Unit 5: Evolution through Natural Selection and other Mechanisms

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Unit 5 Evolution through Natural Selection and
other Mechanisms
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Natural Selection

• Natural selection is the process by which those
  heritable traits that make it more likely for an
  organism to survive and successfully reproduce
  become more common in a population over
  successive generations.
• It is a key mechanism of evolution.

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Genetic variation
-Genetic variation is essential for natural
selection. -The natural genetic variation within
a population of organisms means that some
individuals will survive and reproduce more
successfully than others in their current
environment.
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-Natural selection acts on the phenotype, or the
observable characteristics of an organism, but
the genetic (heritable) basis of any phenotype
which gives a reproductive advantage will become
more common in a population. -Over time, this
process can result in adaptations that specialize
organisms for particular environment.
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Sample Question 1
The idea that evolution takes place at one point
in time, followed by a long period without change
is ______. From Test bank 07-08 A gradualism B p
unctuated equilibrium C artificial
selection D succession
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Sample Question 2

• Charles Darwin proposed his theory of evolution
  based on observations of nature. Which
  observation that contributed to his theory is
  illustrated by this population of beetles?
• Species produce more offspring than can survive.
• Populations remain stable over time.
• C. Individuals within a population may vary
  widely.
• D. Environmental resources are limited.

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Genetic Drift

• -Genetic drift, along with natural selection,
  mutation, and migration, is one of the basic
  mechanisms of evolution.
• -In each generation, some individuals may, just
  by chance, leave behind a few more descendents
  (and genes, of course!) than other individuals.
  The genes of the next generation will be the
  genes of the lucky individuals, not necessarily
  the healthier or better individuals.

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Genetic drift affects the genetic makeup of the
population but, unlike natural selection, through
an entirely random process. So although genetic
drift is a mechanism of evolution, it doesnt
work to produce adaptations.
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Bottleneck and Founder Effects
-Genetic drift can cause big losses of genetic
variation for small populations. -Population
bottlenecks occur when a populations size is
reduced for at least one generation. -A founder
effect occurs when a new colony is started by a
few members of the original population.
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Sample Question 3

• A small population of chimpanzees lives in a
  habitat that undergoes no changes for a long
  period. How will genetic drift probably affect
  this population?
• A It will accelerate the appearance of new
  traits.
• B It will promote the survival of chimpanzees
  with beneficial traits.
• C It will increase the number of alleles for
  specific traits.
• D It will decrease genetic variation.

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Sample Question 4

• Genetic drift is a ______________ process.
• A. directed
• B. selection-driven
• C. random
• D. co-evolutionary

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Speciation
-A species is often defined as a group of
individuals that actually or potentially
interbreed in nature. In this sense, a species is
the biggest gene pool possible under natural
conditions. -Speciation is a lineage-splitting
event that produces two or more separate species.

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Causes of Speciation

1. Geographic Isolation
2. Reduction of Gene Flow

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Kinds of Speciation
Sympatric speciation refers to the formation of
two or more descendant species from a single
ancestral species all occupying the same
geographic location.
Allopatric speciation is just a fancy name for
speciation by geographic isolation
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Sample Question 5
River
This diagram illustrates what kind of
speciation? A Sympatric B Allopatric C Geographi
c D Catastrophic
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Cladogram

• A cladogram is a diagram used to show ancestral
  relations between organisms, to represent the
  evolutionary tree of life.

These are two identical cladograms.
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• Analyzing a cladogram
• Which species are most closely related?
• Which is the common ancestor of flies and bees?
• Which insects share the most recent ancestor?

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Heres a more complicated one
What conclusions can you draw from this tree of
life?
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Sample Question 6
According to the cladogram above, which two
species are most closely-related? A clubmosses
and spikemosses B spikemosses and ferns C ferns
and flowering plants D flowering plants and
clubmosss
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Fossil Record

• Fossils are the preserved remains or traces of
  animals, plants, and other organisms from the
  remote past. The totality of fossils, both
  discovered and undiscovered, and their placement
  in fossiliferous (fossil-containing) rock
  formations and sedimentary layers (strata) is
  known as the fossil record.

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• Fossil evidence indicates that over time
  organisms of increasing complexity appeared on
  Earth. Bacteria and blue-green bacteria are the
  first fossils that were preserved from the
  Precambrian era.
• The fossil record contains many examples that
  could be interpreted to mean that species evolved
  from more ancient organisms.

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Sample Question 7
In a section of the Grand Canyon, scientists have
found the fossil remains of several different
groups of organisms. The diagram on the left
represents the number and age of the fossils the
scientists found. The width of each shaded area
in the diagram below indicates the relative
number of fossils found.
Which of the following statements is supported by
the fossil record? A Group C is the only group
that no longer exists for its now
extinct. B Group B was the most numerous organism
10 million years ago. C Group D has been in
existence the longest. D Group A is the most
recent organism to come into existence.
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Sample Question 8

• Which of the following is usually most helpful in
  determining the age of the fossils found in rock
  strata?
• A the size of the fossils
• B the color of the fossils
• C the water content of the rock layer where the
  fossils were located
• D the depth of the rock layer where the fossils
  were found

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Sample Question 9

• Which of the following best explains how the
  fossil record provides evidence that evolution
  has occurred?
• A It indicates the exact cause of structural and
  behavioral adaptations of organisms.
• B It shows that the form and structure of groups
  of organisms have changed over time.
• C It shows how the embryos of many different
  vertebrate species are very similar.
• D It indicates that forms of life existed on
  Earth at least 3.5 billion years ago.

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Sample Question 10

• Orchids were studied to determine if the amount
  of humidity affected the flowering of these
  plants. Which of these was the independent
  variable in this study?
• A The percentage of humidity
• B The length of time required for flowering
• C The amount watered
• D The number of flowers on each plant

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Remember
DEPENDENT VARIABLE -is the variable that is
measured at the end of the experiment.
INDEPENDENT VARIABLE -is the manipulated variable
in the experiment.