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5.2 Natural Selection
Understanding: Natural selection can only occur if there
is variation amongst members of the same species
If all individuals of a population were identical, there would be no way of
some individuals being favored over others
Natural selection depends on variation within a population
Understanding: mutation, meiosis & sexual reproduction
cause variation between individuals in a species
Variation can be caused by:
• CO of NS HC in PO of M
RO of HC in MO of M
RF of E by S
Which one(s) occur via mutation?
Which one(s) occur via meiosis?
Which one(s) occur via sexual reproduction?
What is the only source of variation for asexual species?
Understanding: adaptations are characteristics that make
an individual suited to its environment and way of life
Structure is related to function
e.g. structure of bird’s beak is related to its diet
e.g. structure of small intestine (thousands of villi) is related to its function (increases
surface area for a sufficient amount of nutrient absorption)
Adaptations = process where a population becomes better suited to survive in its
environment over generational time
develop by natural selection
do not develop during the lifetime of one individual (individuals do not ADAPT or TRY to
adapt to their environment
Populations adapt, not individuals
…not to be confused with
•Acquired characteristics = traits that develop during
the lifetime of one individual; cannot be inherited
•a no longer accepted explanation for evolution. It refers to offspring inheriting
some trait developed or modified during the lifetime of the parents (Lamarck)
… not to be confused with
Acclimation = process where an organism adjusts
physiologically to changing environmental conditions over
a short period of time. No change in the genes of the
individual occur, but the expression of genes may be
e.g. humans adjusting to high altitude, cold temps, or
humid summers
e.g. adjusting fish to a new tank
Understanding: species tend to produce more
offspring than the environment can support
Organisms vary in the number of offspring they produce
slow-breeding, e.g. southern ground hornbill – 0.3
fast-breeding, e.g. bacteria double every 20 min; fungus
produces giant puffball with 7 trillion spores
Despite this variation, overall trend: more offspring
produces than environment can support
Leads to competition for resources and struggle for
Understanding: individuals that are better adapted
tend to survive & produce more offspring while less
well adapted tend to die or produce fewer offspring
This is the definition of
Natural Selection!
Example: in dry season, when only available leaves on
trees are up high, giraffes with longer necks are better
adapted than those with shorter necks, so they will
survive and have a chance to reproduce offspring also
with longer necks
Understanding: individuals that reproduce
pass on characteristics to their offspring
Much of variation between individuals is heritable (can be
passed from parents to offspring through genes)
Examples: skin color in humans and behavior such as
direction of migration & birdsong
Non-examples: acquired traits, such as dark skin obtained
through tanning
Understanding: natural selection increases the frequency of
characteristics that make individuals better adapted &
decreases frequency of other characteristics leading to
changes within the species
 This
is the definition of
evolution by natural selection!
 Over
generations, the characteristics of the POPULATION
gradually change
 Evolution
of melanistic moths in polluted areas
 Evolution
of Finch beaks on Galapagos Islands
 Evolution
of antibiotic-resistant bacteria
Summary of Natural Selection:
(Study this slide!)
There is variation among members of a species.
Species tend to produce more offspring than the environment can
Individuals that are better adapted tend to survive & produce
more offspring while less well-adapted tend to die or produce less
Individuals that reproduce pass on favorable traits to their
Frequency of more favorable traits increase while less favorable
traits decrease, leading to changes within species.
Application: Changes in beaks of finches on
Daphne Major
Darwin studied finches (“Darwin’s finches”) on Galapagos Islands in 1835
Published Origin of Species in 1859
Darwin observed 14 different species with different beak sizes & shapes and different diets
Darwin did not think one could observe evolution happening
Peter & Rosemary Grant studied Darwin’s finches in 1970s-1980s and observed evolution happening!
Studied Geospiza fortis on the island Daphne Major
Eats small and large seeds – small body and beak size
1977 drought = shortage of small seeds = many G fortis died
1982 severe El Nino rain = more small seeds = increase in G fortis
1983 dry & less small seeds
1987 G fortis had longer and narrower beaks than 1983
Nature of Science: use theories to explain natural
phenomena: the theory of evolution by natural selection can
explain the development of antibiotic resistance in bacteria
These trends show evolution by natural selection taking place:
When new antibiotic used on patients, bacteria become resistant within a few years.
Antibiotic resistance spreads to more species of pathogenic bacteria.
In each species, proportion of infections caused by resistant strain increases.
Application: evolution of antibiotic
resistance in bacteria
Rapid (a few decades)
Causes of antibiotic resistance:
Widespread over-use of antibiotics (treating diseases & in animal
Bacteria reproduce rapidly (every 20 min)
Populations of bacteria are usually huge
Antibiotic resistance form either by MUTATION or one bacterial
species PASSING ITS PLASMID containing antibiotic resistance gene
on to another bacterial species