Simply put evolution is the process of small change over long periods of time. Over millions of years specific traits and characteristics in organisms have been selected for, while others have faded way, or in other words the traits have been selected against. The traits that help an organism best adapt to its environment are those that are selected for. Eventually, over millions of years the traits that were selected make individuals so different from the original ancestor that a whole new species is formed.


Evolutionary tree showing how small changes created new organisms.
Image credit: ThinkQuest Library

How did scientists first develop the theory of evolution? In Europe during the 19th century people began tinkering with the traits of animals and plants to achieve specific combination of traits. This unnatural selection of traits is referred to as artificial selection. Many of the dog breeds we know of today came from this artificial selection. The original wild dog was much like today's wolves, and over thousands of years of artificial selection and domestication we end up with the thousands of dog breeds we have today.


Dog evolution through artificial selection.
Image credit: Dr. Bruce Railsback, University of Georgia

Domesticated animals were not the only organisms undergoing artificial selection. Plants have been undergoing this same process since humans first started cultivating crops after switching from a hunter gather lifestyle to an agricultural based living. One stark example is the artificial selection of wild mustard into many of the vegetables we eat today.



Wild Mustard Artificial Selection into various common vegetables.
Image credit: University of California, Berkeley. Evolution Department

The work that breeders and farmers had been working on for decades had gone unconnected to evolution for many years. That was until Darwin returned from his voyage and began to dissect his findings. Upon returning to England, Darwin observed this artificial selection in action and it and his findings helped him develop his theory of natural selection. When most people think of evolution they think of Charles Darwin, but Darwin's work along with that of many other scientists simply scratched the surface of explaining why evolution matters.

What is Embryology?

While Darwin was working on his findings a scientist named Ernst von Baer was developing his own ideas about how evolution was affecting organisms. Von Baer's theory was that the more closely related two organisms were the more closely their development from zygote (fertilized egg) to, fetus, to adult would be. So von Baer's idea was to look at the development of organisms from early stages to adulthood. Von Baer choose to start with mammals and compare numerous animals at various stages of development.


Drawings of the embryologic development of a fish, turtle, salamander, and human. Various similarities among the different vertebrate organisms are observable.
Image credit: Bruce Walsh EEB Dept. University of Arizona


The image above illustrates the comparison between three stages of development among various vertebrate organisms (fish, salamander, tortoise, chick, dog, cat, rabbit, and human).
Image credit: Biologyreference.com

It can easily be seen through the above images that at the early stages of development vertebrate embryos are almost identical in anatomy. In the second stage of development some of the embryos start to differ more greatly. It should also be noted that in this second stage of development the mammals (dog, cat, rabbit and human) still seem to be very similar in anatomical appearance. It is not until the final stage where the individual characteristics of each animal are apparent. These images are similar to those that Ernst von Baer used to come up with his theory that the more closely related two organisms are, the more closely their development from zygote (fertilized egg) to fetus to adult will be.

How do genetics and embryology work together to help better understand evolution?

Until recently evolution was simply a way of explaining how organisms changed over time due to changing habitat, competition, predation, and random mutation. However, scientists did not truly understand the mechanisms or "the how" of evolution. It was not until scientists were able to look at an organism's genes were they really able to explain why and how evolution occurs.

Evolution: Great Transformations Video Clip.

This clip comes from the PBS video series Evolution. The Great Transformations section of the video includes in-depth review of William McGinnis' research as well as other scientist's use of genetics and embryology to understand evolution.

Which of the following organisms are more closely related; a wolf, a dolphin, or a shark? The following link will help you answer this question and find out more about which organisms evolved from each other. The results may be surprising. All In The Family: Animal Relationships

Once scientists realized that genes were the mechanism for evolution they could then study how organisms changed over time and see the connections based on numerous physical (phenotypic) and genetic (genotypic) factors. Through the understanding that genes provide the direction for an organism's development, scientists could connect this back to von Baer's work on comparative embryology. We saw in the "Great Transformation" video that certain genes develop specific sections of the body in all organisms, this was the proof needed to back up von Baer's theory. We can now understand why organisms that are more closely related have more similar embryonic anatomies.

How does Embryology help support Evolution?

Embryology helps support evolution because it shows that the more closely related two organisms are, the more closely their embryonic development is. This idea has been around for close to 200 years but only until recently have scientists been able to explain why. Scientists now understand that an organism's genes dictate their development in the early embryonic stages of life. The more closely two organisms are on the evolutionary tree the more closely they share a common ancestor. That common ancestor has many of the same genes that the two modern organisms had. When that organism is developing we see those same genes being expressed, especially in the early stages of development. It is not until the late stages do we see the genes that the organisms do not have in common being expressed. This supports the earlier images that vertebrate organisms in stage one have almost identical anatomies, then in stage two the mammals continue to look very similar, even though the fish, reptile, amphibian, and bird start to differentiate from the mammals. Then by the third and final stage we see that each organism is taking on the characteristics of their adult animals.