Topic 7.11, Variations in Populations

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.

1. Phenotypic Variation is essential for evolution

At this point in your study of biology, you’ve learned about a variety of examples of evolutionary change.

  • Rock pocket mice (Chaetodipus intermedius) that evolved dark coloration to survive on dark lava flows.
  • Peppered moths (Biston betularia) that 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
  • Three spined sticklebacks that lost their pelvic spines as they adapted to life in freshwater lakes

In every one of these cases, the process of adaptation to environmental change was only possible because of pre-existing genetic variation in the population. That’s one of the most important — and most often misunderstood — principles in evolution.

Here’s an example of a common misconception:

“Ancestors of birds evolved wings so they could fly.”

No! Evolution doesn’t work that way. The formulation “so that they could fly” implies that the ancestors of birds could anticipate a future need, and evolve toward that need. But evolution has no foresight.  As Richard Dawkins has written in “Natural selection is the blind watchmaker, blind because it does not see ahead, does not plan consequences, has no purpose in view.”

Here’s a reformation of the misconception above:

The ancestors of birds — small, feathered dinosaurs — had a variety of forelimb structures. Some individuals had forelimbs that gave them a survival or reproductive advantage: perhaps better balance while running, or the ability to glide short distances to escape predators.

In every generation, surviving individuals passed on the gene variants that led to those structures. Over many generations, natural selection favored further changes that enhanced gliding. Later, small modifications in muscle coordination and feather structure enabled powered flight.

At every step, selection acted on variation that was already present — and each change had to provide some advantage for survival or reproduction. That’s how wings — and flight — evolved.

 

[qwiz generative_feedback=”true” prompt_intro=”Here is a question and my answer to the question. Evaluate my answer in comparison to the answer guide with regard to content and ideas, not with regard to presentation or punctuation. If the answer guide provides alternative response items, consider my answer complete if I provide at least the number of items specified in the question. However, if the question asks for specific points, **evaluate if my answer covers the core biological concepts of those points. If my answer expresses the same concept in different but accurate and reasonable language, consider it covered. Only tell me about missing core concepts or significant inaccuracies.** If I did not include specific items, tell me what the answer guide says for those items. If my answer is incorrect or incomplete, explain how I can improve my answer. If I use synonyms or alternative terms that is fine; the answer guide terms are not more accurate and are not required. Make the comparison between my answer and the answer guide in qualitative terms such as accurate, good, correct, or missing items – not in quantitative terms such as credit or points. In the comparison, do not repeat the question, my answer, or the answer guide. Only show the comparison and explanations about incomplete and incorrect answers. Explain how incomplete and incorrect answers can be improved. Format as HTML. Do not explain HTML.”]

[h]Explaining the importance of variation

[q multiple_choice=“false”]Here’s another misconception: Mammalian stomachs evolved to be highly acidic in order to kill any pathogens that are swallowed during eating.

Write a few lines explaining why this is wrong, and offer an alternative (and more evolutionarily correct) account of how the acidity of the mammalian stomach might have evolved.

[c]U2hvdyB0aG UgYW5zd2Vy[Qq]

[f]VGhlIHNhbXBsZSBhbnN3ZXIgc2hvdWxkIGluY2x1ZGUgdGhlIGZvbGxvd2luZyBpZGVhcw==

Cg==
    Cg==
  • Q3JpdGlxdWU6IFRoZSBzdGF0ZW1lbnQgaW1wbGllcyBmb3Jlc2lnaHQgb24gdGhlIHBhcnQgb2YgdGhlIGFuY2VzdHJhbCBwb3B1bGF0aW9uLiBCdXQgZXZvbHV0aW9uIGhhcyBubyBmb3Jlc2lnaHQ=
    • Cg==
  • [Qq]Alternative scenario
    • Populations of early mammals had stomachs with a variety of secretions: some more acidic, some less acidic.
    • Those individuals with more acidic stomach secretions tended to experience fewer parasitic infections, increasing survival and reproductions.
    • The survivors passed along the genes for more acidic stomach secretions to their offspring
    • Repeat of this process over time led to the optimal pH that’s found in stomach secretions today

[/qwiz]

 

To visualize this, consider this diagram of directional selection

Populations are only able to shift the mean trait value if there is

  • pre-adaptive variation that nature can select from
  • renewal of variation through mutation and recombination so that selection can continue in subsequent generations.

What does pre-adaptive variation mean? It means that the variation is already there. The environmental context makes it adaptive.

2. 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.

 

 

 

 

2. Study this Outline

Note that this outline summarizes the key information explained above, but also goes beyond it.

Phenotypic and Genetic Variation: The Foundation of Evolution

  • Phenotypic variation is essential for evolution
    • Phenotypes are the observable traits of organisms, shaped by their genotypes and environment.
    • Natural selection acts on phenotypes — not on the hidden genetic code itself.
    • Organisms with traits that help them survive and reproduce are more likely to pass on their genes to the next generation.
    • Over time, helpful traits become more common, driving evolutionary change.
  • Phenotypic variation comes from genetic diversity
    • Differences in DNA — from mutations, sexual reproduction, and gene flow — create genetic variation.
    • This genetic variation is the source of the differences in traits we see within a population.

Why Variation Matters for Survival and Evolution

  • Populations with low genetic diversity are more vulnerable
    • When all individuals are genetically similar, a single environmental change can wipe them out.
    • Example: During the Irish potato famine, most potatoes were genetically identical. When a fungus attacked, the entire crop failed.
  • Diverse populations are more likely to survive change
    • If a population has many genetic variants, some individuals may have traits that help them survive when conditions shift.
    • Example: Rock pocket mice come in light and dark colors. On desert sand, light-colored mice are better camouflaged. On dark volcanic rock, dark-colored mice survive better. Populations with both colors can thrive in both habitats.
  • What helps in one environment might hurt in another
    • Traits that are useful in one setting can be harmful in another.
    • Example: People with one copy of the sickle cell allele are more resistant to malaria — a big advantage in regions with the disease. But two copies cause sickle cell disease, which can be harmful or deadly.

Additional Examples of Adaptive Variation

1. Cold-Climate Adaptations in Mammals

  • 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.

2. 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.

3. 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.

Key Takeaways

  • Variation in traits allows evolution through natural selection.
  • Genetic diversity helps populations survive changing environments.
  • What’s helpful in one environment may be harmful in another — context matters!