One of my favorite poems:

When I Heard the Learn’d Astronomer
by Walt Whitman

When I heard the learn’d astronomer,
When the proofs, the figures, were ranged in columns before me,
When I was shown the charts and diagrams, to add, divide, and measure them,
When I sitting heard the astronomer where he lectured with much applause in the lecture-room,
How soon unaccountable I became tired and sick,
Till rising and gliding out I wander’d off by myself,
In the mystical moist night-air, and from time to time,
Look’d up in perfect silence at the stars.

The theory of evolution: there is probably no greater disconnect between what scientists seek to explain and what the general population of non-scientists thinks we explain. Moreover, one should note that this contention is only between science and the public, not between scientists themselves. In fact, not only do the overwhelming majority of biologists accept that evolutionary theory is the only explanation that can fully account for the diversity of life, but so too does every other major scientific association around the world [1]. It has been a long-standing practice and goal of mine (and an aim of this blog) to ferret out scientific misconceptions while helping others learn. Therefore, in this blog post I hope to explain, in a very general sense, what the theory of evolution is and then give an example that I feel will be accessible to those who are unfamiliar with biology.

Firstly, we need to define what evolution is. Put simply, evolution is the change in populations of organisms over time, and starting from this observation, we can explain the diversity and panoply of life. (I should note here that this definition isn’t as precise as it should be, and we will add on to it later on when we have fleshed out some details.) Every individual within a population of organisms has a genome (the complete set of all of its DNA) that can be passed, in part (in sexually reproducing organisms), on through generations from parents to offspring. DNA itself carries all the instructions needed to carry out life-processes and it does this through genes. A gene is a stretch of DNA that carries the instructions to make a protein, and proteins are the “work-horses” of life. You can think of a gene like a sentence, and a protein as a message or idea that the sentence coveys. There is one other thing I must mention about genes: there exists alternate forms of each gene, called alleles. These alleles have slightly different gene sequences, and hence, they have instructions to make proteins with slightly different structures and characteristics. To continue with the analogy, “the boy kicked the ball” and “the boy kicks the ball” are similar to alleles in the sense that they are slight variants of a sentence that provide slightly different information. 

Furthermore, most plants and animals have two sets of chromosomes (packaged DNA within the cell); one set comes from the mother and the other set comes from the father. In addition, each chromosome contains one set of alleles; these organisms are said to be diploid. So, ultimately the offspring receives two alleles for any given gene, one allele each from the parents. The specific combination of alleles across the genome will lead to a plethora of various phenotypes – characteristics, or traits – expressed by an individual. Let’s look at a specific, and contrived, example of eye color in animals. Let’s say that the father passes along his eye color allele to his offspring, and it provides the instructions for making a protein that gives his child brown eyes, and the mother passes along her eye color allele that provides instructions for making brown eyes as well. In this case, the child will end up with brown eyes. Now let’s say the mother passes along an allele for blue eyes instead. Somewhat surprisingly, the offspring will still have brown eyes. In both of these cases the brown eyed allele is said to be dominant; one brown eye allele is sufficient for brown eyes regardless of what the other allele is – it “masks” the trait that the other allele would provide. Now, let’s assume both parents pass along an allele for blue eyes to the offspring. As you probably would have guessed, the offspring will now have the blue eye phenotype. Now we get a sense that the blue eye allele is recessive – in order for the blue eye trait to show in the offspring there can be no brown eyed allele present. To put all of that simply, the combination of alleles that an organism has will have a huge impact on its traits and characteristics.

To expand upon this, let’s delve a little deeper. All individuals within a population of organisms are not genetically identical. This is intuitive but it is central to evolution. We only need to look toward the huge phenotypic diversity in humans to understand this: we have many different skin colors, hair colors, eye colors, sizes, shapes, metabolisms, attached/detached earlobes, hereditary diseases, propensity for athleticism, etc.; no two humans are the same (except in the case of identical twins). Populations of organisms, whether they are bacteria, humans, dogs, fish, palm trees, you name it, all have genetic variation. Genetic variation within a population occurs when there is more than one allele present at a given locus (location within the genome) within a population. This genetic variation allows for the differential expression of traits between individuals, and it is the raw material that evolutionary processes act on.

This is where natural selection comes into the fold. Natural selection is the process where given phenotypes become more (or less) widespread within a population of organisms over time if those phenotypes increase (or decrease) an organism’s survivability and reproductive success (fitness) within that environment. This makes intuitive sense: if organism A has a trait that makes it better adapted to survive and reproduce in its environment than organism B, then organism A will pass along that trait (and the alleles that cause it) to its offspring more readily than organism B. In other words, organism B is more likely to be out-competed by organism A. Over time, alleles that confer a greater fitness advantage to organisms within their environment will increase in frequency within the population, whereas alleles that confer neutral or diminished fitness will decrease in frequency (frequency here simply means proportion). The effect of natural selection is that, over time, populations become better adapted to their environment. Now we can finally get to the precise definition of evolution: evolution is the change in allele frequencies within populations over time.

Now, let’s try to tie this all together using a simplified example. Let’s say a fish acquired a mutation in an allele and she passed that allele onto her daughter. Let’s also say that the mutated allele is dominant and causes small fins (phenotype). As a consequence of her small fins, she will struggle to swim as fast or as well as her siblings, and she will be less adept at catching prey and more vulnerable to predation. Unfortunately due to her phenotype she will have a greater chance of not surviving to reproductive age where she would pass on the mutated allele to her offspring. If she dies before she reproduces, that allele will be purged, or eliminated, from the gene pool.

Conversely, a fish that acquires a mutation in an allele that gives her larger fins will have a higher probability of escaping predators and catching prey. Therefore, she will be more likely to survive to reproductive age and pass along that mutated allele to her offspring. Furthermore, her offspring will now have an advantage over those fish without the allele, and they, again, will be more likely to survive and reproduce. Over successive generations, that allele will raise in frequency within the gene pool, so over time more and more fish within the population will have larger fins. The population as a whole will be more fit within that environment. Notice how the environment selected for those with larger fins and against those with smaller fins - this is natural selection. Notice how inaccurate it is to call this process of differential reproductive success, “random”, as many creationists tend to do. In fact, it is completely non-random.

Lastly, and as more of a side note, most creationists will say that the example I provide above describes the process of microevolution, or change in allele frequency within a population over time – and they would be right in making this assertion. As a general rule, they tend to agree that microevolution occurs; even they can’t deny that evolutionary phenomena such as antibiotic resistance in bacteria is a large problem. That being said, they generally do not accept macroevolutionary changes above the species level, such as speciation events (the formation of new species), common ancestry, descent with modification, etc [2]. However, this is silly because macroevolutionary changes occur via the same mechanisms as microevolutionary changes, just on a larger time scale. Saying that macroevolution can't happen while microevolution can is like saying I can take a few steps forward but I will never reach the other side of the room: large changes occur by the accumulation of small intermediate changes. 

References:

1.       Statements from Scientific and Scholarly Organizations. Retrieved from http://ncse.com/media/voices/science

2.       Theobald, D. (2012). “29+ Evidences for Macroevolution: The Scientific Case for Common Descent.” The Talk.Origins Archive. Retrieved from http://www.talkorigins.org/faqs/comdesc/