1. Genetically influenced phenotypic variation is essential for evolution

Natural selection — the process that explains how adaptations arise and undoubtedly the most important concept in biology — can only work in populations that have genetically influenced phenotypic variation.

1a. Examples of natural selection working on traits caused by single genes

Here are a few examples of this that you’ve already seen as you’ve progressed through this course.

In all of these cases, natural selection was only possible because of pre-existing phenotypic variation in the population. In other words, the environmental challenge doesn’t cause the adaptation. Rather, phenotypic variation — generated by underlying random mutation and genetic recombination — comes first. Natural selection then weeds out variants with unsuccessful phenotypes, while those variants with phenotypes that allow them to survive and reproduce get to 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.

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

• Those three-spined sticklebacks that lack a pelvic spine have a loss-of-function mutation in the pelvic enhancer region of their DNA (click here to review).
• Dark colored rock pocket mice have a mutation in a gene called MC1R, 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).
• In peppered moths, a mutation in the cortex gene results in the darker-winged variant.

1b. Selection based on variation is also at work with polygenic traits

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, one consequence is directional selection.

For directional selection to work, there must be

• pre-existing genetically influenced phenotypic variation that nature can select from
• renewal of variation through mutation and recombination so that selection can continue in subsequent generations.
• Continuous selective pressure in the same direction, generation after generation.

1c. Insular dwarfism and gigantism show how the value of an allele depends on selectivepressures

When a population from a large animal species colonizes an island, the new island population is confronted by limited resources, leading to selective pressure to reduce energy demands. In this context, smaller body sizes are advantageous. That’s because

• Smaller bodies require less food and water.
• Smaller offspring require fewer resources
• Smaller bodies can be better adapted to move through the dense vegetation and rugged terrain often found on islands.

These pressures lead to insular (island) dwarfism. Insular dwarfism can be found in islands around the planet and in diverse clades. This includes

• Dwarf elephants (all extinct) that existed on mediterranean islands such as Sicily, Malta, and Cyprus. These were approximately one meter in height.  Today, the world’s smallest elephant population can be found on the island of Borneo
• Dwarf hippos (all extinct) that lived in Cyprus and Madagascar
• Homo floresiensis. This extinct hominid (a cousin of ours on the human evolutionary tree) stood only a foot tall. It lived on the island of Flores in Indonesia, and became extinct about 50,000 years ago.

While these examples are all mammals, island dwarfism has been documented in birds, dinosaurs, lizards, and snakes.

Note that in the case of insular dwarfism, it’s alleles for smaller size that promote survival. But that’s completely contextual. With different selective pressure, alleles for larger size might be selected. That often happens when smaller species colonize islands.

On islands there’s often

• Fewer predators
• Less competition

With these conditions prevailing, populations from small species often increase in size. Examples include

• Giant tortoises on the Galapagos islands
• The Dodo bird: an giant flightless pigeon (now extinct) on the island of Mauritius.
• Giant rats on the Solomon islands and the Indonesian island of Flores
• The Komodo dragon: the largest living lizard, found on islands in Indonesia.

2. Natural selection and Variation: Checking Understanding

2a. Flashcards

2b. Quiz

3. Molecular Variation

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

3a. 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 healthy circulation and oxygen delivery in freezing environments.

3b. 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.

3c. 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.

4. 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.

Objectives: Delete later

Topic 7.11:

CB Learning objective:

• Explain how the genetic diversity of a species or populationaffectsitsability to withstand environmental pressures.

CB Essential Knowledge

• Thelevelofvariationinapopulationaffects population dynamics.
  • i. The ability of a population to respond to changesintheenvironmentisinfluenced by genetic diversity. Species and populations with little genetic diversity are at risk of decline or extinction.
  • ii. Genetically diverse populations are more resilient to environmental perturbation because they are more likely to contain individuals that can withstand the environmental pressure.
  • iii. Alleles that are adaptive in one environmental condition may be deleterious in another because of differentselectivepressures.