living organisms
have a huge reproductive potential
yet in most cases populations do not
grow exponentially instead
they are limited by the constraints
of the environment and its carrying
capacity
natural or wild populations
are limited by the availability
of food as these
pups battle for positions
to nurse on their mother's milk but the
struggle goes on
throughout life and the battle for survival
also includes escaping
from predators with
the fittest or the best adapted
organisms being able to escape the
clutches
of the predator struggle extends itself
to climatic conditions
like harsh cold and here
these plants are in a struggle for light
space and nutrients
but this
competition or struggle for existence
is not a fair one because
there is genetic variation which can be
continuous showing a
smooth movement from one extreme to the
next
with respect to a particular trait
like skin color or it can be
discontinuous with distinct
types or varieties
of a particular trait this variety
results from independent assortment and
segregation
in meiosis and it results from
crossing over in meiosis
each gamete or sex sell that is formed
is unique and fertilization
is random but in addition to these
built-in forces have variation
nature has its own surprise packet
coming out of genetic mutation
you could get favorable traits
or unfavorable traits the favored ones
have the potential to make significant
changes
in populations sometimes
it can open entirely new areas
for that new types to colonize
as a favorable mutation may
allow a group of individuals
to move into a slightly different
type of habitat or into
and new Niche but for any of these
favored traits to be able to
contribute
to the improvement of a species
population
it is essential that
individuals cross the final hurdle in
the struggle for existence
and that they're able to find a mate
to reproduce and
when they reproduce their offspring must
be
able to reproduce and it is only in this
way
that the fruits of the struggle provide
the seeds
of the next generation so important is
this
aspect of the struggle for existence
that many species
have evolved elaborate displays
and mating rituals designed to ensure
successful
reproduction and
to ensure the safe delivery of their
genetic
package to the future generations
this type of descent with modification
summarized here is more popularly known
as this theory
evolution evolution by
natural selection the most
powerful piece of evidence in support of this
descent with modification or evolution
by
natural selection is the fossil record
here in this artists recreation
of the evolution of Eqqus ferrus-Prezwalski's horse
you can see the smooth gradation
over time the descent with
modification re-creations of this kind
and others are made possible because
of the stories in the rocks the fossil
record
here at the Grand Canyon in Arizona the
Colorado River
winds its way from an elevation
1000 meters at Lee's Ferry some
400 kilometers to Lake Mead
which is just 400 meters above sea level
as the river cuts through the canyon it
exposes the rock
that's able to take us back in time
cutting through some 1.7
billion years of geologic
and Paleontologic history every few
kilometers
reveals another episode in the stages of
descent with modification improvement in
one
type overtime is one thing how can one
species
give rise to others while at the same time
continuing itself to exist this can
happen
when populations are isolated and
subjected
to different environmental conditions
if this isolation is geographic
and brought about by volcanic eruptions
earth movements
or floods then it's referred to as
Allopatric Speciation the marine
iguanas
the Galapagos provides
a classic example of allopatric speciation
