CK-12 Biology I - Honors
Page 56
Figure 12.33
The peppered moth population changed from mostly light (left) to mostly dark (right) as the lichen-covered trees in Englands forests absorbed soot from the Industrial Revolution. Now, as pollution is being cleaned up, the moth population is returning to its former proportion of light moths. These changes illustrate what famous idea?
Much more intentionally, biologists Peter and Rosemary Grant have devoted more than 30 years to a study of two species of Darwin’s finches on one of the Galapagos islands (Figure below). Catching, weighing, and recording the seed species eaten by hundreds of these birds, they have witnessed changes in beak size which clearly correlate with changes in weather and availability of food. A severe drought and food shortage in 1977 led to a significant change. Birds whose small beaks could not crack the tough remaining seeds died, and the larger-beaked individuals who survived reproduced. The following year, offspring were larger bodied and larger-beaked, showing that natural selection led to evolution. A rainy winter in 1984-1985 reversed the trend; more soft seeds were produced, and the smaller beaked finches survived and reproduced in greater numbers than their large-beaked cousins.
Figure 12.34
A large cactus ground finch crushes a seed on the island of Espanola in the Galapagos archipelago. Peter and Rosemary Grant studied two closely related species of Darwins finches and recorded changes in beak size and body size which paralleled changes in weather. How fitting that they should demonstrate natural selection in action something Darwin did not think possible using one of the species he made famous!
Jonathan Winter eloquently describes the Grants’ work and discoveries in his Pulitzer Prize-winning The Beak of the Finch, A story of Evolution in our Time. His words urging that we see evolution as ongoing for all life make a fitting conclusion to this lesson and chapter:
“For all species, including our own, the true figure of life is a perching bird, a passerine, alert and nervous in every part, ready to dart off in an instant. Life is always poised for flight. From a distance it looks still, silhouetted against the bright sky or the dark ground; but up close it is flitting this way and that, as if displaying to the world at every moment its perpetual readiness to take off in any of a thousand directions.”
(Source: http://en.wikiquote.org/wiki/Beak_of_the_Finch)
Lesson Summary
The process of evolution by natural selection continues to change our world and our selves, both despite and because of our best efforts to control it.
Beyond Darwin’s expectations, we have added direct observation of natural selection to the overwhelming evidence for evolution.
Humans have designed and produced crops, work animals, and companions through artificial selection.
Cultivation of crops gave us the freedom to develop civilization.
Hybridization improves the yield of crop species and adapts them to various environments.
Habitat destruction is destroying raw materials for hybridization, and “escape” of “artificial” genes is “polluting” wild species.
Cloning has the potential to reproduce exact copies of selected individuals, but it goes against the principles which govern natural selection.
Genetic engineering, like traditional methods of breeding and domestication, designs medicines, plants, and animals to suit our goals.
Unlike traditional breeding, genetic engineering chooses single genes and can transfer them from one species to another completely unrelated species – making it faster, more precise, and far more powerful.
In both GE and traditional breeding, the potential for genetic pollution remains. Pollution is probably more likely for genetic engineering because developments proceed so quickly.
Products of genetic engineering include insulin and growth hormone, vaccines in milk and bananas, produce with longer growing season and shelf life and more nutrition.
Michael Pollan suggests that we are coevolving with our domesticated crops, animals, and pets, rather than producing them – in other words, that our products are domesticating us as we domesticate them!
Bacteria have developed serious levels of resistance to antibiotics because humans have introduced a new selective force into their environments (our bodies).
An individual bacterium has its own set of genes. If these genes do not confer resistance to antibiotics, the bacterium by itself cannot develop resistance. A population can develop resistance if some of its members have, by chance, the gene for resistance.
The evolution of antibiotic resistance has already resulted in a number of bacteria resistant to most known antibiotics; these are sometime called “superbugs.”
Actions you can take to prevent or slow the evolution of antibiotic resistance include:
Don’t take antibiotics for viral infections.
Take prescribed antibiotics exactly as prescribed.
Never take antibiotics which are left over or belong to someone else.
Consider purchasing meats from animals not treated with antibiotics.
Consider purchasing organic produce.
Resist the use of pesticides in your own gardens.
Viral epidemics occur when chance viral mutations adapt the virus to new hosts or new methods of transmission.
Peppered moth populations changed color as the Industrial revolution changed the color of their habitat.
Peter and Rosemary Grant studied two closely related species of Darwin’s finches and recorded changes in beak size and body size which paralleled changes in weather.
Review Questions
List the ways in which we have directly observed evidence for evolution and/or natural selection.
Describe the importance of artificial selection to human life.
What is genetic pollution and why does it matter?
Compare cloning to natural selection.
Give examples of useful products of genetic engineering.
Explain Michael Pollan’s ideas about our relationship with our domesticated crops, animals, and pets, and give your opinion about them, using examples from your own experience.
Use the concept of natural selection to explain the resistance of bacteria to antibiotics and insects to pesticides.
Explain why an individual bacterium cannot on its own change from sensitive to resistant to antibiotics.
Choose two actions you think would be most likely to control the increase in antibiotic resistance, and support your choices with examples from your own experience.
In what way do viral epidemics demonstrate evolution?
Further Reading / Supplemental Links
Michael Pollan, 2001. The Botany of Desire, Random House, 2002.
David Quammen, 1997. The Song of the Dodo: Island Biogeography in an Age of Extinctions. Scribner.
Carl Sagan, 1980. Cosmos. Random House New Edition, May 7, 2002, 384 pgs -- also available in video and DVD, as Cosmos: A Personal Voyage.
Jonathan Weiner, 1994. The Beak of the Finch: A Story of Evolution in Our Time. Alfred A. Knopf.
http://www.fda.gov/oc/opacom/hottopics/anti_resist.html
http://www.cdc.gov/drugresistance/community/
http://whyfiles.org/038badbugs/
http://www.niaid.nih.gov/factsheets/antimicro.htm
http://www.who.int/mediacentre/factsheets/fs194/en/
http://www.who.int/whopes/resistance/en/
http://library.thinkquest.org/19697/
http://www.fda.gov/fdac/features/2003/603_food.html
http://www.msichicago.org/exhibit/genetics/engineering.html
http://www.cdc.gov/flu/avian/
http://www.who.int/csr/disease/avian_influenza/en/
http://www.biologycorner.com/worksheets/peppermoth_paper.html
http://www.biologycorner.com/worksheets/pepperedmoth.html
http://bsgran.people.wm.edu/melanism.pdf
http://www.millerandlevine.com/km/evol/Moths/moths.html
http://crustacea.nhm.org/people/martin/publications/pdf/103.pdf
&n
bsp; Vocabulary
artificial selection
Animal or plant breeding; artificially choosing which individuals will reproduce according to desirable traits.
cloning
The process of creating an identical copy of an organism.
coevolution
A pattern in which species influence each other’s evolution and therefore evolve in tandem.
genetically modified organism (GMO)
An organism whose genes have been altered by genetic engineering.
genetic engineering
The manipulation of an organism’s genes, usually involving the insertion of a gene or genes from one organism into another.
genetic pollution
The natural hybridization or mixing of genes of a wild population with a domestic or feral population.
geologic time
Time on the scale of the history of Earth, which spans 4.6 billion years.
mutation
A change in the nucleotide sequence of DNA or RNA.
natural selection
The process by which a certain trait becomes more common within a population, including heritable variation, overproduction of offspring, and differential survival and reproduction.
transgenic animal
An animal which possesses genes of another species due to genetic engineering.
Points to Consider
To what extent do you think that humans have removed themselves from natural selection?
In what ways do you still feel subject to “natural” selective pressures?
How effective do you think the measures to limit evolution of antibiotic resistance will be? Are you willing to support them?
Do you think the benefits of genetic engineering outweigh the risks? Are there certain products you support, and others you oppose? Which ones, and why?
Chapter 13: Evolution in Populations
Lesson 13.1: Genetics of Populations
Lesson Objectives
Analyze the relationship between Darwin’s work and Mendel’s discoveries.
Explain the goal of population genetics.
Describe the relationship between genes and traits.
Differentiate between genes and alleles.
Connect alleles to variations in traits.
Distinguish environmental effects on gene expression from allelic variations in genes.
Describe the relationship between mutations and alleles.
Explain the causes and random nature of mutation.
Compare rates of mutation in microorganisms to those in multicellular organisms.
Analyze the ways in which sexual reproduction increases variation.
Relate mutation and sexual reproduction to natural selection.
Explain why populations, but not individuals, can evolve.
Define a population’s gene pool.
Distinguish between a population’s gene pool and a gene pool for a single gene.
Analyze the usefulness of the gene pool concept.
Explain how to determine allele frequencies.
Define evolution in terms of allele frequencies.
Discuss what is meant by a population which is fixed for a certain gene.
Show how allele frequencies measure diversity.
Evaluate the significance of a change in allele frequency.
Introduction
If you have ever taken something apart to find out how it works, you will understand biologists’ delight in exploring, since Darwin's findings, how evolution actually works. Darwin gave us the keys to unlocking this mystery by describing natural selection and common ancestry as a scientific explanation for the similarities and differences among the millions of Earth’s species – living and extinct. However, his theory depended on the ideas that traits could vary, and that variations were heritable – and even Darwin was puzzled as to how this might work. Another biological giant, Gregor Mendel, was a contemporary of Darwin’s, but his now-famous work with pea plant inheritance was not widely known or appreciated until after both scientists had died. During the 20th century, “re-discovery” of Mendel’s work stimulated extensive research in genetics, and the identification of DNA as the universal genetic material of life, brought “heritable variation” into sharp focus. A branch of the new field of Population Biology (discussed in the Populations Chapter) finally combined what Mendel and Darwin had begun separately - the exploration of population genetics and the evolution of populations.
Genetics of Populations
You have studied genetics, DNA, and Darwinian evolutionary theory in previous chapters. You have had the opportunity to learn far more about biology than either Darwin or Mendel could imagine. You are prepared, then, to join biologists in exploring how evolution works. You know that biologists have massive amounts of evidence that natural selection and evolution happen. This chapter will explore what we know about how molecules, genes, and populations change – with the ultimate goal of understanding how the “Origin of Species” produced the vast diversity of life on Earth.
Genes and Alleles
As you’ve learned, each cell of an organism contains all the information needed to build the entire organism in the DNA of its chromosomes (Figure below). A gene is a segment of DNA which has the information to code for a protein (or RNA molecule) (Figure below). For many genes, the protein product controls or at least contributes to a particular trait. For example, enzymes are proteins which are important in the biochemical reactions controlling many cellular processes.
Figure 13.1
The source of variation required for natural selection is not traits or characters, as Darwin predicted, but the underlying genes, which were not discovered until Mendels work became known. Genes are segments of DNA located at a particular place on a chromosome. The genes sequence of bases codes for a protein - often an enzyme that catalyzes a particular chemical reaction in the cell. The chemical reaction determines or contributes to a physical trait or behavior.
Figure 13.2
A gene is a segment of DNA which codes for a protein. Proteins, in turn, determine traits. Changes in genes (mutations) cause changes in proteins, which in turn produce variation in traits.
For example, the gene for the enzyme tyrosinase controls the chemical reaction which makes melanin, a brown-black pigment which colors the skin and hair of many animals, including humans. The gene is the “recipe” for tyrosinase; tyrosinase is the protein, and the trait is coloring of the skin or fur. The rabbit in Figure below has brown fur because its DNA contains a gene for tyrosinase.
Figure 13.3
Most wild rabbits have brown fur because their DNA contains a gene which codes for tyrosinase an enzyme which makes the pigment, melanin.
Genes often have different forms – slightly different nucleotide sequences – known as alleles. In some rabbits, an alternative form of the gene for fur color makes non-functional tyrosinase. A change in the sequence of As, Ts, Cs, and Gs changes the sequence of amino acids in the protein and alters or destroys its activity. This allele is recessive, and can cause lack of pigment – an albino rabbit (Figure below). We will refer to the dominant gene for brown fur as B, and the recessive gene for albinism as b.
Figure 13.4
If a rabbit receives two copies of the mutant allele for tyrosinase, it will lack pigment altogether a condition known as albinism. Although these rabbits are obviously domesticated, mutations leading to albinism do occasionally occur in wild populations.
Recall that Mendel showed that humans and rabbits have two copies of each gene, or "heritable unit" – one from each of our parents. The two alleles we received make up our genotype. For simple Mendelian traits, our genotype determines our physical appearance, or phenotype, for that trait. A rabbit having two copies of the mutant, recessive gene cannot make tyrosinase – or melanin; its genotype is bb, and its phenotype is albino. A rabbit having either one or two copies of the gene for tyrosinase makes melanin; its genotype is either Bb or BB, and in either
case, because B is dominant, its phenotype is brown. Brown rabbits which have different alleles (genotype Bb) are heterozygous, and brown rabbits with identical alleles (BB) are homozygous. Albino rabbits with identical recessive alleles (bb) are also homozygous.
Keep in mind that the environment can influence the expression of a gene – its transcription and translation to produce a protein. By preventing or promoting the expression of certain genes, the environment can cause variation in traits, but note that this influence is not at all goal-directed and does not result in heritable change. If you do not water or fertilize your garden, the plants will be short and stunted, but their small size is not a heritable change. Let’s return to rabbits to see how this kind of variation happens at the level of genes and molecules.
A third allele for rabbit fur coloration codes for a form of tyrosinase which is temperature-sensitive. At low temperatures, the enzyme makes melanin normally. However, higher temperatures denature the enzyme in the same way that cooking changes egg white; at higher temperatures, the enzyme is misshapen and cannot make melanin. The rabbit in Figure below is homozygous for temperature-sensitive tyrosinase. The main part of its body, which has a higher temperature, is white because its tyrosinase does not work at high temperatures. The tips of the rabbit’s ears, nose, and feet, however, are black, because the enzyme can work at the slightly lower temperatures of these extremities. What do you think such a rabbit would look like if it developed in the arctic? The tropics?
Figure 13.5
Environment can influence the expression of a gene. California rabbits are bred for a temperature-sensitive allele of the pigment-producing enzyme tyrosinase. Cooler extremities produce melanin, but warmer body parts do not.