Making Sense of Evolution

Evolution: An Overview

In The Origin of Species by Means of Natural Selection, Darwin provides four postulates:

1. Individuals within species are variable
2. Some of these variations are passed on to offspring
3. In every generation, more offspring are produced than can survive
4. Survival and reproduction are not random: The individuals that survive and go on to reproduce, or who reproduce the most, are those with the most favorable variations. They are naturally selected.¹⁶

Variations within species are essential for natural selection to occur and drive evolution. While Darwin had no understanding of the underlying genetics to support his theory in the 19^th century, modern genetics has allowed scientists to find these genetic variations and mutations. Even sexual reproduction itself is important for creating variations that can allow for changes in species, because genes from parents are combined to make unique combinations in the offspring. Obviously, these variations must be passed on to offspring in order for these variations to drive natural selection, and simply by necessity more offspring must be created than can survive. The most “fit” individuals of a species are the ones to survive and to pass on their useful variations and characteristics on to the next generation. This is as a result not a random process. Natural selection is a process with a definitive and logical steps that are driven by successful reproduction and passing on of traits by organisms that are better adapted to survive.

There are a few key ideas of Darwin’s that should also be explained, as they continue to endure to this day. One of Darwin’s enduring ideas is common ancestry. Common ancestry, as previously noted, explains that all species must have come from common ancestors. Obviously this helps to explain homologous structures in more closely related species, but it also explains why the vast majority of all living things use the same DNA code to store genetic information. As a result of this common ancestry, all of life can be displayed as branches on a common family tree. Another key concept handed to us by Darwin is gradualism. Gradualism states that changes in species and adaptations come slowly, in small steps.¹⁷ Evidence for this can be found in many transitional forms, which are intermediate steps between two related species that have been preserved as fossils.

Natural selection is the central process driving evolution, as it provides the mechanism by which variations are passed across generations to better match organisms to their environments and to ultimately cause species to change over time. Natural Selection involves a process whereby variations in individuals determine their relative ability to survive and reproduce, and only the most successful variants pass along these traits to the next generation. If these variations provide a better match to the environment, the resulting traits are called adaptations. Adaptations may give the appearance that a species might have been specially designed to survive in a given environment, but in reality the natural selection process that occurred over sometimes millions of years is responsible for these adaptations.¹⁸ The ability to fly was most certainly an adaptation for some ancestor of the ostrich. However, at some point the ostrich no longer needed to fly in order to be successful in survival and reproduction and the pressures of natural selection did not favor individuals with particularly good flying ability. As a result, the ostrich has since lost the ability to fly. The same is true for other flightless birds; though their wings may be adapted for new uses, they no longer serve the purpose for which they were originally selected.¹⁹

Vestiges & Atavisms

Because evolution occurs over great spans of time and traits are heritable, sometimes throwbacks appear either in the form of vestigial traits and atavisms. Vestigial traits are structures that an organism still exhibits but are now useless for their original purpose. As mentioned, an example of a vestigial structure is the wings on a flightless species of bird.²⁰ Similarly, true moles have lost the ability to see along with much of the structure of the eye. Similar pressures have occurred with certain species of burrowing snakes and many cave dwelling species that have lost the ability to see. Sight is not essential for cave dwelling species, and therefore natural selection did not favor individuals with the trait.²¹

One famous example of a vestigial organ in humans is the appendix. The length of the human appendix varies greatly, and in some people it is even absent; in animals with a plant based diet, the appendix is considerably larger and more developed, serving to house bacteria or for fermenting plant matter in order to digest it fully.²² It no longer serves the same purpose in humans, and is therefore considered to be vestigial. However, the appendix may not be completely vestigial, as it could harbor helpful bacteria or serve other purposes. For another example, we also have the remnants of our ancestral tail, the coccyx, and when the genetics for that tail are expressed and a human baby is born with a tail, the trait goes from being a vestigial bone at the base of our spinal column to an expressed atavism, or throwback to the ancestral form.²³

Whales, which will be a central focus of this unit due to the rich documentation of their ancestral transitional forms, harbor many examples of vestigial traits that illustrate their ancestry. Despite now being an entirely aquatic species, whales retain evidence of having ancestors that once lived on land. These vestiges include hind limbs and pelvic bones that are not even connected to the rest of the skeleton.²⁴