Download Natural Selection - Teacher **DRAFT

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Natural Selection Instructional Case: A series of 3 student-centered science lessons
Teacher background knowledge for the Natural Selection unit
Why study
Natural
Selection?
All of the great diversity
of life shares one thing
in common – all of life
(past, present, and
future) has descended
(and will descend) from
a common ancestor.
Such descent with modification over many generations is what defines biological evolution. Yet,
biological evolution is far more than change over
time—it involves descent through genetic inheritance. When we think about how evolution occurs,
we can consider it on different scales. On a very
small scale, microevolution involves changes in
gene frequency in a
population from one
generation to the next;
and, on a very large
scale, macroevolution
involves the descent of different species from a
common ancestor over many generations.
A common misconception is that changes that
occur within a lifetime of an individual are a part
of evolution. However, individuals cannot evolve.
The smallest biological unit that can evolve is the
population—a group of individuals of the same
species, living in the same area at the same time.
Evolution acts on genetic diversity, which is
present in populations and absent in individuals.
The genetic diversity in populations is contained in
the gene pool—the total collection of alleles
(alternate forms of a gene) in a population at any
one time. Microevolution occurs when the relative
frequency of alleles in a population changes over
many generations. It is important to remember
that while there is variation amongst individuals in
a given population, not all variation in a population
is heritable, and only the genetic component of
variation is relevant to evolution. Genetic variation
in populations comes primarily from mutations,
genetic recombination (primarily due to sexual
reproduction), and gene flow (migration).
How does evolution work? The mechanisms of
evolutionary change include gene flow
(“migration” of genes from one population to
another, i.e., the genetic exchange between
populations), genetic drift (change in
a population’s gene pool due to
chance), mutation (change in the
nucleotide sequence of a gene), and
natural selection (detailed below).
A common misconception is that
natural selection creates variation. However,
natural selection cannot produce variation and
can only act on genetic variation that already
exists in a population. Natural selection leads to
evolutionary adaptions in organisms—heritable
traits that confer an advantage in the current
environment. This advantage is increased genetic
fitness—how productive a particular genotype is in
a population, i.e., the number of offspring with
that genotype that survive to reproduce. An
organism’s genotype is the full set of genes it
carries, while its phenotype is the expression of
the genotype, i.e., its physical traits. A genotype’s
fitness depends on the environment at that given
time, and the fittest does not necessarily mean
the strongest or fastest, but rather the fittest are
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Natural Selection Instructional Case
Te a c h e r B a c k g r o u n d K n o w l e d g e
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Teacher background knowledge for the Natural Selection unit
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those individuals with heritable traits best suited
to the local environment that survive to reproduce
and leave a larger share of surviving fertile
offspring. Thus, natural selection drives evolution
and is essentially differential survival and reproduction: the more advantageous the trait
(adaptation), the more common that adaptation
will be in the population.
Common misconceptions are that the environment
does the selecting in natural selection and that
species evolve because of
want or need; evolution is goal
directed and leads to perfectly
adapted organisms. To dispel
these misconceptions, it must
be emphasized that biological
diversity exists, and selective
pressure from the environment
determines which individuals
(and their genotypes) survive to
reproduce. Evolutionary change
is a consequence of immediate
advantage, not a distant goal.
Evolutionary change only reflects
improvement in the context of the
immediate environment, and what
is good today may not be so tomor-
row. Thus, species do not steadily get better, they
respond evolutionarily to the environment or they
go extinct.
An excellent resource on evolution is the Understanding Evolution website of UC Berkeley’s
Museum of Paleontology,
http://evolution.berkeley.edu/evolibrary/home.ph
p . Here, you will find Evolution 101 (essentially a
comprehensive course on the science of evolution), a wealth of teaching materials, and a
comprehensive digital library of articles, tutorials,
and other resources.