Download Darwin, Malthus, and Limiting Factors

Darwin, Malthus, and Limiting
Factors
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In 1798, Economist Thomas Malthus noted
people were being born faster than people
were dying, causing overcrowding.
The forces that work against population
growth, Malthus notes, were War, Famine, and
disease.
Darwin realized Malthus’s thinking applied
even more to other organisms than to humans.
As an example, a Maple tree produces
thousands of seeds, but only a few survive to
reproduce. Darwin wondered why.
Artificial Selection
• In order to explain what he was observing
in Nature, Darwin turned to the idea of
Artificial Selection.
• Plant and animal breeders knew that
individual organisms within a given
population varied in attributes like size,
hardiness, and the amount of milk
produced.
• The farmers told Darwin that some of this
variation could be passed on to offspring in
order to improve the stock.
• In Artificial Selection, nature provides the
variations, and humans select those
variations they find useful.
• Darwin had no idea how heredity
worked, or about heritable variation, but
he did know that variation occurred
within natural populations just as in
domesticated plants and animals.
• Darwin’s breakthrough came when he
realized that this variation provided the
raw materials for evolution.
Natural Selection
• Darwin’s great contribution was to describe
a process in nature – a scientific mechanism
– that could operate like artificial selection.
• The struggle for existence – which
organisms, within a species, survive and
reproduce? Darwin hypothesized some of
these organisms are better suited to survive
in their environment than are others.
• These better suited organisms (ex. faster,
stronger, or better camouflaged) would be
the ones to pass on these ‘desirable’ traits.
• Please note that physical attributes aren’t
the only ones passed on – behaviours
that favour reproductive success are also
selected for and re-enforced in
subsequent generations.
• Any heritable characteristic that
improves the organism’s ability to
survive and reproduce in an environment
is called an adaptation.
• Darwin coined the phrase Survival of the
Fittest to illustrate how differences in
adaptions to the environment determine
an organism’s fitness to survive and
reproduce in that environment.
• Natural Selection is the process by which organisms
with variations most suited to local environments
survive and leave more offspring.
• In both artificial and natural selection, only certain
individuals in a population produce new individuals.
In the case of Natural Selection, the environment –
not a farmer or breeder – influences fitness.
• Natural selection occurs in any situation in which
more individuals are born than can survive.
• There is natural heritable variation (variation and
adaptation), and there is variable fitness among
individuals in a given population (survival of the
fittest). This is the long way of saying that Evolution is
not random.
• Common descent – According to the principle of
common descent, all species, whether living or
extinct, are descended from ancient common
ancestors.
Things natural selection doesn’t do
It doesn’t make organisms ‘better’
It doesn’t make organisms perfect
It doesn’t move in a fixed direction
It doesn’t even guarantee the continued
existence of a species
• Natural selection is also not the only
mechanism driving evolution.
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Evidence for Evolution
• Biogeography – This is the study of where
organisms live now and where they, and
their extinct ancestors, lived in the past..
Patterns in distribution of living and fossil
species tell us how modern organisms
evolved from their ancestors..
1) The first is a pattern in which distantly
related species develop similarities in similar
environments.
2) The second is a pattern in which closely
related species differentiate in slightly different
climates.
• To Darwin, the biogeography of the
Galapagos species suggested the
populations on the islands had evolved from
mainland species.
• Over time, natural selection on the islands
produced variation among the populations
that resulted in different, but closely related,
island species.
• On the other hand, similar habitats around
the world are often home to organisms that
fill similar niches, but are distantly related –
the Rhea and Ostriches are examples. Both
show similar characteristics and fill the
same niches in their respective habitats, but
are completely unrelated.
The Age of the Earth and the Fossil record
• Evolution takes time, a long time, and
Darwin thought this pointed to an old
Earth. Recent developments in the use of
radiometric dating has shown the Earth
is at least 4.5 billion years old.
• The fossil record shows that modern
species, like whales, have evolved from
ancient extinct ancestors.
Anatomy
• Homologous structures – Structures shared by
related species and that have been inherited from a
common ancestor are called homologous structures.
• Analogous structures, on the other hand, have similar
functions but are inherited from unrelated species.
Bee wings and Bird wings have similar function but
completely different evolutionary pathways.
• Vestigial structures are structures that have been
inherited from ancestors but have lost much or all of
their original use due to different selection
pressures acting on the descendants. The hipbones
of bottlenose dolphins or boa constrictors are
examples of vestigial structures.
Embryology & Genetics
• Similar patterns of embryological
development provide further evidence
that organisms have descended from a
common ancestor.
• DNA, the universal genetic code,
provides evidence of common descent.