A central question many have asked: Why are there so many species? In
this section on evolution we explore answers to this question. But as
many species as there are, remember, we estimate that 99% of all species
that have ever lived are now extinct. Each year more species go extinct.
Where do new species come from?
What is evolution?
Accumulation of variation over many generations is evolution
All living things are descended from common ancestors
Groups of living things can change over time, sometimes leading to a new
Some key points on evolution:
Usually the characteristics of a species change very slowly, over
100s, if not 1000s of generations. The characteristics may become so different
that we classify it as a different species.
The theory of evolution is well accepted by virtually all scientists and
Some misconceptions about evolution:
Man did not evolve from monkeys, apes, or any other organisms alive
today! However, man and all other primates have a common ancestor.
Evolution; that groups of similar organisms change over time, is well
documented. Whether all species on earth today, humans included, arose
by the process of evolution is the issue that some debate.
Limiting factors, adaptations & natural selection.
Not everything an organism needs to survive, grow and reproduce is
present in abundance, some factors or resources are limited. Ecosystems
differ in the particular factors that are limiting. For example, water
is generally not limiting in the tropics, but is in the desert. Characteristics
of individuals within a population vary. Those individuals within a population
that have characteristics which increase survival and reproduction are
more likely to pass on those characteristics to the next generation. Thus
over time these favorable variations accumulate resulting in (i) organisms
that are better adapted, and (ii) evolutionary change.
Darwin proposed natural selection as the mechanism that causes species
to change over time (evolution): "This preservation of favorable variation
and rejection of injurous variation, I call NATURAL SELECTION" (Darwin).
More Recent and more direct evidence:
Hawaii: 6 million years ago, the islands had just formed, they became colonized from the mainlands. These colonizers changed by evolution resulting is species that unique and only found on Hawaii.
Actual speciation events: we have documented circumstances where new species have evolved.
Does evolution always occur?
Under what conditions would evolution almost certainly not occur or
occur very slowly?
Think about the kind of environmental conditions and the role of variation
Living fossils include the coelocanth and the ginkgo tree
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