Natural Selection Leads to Adaptation

​How does natural selection lead to adaptation of populations?

Identify evidence supporting a claim that natural selection leads to adaptations in populations and describe that a change in gene frequency over time leads to changes in the frequency of a trait in a population.

Vocabulary

1. Adaptation— an inherited trait or characteristic that helps an organism survive in its habitat or environment

2. Co-evolution — the influence of closely associated species on each other in their evolution

3. Genetic Variation — the variety of gene combinations that exist in a population

4. Mutation — a change to the DNA sequence of a gene, which may or may not alter the trait.

5. Natural Selection — a mechanism for evolution where organisms with traits that are better suited to the environment are more likely to survive, reproduce, and pass on their traits

6. Selective Pressures — external agents which affect an organism's ability to survive in a given environment

Open Ended

How does natural selection lead to adaptation of populations?

Study the image of body color variants seen in the peppered moth (Biston betularia).

How did body color variation in peppered moths arise? Under what conditions might it be favorable for moths to have the typical form over melanic form? Under what conditions might it be favorable for moths to have the melanic form over the typical form? How does a changing environment affect the natural selection of traits, such as body color?

Charles Darwin described natural selection as the mechanism for evolution.

Organisms are genetically diverse. Mutations give rise to traits that may be selected for (or selected against) depending on the environment.

Organisms that are best adapted to their environment will survive and reproduce, which increases the frequency of their traits in a population. Over time, populations of organisms either adapt to their environment or they may become extinct.

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Multiple Choice

A new brand of insecticide killed most of the insect pests that were eating a farmer's crops. However, the next growing season, the same insecticide killed only a small portion of the insects. Why did this occur?

Select the best answer.

• The insecticide was effective against insect pests with many different traits.

• 
  

  The crops adapted and developed a way to protect the insects from the insecticide.

• The insects that survived exposure to the insecticide the first time failed to reproduce.

• The small number of insects that survived the first insecticide application passed on their resistance to the next generation.

Sometimes mutations occur in the segments of DNA that make up our genes. A mutation is a change to the DNA sequence, which changes the structure of a gene. Mutations are the source of genetic variation, or the variety of gene combinations that exist in a population. Mutations result in a variant form of the gene that may be transmitted to subsequent generations.

Mutations are caused by the alteration of single base units during DNA replication or the deletion, insertion, or rearrangement of larger sections of genes or chromosomes during cell division.

When mutations occur in the DNA sequence, the protein the gene codes for may still function normally, it may function abnormally, or it may not form at all. Mutations may be acted upon by natural selection.

While the term mutation has a negative connotation, mutations may have harmful, beneficial, or neutral effects on the structure and function of organisms.

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Multiple Select

A loon is an aquatic bird that dives underwater to eat fish. In a certain loon population, half the loons have a gland near their tails that produces oil that they spread on their feathers. The other half of the loon population does not have a functioning oil gland due to a gene mutation. Researchers believe that the oil helps insulate the loons' bodies.

Select the correct statement(s).

• Mutations are a source of genetic variation that always result in desirable traits.

• The gene mutation resulting in a non-functional oil gland is beneficial to loons living in cold environments.

• Natural selection would select against the gene mutation in cold environments.

• 
  Mutations are a source of genetic variation that always result in undesirable traits.

• 
  Loons with oil glands are more likely than loons with non-functioning oil glands to survive and reproduce in cold environments.

An adaptation is an inherited trait or characteristic that helps an organism survive in its habitat or environment.

Organisms have adaptations that help them survive in their habitats. Organisms that don't have adaptations for their habitats survive less well or not at all.

Polar bears have white fur that blends in with snow and ice. They have thick fur that doesn't absorb water and a thick layer of fat to protect them from cold temperatures.

Ice grass has small leaves and grows close to the ground. This allows it to conserve water and heat. It also protects it from the wind. Ice grass produces flowers and seeds during the long, summer days in the polar regions.

Both polar bears and ice grass have adaptations to survive in a polar habitat.

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Multiple Select

Select the correct statement(s).

• Organisms without adaptations may not survive at all in their habitat or environment.

• Adaptations are inherited traits that help organisms survive in their habitat or environment.

• Organisms with adaptations are the least likely to survive in their habitat or environment.

• 
  Adaptations are learned behaviors that help organisms survive in their habitat or environment.

• Organisms with adaptations are the most likely to survive in their habitat or environment.

​The environment that any organism lives in creates selective pressures. Selective pressures work on the variations within a population. Though each species has a distinct set of genes that make most individuals very similar, each individual has numerous small genetic mutations.

These mutations, when translated into proteins, cause changes in the functions of cells and ultimately cause the visible variation seen in a population. While these traits can range from feather color, beak size, to an animal’s ability to digest plant material, every trait has a slight selective advantage or disadvantage.

Gradually, advantageous heritable traits increase in frequency, or proportion of individuals, in future generations and the proportion of individuals that do not exhibit the trait decreases. If selective pressures remain the same, the population will eventually be dominated by organisms that exhibit an anatomical, behavioral, or physiological adaptation well suited to survive and reproduce in a specific environment.

Multiple Select

Analyze the diagram of the giraffes.

Select the correct statement(s).

• Giraffes inherited longer necks over many generations.

• Giraffes acquire a longer neck during their lifetime by stretching it up towards the leaves.

• The leaves of tall trees acted as a selective pressure on giraffes to evolve a longer neck.

• All giraffes are born with the same neck length.

• Giraffes born with longer necks are more likely to reach leaves, survive, and then reproduce.

Changes to the biological (biotic) and physical (abiotic) components of ecosystems affect populations of organisms. These changes may impact growth rates, reproductive rates, alter competition for limited resources, and contribute to a change in gene frequency, leading to adaptation in a population over time.

For example, researchers studying finches in the Galápagos Islands determined that a major factor influencing survival of the medium ground finch (Geospiza fortis) is the weather, which affects the availability of food. During a 551-day drought, the plants on the island began to wither and the tiny ground seeds the finches were accustomed to eating started to disappear. Medium ground finches with larger beaks could take advantage of alternate food sources because they could crack open larger seeds. The smaller-beaked birds couldn't do this, leading many to die of starvation. The researchers measured the offspring and compared their beak size to that of the previous (pre-drought) generations. They found the offsprings' beaks to be 3-4% larger than their grandparents'.

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Multiple Choice

Why were the medium ground finches (Geospiza fortis) a good subject for a study of evolution?

Select the best answer.

• Medium ground finches eat a specific food type.

• Medium ground finches are small and easy to catch and release.

• Medium ground finches have beaks that are variable in size and shape.

• Medium ground finches are closely related to other species of Galápagos finches.

Multiple Choice

A population of butterflies exhibits a normal distribution of the shade trait. Some are completely white, and some are completely black, but most fall in the gray range.

This population of butterflies has just been introduced to a new species of bird that only likes to eat gray butterflies.

After a few generations, how would the graph representing the distribution of shade of butterfly most likely change?

Adaptability is essentially the ability of a population to survive and reproduce in changing environmental conditions. Less adaptable populations not only will not tend to thrive given changing environmental conditions, but will also have a greater potential to go extinct.

Highly specialized organisms, also called specialist species, can thrive only in a narrow range of environmental conditions, or they have a limited diet. Koalas, for example, are herbivorous marsupials native to Australia. They have a strict diet of primarily leaves from the eucalyptus tree. Koalas are not very adaptable to changes in their environment.

In contrast, generalist species are organisms that can eat a variety of foods and survive in different habitats. An example of a generalist species would be raccoons. They are found all over the world. Whether in trash bins or parks, raccoons can easily adapt to changes in their environment.

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Multiple Choice

The Berry Cave salamander (Gyrinophilus gulolineatus) is found in only nine caves in eastern Tennessee. It is in danger of extinction because of urban development near its habitat. Most of the surviving salamanders have characteristics that allow them to tolerate drier and warmer conditions outside the caves.

Which of the following would have the greatest effect on natural selection in these salamanders?

• 
  

  ability to cope with changing environment

• 
  competition for food

• availability of clean water

• presence of predators

Multiple Choice

Lichens are an unusual organism because they are a symbiotic relationship between a fungus and a photosynthetic alga called cyanobacteria. Lichens get their water and nutrients by absorbing them from their surroundings. They get carbohydrates to use for energy from the cyanobacteria.

The table in Figure 2 shows the distribution of various lichen species based on light levels in a particular habitat.

Select the statement that best describes the data.

• Both species A and D are specialist, while species B and C are generalist.

• All the species are generalist and able to survive in any light level.

• Only species A is a specialist, while species B, C and D are generalist.

• Species A, B, and C are generalist, while species D is a specialist.

Multiple Select

A tall building goes up near the lichens' habitat that blocks the majority of all direct light.

Select the statement(s) that describes the effect the building will most likely have on the lichen species.

• The population size of species D will increase rapidly.

• Species C has the highest chance of going extinct.

• 
  Species A has the highest chance of going extinct.

• 
  Species C will become more light tolerant over time.

• 
  For species B, the frequency of low light tolerant individuals will increase over time.

Multiple Choice

The coal burning practices of the early 1800s produced hazardous air pollution that contained toxins such as sulfur dioxide that led to soil acidification and the disintegration of tree bark. Tree dwelling lichens could not establish on the bark of heavily polluted trees and were therefore unable to get the nutrients they needed to live.

Hundreds of years later, researchers investigated how the residual pollution found in trees affects the current distribution of lichen species.

This graph shows the number of lichen species on trees that contain various levels of residual pollution.

If no more pollution is added to this habitat, how will this lichen community most likely change after another hundred years?

• More lichens will go extinct, leaving trees with higher levels of pollution bare.

• The lichens with no pollution tolerance will outcompete the few species that have evolved to be pollution tolerant.

• Fewer lichens will evolve pollutant-tolerant adaptations and inhabit trees with lower pollution levels.

• 
  More lichens will evolve pollutant-tolerant adaptations and inhabit trees with higher pollution levels.

Evolutionary changes are sometimes influenced by relationships between organisms of different species. This is called co-evolution.

Plant–pollinator, predator–prey, and host–parasite are all examples of closely associated populations of different species that can lead to changes in gene expression over time. For example, a hummingbird's bill shape has evolved to match the shape and depth of the flower it feeds from, just as the flower has evolved to accommodate the hummingbird by being more attractive and providing access to its nectar.

Such relationships can also lead to an evolutionary arms race, which is a term used in biology to describe an ongoing struggle between competing sets of co-evolving genes. In other words, two organisms rapidly evolve phenotypic or behavioral traits that develop escalating adaptations and counter-adaptations against each other. This is often seen in predator-prey relationships, where the prey organism increases its defenses against the predator, which in turn makes the predator more effective at hunting the prey.

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Multiple Choice

In 1950, a single strain of myxoma virus (MYXV) was released as a biocontrol agent against Australian rabbit populations. To understand how MYXV in Australia has continued to evolve in wild rabbits, researchers compared viral samples from the 1950s as well as from a 1990s lab.

The table shows the case fatality rate (CFR) of the viral samples from various years.

Select the statement that best explains the changes observed in the data.

• The MYXV developed a mutation after the year 1953, as a result the virus became increasingly more effective at killing Australian rabbits.

• The fluctuations in CFR are the result of adaptations in the rabbit population's immune response, as well as mutations in the MYXV.

• The viruses' CFR did not change the longer it was contracted by the rabbit population, as a result the rabbit population remained stable.

• The rabbit population developed an immunity to MYXV after the year 1953, as a result the virus was no longer effective.

Multiple Select

Recall that peppered moths have two body color variants, as shown in Figure 1. The light-colored moth is the typical form, while the dark-colored moth is the melanic form. Melanism is the result of a mutation causing body color variance.

Figure 2 shows the number of typical and melanic peppered moths in a population between the years 1848 and 1857.

Select the correct statement(s).

• The frequency of melanic peppered moths decreased from 1848-1857.

• 
  The frequency of typical peppered moths decreased from 1848-1857.

• The frequency of melanic peppered moths increased from 1848-1853 and decreased from 1854-1857.

• 
  The total number of peppered moths remained around the same each year.

• The total number of peppered moths decreased from 1848-1857.

Open Ended

How does natural selection lead to adaptation of populations?

Ten Italian wall lizards were introduced to a new island in 1971. The population included 5 breeding pairs - 5 females and 5 males. Researches revisited the island 36 years later and within that time span, the lizards were able to reproduce ~30 generations. The researchers recorded morphometric data describing head size, shape, and resulting bite force of the wall lizards from the new population and compared it to measurements from the original population.

Their measurements are in the table below:

Population / Average Head Length (mm) / Average Head Width (mm) / Average Head Height (mm) / Bite Force (N)

Original / 14.67 / 8.09 / 6.29 / 9.85

New / 16.33 / 9.13 / 7.35 / 11.25

Construct an explanation connecting the data in the table to the graphs in the slider.

In your response, include:

- Describe the differences in morphology between the two populations

- Identify what factor most likely lead to the changes observed

- Explain the general process of natural selection that led to this adaptation in the new wall lizard population

Multiple Choice

In the mid 1800s, researchers noticed a correlation between the decline of lightly-colored tree lichen and the changes observed in peppered moth populations within the same habitat.

Figure 3 shows a light-colored peppered moth on lichen.

Which statement best explains this observation?

• Birds are more likely to see light-colored moths on trees without lichen than dark-colored moths.

• 
  

  Lichens were predators of dark-colored peppered moths, once they disappeared the dark moth variant flourished.

• 
  The same pollutant that killed the lichen also killed the light-colored peppered moths, but the dark-colored moths were immune.

• Light-colored peppered moths rely on tree lichen for reproduction, but dark-colored peppered moths do not.