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How to Explain Darwin's Theory of Evolution

Updated on December 8, 2013
Is this the most important book ever written?
Is this the most important book ever written? | Source

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!

Key Definitions

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:

  1. Animals in a population must show variation (evolution cannot occur amongst clones)
  2. The environment selects those variations that give an advantage (long legs, sharp eyesight, sharper beaks)
  3. Individuals with an advantage survive and reproduce
  4. Offspring of these survivors inherit advantageous characteristics
  5. 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 Finches

Click thumbnail to view full-size
Geospiza fortis, a seed eating finch. This species has been seen to evolve in real time. Beak size has increased 10% due to a soft-seed shortage in the GalapagosCerthidea olivacea has a much narrower and pointed beak to catch insectsGeospiza scandens has a long and sharp beak. This helps the finch to tear and eat cactus flowers and pulpDarwin's original sketches, as documented in 'Journal of Researches into the Natural History and Geology...' These different species of finch all evolved from a single ancestral species stranded on the Galapagos.
Geospiza fortis, a seed eating finch. This species has been seen to evolve in real time. Beak size has increased 10% due to a soft-seed shortage in the Galapagos
Geospiza fortis, a seed eating finch. This species has been seen to evolve in real time. Beak size has increased 10% due to a soft-seed shortage in the Galapagos | Source
Certhidea olivacea has a much narrower and pointed beak to catch insects
Certhidea olivacea has a much narrower and pointed beak to catch insects | Source
Geospiza scandens has a long and sharp beak. This helps the finch to tear and eat cactus flowers and pulp
Geospiza scandens has a long and sharp beak. This helps the finch to tear and eat cactus flowers and pulp | Source
Darwin's original sketches, as documented in 'Journal of Researches into the Natural History and Geology...' These different species of finch all evolved from a single ancestral species stranded on the Galapagos.
Darwin's original sketches, as documented in 'Journal of Researches into the Natural History and Geology...' These different species of finch all evolved from a single ancestral species stranded on the Galapagos. | Source
Apply a selection pressure to a varied group of the same species and you have natural selection.
Apply a selection pressure to a varied group of the same species and you have natural selection. | Source

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

Speciation

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.

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    • Michael J Rapp profile image

      Michael J Rapp 5 years ago from United States

      Outstanding Hub! Your example of the finches makes the Natural Selection concept easy to understand. Voted up and awesome!

    • alliemacb profile image

      alliemacb 5 years ago from Scotland

      Very comprehensive hub. Interesting to read and easy to understand. Voted up.

    • TahoeDoc profile image

      TahoeDoc 5 years ago from Lake Tahoe, California

      Excellent! The whole idea of 'evolution' has become controversial. I think part of that is a complete misunderstanding of the whole concept. The step-by-step explanation of what is indicated by evolution over time and survival of the fittest is very logical and easy to understand. I hope lots and lots of people read this!

      Another common mistake people make is taking the word 'theory' to mean in science what it means in every day life. Really, these definitions are different. A non-scientific theory can be loosely defined as just an assumption or presumption, that may be abstract or speculative or may have some basis in evidence. In science, a theory is quite different. A scientific theory refers to the explanation or model supported by repeated observation, tests or experiments. (These are not official definitions, just a description of the differences in definition.)

      Anyway, this hub should be used a reference for anyone who want to truly understand the scientific theory of evolution. Great job with a complex topic!

    • poowool5 profile image

      poowool5 5 years ago from here in my house

      Well, I'm married to a geneticist but not an expert in this field myself. Nonetheless, could make head and tail of it, and really enjoyed your hub.

      It is such commonly accepted knowledge now, it seems incredible that there are people who still kick back against it, especially those campaigning to have the teaching of Natural Selection banned from American high school biology text books. Unbelievable...

      Thanks for making an important subject clear and accessible to all. Voted up

    • TFScientist profile image
      Author

      Rhys Baker 5 years ago from Peterborough, UK

      @ Michael and Allie: thanks for the feedback - I was worried I was making it too technical, particularly talking about speciation events.

      @TahoeDoc: The use of the word 'theory' by anti-evolutionists is also a pet hate of mine. As I said in the hub, the controversy between biologists was settled more than a century ago!

      @poowool5: I'm glad you enjoyed the hub - Fortunately we don't have that much credible and serious opposition to teaching evolution in the UK. I teach in a Catholic school and have never had a problem with it

      Thanks to all for the feedback!

    • Austinstar profile image

      Austinstar 5 years ago from Somewhere in the universe

      Evolution explained in two minutes! That's about 1.5 minutes more attention span than the religionists have!

      At any rate, I thought this hub was very succinct and accurate. James Watkins himself might be able to understand this one.

      Any geneticist worth his pay will agree with this.

    • Marcy Goodfleisch profile image

      Marcy Goodfleisch 5 years ago from Planet Earth

      I love the way you summarized the salient points of Darwin's theories in a brief and comprehensive way, and your examples are excellent! Thanks for publishing this - voted up, useful, interesting and shared!

    • Robie Benve profile image

      Robie Benve 5 years ago from Ohio

      I love the natural selection theory. I am catholic, but to me believing in God does not mean being narrow minded, so I totally embrace the evolution theory.

      Great hub! voted up and awesome! :)

    • TFScientist profile image
      Author

      Rhys Baker 5 years ago from Peterborough, UK

      I am glad that everyone seems to understand the content of this hub. I have entire textbooks on the issue so I was worried that by cutting down so dramatically I was losing important points.

      @Marcy: Thank you for the specific feedback - I am very pleased you enjoyed it. Thanks for the share! :)

      @Robie: I work in a Catholic School and have had some very heated and enjoyable debates regarding the teaching of evolution. I have argued in the forums that if the Pope and Archbishop of Canterbury accept Evolution, then so should their congregations: Religion and Evolution are not incompatible, but two sides of the same coin.

    • donabhatt profile image

      donabhatt 4 years ago from Hyderabad

      Very informative......

    • TFScientist profile image
      Author

      Rhys Baker 4 years ago from Peterborough, UK

      I would remind any that once you insult me on a forum or on one of my hubs I will no longer accept biased, narrow-minded non-evidence based rants. Civility does not appear in some people's vocab-particularly when they then insult me for not publishing their comment.

      Give up. Your comments will never be approved. You don't know what debate is. You insult, belittle and demean any who disagree with you. We tried the whole back and forth and you just used it to annoy me.

    • osaeoppongde profile image

      Deborah L. Osae-Oppong 4 years ago from Chicago, IL

      Thank you for sharing this! I appreciate the clarity with which you broke down Darwinian evolution here! The term "evolution" carries so many meanings! This post was very informative!

    • nicomp profile image

      nicomp really 23 months ago from Ohio, USA

      The finches did not evolve in 'real time' -- some of the birds simply inherited traits that were already present in their ancestors.

      By your logic tall people 'evolved' from shorter people because environmental pressures required them to play basketball. When we see a tall person we don't say "wow that guy is really evolved."

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