Source: Introduction to Biology I Midterm Examination, University of Washington
Difficulty: Introductory | Prerequisites: Evolution Fundamentals and Natural Selection study notes.
Once you understand what natural selection is and how it works, the next question is: what evidence shows that evolution has occurred? This topic covers four major lines of evidence that come up repeatedly on the midterm: artificial selection, homology (structural and genetic), vestigial structures, and transitional forms. It also covers the sickle cell trait as a case study in heterozygote advantage. You need a solid grasp of Darwin's postulates before this material will make sense.
Multiple independent lines of evidence support evolution: humans have directly shaped species through artificial selection, shared ancestry is visible in homologous structures and genetic sequences, vestigial structures show traits that have lost their original function, and transitional fossils show intermediate forms between major groups. The sickle cell allele persists in malarial regions because heterozygous carriers gain a survival advantage.
Artificial selection
The deliberate breeding of organisms by humans to produce desired traits. In simple terms, humans play the role of the environment, choosing which individuals reproduce. The classic example is the selective breeding of domestic dogs from wolves.
Homologous structures
Structures in different species that share a common evolutionary origin (ancestry) but may serve different functions. Think of it as: the same underlying blueprint, modified for different jobs. A human arm, a whale flipper, and a bat wing all share the same bone arrangement.
Analogous structures
Structures that perform the same function in different species but have different evolutionary origins. In simple terms, these are cases of convergent evolution, not shared ancestry. A bird wing and an insect wing both enable flight but evolved independently.
Vestigial structures
Structures that were functional in an ancestor but are reduced or have lost their original function in a descendant species. Think of it as: evolutionary leftovers. The human tailbone (coccyx) and the appendix are common examples.
Transitional forms (transitional fossils)
Fossil organisms that display features intermediate between an ancestral group and its descendants. In simple terms, they show evolution "caught in the act." Tiktaalik, with features of both fish and tetrapods, is a well-known example.
Heterozygote advantage
A situation where individuals with one copy of each allele (heterozygous) have higher fitness than individuals homozygous for either allele. Think of it as: carrying one copy gives you the best of both worlds.
Sickle cell trait
The condition of being heterozygous for the sickle cell allele. In malarial regions, carriers have a selective advantage: they gain some resistance to malaria while experiencing only mild symptoms of sickle cell disease.
Humans have produced dramatic changes in species over short timescales by selectively breeding individuals with desired traits.
The breeding of domestic dogs from wolves is the standard exam example. All dog breeds share a common wolf ancestor, yet they vary enormously in size, shape, and behaviour, all within a few thousand years.
Artificial selection demonstrates that heritable variation exists and can be acted on to change populations. If humans can do it deliberately, natural selection can do it over longer timescales.
Structural homology: Shared bone structures across vertebrates (the human arm, whale flipper, bat wing, horse leg) reflect a common ancestor. The structures serve different functions but share the same underlying skeletal layout.
Genetic (molecular) homology: Similarities in DNA sequences, gene order, and protein structure across species. The more closely related two species are, the more similar their genetic sequences tend to be.
Homologous structures are distinct from analogous structures. Analogous structures (e.g. bird wings and insect wings) look similar and serve similar functions but evolved independently, reflecting convergent evolution rather than shared ancestry.
Vestigial structures were functional in ancestors but are reduced or non-functional in descendants.
They provide evidence for evolution because they only make sense if the species descended from an ancestor in which the structure was useful.
Examples: the human tailbone (coccyx), the appendix, hind limb bones in some whale species, wings in flightless birds.
Vestigial structures are found in living organisms, not only in fossils.
Transitional fossils display intermediate features between ancestral and descendant groups.
They provide direct evidence that major groups of organisms are linked by shared ancestry.
Example: Tiktaalik, which has features of both lobe-finned fish (scales, fins) and early tetrapods (flat head, proto-wrist, ability to support itself).
The sickle cell allele causes red blood cells to deform under low-oxygen conditions.
Individuals homozygous for the sickle cell allele (HbS/HbS) suffer severe sickle cell disease.
Individuals heterozygous for the allele (HbA/HbS) gain resistance to malaria with only mild sickle cell symptoms. This gives them a selective advantage in regions where malaria is prevalent.
The allele persists at relatively high frequencies in malarial regions because of this heterozygote advantage, even though it is harmful in the homozygous state.
Scientific evidence (radiometric dating of rocks and meteorites) places the age of the Earth at approximately 4.5 billion years. This is a straightforward factual recall question on the exam.
"Homologous structures always serve the same function." They do not. Homologous structures share ancestry but may serve very different functions in different species (e.g. a human arm for manipulation vs. a whale flipper for swimming).
"Vestigial structures are completely useless." Some vestigial structures retain minor secondary functions, but they have lost their original primary function. The point is that they no longer serve the purpose they served in the ancestor.
"Vestigial structures appear only in fossils." Vestigial structures are found in living organisms. The human coccyx and appendix are vestigial structures you carry right now.
"The sickle cell allele is always harmful." In the heterozygous state, it confers a survival advantage in malarial regions. It is harmful only in the homozygous state.
⚠️ Homologous vs. analogous structures is a common multiple-choice trap. Homologous = shared ancestry, possibly different function. Analogous = same function, different ancestry.
⚠️ Know the definition of vestigial structures precisely: traits functional in ancestors but reduced or lost in descendants.
⚠️ The sickle cell/malaria relationship is tested as a case study of heterozygote advantage. Know why the allele persists and which genotype benefits.
⚠️ Artificial selection (dogs from wolves) is the go-to example. Know why it supports evolution: it demonstrates that heritable variation can shift a population.
⚠️ The age of the Earth (4.5 billion years) is a direct recall question.
True or false: Homologous structures always serve the same function in different species. (False. They share ancestry but may serve different functions.)
True or false: Vestigial structures indicate that species have evolved from common ancestors. (True.)
True or false: Artificial selection by humans can produce significant changes in species over relatively short time periods. (True.)
Fill in the blank: A trait inherited from a common ancestor but potentially serving a different function in descendant species is called a _______. (homology / homologous structure)
What is the age of the Earth as determined by scientific evidence? (Approximately 4.5 billion years.)
Q: Which of the following is an example of artificial selection providing evidence for evolution? (a) Antibiotic resistance in bacteria, (b) Selective breeding of domestic dogs from wolves, (c) Formation of new species by geographic isolation, (d) Vestigial tailbones in humans.
A: (b). Selective breeding of dogs from wolves is the textbook example of artificial selection. Antibiotic resistance is natural selection, not artificial.
Q: Homologous structures are best defined as: (a) Structures with the same function but different origins, (b) Structures with shared ancestry but possibly different functions, (c) Unrelated structures in different organisms, (d) Structures with no purpose.
A: (b). Shared ancestry is the defining feature. Different functions in different species are expected.
Q: Which statement correctly explains the relationship between sickle cell trait and malaria?
A: Heterozygous individuals (HbA/HbS) have a selective advantage in malarial regions because they gain resistance to malaria while experiencing only mild sickle cell symptoms. This is heterozygote advantage.
Q: Define homology and discuss two types of homology used as evidence for evolution.
A: Homology refers to similarities between species that are due to shared ancestry. Structural homology involves shared anatomical features (e.g. the vertebrate forelimb: same bone arrangement in humans, whales, bats, and horses, despite different functions). Genetic (molecular) homology involves shared DNA sequences, gene arrangements, or protein structures across species. Both types support the conclusion that the species in question descended from a common ancestor.
Q: Critically evaluate the evidence supporting evolution from four distinct lines of evidence: artificial selection, homology, transitional forms, and vestigial structures.
A: Artificial selection shows that heritable variation can be acted on to produce major changes in a population within a few generations (dogs from wolves). Homology (structural and genetic) shows that diverse species share underlying features traceable to common ancestors. Transitional fossils (e.g. Tiktaalik) show intermediate forms linking major groups, providing direct fossil evidence of evolutionary transitions. Vestigial structures (e.g. the human coccyx, whale hind limb bones) show remnants of features that were functional in ancestors, making sense only in light of descent with modification. Together, these four lines converge on the same conclusion from independent types of evidence.
This material sits between Evolution Fundamentals (the mechanism) and Phylogenetics (how we reconstruct evolutionary history). The evidence covered here is what allows biologists to build phylogenetic trees with confidence. The sickle cell case also connects to genetics and population biology, which you will encounter later in the course.
Artificial selection, natural selection, homologous structures, analogous structures, convergent evolution, vestigial structures, transitional fossils, Tiktaalik, sickle cell, malaria, heterozygote advantage, HbA, HbS, coccyx, tailbone, appendix, dogs from wolves, selective breeding, age of Earth, 4.5 billion years, radiometric dating, structural homology, molecular homology, genetic homology, evidence for evolution, BIO I, Introduction to Biology, University of Washington, midterm review