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Selection
“This preservation of favourable
individual differences and
variations, and the destruction
of those that are injurious, I
have called Natural
Selection….”
Charles Darwin
The Origin of Species 1859
Remember the idea….
….more organisms born than can survive and reproduce
….variation among individuals in the population
….some of this variation is heritable
….variation in form leads to variation in survival.
Therefore, over generations, the population changes in
response to the environment.
Some call it “survival of the fittest”.
Darwin called it natural selection.
The environmental factors leading to selection may be
either…..
…biotic
…or abiotic.
The term “survival of the
fittest” was actually
introduced by Herbert
Spencer in a biology text
he wrote in 1864 (after
reading Darwin).
It’s important to note that
“survival” alone isn’t
enough to lead to
adaptation. Evolutionary
success requires
reproduction.
Herbert Spencer
A key concept….
Fitness – the relative
genetic contribution of
an organism to future
generations.
Determined by survival
and reproduction.
A gazelle that does not survive, can
not contribute to future
generations….
…but survival alone does not assure a
genetic contribution. The gazelle must
reproduce.
Artificial Selection
Exactly the same process as
natural selection, except that
WE make the choices about
which individuals are allowed
to contribute.
Pigeons
Brought into
domestication very
early.
Aristotle describes
five different
breeds.
All are descended
from the wild rock
dove.
• Artificial
selection has
produced
numerous
breeds of
dogs, which
diverged from
wolves about
15,000 years
ago.
The first signs of
cat domestication
show up in
Mediterranean
culture some 7500
years B.C.
Like dogs, cats first were symbiotic with humans.
Then, capture led to artificial selection for
coexistence with humans and control of vermin.
Agriculture
Artificial selection
has had a
tremendous effect
on agriculture in
providing enhanced
varieties of food
and other crops.
Corn has been developed from the ancestral wild grass, teosinte.
Artificial selection has produced the male tassel and female ear of
modern corn. Ears are larger in modern corn, and tassels and ears are
on different parts of the plant compared to the ancestral teosinte.
• From left to right: popcorn, sweet corn, flint corn, dent corn, and pod
corn.
• Through selective
breeding, tomatoes
with different shapes
have been
produced; within the
rose family, different
colors and flower
structures have been
artificially selected.
• Artificial selection
of begonias for
flower shape, size,
and color has
produced distinct
varieties.
• Tulips come in a variety of colors and stripes, but never in a deep
color of completely black. This variety does not exist, at least so far,
within the species.
Natural Selection
Organisms less suited to their environment die, or
contribute fewer offspring. Better suited individuals are
more likely to survive and reproduce.
There is no “guarantee”. It is a statistical phenomenon.
Natural selection acts on the phenotype.
Ultimately, this acts on the genotype.
Where can we find examples of natural selection?
Peppered moth – Biston betularia
The moth occurs in two color phases, peppered and melanic.
(a) Both phases are displayed against an unpolluted, lichencovered tree. (b) Both phases are displayed against a dark
tree, on which the lichen were killed by pollution.
In 1996, cliff swallows
returning to nesting
sites in North America
were met by cold, rainy
weather. Many died.
Researchers found
significant differences
between survivors and
the ones that died.
Survivors were larger,
and had a more
symmetric body form.
Cepaea
This land snail shows
significant variation in
its shell color.
The background
colors are brown,
pink, and green.
Researchers found
that brown
background
coloration was
common in
woodland-beech
habitats, while green
was prevalent in
meadows.
Birds, such as this song thrush, hunt snails and break their shells
open against “anvil rocks” where debris collects. The shards of
shells at the anvil rocks indicated that the green snail were being
eaten in the woodland habitat, while the brown shells were being
taken in the meadows.
The researchers further demonstrated that, in deciduous
woodlands, the frequency of the color phases changes over the
year.
Nerodia sipedon
The northern water snake (Nerodia sipedon) occurs throughout
eastern North America. It is darkly patterned, except on some
islands in Lake Erie where many are light colored.
The Lake Erie water
snake is unbanded,
and is usually pale
grey or brownish.
Scored color phases A
(light) to D (banded). In
young born on the islands,
most are C or D (dark and
banded). But by the time the
snakes reach adult stage,
most are A or B (light
colored).
On the islands, predaceous
gulls feed on young snakes,
usually spotting and eating
the more conspicuous
banded snakes, producing
differential survival of mostly
unbanded snakes.
Occasional immigrants from
the mainland return some of
the genes for banded color.
Types of Selection
We can look at the evolution of resistance to
insecticides as an example of natural selection
DDT is very effective in its first use. Over time, however,
DDT-resistant mosquitoes begin to appear. Can we
calculate the fitness of the resistant strains?
Assume that resistance to DDT is the result of a single
dominant allele that we will call R, with the susceptibility
allele being designated r.
Resistance of Culex to permethrin is due to a
resistance allele, R, which acts in a semi-dominant
fashion.
Sometimes the
selective value of
a trait is
determined by
complex factors.
Sickle cell anemia is an
example of a polymorphism
with a heterozygous
advantage.
Sickle cell anemia
kills about 100,000
people per year.
About 80% of the
homozygotes die
without reproducing,
yet the gene
frequency is almost
10% in some
populations.
Why?
The incidence of SC
coincides closely
with the occurrence
of malaria.
Malaria is probably the
deadliest disease in history,
currently killing some 3.5
million people per year,
mostly children aged 1-4
years.
It is caused by protozoans
in the genus Plasmodium.
It appears that individuals that are heterozygous
for the sickle cell gene have an increased resistance
to malaria, providing them with a heterozygous
advantage.
Evolution: Sickle Cell Anemia
The fitness of the heterozygote is higher than either
homozygote
In some cases, the
fitness of a genotype
may depend on its
frequency.
This is true in the
case of mimicry.
Coral snake
Scarlet king
snake
Batesian mimicry
Mullerian mimicry
Queen
Monarch
Sexual Selection
Why do we often find
major differences among
individuals?
In particular, why do we
find major morphological
differences between the
sexes within the same
species.
Secondary Sexual Characteristics
Male Stellar’s sea lion
females
How do we explain
the selection of
individuals as
mates who seem to
have traits that are
disadvantageous.
One possibility:
Zahavi’s Handicap Principle
Females may choose to mate with healthy,
unparasitized males
Bird species subject to higher degrees of
parasitism tend to have more brightly colored
males.
Brown creeper
Subfamily Certhiinae
Painted bunting
Subfamily
Emberizinae
Another possiblity:
The Runaway Principle