1.13: Evolution

Evolution: Mechanisms and Evidence

Overview

• Introduction
• Evolution on the Large and Small Scale
• Modeling Natural Selection
• Evidence of Evolution

Introduction

When looking at the great diversity of life, there are clear differences between many species. Some organisms are photosynthetic, others have scales, some are capable of flying. Although these differences seem great, they are ultimately due to smaller, less noticeable changes that accumulate over time. This change over time is called evolution. There are several mechanisms that can lead to change, such as natural selection, genetic drift, and gene flow. Natural selection is a process where change is driven by selective pressures from the environment, genetic drift is a change in a population’s allele frequency, and gene flow is the movement of alleles in or out of a population as seen in the image below. However, the ultimate driver of all change is mutation (random genetic changes), as all other mechanisms of change rely on having a varied genetic pool to work. For this lab we will model two of these mechanisms of evolution -- natural selection and the bottleneck effect.

Figure \(\PageIndex{1}\): Example of Gene flow two distinct populations green beetles and brown beetles with their own allele frequencies. When an individual migrates to a different population it brings with it potentially novel genes and alters the allele frequencies. ("Gene Flow” by OpenStax, Rice University, CC BY 4.0, via Wikimedia Commons)

Evolution on the Large and Small Scale

When most people think of evolution, they often think of large, obvious changes such as the differences between a mouse and an elephant. The divergence seen between such drastically different organisms is due to macroevolution -- larger changes that have occurred over millennia. However, evolution does not always lead to such obvious differences. Evolution can also occur on a smaller scale. This smaller scale is called microevolution -- smaller changes that occur within a species over a few generations.

Macroevolution is seen when two species diverge, such as the divergence seen in early hominins and other primates. The image below demonstrates how macroevolution led to the speciation (development of new species) of the many plants we see today. The flowering plants seen on the right are more closely related to the gymnosperms and more distantly related to the mosses, liverworts, and charophytes.

Figure \(\PageIndex{2}\): Cladogram of the Plants including beginning from the common Green Algal Ancestor. Cladogram includes Charophytes, Liverworts, Mosses/Hornworts, Club Mosses, Ferns, Gymnosperms, and Angiosperms (Flowering plants), as well as the derivative trait that defines those different groups. (Laurenprue216, CC BY-SA 3.0, via Wikimedia Commons)

Microevolution is seen when a population changes, such as a change in beak size in finches due to changes in food source. The population is still considered part of the same taxonomic group, but has accumulated some differences when compared to previous generations. The image below demonstrates the development of antibiotic resistance an example of microevolution that is impacting our ability to fight bacterial infections in modern medicine.

Figure \(\PageIndex{3}\): Bacterial evolution of antibiotic resistance, from left to right non-resistant cultures, cultures including two that have a mutation for resistance, non-resistant cultures die after introduction of antibiotics, resistant cultures spread.("Evolution – Antibiotic Resistance" by Karen Marks, Reedley College is licensed under CC BY 4.0)

Modeling Natural Selection

While in the Galapagos Islands, Darwin noticed that the beaks of various finches were of different lengths, shapes, and sizes. The differences he observed led him to several of his most important ideas about the mechanisms and processes that are responsible for descent with modification, including natural selection.

In order for Natural Selection to occur:

• There must be variation in a population
• The variation must be heritable
• There must be competition (not all individuals will be successful at passing on their traits)

Experiment: Best Beaks

Sialia Mexicana (the Western Bluebird) is a small passerine bird of the family Turdidae. It has a thin, straight beak and feeds primarily on worms, berries, and insects, but it is also known to be a voracious consumer of grapes and raisins. In this exercise, you will simulate the functional consequences of variation in beak shape (short, stout beaks compared to long, thin beaks) of the Western Bluebird and how these might be reflected in subsequent generations. For the purposes of this activity, let’s assume:

• There is variation in beak length among individuals in a population.
• Variation in beak length is heritable.
• Individuals with one type of beak length survive or reproduce better than others.

Materials per group:

• 4 wooden craft sticks 2 rubber bands
• 10 pennies Timer Ruler
• 25 large grapes
• 25 raisins
• 1 plate
• 2 bowls

Procedure

1. Make your short, stout beak (Figure 4) by placing three pennies between two of the wooden craft sticks about one inch (2.54 cm) from one end.
2. Secure the wooden craft sticks together with one of the rubber bands.

Figure \(\PageIndex{4}\): The three-penny short, stout beak is made with two popsicle sticks with 3 pennies stacked near one end with a rubber band wrapped around at the pennies. (Image from 10.2: Best Beaks is shared under a CC BY-SA 4.0 license and was authored, remixed, and/or curated by Leah Howell (Dalton State College)

3. Now, make your long, thin beak (Figure 5) by placing seven pennies between the remaining two wooden craft sticks, this time approximately 2.5 inches (6.35 cm) from one end, and secure with a rubber band.

Figure \(\PageIndex{5}\): The seven-penny long, thin beak. Made with 2 popsicle sticks with 7 pennies stacked near the middle, and a rubber band wrapped around at the stack of pennies. (Image from 10.2: Best Beaks is shared under a CC BY-SA 4.0 license and was authored, remixed, and/or curated by Leah Howell (Dalton State College).)

Before continuing make some predictions about the two different beak sizes.

Which beak do you predict will be better suited to eating the typical grapes?

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Which beak do you think will be better suited to eating the drought year food raisins?

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4. Spread the raisins evenly over the surface of the plate and set your timer for 30 seconds.
5. For 30 seconds, attempt to pick up as many raisins as possible with your small beak and place them in the bowl.
6. Record the number of raisins picked with the small beak in Table Number of Raisin by beak size.
7. Return the raisins to the plate.
8. Complete a second trial, attempting to pick up as many raisins as possible with the small beak in 30 seconds. Record your results in Table Number of Raisin by beak size.
9. Now, attempt to pick up as many raisins as possible with the large beak and place them in the bowl in 30 seconds.
10. Record the number of raisins picked up with the large beak in Table Number of Raisin by beak size.
11. Clear the plate.
12. Spread the grapes out evenly on the plate, and again, set your timer for 30 seconds.
13. Using your small beak, attempt to pick up as many of the grapes as possible in 30 seconds and place them in the bowl.
14. Record the number of grapes picked up with the small beak in Table Number of Grapes by beak size.
15. Now, using the large beak, attempt to place as many of the grapes as possible in the bowl in 30 seconds.
16. Record the number of grapes picked up with the large beak in Table Number of Grapes by beak size.
17. Return the grapes to the plate.

Table 1. Number of Raisin by beak size

Beak Length	Trial #	Trial time	Number of Raisins
Short, stout	1	30 sec.
Short, stout	2	30 sec.
Long, thin	1	30 sec.
Long, thin	2	30 sec.

Table 2. Number of Grapes by beak size

Beak Length	Trial #	Trial time	Number of Grapes
Short, stout	1	30 sec.
Short, stout	2	30 sec.
Long, thin	1	30 sec.
Long, thin	2	30 sec.

Follow-up questions

1. Which beak was better for eating the grapes, and was your hypothesis correct? ________________________________

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2. Which beak was better for eating the raisins, and was your hypothesis correct? _________________________________

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3. If grapes are the typical food source during a normal year, how would you expect the ratio of beak size to change from year to year?_____________________ _____________________________________________________________________________________________________________________________________

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4. How would you expect the ratio of beak size to change during a drought year? ________________________________

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5. If droughts are relatively infrequent, how is there variation in the population? _________________________________

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Evidence of Evolution

The evidence for evolution is compelling and extensive. Looking at every level of organization in living systems, biologists see the signature of past and present evolution. Darwin dedicated a large portion of his book, On the Origin of Species, identifying patterns in nature that were consistent with evolution and since Darwin our understanding has become clearer and broader.

Fossils

Fossils provide solid evidence that organisms from the past are not the same as those found today; fossils show a progression of evolution. Scientists determine the age of fossils and categorize them all over the world to determine when the organisms lived relative to each other. The resulting fossil record tells the story of the past, and shows the evolution of form over millions of years (Figure 6). For example, highly detailed fossil records have been recovered for sequences of species in the evolution of whales and modern horses. The fossil record of horses in North America is especially rich and many contain transition fossils: those showing intermediate anatomy between earlier and later forms. The fossil record extends back to a dog-like ancestor some 55 million years ago that gave rise to the first horse-like species 55 to 42 million years ago in the genus Eohippus. The series of fossils tracks the change in anatomy resulting from a gradual drying trend that changed the landscape from a forested one to a prairie. Successive fossils show the evolution of teeth shapes and foot and leg anatomy to a grazing habit, with adaptations for escaping predators, for example in species of Mesohippus found from 40 to 30 million years ago. Later species showed gains in size, such as those of Hipparion, which existed from about 23 to 2 million years ago. The fossil record shows several adaptive radiations in the horse lineage, which is now much reduced to only one genus, Equus, with several species.

Figure \(\PageIndex{6}\): This illustration shows an artist’s renderings of these species derived from fossils of the evolutionary history of the horse and its ancestors. The species depicted are only four from a very diverse lineage that contains many branches, dead ends, and adaptive radiations. One of the trends, depicted here is the evolutionary tracking of a drying climate and increase in prairie versus forest habitat reflected in forms that are more adapted to grazing and predator escape through running. Przewalski's horse is one of a few living species of horse. (Image from “BIOL 310: General Biology” by Eric Wada for Folsom Lake College, “Chapter 7: Evolution, section 7.3 Evidence of Evolution” is shared under CC BY 4.0 License and was authored, remixed, and/or curated by Openstax.)

Fossils Observation Exercise

1. There are several fossils in the lab room, observe the various invertebrate, plant, and vertebrate fossils.
2. In the space below take notes on how old the fossils are and how different or similar they look to modern organisms.

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Anatomy and Embryology Evidence

Another type of evidence for evolution is the presence of structures in organisms that share the same basic form. For example, the bones in the appendages of a human, dog, bird, and whale all share the same overall construction (Figure 7). That similarity results from their origin in the appendages of a common ancestor. Over time, evolution led to changes in the shapes and sizes of these bones in different species, but they have maintained the same overall layout, evidence of descent from a common ancestor. Scientists call these synonymous parts homologous structures. Some structures exist in organisms that have no apparent function at all, and appear to be residual parts from a past ancestor. For example, some snakes have pelvic bones despite having no legs because they descended from reptiles that did have legs. These unused structures without function are called vestigial structures. Other examples of vestigial structures are wings on flightless birds (which may have other functions), leaves on some cacti, traces of pelvic bones in whales, and the sightless eyes of cave animals.

Figure \(\PageIndex{7}\): The similar construction of these forelimbs indicates that these organisms share a common ancestor. The bones are all analogous and allow for wide array of forms and functions with small modifications of the same structures. (Image from “BIOL 310: General Biology” by Eric Wada for Folsom Lake College, “Section 7.3: Evidence of Evolution” is shared under CC BY 4.0 License and was authored, remixed, and/or curated by Openstax.)

Another evidence of evolution is the convergence of form in organisms that share similar environments. For example, species of unrelated animals, such as the arctic fox and ptarmigan (a bird), living in the arctic region have temporary white coverings during winter to blend with the snow and ice (Figure ). The similarity occurs not because of common ancestry, indeed one covering is of fur and the other of feathers, but because of similar selection pressures—the benefits of not being seen by predators.

Figure \(\PageIndex{8}\): The white winter coat of (a) the arctic fox and (b) the ptarmigan’s plumage are adaptations to their environments. (credit a: modification of work by Keith Morehouse). Image from “BIOL 310: General Biology” by Eric Wada for Folsom Lake College, “Section 7.3: Evidence of Evolution” is shared under CC BY 4.0 License and was authored, remixed, and/or curated by Openstax.

Embryology, the study of the development of the anatomy of an organism to its adult form also provides evidence of relatedness between now widely divergent groups of organisms. Structures that are absent in some groups often appear in their embryonic forms and disappear by the time the adult or juvenile form is reached. For example, all vertebrate embryos, including humans, exhibit gill slits at some point in their early development. These disappear in the adults of terrestrial groups, but are maintained in adult forms of aquatic groups such as fish and some amphibians. Great ape embryos, including humans, have a tail structure during their development that is lost by the time of birth. The reason embryos of unrelated species are often similar is that mutational changes that affect the organism during embryonic development can cause amplified differences in the adult, even while the embryonic similarities are preserved.

Comparing Vertebrate Skeletons Procedure

1. Look at the skeleton model of Ichthyostega, the ancestral tetrapod or terrestrial vertebrate. In particular look at the forelimb (Figure 7.) made from a humerus (upper arm bones), radius and ulna (forearm bones), carpal bones (wrist bones), metacarpals (hand bones), and phalanges (finger bones)
2. Then compare the same forelimb bones of modern tetrapods: human, lizard, bat, bird, cat, and amphibian.
3. Fill in the table below which of the bones for each organism are still similar to the ancestral form and which ones are different (modified) from the ancestral form.

Table 3. Vertebrate Forelimb comparison table

Animal	Bones resembling ancestral form	Bones differing from ancestral form
Human
Lizard
Bat
Bird
Cat
Amphibian

Follow-up Questions

1. When comparing forelimb skeletons of vertebrates, which organisms seemed more ancestral (more similar to the ancestral tetrapod)?

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2. Order the different organisms on a spectrum of ancestral to derived (more modified).

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3. What could you say about the organisms based on where along that spectrum they fall?

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Embryonic Evidence

The embryonic development of all vertebrates shows remarkable similarities as you can see from these drawings (Figure 9. supplied by Open Court Publishing Company). The drawings in the top row are of the embryonic stage called the pharyngula. At this stage ("I") they all contain a notochord, dorsal hollow nerve cord, a tail extending behind the anus, and a series of paired branchial grooves.

Figure \(\PageIndex{9}\): Embryos of numerous taxa going through similar developmental stages of embryonic development. The branchial grooves are matched on the inside by a series of paired gill pouches. In fishes, the pouches and grooves eventually meet and form the gill slits, which allow water to pass from the pharynx over the gills and out the body. In the other vertebrates shown here, the grooves and pouches disappear. In humans, the chief trace of their existence is the eustachian tube and auditory canal which (interrupted only by the eardrum) connect the pharynx with the outside of the head. George Romanes's 1892 copy of Ernst Haeckel's controversial embryo drawings. (Public Domain)

Comparing Vertebrate Embryos procedure

1. Your professor will have setup three compound microscopes with three different embryos at similar stages as well as have an image of a human embryo up on the screen for you to compare.
2. Look at four embryos and try to identify the common structure found in all of them, and any different structures.
3. Examples include pharyngeal pouches, tail, heart, eyes, yolk stalk, ear. . .
4. Using this information fill in the below table.

Table 4. Embryo Comparison Table

Animal	Features in common	Feature not in common
Human
Fish
Bird
Frog

Human Evolution procedure

1. In lab there are both a human skeleton and chimpanzee skeleton, list their similarities and their differences.

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2. In the lab there a number of skulls including a Gorilla, Human, and other Hominids. While observing them find and list any traits that seem more ancestral and traits that are more derived.
   1. Ancestral traits:

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   2. Derived traits:

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Molecular Evidence of Evolution

Like anatomical structures, the structures of the molecules of life reflect descent with modification. Evidence of a common ancestor for all of life is reflected in the universality of DNA as the genetic material and of the near universality of the genetic code and the machinery of DNA replication and expression. Fundamental divisions in life between the three domains are reflected in major structural differences in otherwise conservative structures such as the components of ribosomes and the structures of membranes. In general, the relatedness of groups of organisms is reflected in the similarity of their DNA sequences—exactly the pattern that would be expected from descent and diversification from a common ancestor.

DNA sequences have also shed light on some of the mechanisms of evolution. For example, it is clear that the evolution of new functions for proteins commonly occurs after gene duplication events. These duplications are a kind of mutation in which an entire gene is added as an extra copy (or many copies) in the genome. These duplications allow the free modification of one copy by mutation, selection, and drift, while the second copy continues to produce a functional protein. This allows the original function for the protein to be kept, while evolutionary forces tweak the copy until it functions in a new way.

Protein similarity experiment procedure

For this experiment a rabbit is injected with human blood, causing the rabbit’s immune system to produce antibodies for that human blood. Those antibodies are recognizing proteins found on the surface of the blood cells, surface antigens. The more closely related an organism is the more similar these proteins will be to each other, allowing us to now use this rabbit serum to tell in relative terms how closely related organisms are to humans. This is a process called Serology, and we are going to use it to identify 5 mystery serums.

1. For this test there are 5 test serums each one could be Human, Cow, Chimpanzee, Monkey, and Orangutan.
2. Knowing what you’ve learned so far about evolution construct a list starting with Human ending with the animal least related to Human.
   1. Human
3. Obtain a chemplate, several stir sticks (enough that you wont contaminate samples), one Rabbit serum, one human serum, one of each of the test serums labeled I, II, III, IV, and V.
4. Add a couple drops of Rabbit serum to wells 1 through 6
5. Add a couple drops of Human serum to wells 1 through 6 as well. The Rabbit serum is now sensitized to Human serum and will react to human proteins.
6. Add 4 drops of serum I to well 1 and stir
7. Repeat step 6 for serums II-V adding them to wells 2-5. Well 6 will be a control showing no reaction, completely unrelated to Human.
8. Wait at least 10 minutes for a reaction to occur.
9. Compare the reactions in the wells and fill in the below table with the amount of precipitate and with which animal you suspect it is.

Table 5. Serology Results

Serum #	Amount of precipitate	Animal
I
II
III
IV
V

Licensing and Attributions

This Lab titled “Evolution: Mechanisms and Evidence” is shared under CC BY-NC-SA 4.0 License and was authored, remixed, and/or curated by Tyler Wilson (Oxnard College)

Citations

10.2: Best Beaks is shared under a CC BY-SA 4.0 license and was authored, remixed, and/or curated by Leah Howell (Dalton State College).

“BIOL 310: General Biology” by Eric Wada for Folsom Lake College, “Chapter 7: Evolution, section 7.3 Evidence of Evolution” is shared under CC BY 4.0 License and was authored, remixed, and/or curated by Openstax.

11: Living Things Change - Mechanisms of Evolution is shared under a CC BY-NC-SA 4.0 license and was authored, remixed, and/or curated by Karen Marks and Valeria Hochman Adler.

This page titled Embryonic Evidence was last modified on Mon, 17 Mar 2025 11:54:56 GMT and is shared under a CC BY 3.0 license and was authored, remixed, and/or curated by John W. Kimball via source content that was edited to the style and standards of the LibreTexts platform.