1. Rock pocket mice (Chaetodipus intermedius) evolved dark coloration to survive on dark lava flows.

Dark colored rock pocket mice have a mutation in a gene called MC1R (the Melanocortin 1 Receptor gene), which codes for a membrane receptor. Mutations in the gene lead to more expression of dark colored melanin (a pigment). More dark-colored melanin leads to the dark fur color on the back of the mice that enhances survival on dark environments (like lava flows: you can read an article about this here).

2. Peppered moths (Biston betularia) evolved dark coloration to blend in with darkened tree trunks in polluted forests, and then evolved light coloration as pollution abated and trees were once again covered with whitish lichens.

A mutation in the cortex gene results in the darker-winged variant.

3. Three-spined sticklebacks lost their pelvic spines as they adapted to life in freshwater lakes.

The form on the left is the marine form, with a pelvic spine. The form on the right is the freshwater form, without a pelvic spine. In marine sticklebacks, the pelvic spine provides protection against predatory fish.

In the freshwater form, these predatory fish are absent (making the pelvic spine superfluous). In addition, the pelvic spine provides a way for predatory dragonfly larvae to grab onto sticklebacks, making the pelvic spine a liability.

The difference is caused by a mutation in the Pitx1 pelvic enhancer.

4. The sickle cell allele (HbS) allows heterozygotes (HbA/HbS) to to survive malaria infections with reduced severity, providing a selective advantage in regions where malaria is common.

The sickle cell allele (HbS) results from a point mutation in the β-globin gene of hemoglobin, the blood-carrying protein in red blood cells. The mutation causes a substitution of valine (a nonpolar amino acid) for glutamic acid. The valine creates a hydrophobic region on the outside of hemoglobin, causing hemoglobin molecules to chain together, causing red blood cells to sickle.

In heterozygotes, only a small amount of sickling occurs, but the presence of the mutated protein creates a less hospitable environment to the plasmodium parasite that causes malaria.

2. Natural Selection Depends on Preexisting Variation

In each case, natural selection was only possible because of preexisting, genetically influenced phenotypic variation in the population. Note that this doesn’t imply any foresight, intention, or planning on the part of the evolving population. Rather, it’s part of the very nature of life. DNA is always changing because of mutation and, in sexual reproducing species, genetic recombination. That creates phenotypic variation. Natural selection then weeds out variants with unsuccessful phenotypes, while those variants with phenotypes that allow them to survive and reproduce pass their genes onto the next generation. Over time, allele frequencies in the population shift. That shift in allele frequencies is, from a population genetics perspective, the definition of evolution.

3. Selection based on variation is also at work with polygenic traits

In the examples above, the successful phenotype is linked to single underlying gene.

Natural selection also works with polygenic traits (traits caused by many genes). Polygenic traits, which we learned about in unit 5, are much more common than traits caused by single genes. In humans, examples of polygenic traits include height, skin color, eye color, hair color, weight, intelligence, athletic ability, blood pressure, cholesterol levels, body shape, facial features (just to name a few). And because they’re caused by many genes, these traits have continuous variation, defined by a bell-shaped curve, as you can see below.

When natural selection works upon a trait like height, the entire curve of variation is pushed in one direction, resulting in directional selection.

In polygenic traits, directional selection works just as it does with traits controlled by single genes.

• There’s preexisting genetically influenced phenotypic variation that nature can select from
• There’s selection and differential survival of individuals with better adapted phenotypes.
• There’s renewal and enhancement of variation through mutation and recombination so that selection can continue in subsequent generations.
• There’s continuous selective pressure in the same direction, generation after generation.

Think of any complex, multi-faceted adaptation. Directional selection is how it came to be.

5. Natural Selection and Genetic Variation Flashcards

6. Variation on the molecular level also influences fitness

In the context of evolution, fitness is measured by the an organism’s probability of surviving and reproducing.

The variations discussed above mostly focused on the observable phenotype: fur color, wing color, presence or absence of pelvic spines, height, etc. But molecular variation can also be crucial for an organism’s fitness and a population’s evolutionary success. Here are a few examples:

6a. Phospholipid variation and cold weather adaptation

• Cell membranes are made of phospholipids with fatty acid tails — these can be saturated or unsaturated.
  
• In cold-weather mammals:
  • Cells in the body core have more saturated fatty acids — these make the membranes more rigid.
  • Cells in the legs and ears have more unsaturated fatty acids — these keep membranes fluid in cold temperatures.
• This variation helps maintain maintain the correct level of diffusion. healthy circulation and oxygen delivery in freezing environments.

6b. Hemoglobin Changes Before and After Birth

• Hemoglobin is the protein that carries oxygen in red blood cells.
• Before birth, human babies use a special type of hemoglobin that grabs oxygen more tightly — helping draw oxygen from the mother’s blood in the placenta.

  

• After birth, this fetal hemoglobin is replaced with adult hemoglobin, which works better in the lungs.
  
• This switch is a great example of developmental variation that supports survival at different life stages.

6c. Different Types of Chlorophyll in Plants

• Plants use chlorophyll to absorb light for photosynthesis.
• There are two main types:
  • Chlorophyll a: Absorbs red light — great for direct sunlight.
  • Chlorophyll b: Absorbs blue light — helpful in shade or indirect light.

    

• Some plants are adapted to shady environments and have more chlorophyll b. Others thrive in bright light and rely more on chlorophyll a.
• Many plants have both types, helping them use a wider range of light throughout the day or year.

7. Species and populations with little genetic diversity are at risk of decline or extinction

The extinction vortex is a positive feedback loop that accelerates the decline of small, isolated populations towards extinction. Note that in this context, there’s nothing positive about a positive feedback loop. What we’re talking about is an extinction-causing vicious cycle that’s rooted in a population’s lack of phenotypic and genetic variation. Here’s how it works.

1. For a variety of reasons (environmental changes, climate shifts, arrival of new competitor or predator, etc.), a population’s birth rate falls below its death rate, and its population starts to decline.
2. This decline in numbers leads to genetic drift and loss of genetic diversity.
3. Reduced genetic diversity results in lower fitness and adaptability.
4. Further population decline ensues, perpetuating the cycle.

Resilient populations — populations that can bounce back from difficult conditions — are populations that have the phenotypic and genetic variation that allows some individuals to thrive (or survive) while other individuals are struggling.

8. Variation and Fitness: Checking Understanding

This tutorial ends Topics 7.10 – 7.11, Speciation and Variation. Return to the Unit 7 Main Menu to access  the next topic (Topic 7.12, The Origin and Early Evolution of Life)