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Topic 5.2: nAturAL SELECTIOn
Natural Selection
The process of natural selection occurs in response to certain conditions:
• There is genetic (inheritable) variation within a population (caused by mutations, meiosis and sexual reproduction)
• There is competition for survival (species tend to produce more offspring than the environment can support)
• Environmental selection pressures give rise to differential rates of reproduction
• Organisms with beneficial traits are likely to survive and reproduce, while those less well adapted produce less offspring
• Over generations, these beneficial traits become more common (evolution = a change in allele frequency in a gene pool)
dull
bright
Variation
Competition
Adaptations
Selection
Overview
Selection Pressures
The key components to the process of natural selection are:
Examples of environmental selection pressures include:
•
•
•
•
•
•
Inherited variation
Competition
Environmental selection
Adaptations
Genotype frequency changes
Evolution occurs
•
•
•
•
•
•
Predator / prey dynamics
Abiotic factors (e.g. climate)
Nutrient supply (food source)
Diseases / pathogens
Available resources (e.g. light)
Space requirements (habitat)
Adaptations
Adaptations are traits that make an individual suited to its environment and way of life
• Adaptations can be structural, behavioural, physiological, biochemical or developmental
Populations will evolve different adaptations according to environmental conditions
• The functional position of an organism in the environment is its ecological niche
When members of a species occupy a variety of different ecological niches, it will lead to
the rapid diversification of the original ancestral line (this is called adaptive radiation)
• An example of this can be seen in the changes in beaks of finches on Daphne Major
insects
leaves
seeds
nectar
Ancestor
grubs
tool use
Examples of Evolution
Certain types of bacteria have developed antibiotic resistance
• These strains are more prevalent where antibiotics are
commonly used (e.g. hospitals)
1
2
3
4
Both strains
exist normally
Antibiotics kills
susceptible strains
Resistant
strains thrive
Conjugation may
transfer trait
The peppered moth displays two distinct melanic forms
• The frequency of these forms has evolved with pollution
levels (dark colors thrive when trees are covered in soot)
Industrial period
(black moths more common)
Post-Industrial period
(white moths more common)
5.2 Natural Selection
Selection Process
Explain how natural selection leads to evolution
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Identify the three primary sources of genetic variation
1. …………………………………………………………………………………………………………………………………………………...
2. …………………………………………………………………………………………………………………………………………………...
3. …………………………………………………………………………………………………………………………………………………...
List three examples of selection pressures
1. …………………………………………………………………………………………………………………………………………………...
2. …………………………………………………………………………………………………………………………………………………...
3. …………………………………………………………………………………………………………………………………………………...
Give examples of the different types of adaptations
Structural: ………………………………………………………………………………………………………………………………………
Behavioural: ……………………………………………………………………………………………………………………………………
Physiological: ………………………………………………………………………………………………………………………………….
Biochemical: ……………………………………………………………………………………………………………………………………
Define adaptive radiation
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Selection Examples
Using the examples provided, explain how evolution occurred in response to environmental change
Peppered Moth:
Polymorphic Variants: ……………………………………………………………………………………………………………………..
Selection Pressure: ………………………………………………………………………………………………………………………….
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Antibiotic Resistance:
Polymorphic Variants: ……………………………………………………………………………………………………………………..
Selection Pressure: ………………………………………………………………………………………………………………………….
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Describe the evolutionary changes to the beaks of Darwin’s finches
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Name _________________________________ Per____
Patterns of Natural Selection Worksheet
Define the following terms:
1. stabilizing selection________________________________________________________________________________________
__________________________________________________________________________________________________________
2. directional selection _______________________________________________________________________________________
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3. disruptive selection ________________________________________________________________________________________
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Write the correct type of selection below (Disruptive, Directional, or Stabilizing). Then draw what you would expect on the graph.
___________________________Starlings produce an average of five eggs in each clutch. If there are more than five,
the parents cannot adequately feed the young. If there are fewer than five, predators may destroy the entire clutch.
As a result, five eggs becomes the most common clutch size.
___________________________Seed cracker birds have either large beaks or small beaks. They do not have medium
sized beaks because medium sized beaks do not allow for adequate cracking of seeds.
___________________________A scientist measures the circumference of acorns in a population of oak trees and
discovers that the most common circumference is 2 cm. There are very few acorns with circumferences of 3 cm or
circumferences of 1 cm. Only the 2 cm circumference acorns survive.
___________________________A population of Madagascar hissing cockroaches lives in a woodpile. The
cockroaches are eaten by lizards. Because the lizards have small heads, the lizards are unable to eat the very largest
adult cockroaches, and instead prey upon small and medium sized adults. Over time, only the large headed lizards
survive.
___________________________ Female birds that lay close to the optimum number of eggs have the most surviving
offspring. Those that lay fewer or more eggs have lower relative fitness.
____________________________A population of birds with various size beaks eats seeds. Small seeds can be eaten
by birds with small beaks. Larger, thicker seeds can only be eaten by birds with larger, thicker beaks. During a
drought, only large thick seeds exist so only the large, thick-beaked birds survive.
____________________________Black rabbits (BB) and white rabbits (bb) are both able to survive because they can
camouflage into the white and black rocks in their environment. However, the intermediate gray rabbits (Bb) do not
survive. This results in only white and black rabbits.
____________________________Small lizards have difficulty defending territories so they end up dying out. Large
lizards are more likely to be preyed upon by owls. Over time, only medium sized lizards end up surviving.
____________________________Women often have complications during labor while giving birth to very large
babies, whereas very small babies tend to be underveveloped. As a result, medium sized babies are the ones that
end up surviving to adulthood.
Label the three types of selection illustrated by the graphs below.
Name _________________________________ Per____
For your study, you will be examining a specific population of spiny cacti, which is originally in genetic equilibrium. A road is built
quite close to the study site, which keeps away the treacherous and parasitic insects, but with the road comes the tourists. In many
desert areas of the United States, passing cactus lovers like to pick up a souvenir cactus to take home with them after a day-trip out
into the desert. This is a serious problem in some areas because the tourists always take the better looking cacti, and these happen
to be the ones with the middle-spine-numbers. Years of collecting have left their toll on the roadside cacti. Using this background
knowledge and the data collected below; create a graph of both the original population of cacti and the current population on a
piece of graph paper. Place the number of cacti on the Y axis (count by 1’s) and the number of spines on the X axis (start at 70 and
count by 5’s). Remember to label your x and y axis and give your graph a title.
What type of selection does this graph represent?_______________________________
AP Biology
N am e/H r: _________________________ Evolution by Natural Selection
What are some differences you have noticed in the size and shape of dogs in your neighborhood?
Do their differences make them better at some things, but not well suited for others?
What would happen if the world suddenly changed, so the only thing that dogs could eat was deer and there was
absolutely no way for a dog to eat if it wasn’t big or strong enough to catch and kill a deer?
Next, think about the mice shown below. Using the space below, describe what is happening in figures 1-3.
Living things that are well adapted to their environment survive and reproduce. Those that are not well adapted don’t
survive and reproduce. An adaptation is any characteristic that increases fitness, which is defined as the ability to
survive and reproduce. What are some characteristics of animals or plants that affect their fitness?
Below are descriptions of four female mice that live in a beach area that is mostly tan sand with scattered plants.
According to the definition given for fitness, which mouse would biologists consider the fittest? Explain why this mouse
would be the fittest.
Color of fur
Age at death
# pups produced by each female
Running speed
Brown
2 months
0
8 m/min.
Tan
8 months
11
6 m/min.
Tan and Brown
4 months
3
7 m/min.
Cream
2 months
0
5 m/min.
A more complete definition of fitness is the ability to survive and produce offspring who can also survive and reproduce.
Below are descriptions of four male lions. According to this definition of fitness, which lion in the table below would
biologists consider the “fittest”? Explain why.
Name
George
Dwayne
Spot
Tyrone
Age at death
13 years
16 years
12 years
10 years
# cubs fathered
19
25
20
20
# cubs surviving to adulthood
15
14
14
19
Size
10 feet
8.5 feet
9 feet
9 feet
(Adapted from Michigan State University, Occasional Paper No. 91, Evolution by Natural Selection: A Teaching Module
by Beth Bishop and Charles Anderson, 1986)
Suppose that Tyrone had genes that he passed on to his cubs that helped his cubs to resist infections, so they were more
likely to survive to adulthood. These genes would be more common in the next generation, since more of the cubs with
these genes would survive to reproduce. A characteristic that is influenced by genes and passed from parents to offspring
is called heritable.
Over many generations heritable adaptive characteristics become more common in a population. This process is called
evolution by natural selection. Evolution by natural selection takes place over many, many generations.
Evolution by natural selection leads to adaptation within a population. The term evolution by natural selection does not
refer to individuals changing, only to changes in the frequency of adaptive characteristics in the population as a whole.
For example, for the mice that lived in the beach area with tan sand, none of the mice had a change in the color of their
fur; however, due to natural selection, tan fur was more common for the pups than for the mother mice.
In summary, a heritable characteristic that helps an animal or plant to have more offspring that survive to reproduce will
tend to become more common in a population as a result of evolution by natural selection.
Questions
1. Explain why a characteristic that helps an animal to live longer will generally tend to become more common in
the population as a result of evolution by natural selection.
2. Not all characteristics that contribute to longer life become more common in the population. Some characteristics
contribute to long life, but not more offspring. For example, a female cat that is sterile and cannot have any
offspring may live longer because she will not experience the biological stresses of repeated pregnancies. Explain
why a characteristic like this which contributes to a long life, but with few or no offspring, would not become
more common as a result of evolution by natural selection.
Below is a series of pictures representing changes in a population of cacti over many generations.
1
2
3
4
3. Why would a deer be more likely to eat the left cactus than the right cactus?
4. In figure 3, the right cactus has flowers, but the cactus that has been eaten by the deer is too damaged to make
flowers. Figure 4 shows the situation several months later. What has happened? Do you think that evolution by
natural selection is occurring in this cactus population? Explain why or why not.
5. What adaptation seems to increase the fitness of the cacti? How do you think this adaptation increases the ability
of cacti to both survive and reproduce?
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