Biology 100/101
Lectures 23 and 24
Microevolution: The Forces of Evolutionary Change
(Print Version)

Announcements

Objectives

Web Resources

What is Evolution?

Darwin's Ideas

Examples of Microevolution

Conditions Causing
Evolutionary Change

Natural Selection

Balanced Polymorphism

Artificial Selection

Nonrandom Mating

Mutation

Genetic Drift

Lecture Syllabus

IB 100/101 Home Page

Text Readings in Lewis et al.	Testing Your Knowledge	Thinking Scientifically
Ch. 14, The Evolution of Evolutionary Thought Ch. 15, The Forces of Evolutionary Change	Pg. 283, Questions 2, 4, 6 Pg. 298, Questions 1, 5, 6	Pg. 283, Questions 1, 4, 6 Pg. 298, Questions 1-3

Much of the material cited in this lecture outline came from your textbook (Lewis et al., 2004, Life Fifth Edition). It is highly beneficial to read these chapters carefully before your final exam.

You may also ask questions and see answers to your classmates' questions in Web Crossing in the "Talk to Ross and Ed" discussion.

Objectives:

After studying this material you should be able to:

1. Describe biological evolution in terms of change in allele frequency in a population.
2. Explain Darwin's main ideas concerning evolution by natural selection.
3. Describe what is meant by artificial selection and how it compares to natural selection.
4. Describe the role of nonrandom mating and sexual selection in the process of microevolution.
5. Explain how genetic drift (including bottleneck events and founder effects) and gene flow via migration could change allele frequencies of a population.
6. Describe the evolutionary mechanism leading to the rise of antibiotic resistant strains of bacteria, or the development of heavy metal-tolerant invertebrates in Foundry Cove, NY, or industrial melanism.
7. Describe an example in which natural selection has affected the virulence and/or spread of a human disease.
8. Describe the results of directional, disruptive, and stabilizing selection.
9. Explain the concept of balanced polymorphism and give an example of it.
10. Understand the relationships among these terms:

biological evolution	artificial selection	natural selection
microevolution	macroevolution	allele frequencies
directional selection	stabilizing selection	disruptive selection
genetic variation	balanced polymorphisms	mutation
genetic drift	Hardy-Weinberg equilibrium	bottleneck
founder effect	gene flow	random mating

Web Resources:

These are excellent starting points for Web Crossing Assignment #4.

• PBS Evolution Series
• PBS Evolution Library
• PBS Teaching and Learning Guide for Evolution
• Evolution Web Sites from the University of Virginia.
• Evolutionary Biology from the Talk.Origins archive.
• Evolution, from Wikipedia Encyclopedia
• EvoDots, Evolution Simulation Software, by Jon C. Herron, to explore evolution by simulating natural selection in a population of "dots."

Lecture Activity

1. Get together in small groups.
2. Each person print and sign your name on one piece of paper. Also include your TA's name(s) and/or section(s).
3. Discuss and answer (briefly!) the following questions:

What is (biological) evolution?

What factors act to increase genetic variation in a natural population?

What factors act to decrease genetic variation in a natural population?

Give one example of evolutionary change.

What is Evolution?

• Evolution is genetic change in a population over time. Specifically, a change in allele frequency (percentage) from one generation to the next. This has been called microevolution, and such changes can take place over a relatively short time periods.
• Recall, a population is a group of organisms of the same species in a given geographic location (Lewis et al., Life, pg. 838). The Glossary on pg. 938 gives a slightly different wording of the same concept, "A group of interbreeding organisms living in the same area." Because an individual cannot change his or her genes, we focus on populations as the targets of evolution.
• Macroevolution, the subject of our last set of lectures, represents accumulated changes in allele frequency in two populations that preclude their interbreeding leading to the formation of new species (or their extinction). Often, macroevolutionary changes tend to span very long time periods.
• The "raw material" of evolution is inherited variation (through sexual reproduction and mutation, with the latter having potential to introduce new alleles/traits into the population).
• All the genes and their alleles in a population constitute the population's gene pool.
• The proportion of different alleles for each gene determines the characteristics of that population.
• Changing allele frequencies (percentages) within populations over time "are the small steps of change that collectively drive evolution."
• Figure 15.6f, Diagram representing changing allele frequency in a population
• Many factors can alter allele frequencies.
• Evolution is an ongoing process.

Darwin's Main Ideas on Evolution

or, Natural Selection as the Mechanism for Biological Evolution

Darwin's Finches (14 of them, each slightly different, on islands that differ slightly in habitat).
Fig. 14.5, Finch beak shape reflects natural selection.

Adaptive radiation--the divergence of several new types of organisms from a single ancestral type.

Table 14.1, Darwin's Main Ideas (Listed below)

Darwin's Observations of Nature

• Organisms vary and some variations are inherited. Within a species, no two individuals are exactly alike
• More individuals are born than survive to reproduce
• Individuals compete with one another for limited resources that enable them to survive

Darwin's Inferences Based on these Observations

• Within populations, the inherited traits of some individuals make them more able to survive and reproduce than others under certain environmental conditions
• As a result of the environment's selection against nonadaptive traits, only individuals with adaptive traits live long enough to transmit traits to the next generation. Individuals with adaptive traits are more likely to reproduce and increase the frequency of adaptive alleles in the population. Over time, natural selection can change the characteristics of populations (and even mold new species)
• Lewis et al. (page 935) define natural selection as "the differential survival and reproduction of organisms whose genetic traits better adapt them to a particular environment"

Some Examples of Microevolutionary Change

1. Industrial Melanism

Two peppered moths, one typical and one melanic, resting on: the dark bark of an oak tree and the algae covered bark of another.

• Peppered moths and industrial melanism
• Recent history of melanism in American peppered moths, a PDF file

2. Evolution in Disease Organisms

The study of infectious disease origin and transmission.

Emerging and reemerging infectious diseases, epidemics, and antibiotic resistance all reflect selective pressures. Microorganisms and viruses that cause infectious diseases are ever-evolving. Their very short generation times allow for a more rapid emergence of variants.

Earlier we talked about the mechanism of antibiotic resistance in bacteria. Remember the resistance genes on bacterial plasmids that arise by mutation and can be passed from bacterium to bacterium? Here we are talking about how individual bacteria carrying these resistant genes can become more frequent in the population of bacteria, "swamping out" the non-resistant bacteria.

• Evolution of Antibiotic Resistance, from PBS Evolution
• What Ricki Lewis has to say about antibiotic resistance
• Battle of the bugs: Fighting Antibiotic Resistance, from the US FDA.
• More on antibiotic resistance, from P&S Medical Review.

• HIV Evolution, from PBS Evolution
• Domesticating Diseases: Cholera and HIV, from PBS Evolution.
• Evolution and the way our bodies deal with disease, a PDF file.

Conditions that Cause Evolutionary Change in Natural Populations

Microevolution occurs when the frequency of an allele in a population changes. This may happen through:

1. Natural selection
and artificial selection for comparioson

2. Nonrandom mating

3. Mutation

4. Genetic drift

1. Natural Selection

Natural Selection is "the differential survival and/or reproductive success of individuals with particular genotypes in response to environmental challenges." (Lewis et al., page 270)

• What do you think Gary Larson is trying to illustrate here? and the real reason dinosaurs became extinct.
• Mice, men share 99 percent of genes from CNN.com. The article says "Scientists say mice, humans and many other mammals descended from a common ancestor about the size of a small rat from 75 to 125 million years ago. That creature lived alongside the dinosaur. While mice and humans certainly don't look much the same these days, their genetic blueprints are startlingly similar."
• If a population contains variation, and
  if the variation is at least partly heritable, and
  if some variants survive to reproduce at higher rates than others,
  then the population will evolve. (EvoDots Tutorial, Jon C. Herron 2002)
• Simply, some phenotypes are better adapted to a particular environment than others. Natural selection favors some phenotypes and the alleles that produce them and removes others from the population. Therefore, allele frequencies will change in response to environmental change.
• Two fundamental forces are operating: genetic variation and environmental change. Both are constantly occurring at random in every natural population. Those with more adaptive traits survive in the new environment.
• Natural selection reflects adaptation to a prevailing environmental condition. The direction of natural selection can change. A phenotype that is adaptive in one set of circumstances may be a liability in another.
• Over time, the population would change so that it could no longer breed with the original group. Eventually, a new species would arise.
• Figure 15.6f, Natural Selection

Types of Natural Selection:

• Figure 15.4a, Directional Selection. This is selection against one extreme phenotype, allowing another to gradually become more prevalent. An example of this is industrial melanism, where some 100 species of insects have undergone color changes enabling them to blend into polluted backgrounds. Another example is the rise of antibiotic resistance.
• Figure 15.4b, Disruptive Selection. Two extreme expressions of a trait each have a selective advantage, so both persist. An example is white and tan snails living among white barnacles on tan colored rocks; green colored snails are more often seen and eaten by predatory shorebirds.
• Figure 15.4c, Stabilizing Selection. Selection is for an intermediate form of a trait, as it has greater survival and reproductive success. Extreme phenotypes are less adaptive.

Balanced Polymorphisms

• Is a form of stablizing selection that maintains deleterious recessive alleles because heterozygotes are protected against another medical condition.
• Maintains a potentially lethal genetic disease in a population even though the illness diminishes the fitness of affected individuals.
• The inherited disease persists because carriers (heterozygotes) have some health advantage over those who are homozygous dominant (and don't have the disease).
Malaria and Sickle Cell Disease. Also, see Figure 15.5a in your text. SCD carriers, who do not have symptoms, are resistant to malaria. Bursting normal RBC's in those individuals who do not have or carry SCD. The malaria parasite enters the cells, reproduces, and eventually bursts the cells. In SCD carriers, about half the cells are sickled shaped. These cells are inhospitable to the parasite. Over 35 generations, the frequency of the sickle cell allele in East Africa rose from 0.1% to 45%.
• Table 15.2, Other Examples of Balanced Polymorphisms
• Further explanation of balanced polymorphism, from PBS Evolution and Ricki Lewis.

Artificial Selection: Proof of the Power of Selection

• Also called selective breeding, domestication, or selection by humans.
• The process of intentional or unintentional modification of a species through human actions which encourage the breeding of certain inherited traits over others. The breeding potential of individuals who possessed desirable characteristics is intentionally encouraged, whereas the breeding of individuals with less desirable characteristics is discouraged.
• The many breeds of domestic dogs and cats existing today are a consequence of artificial selection. Darwin raised pigeons and artificially selected several new breeds.
• Artificial Selection in Dogs
• The domestication of maize and additional background information
• Darwin used his observations of artificial selection in animals and plants to help him think about his observations of what was going on in nature.
• The same process can occur naturally. "Individuals in the wild who possess characteristics that enhance their prospects for having offspring would then undergo a similar process of change over time; although in this case "desirable" characteristics would be not those which specifically satisfy human needs, but those which enhance survivability. This natural process forms the basis of Darwinian evolution." From Wikipedia, the Free Encyclopedia.
• Artificial selection underscores the power of selection in generating evolutionary change.

2. Nonrandom Mating

(Might be considered a type of Natural Selection)

• Completely random matings (where each individual has as equal chance of mating with every other member of the population) are nearly impossible to achieve.
• Individuals seek mates within similar subpopulations within a larger population.
• Isolated populations, such as those on islands, have no choice but to mate among themselves.
• Sexual selection: the natural selection of traits that increase an individual's reproductive success. These traits contribute to attraction, courtship, or mating. Most species exhibit some sort of preferences in mate choice; the alleles for these desired traits will become more common in future generations.

Examples: Stag's anthlers; rhinoceros's horn; male insect's genitalia; elaborate feathers in male birds; courtship songs of birds.

Male and female ringed pheasants

Sex and the Single Guppy from PBS Evolution Series

and just for fun The mating Game from PBS Evolution Series

• Figure 15.6b, Nonrandom mating

3. Mutation

• Review our lecture on mutations. Remember, most mutations are neither beneficial nor useful, with no effect on phenotype. Some are harmful, resulting in defects in protein production that can lead to genetic disease.
• Sickle cell disease results from a single base change (See Figure 13.17, in Lewis et al., page 256) and Hemoglobin mutants.
• Mutations introduce new alleles (new traits) into a population by altering old alleles.
• White Lion, White Deer, and White Penguin

• Figure 15.6e, Mutation
• The genetic makeup of populations, and ultimately species, changes as natural selection permits differential survival of variants that are adapted to a particular environment.
• Other examples: A random mutation in the DNA of one bacterium that confers antibiotic resistance will permit that bacterium to live and reproduce. Eventually, all bacteria without the mutation die, and the mutant offspring thrive and reproduce. Mutations in the genes that encode certain receptors on T cells in humans protect against HIV infection.

4. Genetic Drift

• Changes in allele frequency in a population that result from RANDOM survival or reproduction of individuals with certain characteristics.
• Survival or reproduction of those individuals in the face of some environmental change is just a matter of CHANCE, not because of their phenotype or genotype.

For example: if a Florida Panther is killed by a truck on a highway, that is bad luck. The panther did not get hit because of some allele it carried.

• This contrasts with selection. In selection the environmental events that affect a population may be random, but the survival or reproduction of the individuals depends on their phenotypes and genotypes.

An example of selection: If the panther population is infected with FIV (feline AIDS), individuals with alleles that give them resistance to the disease will survive. The introduction of the virus is a random event, but survival is based on genes.

Figure 15.6d, Genetic Drift

Random Genetic Drift from the Talk.Origins archive.

Types of Genetic Drift:

Gene Flow Resulting from Migration

• Individuals migrate between populations.
• Immigrating individuals introduce new alleles by mating with members of the population they are joining.
• Emigrating individuals remove alleles from the population they leave behind.
• Gene Flow is the movement of alleles from population to population.
• Any advantage given to individuals with new alleles will change the population due to subsequent natural selection.
• Because geographic barriers greatly influence migration patterns, allele frequencies may differ between adjacent but separated geographic regions.
• Figure 15.6c, Migration

Founder Effect:

A type of genetic drift resulting in the establishment of a new, geographically isolated population from a single or very few individuals. It is very unlikely that the gene pool of a founding population is representative of the total genetic diversity of the original population.

Founder effect is different from gene flow because the migrating individuals are establishing a new population where none existed before.

White Deer of the former Seneca Army Depot

Ellis-van Creveld Syndrome. Small groups of people founding new settlements may have different allele frequencies than the original population, and may also have higher incidents of certain traits (such as genetic disorders) because they marry within the group.

Ellis-van Creveld is an autosomal recessive disease and occurs in 7% of the people in the Amish community of Lancaster County, Pennsylvania. The occurrence of the disease is high because these Amish marry among themselves. See page 291, text, for more information.

Figure 15.2a, The Founder Effect

Population Bottleneck

A type of genetic drift occurring when many members of a population die, and a few remaining individuals mate, eventually restoring their numbers. The new population has lost much of the genetic diversity that was present in the larger ancestral population.

Figure 15.3, The Bottleneck Effect, and further information on the cheetah.

Another illustrated example of a bottleneck

The Greater Prairie Chicken and the Illinois Prairie Chicken's Last Waltz

Florida Panther breeding w/Mountain Lions from Texas from Geocities.com

New Hope for the Florida Panther (from the Endangered Species Bulletin, Vol. XXI No. 2)

"Plain Ol'e Genetic Drift"

In the absence of migration into or out of the populaiton or drastic changes in population size resulting from some catastrophic die-off, the allele frequencies of a population can change because of genetic drift.

Chance events affect which individuals survive and/or reproduce in a population independent of the genetic make up of those individuals affected. Small random changes in allele frequency can "build up" over generations and result in a significant change in allele frequency.

Because such changes in allele frequency are not related to phenotype or genotype, they are classified as genetic drift.