Evolution & Natural Selection
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of ideas of Life on Earth
people believed in Creationism, which considered that all life
was created just as it is now. This was not based on any evidence, but
was instead a belief.
began systematic classification systems (especially Linnaeus 1707-1778)
and noticed that groups of living things had similar characteristics and
appeared to be related. So their classifications looked a bit like a
naturalists travelled more widely and discovered more fossils, which
clearly showed that living things had changed over time, so were not
always the same. Extinctions were also observed (e.g. dodo), so
species were not fixed.
Lamark (1809) proposed a theory that living things changed by inheriting
acquired characteristics. e.g. giraffes stretched their necks to reach
food, and their offspring inherited stretched necks. This is now known
to be wrong, since many experiments (and experience) have shown that
acquired characteristics are not inherited, but nevertheless Lamark's
theory was the first to admit that species changed, and to try to
(1859) published "On the
origin of species by means of natural selection, or the preservation of
favoured races in the struggle for life", which has been
recognised as one of the most important books ever written. A very
similar theory was also proposed by Alfred Wallace, and Darwin
and Wallace agreed to publish at the same time.
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theory was based on four observations:
concluded that individuals that were better adapted to their environment compete
better than the others, survive longer and reproduce more, so passing on more of
their successful characteristics to the next generation. Darwin used the
memorable phrases survival of the fittest,
struggle for existence and natural
explained the giraffe's long neck as follows. In a population of horse-like
animals there would be random genetic variation in neck length. In an
environment where there were trees and bushes, the longer-necked animals were
better adapted and so competed well compared to their shorter-necked relatives.
These animals lived longer, through more breeding seasons, and so had more
offspring. So in the next generation there were more long-neck genes than
short-neck genes in the population. If this continued over very many
generations, then in time the average neck length would increase. [Today it is
thought more likely that the selection was for long legs to run away from
predators faster, and if you have long legs you need a long neck to be able to
drink. But the process of selection is just the same.]
wasn't the first to suggest evolution of species, but he was the first to
suggest a plausible mechanism for the evolution - natural selection, and to
provide a wealth of evidence for it.
used the analogy of selective breeding (or artificial selection)
to explain natural selection. In selective breeding, desirable characteristics
are chosen by humans, and only those individuals with the best characteristics
are used for breeding. In this way species can be changed over a long period of
time. All domesticated species of animal and plant have been selectively
bred like this, often for thousands of years, so that most of the animals and
plants we are most familiar with are not really natural and are nothing like
their wild relatives (if any exist). The analogy between artificial and natural
selection is a very good one, but there is one important different - Humans have
a goal in mind, nature does not.
are three kinds of Natural Selection.
occurs whenever the environment changes in a particular way. There is therefore selective
pressure for species to change in response to the environmental change e.g.
peppered moth (studied
by Kettlewell). These light coloured moths are well camouflaged from bird
predators against the pale bark of birch trees, while rare mutant dark moths
are easily picked off. During the industrial revolution in the 19th century,
birch woods near industrial centres became black with pollution. In this
changed environment the black moths had a selective advantage and became the
most common colour, while the pale moths were easily predated and became
resistance to antibiotics.
Antibiotics kill bacteria, but occasionally a chance mutant appears that is
resistant to that antibiotic. In an environment where the antibiotic is
often present, this mutant has an enormous selective advantage since all the
normal (wild type) bacteria are
killed leaving the mutant cell free to reproduce and colonise the whole
environment without any competition. Some farmers routinely feed antibiotics
to their animals to prevent infection, but this is a perfect environment for
resistant bacteria to thrive. The best solution is to stop using the
antibiotic so that the resistant strain has no selective advantage, and may
e.g. Warfarin (poison used to kill rats.
When warfarin was introduced, some populations already contained
rates with a chance mutation that gave them resistance to the poison.
Without warfarin, stabilising selection favours normal rats –
resistant rats are selected against, because they need a lot of vitamin K in
their diet. Warfarin was a new
environmental factor that killed normal rats.
A few resistant rats survived, reproduced and passed on the
resistance gene. They produced
a new population of resistant rats.
do not have to decide to adapt, or mutate, after an environmental change.
The mutation, or combination of alleles giving resistance, have to
already be there by chance, otherwise the population may become extinct.
includes biotic as well as abiotic, so organisms evolve in response to each
other. e.g. if predators run faster there is selective pressure for prey to run
faster, or if one tree species grows taller, there is selective pressure for
other to grow tall. Most environments do change (e.g. due to migration of new
species, or natural catastrophes, or climate change, or to sea level change, or
continental drift, etc.), so directional selection is common.
occurs when the environment doesn't change. Natural selection doesn't have to
cause change, and if an environment doesn't change there is no pressure for a
well-adapted species to change. Fossils suggest that many species remain
unchanged for long periods of geological time. One of the most stable
environments on Earth is the deep ocean e.g.
Coelocanth. This fish
species was known only from ancient fossils and was assumed to have been
extinct for 70 million years until a living specimen was found in a trawler
net off South Africa in 1938. So this species has not changed in all that
example of stabilising can be seen in the birth weight of humans.
The heaviest and lightest babies have the highest mortality and are less
likely to survive to reproduce and pass on their alleles.
occurs where an environment change may produce selection pressures that favour
two extremes of a characteristic e.g.
plants in Welsh Copper mines.
Soil contaminated by copper from the mines is lethal to normal grass plants,
but a chance mutation allowed one plant to grow. This plant prospered and
reproduced, but only on the contaminated soil. On normal soil it grew more
slowly than the normal plants and was easily out-competed. So now there are
two varieties growing close together.
People homozygous for this recessive allele usually die before
reproducing. Their red blood
cells contain abnormal haemoglobin which makes them become sickle-shaped and
stick in their capillaries. People
heterozygous for the allele should be at a disadvantage, because their red
cells can sickle during exercise – the allele should therefore be selected
against and rare, however, its frequency is high in parts of the world where
malaria is common – in some populations over 20% carry the allele (as
heterozygous for sickle-cell anaemia are more resistant to malaria than
people homozygous for the normal allele.
Where malaria is found, people heterozygous for sickle-cell have an
advantage and are likely to survive, reproduce and pass on the allele.
People without the allele also have an advantage, because their red
cells behave normally. This
produces populations with an equilibrium for numbers of people heterozygous
for sickle-cell and non-carriers (balances polymorphism)
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species is defined as a group of interbreeding populations that are reproductively
isolated from other groups. Reproductively isolated can mean that sexual
reproduction between different species is impossible for physical, ecological,
behavioural, temporal or developmental reasons. For example horses and donkeys
can apparently interbreed, but the offspring (mule) doesn't develop properly and
is infertile. This definition does not apply to asexually reproducing species,
and in some cases it is difficult distinguish between a strain and a species.
species usually develop due to:
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Start with an interbreeding population of one species.
The population becomes divided by a physical barrier such as
water, mountains, desert, or just a large distance. This can happen when
some of the population migrates or is dispersed, or when the geography
changes catastrophically (e.g. earthquakes, volcanoes, floods) or
gradually (erosion, continental drift).
If the two environments (abiotic or biotic) are different (and
they almost certainly will be), then the two populations will experience
different selection pressures and will evolve separately. Even if the
environments are similar, the populations may change by random genetic
drift, especially if the population is small.
Even if the barrier is removed and the two populations meet
again, they are now so different that they can no longer interbreed.
They are therefore reproductively isolated and are two distinct species.
They may both be different from the original species, if it still exists
is meaningless to say that one species is absolutely better than another
species, only that it is better adapted to that particular environment. A
species may be well-adapted to its environment, but if the environment changes,
then the species must adapt or die. In either case the original species will
become extinct. Since all environments change eventually, it is the fate of all
species to become extinct (including our own).
Isolation is a type of genetic isolation. Here
the formation of a new species can take place in the same geographical
area, e.g. mutations may result in reproductive incompatibility.
A new gene producing, say, a hormone, may lead an animal to be rejected
from the mainstream group, but breeding may be possible within its own groups of
variants. When this mechanism
results in the production of a new species it is known as sympatric
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