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Darwin's Theory of Natural Selection

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James Norris

on 24 January 2014

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Transcript of Darwin's Theory of Natural Selection

What is it?
Darwin's Theory of Natural Selection
By Gurminder, James and Katherine
Examples of Natural Selection, their long bill and tongue evolved to let them reach deep into a flower or nectar.
A natural process where organisms best withstand their environment survive, producing adapted offspring whilst those less adapted diminish.
Published in 1859 in 'The Origin of Species'.
Forms of Natural Selection
Common factors for natural selection include diseases, climatic conditions, availability of food, and predators.
Directional Selection
When an individual has a favoured and more extreme variation of a trait than the average population.
Common in artificial breeding.
Stabilising Selection
Extreme traits on both ends of the trait distribution are selected against.
Disruptive Selection
Favouring the two extremes rather than the middle.
Sexual Selection
An individual's selection of a mate based on attraction.
E.g. Male cardinals use brightly coloured physical features and song to attract females.
Consequences of Human Influence on Natural Selection
Humans can interact with all other species, both directly and indirectly.
We alter habitats by clearing land for mining, agriculture and urban expansion.
The greenhouse gases we emit are changing the composition of Earth's atmosphere and oceans.
These have the potential to influence future natural selection of species.
Supported by evidence from a variety of scientific disciplines, including palaeontology, geology, genetics and developmental biology.
Example of Directional Selection:
Hummingbirds with the majority having medium-length bills enter a place with large flowers.
Individuals with longer bills will be selected by environment, can get food, survive and reproduce.
Example of Disruptive Selection:
Hummingbirds with a majority of medium-sized bills in a place with two types of plants with different-sized flowers.
Hummingbirds with medium-sized bills fail.
Hummingbirds with either long or short bills are favoured.
Tibetans living at high altitude:
• At this elevation, the oxygen level is only 40 % of that at sea level. Over a period of days and weeks, their red blood cell count increases, helping them obtain adequate oxygen.
• Geneticists have documented more than 30 genes that have been selected within the Tibetan population.
The physical and behavioural changes that make natural selection possible happen at the level of DNA and genes. Such changes are called "mutations."
Mutations can be caused by chemical or radiation damage or errors in DNA replication. Mutations can even be deliberately induced in order to adapt to a rapidly changing environment.
Most times, mutations are either harmful or neutral but in rare instances, a mutation might prove beneficial to the organism. If so, it will become more prevalent in the next generation and spread throughout the population.
In this way, natural selection guides the evolutionary process, preserving and adding up the beneficial mutations and rejecting the bad ones.
Micro-evolution involves small-scale genetic changes in a species over time. One example of this is a colour change undergone by British pepper moths in response to changing levels of air pollution. The acquisition of antibiotic resistance by bacteria and the trend towards tusk-less elephants in Africa are also examples of micro-evolution.
Because it is so well documented, even people who don't believe that evolution can lead to the creation of new species accept that micro-evolution occurs.
Most micro-evolution studies involve change over very short time periods, on the order of decades or a few hundred years. The detection of micro-evolutionary changes over longer time periods has been difficult because it requires that ancient DNA deposits be found together with samples from modern populations of the same species.
Biological fitness is key to natural selection, as it reflects the organism's ability to successfully reproduce.
He believed that:
Organisms show a variation of traits.
More organisms are born than could ever possibly be supported by the planet's resources.
Therefore, all organisms must struggle to live.
Some traits offer advantages in the struggle.
Organisms that have those traits are more likely to successfully reproduce and pass the traits on to the next generation.
Successful variations accumulate over the generations as the organisms are exposed to population pressure (from floods, droughts etc.)
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