Evolution is a Unifying Theme
Evolution, commonly defined as “change over time,” is a unifying theme in biology; it ties all other fields of biology together. It is supported by scientific evidence from many fields of biology and other sciences. Evolution doesn’t just tell us about the past, it also has a number of practical applications in our world today, which we will discuss in this module.
What Is Evolution?
Evolution is formally defined as a heritable change in organisms (specifically, a population) over generations. What does that mean? It means that a species or a population (not an individual) will change over time as traits are passed on from generation to generation.
The Theory of Evolution was initially a formal hypothesis put forth by Charles Darwin. It is now considered a scientific theory, and therefore attempts to explain what we observe in nature with significant evidence, or support. Two main ideas that make up this theory:
- Evolution has occurred to produce the modern life on Earth that we see. Living things here today descended from a common ancestry.
- Evolution occurs through the process, or mechanism, of natural selection. Natural selection is a process that occurs when living things with favorable (inherited) characteristics survive to reproduce, thereby passing on those beneficial characteristics.
History of Evolutionary Theory
Ideas relating to evolution began centuries before Darwin. Aristotle (~300 B.C.) arranged living things on a scale from simple to complex, expressing the idea that life moved “toward perfection.” People discovered fossils in layers of rock, some of which seemed out of place (such as whale bones in layers of rock on a mountain) and may or may not look like the modern organisms they were familiar with.
In the late 1700s, a French Naturalist, Jean Baptiste de Lamarck (1744-1829) was the first scientist to propose that change occurred in species over time (evolution). Lamarck provided an explanation for how he thought evolution would occur. He called his mechanism for evolution The Inheritance of Acquired Characteristics, which is outlined below.
About the same time, there were two other people expressing ideas that would eventually influence Darwin:
Charles Lyell (1797-1875), an English geologist, explained his ideas about how the Earth has been shaped slowly over time by geological processes (volcanic activity, erosion, uplift) in his book, Principles of Geology. This slow pace suggested that the Earth was older than most people at the time believed.
Thomas Malthus (1766-1834), whom we discussed in the Ecology module, explained how populations struggle when resources are scarce.
Finally, Darwin was influenced by his knowledge of artificial selection that was used by breeders and farmers in England. Artificial selection is the process of selecting a desired trait and breeding organisms with those traits to produce a desired offspring. Using this method, breeders or farmers could domesticate an animal or plant to show the desired trait in few generations.
Examine the picture below. You should recognize four common vegetables: broccoli, cauliflower, brussel sprouts and kale. Each is a member of the same species (Brassica oleracea) that has been artificially selected for certain features.
Charles Darwin (1809-1882), developed the basis for the Theory of Evolution after traveling around the world on the H.M.S. Beagle.
Learn more about Darwin and how this trip influenced his ideas about evolution.
During the trip on the H.M.S. Beagle, and for many years after his return, Darwin puzzled through the evidence he collected on his voyage to explain how organisms changed over time. Natural selection is the mechanism that he eventually proposed to explain evolution.
Examine the cartoon below that shows a population of beetles over time. We will use it to explain natural selection below. For some learning fun, navigate to the sidebar and play the Nowhere to Hide game that matches this cartoon.
The outcome: The trait becomes more common in the population.
Let’s return to the example with the giraffe and see how Darwin may have used natural selection to argue against Lamarck:
|Natural Selection Piece||Explanation for neck length using Natural Selection|
|A population has variation in a trait.||A past population of giraffes has members with variation in neck length.|
|Differential reproduction occurs.||The giraffe population increases over time. Giraffes with shorter necks cannot get the food at the tops of trees and compete with other herbivores. The giraffes with longer necks will obtain more food and survive to reproduce.|
|The trait is inherited.||Longer-necked giraffes pass on this advantageous trait. Over many generations, this will increase the amount of individuals with longer necks.|
It is important to note that natural selection is not random. Specific traits (longer neck, orange beetle color) are specifically being targeted by nature and/or the predator because they are advantageous in those unique situations. However, the mutations that create variation are random – they determine the amount of variation in a population.
Survival of the Fittest – or Fit Enough
You may have heard of the phrase, survival of the fittest. It does not mean that only the strongest, most physically fit organisms survive. Survival of the fittest refers to the idea that the individuals that are most fit are those that can make it to reproductive age and pass on their traits. Fitness, in this sense, means reproductive fitness. Another common misconception is that evolution and/or adaptations lead to perfection. This is not the case – an organism need only survive; the phrase is probably better reworded as survival of the fit enough.
Let’s look at two examples of fitness.
Reproductive fitness can be the result of an individual being stronger (such as when two male sheep ram each other to protect their ewes, potential mating partners) or it may not. Take a look at the two examples below:
In summary: Over generations, natural selection would lead to a population made up of individuals with traits that allow organisms to be more “fit” for that environment.
Types of Natural Selection
An adaptation is an inherited trait that enhances or aids the chances of survival and reproduction for an organism. Adaptations increase “fitness.” Adaptations within a population are the result of natural selection over time. Darwin’s finches, by way of example, each had beaks that were adapted for their specific diet:
There are three categories of adaptations. Study the table below:
|Type of Adaptation||Explanation||Example|
|Physical/Structural||Internal or external feature of the anatomy that increases fitness||Camouflage (see below for more on this)
Tongue of an anteater
|Physiological||Functional or biochemical feature that increases fitness||Snake venom
Protein in spider webs
|Behavioral||Response to the environment that increases fitness||Bird migration
Squirrels hiding nuts
Wolves’ ability to track prey
There are a variety of physical adaptations that involve camouflage. Read about each of the types of camouflage below.
- Concealing coloration – Organism displays same colors as its environment to hide from predators, or surprise prey (polar bear)
- Disruptive coloration – Shading or coloring that breaks up organism’s outline so it doesn’t stick out or show as well (zebra)
- Mimicry – looks similar to a poisonous or dangerous organism that is avoided by predators (a Viceroy butterfly is non-poisonous but has the same appearance as the poisonous Monarch)
- Disguise – Looks like another object in its environment (walking stick insect)
Stop and Think: Take a look at the insects in the picture below.Only insects A and B are actual wasps, the others are either beetles or hoverflies. What type of camouflage is being displayed by C-F? Why? (Answer: mimicry, because animals think they sting and stay away)
How do species change over time, or evolve?
Darwin proposed that natural selection could work on organisms over long periods of time, making populations that are different than they were in the past. Darwin explained that this is why species today appear different from their ancestors (not how one species can become another!–added note from EP), because small changes in the population have occurred over time. This principle is called descent with modification. Each species has an ancestry that can be traced back over time.