When the first rains come long dormant seeds germinate. Typically in a square foot of vacant lot soil,
several hundred seeds will sprout. If
you return to the same space in July, you will find that only one or two
plants still to be found. For some its
bad luck and they are eaten by a predator, for others it’s slight difference in
sunlight or soil. But the major deciding
factor is that some have some sort of genetic advantage.
The beauty of sexual reproduction is that it causes
genes from two parents to recombine in a variety of different combinations. Even a small genetic advantage promoting more
rapid growth, resistance to drought, resistance to pests… can give the small
advantage needed to allow better survival that for others in the group.
This is the basis for natural selection. There are always more offspring produced than
can survive. Due to genetic
recombination in sexual reproduction some of the offspring are better equipped
for survival than others. The survivors
live to pass on their beneficial genetic information to later generations.
Sometimes when DNA, the cell molecules that carry genetic
information, duplicates during cell division, small spontaneously changes occur. The changes are included in all descendant
cells. The vast majorities of these
changes (mutations) are detrimental to life – and diminish chances of
survival. Rarely a mutation occurs that
benefits survival.
These genes would be
passed on while others can’t compete as well. Beneficial genes collect in the
survivors. It was once thought that vast
amounts of time were needed for this process – now we see many examples of
changes in organisms that occur as the result of a few mutations over brief time periods - most notably changes in virus, bacteria, insects, and birds.
In any environment – all of those organisms that are suited for survival will survive.
Natural
selection results in survival of new mutations when the environment is
changing. Take the case of White Sharks
– the ocean has provided a stable unchanging environment… There is no advantage
in receiving changes through new mutations.
If there is any limit in the environment, food supply, predators, etc.
then natural selection would be at work… but sharks live in an environment that
has not changed since well before the time of the dinosaurs – and the fossil
record shows that shark design is also largely unchanged. The same is true of any stable species living
in an unchanging environment.
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It was thought until recently that evolution of life was a continuous
process dependent on the introduction of new genetic information as the result
of random mutations. Now evidence suggests that as long as any environment is stable the rate of genetic change
will be very small. Then during times of
rapid environmental change the process of natural selection occurs ‘quickly'. Changes in an individual species may occur
within observable time, but for enough changes to add up to result in new
species would require thousands of generations.*(bottom of page)
The process of periods of
stability followed by times of rapid change are called ‘”Punctuated
Equilibrium”. An example of this would
be the long reign of dinosaurs on earth cut short by a comet or asteroid
colliding with the earth – causing catastrophic climate change. The fossil record shows a number of times of
change that resulting in times of great dying off, followed by the slow emergence of
new life forms suited for the new
Our current pattern of climate change is, in earth history
terms, – very rapid. We are experiencing
the kind of change that would normally require many thousands of years to take
place naturally. This appears to constitute a
change in our environment stability that will cause extinction of species. At least 1000 species have been permanently
lost in the last 500 years...Extinction is forever.
According to punctuated equilibrium, this appears to be a
time when the ‘rapid’ (a few hundreds years) change in the earths environments
will cause a rash of extinctions… followed by the next many thousands of years
with the evolution of life forms suited for the next stable environment. Read
more here; http://www.biologicaldiversity.org/programs/biodiversity/elements_of_biodiversity/extinction_crisis/
*A species is defined as a group of animals that can only
produce fertile offspring by mating with others in that group. So if two similar populations of salamanders,
both descendants of the same group are different enough to no longer be able to
mate and produce fertile offspring - they have become different species.
