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Section 23_2: Observations of Evolutionary Change

Page history last edited by Darlene Walaski 11 years, 1 month ago

Section Summary


Because evolution is a theory and not directly observable, scientists are forced to rely on observations to support the theory.  This section covers the areas in which scientists have been able to identify observable biological occurrences supporting the theory of evolution.  Such observations covered in this section include those found in fossil records, instances of biogeography, convergent evolution, selective breeding, and homologies.  Fossil records support evolution because when placed in chronological order, fossils reveal gradual evolutionary change.  Though incomplete, the fossil record has been expanded through more recent discoveries to illuminate several examples of evolution, including the evolution of tetrapods, or four legged animals. Transitional forms are particularly useful in observing evolution through fossil records because they provide a link between the older and more recent forms of evolutionBiogeography supports evolution through the study of how living and extinct species are distributed geographically.  Like Darwin had observed with the finches of the Galapagos Islands, geographically isolated plants and animals evolve distinct traits and communities.  This gives rise to certain endemic species which are native to only one particular location.  Close evolutionary relatives of such species can often be found on nearby bodies of lands.  Convergent evolution is the observance of similar characteristics in loosely related species due to similar environments.  These similar characteristics are called analogous structures or convergent traits.


Selective Breeding, also called artificial selection, is caused by humans and refers to the sequences and actions taken to alter the traits of a domesticated species.  While natural selection relies on environmental factors to determine which parents are the best choice to pass on their traits, artificial selection is a result of a breeder choosing traits that are advantageous to themselves and controlling which parents pass on genes.  This is possible due to genetic diversity within a species.  Breeders are able to select members of a species with the preferred phenotype.  These differences in phenotypes are a result of different alleles of a species, which are variations of a gene.  This is evident in breeds of dogs which display a wide range of phenotypes though all belonging to the same species, many of which are products of selective breeding.  Another example would be the wild mustard plant which has been bred by humans to occur domestically as broccoli, cauliflower, cabbage, Brussels sprouts, and kale.


Homology is defined as a similarity explained by a common ancestor and can be categorized as anatomical, developmental, and molecular.  Anatomical homologies are called homologous structures because they are structures that are similar due to derivation from a common ancestor.  Vestigial structures are also categorized as anatomical homologies because though they serve no apparent purpose, they can be explained by a similarity to a structure of an ancestor.  Developmental homologies are similarities that can be observed during embryotic stages, such as the bony tail and gill-like ridges of human embryos.  Molecular homologies are homologies observed at the molecular level such as the common biochemical pathways found in many or all species.  An example would be the metabolic pathway which uses oxygen to sustain life in most living things.  The genetic homologies discovered through genetic sequencing makes some of the strongest cases for all life originating from a common ancestor because similar types of genes can be found across a broad range of organisms.


Useful Materials



     Part of a series that outlines the basics of biology, this video provides a clear reiteration of many of the major concepts of the section along with some different examples for those who learn better hearing something explained multiple ways.  The video also mentions several other examples of these concepts that aren't in our textbook and ties it in to what we've learned in th earlier chapters and some of the later concepts we'll be covering in the following weeks, such as speciation.

      Hank green talks about how "evolution is a thing" and how through these methods of observation we can support evolution as a theory and explain how it's almost certain to skeptics, tying everything back to the theory of evolution and the astounding evidence supporting it.

Convergent evolution 

     This page goes more in-depth about common analogous traits and why they're so common.  The page explains how convergent evolution can help animals hide from predators or pray, and sometimes scare them off.   It gives its readers a feel for exactly how widespread convergent evolution is and how organisms are able to benefit from it.

     This page makes understanding convergent evolution a lot easier because not only does is give a more in-depth explanation, but it teaches the scope of convergent evolution and the impact it has on the animal kingdom specifically.  Understanding the theory of convergent evolution is only half of it.  The other half is understanding exactly how relevant these convergent traits are beyond a few text book examples.  Without proper background, convergent traits are easy to mistake for homologies, making them hard to recognize.  This page shows us how just because two species of cacti have similar spikes, they have spikes just because they're cacti and they get their spikes from the same place.  Being aware of just how many traits are convergent out there, we are able to better understand the marvel of evolution and how well we are able to adapt to our environments over time.

Selective breeding

     In our textbook, the only animal examples of selective breeding are the various breeds of dogs.  This page not only gives additional examples of selective breeding in animals, it gives examples of projects breeding for specific traits.  Also, where dogs have been domesticated for years, in some of these examples we see animals being domesticated from the wild type of the species and what kind of effect domesticating from the original bred has.

     In addition, we learn from this page how selecting for one trait can also effect other traits.  We can also see how people are able to use this method to their own advantage and preform experiments telling us how was can manipulate a species and giving insight into how exactly allele variants and genetic diversity works within a species.

Problems with Selective Breeding

     This page works with the above page to put selective breeding into perspective.  Whereas the above page talks about how human kind has benefited from selective breeding, this page points out how selective breeding isn't always advantagous for the organisms bred.  There's a reason that selective breeding doesn't occur naturally--humans and nature have different interests.  Selective breeding is entirely dependent on human intervention which we sometimes have to be careful with.

     This page brings up how breeding dogs with like traits can sometimes lead to problem by allowing recessive gene conditions to cause health problems.  Breeding purebreds can have results similar to inbreeding in humans--the genetic codes are similar enough to cause uncomfortable and risky health conditions.  If anything, it is a lesson in the importance of genetic diversity in a species and why it occurs naturally to protect the species interest rather than the humans' interests--to keep species healthy.



Primary Literature 


                This article first summarizes the debate surrounding homologies.  The argument is made that because two species share similar structures or traits, it doesn’t necessarily prove that such traits were inherited from a common ancestor.  Two closely related species are likely to acquire similar adaptions if exposed to the same environment, so such traits could very well have developed independently of each other via convergent evolution.  The argument then turns to under which circumstances a common trait may or may not be considered a homology.  Over time, several scientists have proposed varies standards in an attempt to outline what may or may not be considered a homology.

                Because the factors relating to whether something is a homology—those factors being evolution and development—are so interrelated, defining a homology is complicated.  It has been proposed that instead of working to define homologies, scientists look into angering the pragmatic questions that accompany homologies as a notion.  This paper goes on to discuss some of the linkages between homologies and the developmental process.

                Researchers now are looking into using perceived molecular homologies to validate what structures are and are not homologies.  The author concludes that there is no clear-cut way to identify a true homology with all the variables that factor into how structures develop in any given organism.  However, the concept of homology is constantly being referred to as “the central concept for all of biology” and the article repeatedly affirms the importance of the notion because it is homologies that are able to support how the theory of evolution works and shows the continued evolution of a species through past ancestors allowing all life as we know it today.


Virtual Lectures





Grading Sheet (Darlene)


A. Learning Objectives:

  • Explain the importance of the discovery of transitional fossils
  • Name the evolutionary trends revealed by study of horse evolution
  • Explain the principle of convergent evolution
  • Explain the evolutionary significance of homologous and vestigial structures 

B. Section Summary:

Fossils make up the bulk of the evidence we have toward past life on earth. They are very hard to work with because it is rare to find a complete skeleton fossilized, but from various fossils we can and have put together what certain organisms look like and created a very large fossil library. From this fossil library, we have classified many species. The main problem with doing this, is that it is very arbitrary to say what is and what isn't part of a species. We overcome this with transitional fossils. Transitional fossils are fossils that show traits of species that came before it and species that came after it. For example, Homo erectus is a transitional phase between Australopithecines and modern humans because it shows characteristics of both species. With a larger fossil library, we are better able to connect lineages and actually see trends that changed as a species evolved.


The way horses evolved reveals much about the ways animals adapt to their environment and natural selection. The horse started out similar to a dog, but because because it lived in open grasslands, it began to have to run more to escape predators that could easily spot them. Its feet began to change, its big to grew and eventually it lost the other four, and lastly a bony structure formed to prevent it from injuring its soles. This wasn't the only change it went through, it's diet influenced its evolution as well. Because it fed mostly on leaves and herbs, it grew large molars with complicated ridge patterns to aid in grinding its food.


Convergent evolution is another way we can learn from evolution. Evolution selects for the best traits, therefore species that are very different evolutionarily, can independently develop similar traits because of similar environments. Some examples of this occurrence are the way bats and birds separated in the evolutionary chain much longer ago than when they developed wings. This is because flying was just more evolutionarily beneficial for their environments. Diet too has an impact on the way animals evolve. Anteaters and echidna both feed on ants, which live in these small volcano looking structures, so they each developed a long snout to eat the ants straight from the ant hole.


Unlike convergent evolution, homologous structures are structures that are similar between species because they have a common ancestor. These structures may have mutated over time to serve a different purpose but are still very similar. There are three types of homologous structures, which are anatomical, developmental and molecular. Anatomical homologies are similarities in the composition of organisms, such as how bats and humans both have a radius and ulna. Developmental homologies are traits that are shown in its embryonic stages that are not expressed in developed organisms. When humans are in their embryonic stages, they develop gills and webbed hands and feet, but these traits are lost well before the fetus is ready to be born because they are not traits that are useful today. Molecular homologies are similarities in the molecular composition of organisms. For instance, humans and chimpanzees are 99% related to each other in their DNA, because we are very close to them ancestrally. These homologies are important because they provide evidence and clues toward lineages and other evolutionary trends. By looking at which organisms share which traits, we can determine common ancestors and connect lineages.


When a homologous structure no longer serve a function to the organism, they are classified as vestigial structures. These are structures such humans as having muscles to move their ears, which we haven't needed to do for an incredibly long time, long enough that it is rare if a human can even operate these muscles slightly. While, these structures do not provide a function to the organism, they tell us a lot about which species an organism may have evolved from. Another example of a vestigial structure is the pelvis in whales, it serves no function, but does tell us that they evolved from a land-dwelling creature, and from that we can use transitional fossils to figure out their gradual progression to what they are now.


C. Useful Materials:

In this short video Ken Miller, a biologist, talks about whale evolution and transitional fossils. He mentions much of what I also discussed in my presentation. He first mentions that they found five definite transitional fossils that indicate the transformation from land-dwelling animal, to an intermediate phase that can go in and out of the water, to what we know the whale to be now. He also discusses the bones that make up the ear and they way that they changed relative to how the certain creature lived at the time. The ears that animals that live on land are much different than the ones used by animals that live in the water because of the differences in the way that sound travels in the air and in the water.

Another big thing he mentioned, which is very controversial, is the use of transitional fossils. Often times, those who don't believe in evolution argue that the transitional fossils aren't enough, they want more to fill the gaps. Ken talks about how if you tried to fill in every gap, it'd be impossible because you'd have to every generation of a species lineage, and fossils are already very rare compared to how many living creatures actually existed.












This video is very informative and a great tool to help you study for this section. It goes into a lot more detail than our book in portraying the evolution of the horse and also talks about the general properties of evolution. It shows transitional fossils that were on display at the museum of natural history that lead up to the horse from its earlier form. The narrator compares the horse to evolutionary cousins of the horse and talks about which traits are primitive and which traits were modified to what they are now.

The narrator mentions two new terms that relate to evolution, which are cladogenesis and anagenesis. These two terms refer to how an organism evolves. Cladogenesis is when a species branches into two or more species, whereas anagenesis is when a species undergoes enough changes to make it a separate species, but does not form two new species. He even mentions a certain fossil of the horse intermediate with fossilized leaves that were found where the stomach of that animal would be, which indicated the diet it may have had and the reason for why its teeth would adapt to that diet.

If that isn't enough to get you to watch the video, he has a pretty cool accent.


This article sheds light on a new discovery about vestigial structures. It refers to a study that was done, where researchers studied the health of World War II veterans, some with spleens, some without. The result were as followed, "those without a spleen were twice as likely to die of heart disease and pneumonia. What this indicate, is that the spleen, which we classified as a vestigial organs may actually serve a purpose.

Another study done at Duke University found that the appendix, once thought to be completely useless, actually provides a benefit to humans. It found that the appendix actually stores bacteria that replenish the stomach after an illness, typically one that affects the digestive system. This may provide a reason for why we still haven't evolved out other vestigial organs and further research may reveal more clues about the other species we or other animals evolved from.


This page is really cool. I found this page and was really impressed by how much of my topic it covers. It provides a very simple yet very informative explanation of different homologies, and even a bit about selective breeding. It first explains how leaves are actually homologous because having leaves is a trait that was passed down from a common ancestor of all plants.

It then goes into some examples which I actually covered in my lecture, such as animals that have a radius and ulna and the auditory bone I talked about. The selective breeding section is covered with the wild mustard plant that the textbook talks about. There are also a few new topics that we haven't discussed, however I'm not going to spoil them because they are pretty interesting.



D. Primary Literature:


This article talks about a very interesting discovery made in a fish, Astyanax fasciatus, where they found a red and some green pigment detectors in their eyes. These fish are actually blind, but they developed a DNA sequence that encodes for this gene and it is almost identical to the one found in humans with some minor mutations. This is a great example of convergent evolution at its finest and it provides another great example of how some organisms can develop similar traits independently of each other.


This finding also reveals much about the complexity of DNA and how the genetic code actually encodes for similar genes by using similar sequences even though they are genes that evolved completely separate of each other. I don't know if it's just me, but I think it's amazing that life has this universal language that no matter how different these two species are, if the genetic code that makes them up has the same or a similar sequence, the organism will develop the same or similar features.


Learning from this, the next step should be learning how to actually decipher the DNA code. If we can actually figure out which sequences encode for which genes and how they vary from species to species, we could actually use other DNA to map entire evolutions because we could be able to know the sequence of a gene that our ancestors had. Also, relating to selective breeding, we might be able to actually genetically engineer certain organisms to do certain functions, where as artificial selection may also select for detrimental genes, such as shortening a life span, we could change the sequence entirely to cause an organism to express all the traits we want them to.


E. Virtual Lectures:


Part 1



Part 2

F. Powerpoint Slides:


Observations of Evolutionary Change


G. Final Exam Questions:



H. Grading Sheet:

Grading Sheet (Noah)

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