What are the main ideas of Darwin's theory?
Natural selection proposes that all the biodiversity of life lead back to one common ancestor - the origin of life. However, when it comes to natural selection, we must be clear that it chooses the fittest organisms and not the organisms evolving to suit the environment. Also, the requirements of natural selection changes with the environment; whatever that is deemed as the advantageous trait now might not remain the same after a period of time.
There are many processes of natural selection, namely,
1. Overproduction
Organisms produce more offspring that can possibly survive, so that if some die due to unforeseen circumstances, there will still be some to ensure the continuity of its species.
2. Variation
Variation is the gene pool shared by a population in the environment. For example, two organisms of the same species can have black and green eyes respectively.
3. Competition
Also, competition is inevitable in a population as and environment has limited resources (eg. Food, mates).
4. Survival of the fittest
Survival of the fittest refers to an organism being successful in competition with their population. But fittest does not necessarily mean the strongest. In other words, it is the suitability to adapt to the environment when there are selection pressures from the environment.
5. Differing reproductive success (based on ability to adapt)
Due to their differing ability to adapt to the environment, the organisms with reproductive advantages (favorable traits) over others are more likely to survive and in turn, can produce more offspring.
6. Inheritance of favorable traits (genetic basis)
As the well-adapted (fittest) organisms are more likely to breed than the less adapted, they are able to pass their favorable genes (that enabled them to be fitter than the rest) to their following generations.
7. Increase frequency in gene pool – microevolution
As a result, the frequency of their genes increased in the gene pool of the population and that particular genotype of organisms will become more common in the environment.
It can be said that microevolution is occurring as there is a change in gene frequency in the gene pool of a population.
8. Speciation
The organisms will develop individually based on the requirements of their respective environment. There are no two similar environments and this will cause the same organisms to descent with modification and in the long run it will lead to speciation. Basically, there are 2 main causes of speciation:
- Geographical distribution
When a population is further divided into smaller subpopulations, natural selection causes them to evolve to adapt to their different surroundings. After some time, even if they interact again, their genetic differences would prevent them interbreeding.
- Reduction in gene flow
If a population were spread across a wide geographical location, it would be impossible for organisms from two ends to interact to breed. Thus in this case, the gene flow would be limited to certain areas and not fully spread out, which may result in speciation.
9. Macroevolution
Macroevolution is a very large-scale evolution, which is also known as the compounded microevolution. It is no longer observing an evolution of a single species, but the diversity of organisms in the world. For example, studying how mammals and reptiles have totally different appearance despite evolving from a common ancestor.
What examples were used to support his theory?
English Peppered Moth
A microevolution example, the English Peppered moth (Biston betularia) has both light and dark coloured phenotypes (variation), A female moth can lay up to 500 eggs to ensure continuity of its species (overproduction).
In England, during the Industrial Revolution period, soot from industries killed the lichens on the birch tree and darkened its bark. The darker moths managed to camouflage; however the lighter moths were easily spotted against the dark background and had a higher predation rate (survival of the fittest). The dark moths had a reproductive advantage over the lighter ones (differing reproductive success) and their advantageous traits were passed on more successfully (inheritance of favourable genes). As a result, the gene pool had an increase in dark phenotype and a decrease in light phenotype due to the failure of reproduction (microevolution).
Later in the 1950s, air pollution had significantly improved and the lichen had grown back on the trees, causing the trees and other backgrounds to be lighter. Then lighter moths could camouflage much better than the darker ones and natural selection made its frequency higher in the population again. (natural selection is never constant)
Darwin's Finches
An example of evolution is Darwin’s only finches. When Darwin visited Galapagos Islands, he observed that there were 13 different species in each island which had different types of beaks based on their diets that ranged from nutrition intake. However, he had knew that one species of finch had existed. Hence, he concluded that all these species led back to one common ancestor on the mainland of South America.
These species were originally all the same species and were distributed to different islands, with different environments (geographical isolation). In this case, each single environment had differing requirements for adaptations (adaptation). Thus, the bird with the best suited beak within its population and environment that could adapt and survive, managed to pass on its selected trait to the next generations (survival of the fittest).
In other words, the different environments encouraged a particular gene to increase its frequency in the gene pool over a long period of time. Eventually, the isolation of the same species on 13 islands resulted in a diversity of beaks ranging in size and shape (microevolution).
Common homologous structures
The study of homologous structures of dolphins and chimpanzees revealed that they have the same underlying arrangement of bones even those they function in different ways. This suggests that they have evolved from a common ancestor and have undergone macroevolution many years ago.
Sources:
Evolution Happens
http://www.evolutionhappens.net/
The Talk.Origins Archive
http://www.talkorigins.orgs
Science and Creationism
http://books.nap.edu/html/creationism/
Wikipedia
http://en.wikipedia.org/wiki/Natural_selection
Evolution 101
http://evolution.berkeley.edu/evosite/evo101/VCCausesSpeciation.shtml
Picture of English Peppered Moth
http://www.bbc.co.uk/schools/gcsebitesize/img/bi06001.jpg
Biology - Picture of Darwin's finches
http://www.bio.miami.edu/dana/250/darwinfinches.jpg
Complete Biology
- by Pickering
Evolution
Essential Science - by John Gribbin