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How to Explain Darwin's Theory of Evolution
What is Evolution? The Unifying Theory of Biology
On the 24th November 1859, a naturalist named Charles Darwin published On the Origin of Species by Means of Natural Selection . This publication was to profoundly change the way humans looked at the world and was to spark a controversy that survives to this day. Having taught evolution in a Catholic school, I can testify to the strength of feeling on both sides of this divide.
It is telling that the word 'evolution' does not appear until the very end of first edition of The Origin of Species. Instead, Darwin referred to 'descent with modification;' Darwin presented evidence that all organisms on Earth are descendants of ancestral species that were different from modern species. Just as importantly, Darwin proposed a mechanism for this descent with modification' which he termed natural selection.
Anticipating the uproar it would cause both theologically and scientifically, Darwin spent 15 years agonising over whether to publish his work. (In this time, another British naturalist called Alfred Russel Wallace published a theory of natural selection similar to Darwin) Despite this, within 10 years of publication, Darwin's book and it's proponents (notably Thomas Henry Huxely) had convinced most biologists that the vast biodiversity on our planet was a result of evolution. How did Darwin succeed in this bringing this paradigm-changing theory to general acceptance? One word - evidence. Darwin collected over 500 pages of evidence supporting his theory. Since then we have added millions more.
What is Evolution? Put simply, Evolution is change over time. But it is not that simple!
Evolution: Gradual change over time
Mutation: Random alterations in the genetic sequence of an organism. Mutations create the diversity and variation between individuals of the same species that is acted upon by Natural Selection
Natural Selection: A proposed mechanism for evolution. The best adapted organisms in a population outcompete those that are less well adapted, reproduce and pass on their favourable genes.
Selection Pressure: An external pressure that drives evolution in a particular direction
Species: A group of animals that can interbreed to form viable (fertile) offspring
Variation: The differences between individuals
How Does Evolution Work?
Evolution works because each species produces more offspring than their ecosystem can support. This leads to a struggle for survival between individuals of a population - members of the same species compete for food, shelter/territory and mates. Put simply, Evolution works as follows:
- Animals in a population must show variation (evolution cannot occur amongst clones)
- The environment selects those variations that give an advantage (long legs, sharp eyesight, sharper beaks)
- Individuals with an advantage survive and reproduce
- Offspring of these survivors inherit advantageous characteristics
- This generation is therefore better adapted to their environment. Over time, the group becomes better adapted.
This differences can be very minute - almost imperceptible - but over millions of years, they add up. A group of animals from a given species are said to have evolved into a new species when the two groups can no longer breed together to make fertile offspring. This usually happens when:
- A group of the species moves to a new environment (different selection pressures)
- An environmental change affects only certain members of the species (different selection pressures)
- Populations of a species become sexually isolated (gene pools cannot mix)
As you can see, this whole process is driven by natural selection.
Beak Variation in Galapagos FinchesClick thumbnail to view full-size
What is Natural Selection? - Survival of the Fittest
It is difficult to explain natural selection much more succinctly than Darwin himself does in 'On the Origin of Species'...after all, he only takes a single paragraph to do it:
As many more individuals of each species are born than can possibly survive; and as, consequently, there is a frequently recurring struggle for existence, it follows that any being, if it vary however slightly in any manner profitable to itself, under the complex and sometimes varying conditions of life, will have a better chance of surviving, and thus be naturally selected. From the strong principle of inheritance, any selected variety will tend to propagate its new and modified form
This boils down to the phrase Survival of the Fittest.
Survival depends, at least in part, on traits that are inherited from parents. Individuals in a population who inherit traits that give them a survival advantage are more likely to reproduce than individuals without these traits. The unequal ability of different individuals to survive and reproduce causes the whole population to gradually change over generations. Eventually the favourable characteristic will become common amongst the group.
The variation in traits between individuals of the same species is brought about by mutation - random alterations in the sequence of DNA of an organism. Mutations can be advantageous, harmful or neutral.
- Harmful mutations are quickly removed from the gene pool
- Advantageous mutations quickly become common place in the gene pool.
- Neutral mutations are neither selected for nor against
When a mutation/trait has gone from rare to common, the population is said to have evolved
Environments are always changing. Thanks to mutations, novel adaptations to environments are constantly being thrown up that can be acted upon by Natural Selection. Over millions of years, these two factors can cause one species to evolve into another - a process called anagenesis. In this case, the species has not undergone a branching event, but individuals from the current species would not be able to reproducewith their ancestors from, say, one million years ago to produce fertile offspring.
More frequently, however, the changing of one species into another is due to a speciation event.
Allopatric speciation occurs when a physical barrier (mountain range, canyon, river) separates two populations of the same species. Isolated from each other, and subject to different selection pressures in their unique environements, each population follows a different evolutionary path. Eventually, these two populations become so different from each other that they cannot reproduce with each other to form fertile offspring - it is at this point that they are said to be separate species.
Sympatric Speciation describes what happened to many of the Galapagos finches. This occurs where new species emerge without physical separation, but where populations of a species adapt to different opportunites in the environment. If these populations then cease to interbreed - Hey presto - a new species is formed.
Despite the differences between how species can be formed, they all work on the same principle. Eventually, the tiny changes acted upon by natural selection add up to an animal that is so different from it's ancestor and/or neighbouring groups that it cannot interbreed with them to form fertile offspring. The key thing is the timescale this occurs over - hundreds of generations must pass for these differences to become significant. This is why evolution is readily seen in organisms with a short generation time (insects, bacteria) and more difficult to view in longer-lived organisms...such as humans.
Where Next? Evolution
- Evolution in Two Minutes | DISCOVER Magazine
Five videos, each tasked with explaining evolution in 2 minutes or less. Fantastic summary resource!
- What is Evolution? NHM
Find out more about evolution, how it works, and how evolutionary theory first developed.
- Galapagos Finches
A resource looking at Darwin's Finches - one of the key pieces of evidence that inspired On the Origin of Species.