Download Lesson 23 Natural Selection: A Mechanism for Change (3

Lesson 23 Natural Selection: A Mechanism for Change (3.4.2)
MECHANISMS OF EVOLUTION
Mechanisms of evolution deal with how evolution occurs. Most scientists consider
natural selection one of the most important mechanisms of evolution. Other mechanisms
including mutations, gene flow, genetic drift and various disease agents are also
significant.
NATURAL SELECTION
Naturalist Charles Darwin proposed the idea of natural selection in 1859. Natural
selection states that organism best suited to the environment are the ones most likely to
survive and reproduce. Some points from The Origin of Species for the basis of natural
selection are:
*Resources are limited in all environments. The availability of food, water and shelter
in an environment is limited. This leads to competition among organisms for food, space,
habitats and mates.
* Most organisms have more offspring than the environment can support. For
example, one tree produces millions of seeds, but only a small number of seeds germinate
and become full grown trees.
* There is natural variation within a population. A variation is a difference in a trait
between organisms with in a population. Not all organisms are exactly alike. For
example not all earthworms are the same length or nor all robins have the same length of
beaks.
* Natural selection is always taking place. Organisms with traits that are the most
desirable are selected to survive. Organisms in any environment have a specific fitness
for the environment. Fitness is the ability of an organism to live, survive and reproduce
in that environment. Because of an organism’s fitness, the organism is by nature selected
for survival. Variations in physical characteristics make some organism better suited or
more fit, to live in their environments. Much of this variation is inherited.
A classic example of natural selection is represented by
two varieties of English peppered moth that underwent
a change in gene frequencies during the industrial
revolution of the 1800’s. A light variety and a dark
variety lived in regions where the landscape was
darkened when industrial pollution killed lichens on
trees and soot accumulated leaving darkened tree
trunks. Dark moths increased in number and the light
colored moth almost became extinct because the
peppered colored moths were more visible to birds that eat moths.
TYPES OF NATURAL SELECTION
Natural selection acts on variation and can affect the frequency of a heritable trait in a
population in three different ways. Selection depends on which phenotypes in a varying
population are favored.
STABILIZING SELECTION
When average individuals in a population are
favored based on their genetic traits, it is known
as stabilizing selection. Medium sized lizards
may be more efficient at finding food as
compared to smaller or larger lizards.
DISRUPTIVE SELECTION
When two extreme traits are favored, it is known
as disruptive selection. Clams with either
extreme shell color are favored on a beach with
both light and dark sand. Clams with medium
colored shells are selected against.
DIRECTIONAL SELECTION
When organisms with one of the extreme
variation traits are favored, it is known as
directional selection. Termites are building
their nests deeper into the ground; therefore ant eaters with the longest snouts are
favored.
PATTERNS OF EVOLUTION
VARIATION
Environmental conditions also
contribute to a range in traits among
individuals of the same species know as
variation. The size of house sparrows
in North America varies depending on
location. House sparrows living in
colder climates are larger than those living in the warmer climates. As a general rule, the
larger the body size of an animal, the more body heat it can trap or conserve.
The size of extremities, such as ears or legs in some animals, also demonstrates
environmental differences. Since extremities give off heat to help cool the animal’s
body, mammals living in hot climates tend to have larger ears and longer legs than their
cousins in cooler climates. An African elephant has much larger ears that an Asiatic
elephant, and a desert jackrabbit has much larger ears than a rabbit found in a temperate
(cooler) climate. Natural variation within a population allows for some individuals to
survive over other individuals in a changing environment. This natural variation can
eventually lead to the formation of new species, which is also called speciation.
GENE FLOW
Gene flow is the change in occurrence of genes in a
population. A population is the group of organisms of
the same species in a given geographic area during a
given time. Gene flow occurs when individuals leave a
population or a new individual join a population.
Immigration occurs when organisms enter into a new
population, and emigration occurs when organisms
leave a population. Gene flow tends to increase the
similarity of individuals from different populations,
since these individuals share their genes with each other
through reproduction. Emigration often leads to the
formation of new species.
Gene flow happens easily in plants that have seeds carried by wind. The wind carries the
seeds of a plat from one population to another population. When these new seeds grow
into plants, the plants can cross-pollinate with the existing plants, and genes from
different populations are shared.
A change in the geographic make up of the earth can
affect natural selection and lead to speciation. If a
natural disaster causes a river to change routes, or an
earthquake causes a break in the earth, or a
mountain, a population of organisms may be
separated. Geographic isolation is when a barrier
(river, mountain, crack in the earth, etc) cause a
population of organisms to become separated from
one another. Because these organisms can no longer
mate and share genes, speciation is often favored. A
good example of this is two squirrel populations on
each side of the Grand Canyon. Over time, erosion has lead to the formation of the
Grand Canyon. As the canyon become too deep and too wide for the squirrels to get
across, they were no longer able to mate with one another. These populations of squirrels
adapted individually to their environment and eventually became two species of squirrels.
Today, there is a population of gray squirrels on one side of the canyon and a population
of red squirrels on the other side. Geographic isolation has caused one population of
squirrels to become two different species.
GENETIC DRIFT
Genetic drift provides random changes in the occurrences of genes through chance
events. It can occur if a large number of the population is killed because of the disease,
starvation, and change in natural environment or a natural disaster. When this happens to
a population, it is called bottlenecking. When bottlenecking occurs, a large population is
reduced to a few individuals, and the genes of subsequent generations become very
similar. Inbreeding between these few individuals leads to populations that have very
few genetic differences. It is believed that African cheetahs went through two
bottlenecks, one about 10,000 years ago and one about 100 years ago. All African
cheetahs alive today are descendents of a few cheetahs. Because cheetahs are genetically
similar, they have become very susceptible to diseases.
CO-EVOLUTION
Co-evolution occurs when two or more organisms in an
ecosystem evolve in response to each other. Co-evolution is
believed to occur frequently with flowers and their
pollinators. Hummingbirds have long narrow beaks, an
attraction to the color red and a poor sense of smell. The
fuchsias plant, whose flowers bloom in various shades of
red, emits little fragrance and have long, narrow flowers.
Fuchsias rely on hummingbirds as their pollinators. Over
time, the red-flowered fuchsias and the long beaked
hummingbirds have had great success as partners in survival.
Fly orchids look and smell similar to female wasp. The male
“mates” with the female and transfers pollen from one flower
to the next.
Bacteria have been evolving in response to antibiotics as well. Bacteria have the ability
to easily adapt to their environment. Penicillin was the first antibiotic to be developed,
and it was used for every bacterial infection. Today penicillin, although still widely used,
cannot be used for a number of bacterial infections. Scientists have developed new
antibiotics, but bacteria are rapidly evolving to become resistant to these antibiotics as
well. There are some bacterial infections that no antibiotics work on. Over use and
misuse of antibiotics are contributing to these problems. When people do not finish the
antibiotics prescribed to them, not all the intended bacteria in their body are killed off.
Bacteria are able to exchange DNA through a process known as conjugation. Bacteria
that are not resistant to a certain antibiotic can become resistant through conjugation.
This is why it is very important to only use antibiotics when it is a known bacterial
infection, and to finish the entire dose. You must kill the entire intended bacteria
population or you run the risk of that antibiotic no longer working. Many pests are also
becoming resistant to the pesticides and fungicides that farmers are using as well. New
pesticides and fungicides must continuously be created to keep up with the pests and
fungi that are so quickly evolving to become resistant.
Activity
Study the graphs and match the two scenarios with each of the graphs. Create a
scenario for the unmatched graph.
1. The average height of humans has shifted to become taller. This has been
supported by studying soldier’s uniforms.
2. Babies that are breast feed get fewer infections in their formative years as
compared to babies that are not breast feed and therefore require fewer antibiotics.
3. Create a scenario for the unmatched pair.
Lesson 23 Review: Mechanisms and Patterns of Evolution
A. Define the following terms.
natural selections
fitness
selection
directional selection
speciation
isolation
immigration
emigration
bottle necking
genetic drift
stabilizing selection
disruptive
gene flow
geographic
co-evolution
variation
conjugation
B. Choose the best answer.
1. What are effects of genetic drift and gene flow?
A. change in gene occurrences
C. change in DNA replication patterns
B. change in vision acuity
D. change in organism size
2. Which of the following are patterns of evolution?
A. structural replication, reproduction homology and special creation
B. metabolic pathways, hormonal indicators and genetic studies
C. modern creationism, fossil theory and punctual model
D. convergent evolution, co-evolution and divergent evolution
3. A classic example of natural selection is supported by evidence that organisms
underwent a
change in gene frequencies as a result of environmental changes.
A. Lamarck’s giraffe
C. Darwin’s finches
B. peppered moth
D. both B and C
4. Darwin identified at least 13 different species of finch during his time on the
Galapagos
Islands. The main difference between the finches was the size and shape of their
beaks.
Which of the following statements best describes these differences?
A. the beaks differ because they are vestigial structures
B. the beaks differ because they are analogous structures
C. the beaks differ because selection acts directly on phenotype
D. the beaks differ because selection acts directly on genotype
5. Which scientist’s theory of evolution had the following two parts: (1) use and disuse,
and
(2) acquired characteristics?
A. Darwin
B. Lamarck
C. Pasture
D. Mendel
C. Complete the following exercises.
1. Explain the theory of natural selection
2. How is variation beneficial to an organism? How is it harmful?