow. Yet if the Siberian Husky had lighter muscles, it would not be strong enough to pull sleds and equipment, so the dog would have little value as a working dog. So stabilizing selection has chosen a norm for the the size of the Siberian Husky.

20. Directional selection makes an organism more fit.
Chapter 15
Evolution
15.3 Shaping Evolutionary Theory
Directional selection makes an organism more fit.
Favors the extremes
Ex. Greyhound Dog
Directional selection favors those individuals who have extreme variations in traits within a population. A useful example can be found in the breeding of the greyhound dog. Early breeders were interested in dog with the greatest speed. They carefully selected from a group of hounds those who ran the fastest. From their offspring, the greyhound breeders again selected those dogs who ran the fastest. By continuing this selection for those dogs who ran faster than most of the hound dog population, they gradually produced a dog who could run up to 64km/h (40mph).

21. Sexual Selection
Sexual selection operates in populations where males and females differ significantly in appearance. (Males are largest and most colorful in the group).
Qualities of sexual attractiveness appear to be the opposite of qualities that might enhance survival. (More likely target for predators yet bodies enhance reproductive success).
Examples: Peacocks

22. Reproductive Isolation
Two types of reproductive isolating mechanism prevent gene flow among populations:
Prezygotic isolating mechanisms – operate before fertilization occurs.
Postzygotic isolating mechanisms – operate after fertilization has occurred to ensure that the resulting hybrid remains infertile.

23. Prezygotic Isolation
prevents reproduction by making fertilization unlikely.
prevents genotypes from entering a population’s gene pool through geographic, ecological, behavioral, or other differences
Eastern and Western meadowlarks similar in appearance but their songs separate them behaviorally.

24. Postzygotic Isolation
Occurs when fertilization has occurred but a hybrid offspring cannot develop or reproduce.
Prevents offspring survival or reproduction.
A Liger is the offspring of a male lion and a female tiger. Ligers are sterile.

25. Speciation
A population must diverge and then be reproductively isolated for speciation to occur.
Two types of speciation:
Allopatric speciation - A physical barrier divides one population into two or more populations. (The Grand Canyon separates the Abert and Kaibab squirrels).
Sympatric speciation - A species evolves into a new species without a physical barrier. The ancestor species and the new species live side by side during the speciation process. (Occurs frequently in plants due to polyploidy).

26. Patterns of Evolution
Adaptive Radiation - Can occur in a relatively short time when one species gives rise to many different species in response to the creation of new habitat or some other ecological opportunity.
Follows large-scale extinction events

27. Coevolution
The relationship between two species might be so close that the evolution of one species affects the evolution of the other species.
Mutualism – occurs when two species benefit each other. (Moth and the comet orchid it pollinates exist in a mutualistic relationship).
Coevolutionary arms race – one species can evolve a parasitic dependency on another species.
Example: A plant and an insect pathogen that is dependent on the plant for food. The plant evolves a chemical defense; the insect evolves a biochemistry to resist the defense; response continues to escalate.

28. Convergent Evolution
Unrelated species evolve similar traits even though they live in different parts of the world.
Occurs in environments that are geographically far apart but have similar ecology and climate.

29. Rate of Speciation
Evolution proceeds in small, gradual steps according to a theory called gradualism.
Punctuated equilibrium explains rapid spurts of genetic change causing species to diverge quickly.