In the 1830s, Charles Darwin embarked on a surveying mission around the world aboard the HMS Beagle. Darwin, a naturalist, observed many organisms during the journey; none more famous than the finches of the Galapagos islands.
Darwin observed that each island’s species of finch had beaks that were uniquely adapted to the type of food that was available on that island. For example, if there were insects, the beaks would be skinny and pointed. If there were hard nuts, the beaks would be thick and strong.
In 1859, Darwin published his findings in The Origin of Species. He argued that evolution had occurred in the past and that natural selection was its primary mechanism. This post will explore that mechanism.
What causes variation in a population? Why are some people short and some people tall? Why do some people have black hair and some people have blonde hair? One factor is mutation. Last week we saw how the software that produces our phenotype is coded in DNA.
For example, ATTACCGGA might be the gene that codes for height. If a mistake is made in copying the DNA, or an environmental factor alters it, the code might randomly change. Most of the time this random change will either do nothing or kill the organism. Cancer is an example of the latter. Unless the mutation happens to occur in egg or sperm cells, it won’t be passed on to offspring. For these reasons, mutation is probably not the most important factor for variations in a population.
Sexual reproduction, far and away, provides the main source of variation within a population. All complex organisms have some form of system where genetic material mixes between different individuals. In humans, a male and female both provide 23 chromosomes to their offspring. So for every trait, the offspring essentially has two versions. When that offspring reproduces, it will randomly pass on some combination of chromosomes that are a mix of the chromosomes it got from its mother and father.
Since there are two versions of every chromosome that each parent can pass on to their offspring, there are 2^46 possible combinations that can be passed on. This is about 70 trillion combinations. Further more, when those pairs of chromosomes line up together a mixing occurs called crossover. This means random chunks of DNA from one chromosome will swap with the equivalent random chunks in a homologous chromosome.
Natural selection is basically variation and selection. What is being selected for though? Some might assume that we are selecting for whatever is better. Humans evolved to be better and smarter and stronger etc. In an evolutionary context, better makes no sense. Natural selection is not the process that things become better.
For natural selection, all that matters is fitness to an environment. If a trait helps an organism to survive and reproduce in a given environment, it will be selected for whether or not we think it is better by our standards. The example that is given in every biology textbook is the peppered moth. In preindustrial England, peppered moths flourished in a wide variety of differing whiteness. In postindustrial England, the whitest of the moths disappeared. As soot covered trees, the white moths stood out and were easy targets for predators. The darker moths were selected for.
Aside from Darwin’s finches and the peppered moths of the world, evidence for natural selection is abundant. In fact, all of the progress that has been made in biology over the years would make no sense without evolution and natural selection.
Structural evidence: When we look at the bone structure of different species, they are shockingly similar. Take the human hand and compare it to the leg of a dog, the wing of a bird, or the fin of a whale. In each case, the fundamental structure is so similar that it might suggest they all share a common ancestor. Look at the structure of embryos from different species and they all seem to start out the same. Look at the fossil record and species separated by millions of years share a gradually changing bone structure.
Genetic evidence: If we look at the DNA of a human, it is 98% the same as a chimp, 92% the same as a mouse, and 18% the same as a plant. Once again, we might conclude that they all share a common ancestor. Furthermore, genetics have given us a window into the process of evolution that Darwin had no idea about. We know genes can be altered to produce different traits and we know these genes vary in a population.
Direct Observational evidence: Whenever we use antibiotics to kill bacteria, the ones that survive go on to reproduce and flourishes. When the flu is defeated by our immune system in a given year, the strains that survive go on to reproduce and return in the following year. Both cases are a direct observation of natural selection in action.
Environmental evidence: Wherever the distribution of species are disrupted by geological change, for example the formation of islands or tectonic shifts, a divergent evolutionary path occurs. Consider the marsupial species of Australia who use a pouch to carry young. This differs greatly from the mammals found in the rest of the world. Similarly, Darwin’s finches where the product of being separated by islands.
The evidence doesn’t stop here. There are mathematical models, populations simulations, DNA and Protein studies, and much more that support the concept of evolution through natural selection. If one day evidence arises that proves this theory wrong, the whole science of Biology will have to be rewritten.
Natural selection is the idea that traits which help an organism survive and reproduce in a given environment will be selected for. Variation occurs through the mixing of genes during sexual reproduction. Selection occurs as the best fit members of a population pass their genes on to future generations.
So far, all of our observations in modern biology are explained by evolution and natural selection. To understand heredity, cancer, biodiversity, etc we use the evolutionary foundation that Darwin laid out. Could this all be wrong? Yes, but that would also mean our whole understanding of modern biology is wrong. The balance of evidence doesn’t suggest that.