Species and evolution
Organisms within any one kingdom of living things are further classified
into groups showing increasing structural similarity.
Phylum
Class
Order
Family
Genus
Species
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greater
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greater
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Species
A species is a population of organisms which closely
resemble each other structurally and physiologically, and which can
interbreed to produce live fertile offspring.
They are therefore said to share the same gene pool, because the genes
(inherited information) of the different individuals can mix through
mating.
In contrast, if a horse and a donkey do breed, the offspring (mule)
is sterile and therefore cannot breed itself, so the gene pools of the
two different species remain separate.
Mutations
There is variation between all individuals in populations, as is obvious
in the human population. Some of these differences are "acquired"
such as tattoos and spiked hair, and these will not be inherited by
the offspring. For inherited features though, the original source of
the variation is due to changes in the genetic code (called mutations)
which occur during the formation of the gametes (sex cells). These changes
are randomly passed on to offspring leading to variation between individuals.
A mutation in a sperm or ovum changes the genetic information, coding
for a different mutant feature in the resulting offspring. Mutations
occur naturally, but their frequency can be increased by factors such
as radiation (e.g. X rays or nuclear fallout) and certain chemicals.
Mutations and evolution
Some mutations are fatal and the offspring never develop. If, however,
the mutation creates a feature that aids survival (i.e. it is an adaptation),
then that organism is more likely to survive and reproduce successfully,
leaving more offspring which carry that mutation also. If the environment
changes, then different features will be advantageous.
The British naturalist, Charles Darwin, travelled to South America
in 1835 and his observations led him to suggest that species changed
over time by a process referred to as "evolution". He hypothesised
that this occurred by the process of Natural Selection or "survival
of the fittest". By
this he meant that there is variation between individuals in a population
and those individuals best adapted in a certain environment will survive
best and pass on those traits to their offspring. Therefore, over the
next generations, the number of well-adapted individuals increases until
most of the population carries the trait.
Speciation
Two separate species can develop from one ancestral species if the two
groups are isolated over a long period of time, during which the environmental
conditions change and natural selection operates.
Darwin's theory of evolution through natural selection was in contrast
to the theory of Jean Lamarck (France, early 1800s) which suggested
that organisms could aquire new characteristics during a lifetime and
these could be passed to the next generation, whereas Darwin observed
that the variation already existed in the population.
Case history
In
Pre-Industrial England, light-coloured pepper moths were common, escaping
predation by birds because of their camouflage against the light-coloured
lichens on the trees. Dark moths were easy prey.
Industrialisation created air pollution and tree-trunks became darkened.
Darker moths seemed to appear and become more numerous. These mutant
forms now had a distinct survival advantage because light-coloured moths
were easily seen on the darker trees and were eaten by birds. The mutant
dark ones survived and had many dark offspring with the result that
the population changed through time. A considerable time later, and
with further environmental change, a new species may well evolve.
Fossil
evidence
Fossil evidence suggests that life forms have changed significantly
from the very simple organisms found in the earliest fossil layers.
Similarities in embryonic forms and DNA (genetic information) also point
to common ancestry.
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