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. Every individual within a population of organisms has a genome (the complete set of all of its DNA) that is passed on through generations from parents to offspring. DNA itself carries all of the instructions needed to carry out life-processes and it performs this task 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 might encode instructions to make proteins with slightly different structures and characteristics. To extend 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: 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 fur color in bears. Let’s say that the father passes along his fur color allele to his offspring, and it provides the instructions for making a protein that gives his child brown fur, and the mother passes along her fur color allele that provides instructions for making brown fur as well. In this case, the child will end up with brown fur. Now let’s say the mother passes along an allele for white fur instead. Somewhat surprisingly, the offspring may still have brown fur. In both of these cases the brown fur allele is said to be dominant: one brown fur allele is sufficient for brown fur 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 white fur to the offspring. As you probably would have guessed, the offspring will now have the white fur phenotype. Now we get a sense that the white fur allele is recessive – in order for the white fur trait to show in the offspring there can be no brown fur allele present. To put all of that simply, the combination of alleles that an organism has can 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 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 in which evolution acts 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 outcompeted 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 disadvantageous alleles 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 a more 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. 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.