Modern synthesis is the combination of Gregor Mendel and Charles Darwin's original ideas. It further solidifies the theory of evolution by explaining how traits are passed down, according to Darwin's ideas of natural selection, and by explaining how traits are passed down according to Mendel's ideas of inheritance. In addition, modern synthesis also uses the ideas of mutation, gene flow between populations, and genetic drift as explained by other scientists over time.
Modern synthesis is different from Darwin's original theory of evolution because it adds the evolutionary forces of mutation, gene flow, and genetic drift whereas Darwin's theory recognized only natural selection as an evolutionary force.
What allows evolution to occur?
In order for evolution to occur you must first start with a population with two or more different traits to be chosen between. If there was only one trait that could be passed on, there would be no genetic variation within a population and therefor, no evolution.
Changes in the gene pool, environment, or population size can speed up the effects of evolution by causing there to be more fitness in one of the characteristics. Changes in the gene pool can result in one characteristic being shared by the majority and thus the population, over time, will shift towards the characteristics of the majority. A change in population size can have the same or a similar effect. If there is a change in the environment, those who have the characteristics to better fit the environment will have more fitness and will be more likely to reproduce and survive.
A perfect example of a population evolving to fit its environment is the population of peppered moths during the Industrial Revolution. Before the revolution, trees were a light color and when the moths landed on those trees, they couldn't be seen by predatory birds. However, after the Industrial Revolution, the trees began to build up a coat of soot which was dark. Therefor the moths with a darker color were favored through natural selection because they could escape from predatory birds.
How does evolution occur?
There are four modes of evolution in a population.
Natural Selection
Nature will select those that have the most fitness for a population it its environment. Those with more fitness will be more likely to reproduce and survive causing the majority of the population to have those "fit" characteristics.
An example of this is the evolution of the Galapagos finches. Finches on different islands in the Galapagos have different sized beaks to fit the seeds that grow on each island.
Genetic Drift
This is often also called the Bottleneck or Founder effect. By random chance usually through a natural disaster, there is a lowering in a population's size leading to a change in allele frequency. There will then be a characteristic that the majority of the population has and that majority will be the most likely to reproduce and ultimately survive. There could also be a recessive gene in the gene pool that, when the population size in decreased, has a better chance of becoming more prominent.
An example of this is shown through a population of Amish settlers in the US and Canada. One of the original two hundred founders of an Amish colony had a disease called Ellis-van Creveld syndrome a type of Dwarfism where those affected have six fingers and toes on each hand or foot. Because a smaller population was created, there was a better chance for that recessive gene to become more prominent in the population. Today, 1 in 14 Amish have this syndrome compared to 1 in 1,000 in the rest of the world.
Gene Flow
Due to emigration and immigration there are new genes added to the gene pool. This causes the population to include that trait in it's "mix" and, if the new trait increases fitness, it will become more prominent in the population through natural selection.
An example of this is if light colored deer emigrated to a population of dark colored deer, the light colored deer would still be able to reproduce and their color characteristic would be observed with the dark colored characteristic. However, if the environment suddenly changed to a snowier environment where a light color would be preferred to hide from predators, the light colored characteristic would offer more fitness to an individual and would be favored through natural selection.
Sexual Selection
The organisms in a population who are the most visually or behaviorally appealing will be more likely to reproduce. As a result, because those with the appealing traits are the ones most often reproducing, the population will evolve to also have these traits making the population a less diverse one.
A perfect example of this is a population of peacocks. A male with a fuller and more colorful tail will be more likely to attract a female to reproduce with. Then, their offspring are likely to have full colorful tails as well.
These modes of evolution can be one of three types of selection.
Disruptive Selection
Disruptive selection is the selection against the mean of the population. In most cases this is due to a dramatic change in environment and can often lead to extinction.
Directional Selection
Directional selection occurs when one characteristic, usually the extreme, is favored over another. This leads to a "new normal" in traits and shifts the population towards these characteristics.
Stabilizing Selection
Stabilizing selection occurs when the mean of the population is favored. This leads to the most common trait becoming the majority of the population and can often lead to limited variation of traits.