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Wize University Biology Textbook > Evolution

Evolution of Populations

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Evolution of Populations

This is a huge section of conservation and evolutionary research that seeks to understand how populations change over time.

Natural selection acts on an individual, but evolution is seen in populations

Watch Out!
Individuals cannot evolve!


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Evolution and Genetics


Photo by CNX OpenStax | CC BY
  • Gene: DNA that codes for a trait
  • in diploids there are 2 copies of each gene = 2 alleles
  • Evolution: Change in allele frequencies of a population over time
  • Fitness: How successful an individual's genotype is in the next generation Example: An individual which produces 2 offspring has 2x the fitness of an individual that only produces 1


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Drivers of Evolution

  • Natural Selection
  • Directional Selection: one extreme value of a trait is most successful
  • The mean population value shifts, but the variance stays the same
  • Stabilizing Selection: an intermediate trait value is most successful
  • the mean population value stays the same, but the variance decreases
  • Disruptive Selection: the two extreme values of a trait are most successful
  • the mean population value stays the same, but the variance increases


Photo by CNX OpenStax | CC BY
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  • Mutation
  • Random change in DNA rarely results in evolution
  • Genetic Drift
  • Change in allele frequency due to chance (stochasticity)
  • exaggerated in small populations
  • Bottleneck Effect: population reduces drastically, leaving only a small subset of the original gene pool
  • Founder Effect: small group breaks off from population and founds a new population with a small subset of the original gene pool
  • Gene Flow
  • Mating occurs between two distinct populations
  • more variation within a population
  • less difference between populations
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Example: Directional Selection

In a forest in the northern latitudes rabbits use their fur color as camouflage. Most rabbits are brown, though there is significant variation in fur color.
If the climate changes and now snow covers the ground, what type of selection would you expect on fur color?
Describe the type of selection and the change in the population's fur color.




We would expect to see directional selection, since now rabbits with whiter fur will have higher survival in the population. This will shift
the mean fur color of the population towards white.













Practice: Genetics

How many alleles of each gene are present in diploid organisms?

Practice: Drivers of Evolution

Which of the following best describes Disruptive Selection?

Practice: Drivers of Evolution

Individuals migrating between two distant populations will result in __________.
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Graphing Allele Frequencies Over Time

Draw a graph that represents the change in allele frequencies in a population overtime in the dominant (H) and recessive (h) alleles in a population undergoing:

Selection against the dominant phenotype with complete dominance
Selection against the recessive phenotype with complete dominance









Selection favoring heterozygotes with incomplete dominance
Selection against heterozygotes where H has an initially high frequency












Selection against heterozygotes where h has an initially high frequency


You are observing a population of pigs where the B locus controls pig colour with two alleles B1 and B2. There are so many pigs in this population that we can ignore the effects of genetic drift. You note that the starting frequencies of B1 and B2 are 0.7 and 0.3, respectively. Under which situation would B2 reach a frequency of 1 and B1 would disappear completely in the population?
  1. wB1B1 > wB1B2 < wB2B2
  2. wB1B1 < wB1B2 < wB2B2
  3. wB1B1 < wB1B2 > wB2B2
  4. wB1B1 = wB1B2 < wB2B2

Extra Practice
Carrying capacity refers to:
Carrying capacity refers to:
It was found that guppies (live-bearing fish), when raised in predator free waters, produce larger sized offspring that mature slowly. This is in comparison to guppies raised in the presence of predators whereby the progeny are smaller, mature quicker and reproduce earlier. What phenomena is occurring?
Evolution cannot occur within a generation.
Which form of natural selection acts against both extreme phenotypes and favours an intermediate variant?
Which of the following changes the genetic structure of a population due to immigration/emigration?
Imagine a new volcanic island is created off the coast of a mainland country. Somehow, a small population of marine lizard manages to swim to the island and start a new colony on the island. What is true about the fate of this new marine lizard population?
How do mutation and selection function together?
Could natural selection be influenced by global warming?
For the following scenarios, identify the type of selection/effect that may be operating
a) A local mining plant has produced extreme amounts of acid rain in the nearby forest. Some species of plants in this environment have been found to have significantly increased acid tolerance.
b) In the praying mantis population, the female invests heavily in finding her breeding nest and which mate she will choose.
What type of selection is occurring in each scenario?
a) Large cockroaches avoid predation by lizards while the medium and small kin do not.
b) Pansies (flowers) vary in colour. Bees search for the most common color and preferentially pollinate them.
Consider the following scenario:
A species of C. elegans produces gametes that have two copies of the gene noggin rather than one. Which of the following mechanisms is at play in each scenario?
a) Selection b) Migration c) Mutation d) Non random mating e) Genetic drift
Consider the following selection scenarios. State whether it is disruptive, directional or stabilizing.
a. Population of bats subject to selection where only bats with large wings or small wings survive:
b. Population of elephants in arid climate where only those with small ears are selected against since they cannot dissipate heat fast enough:
Practice: Selection/Effect
For the following scenarios, identify the type of selection/effect that may be operating.
Practice Problem 2
What type of selection is occurring in each scenario?
a) Large cockroaches avoid predation by lizards while the medium and small kin do not.
b) Pansies (flowers) vary in colour. Bees search for the most common color and preferentially pollinate them.
For the following scenarios, identify the type of selection/effect that may be operating
a) A local mining plant has produced extreme amounts of acid rain in the nearby forest. Some species of plants in this environment have been found to have significantly increased acid tolerance.
b) In the praying mantis population, the female invests heavily in finding her breeding nest and which mate she will choose.
How do mutation and selection function together?
Which of the following does not tend to promote speciation?
A certain community in rural Alberta was identified as being genetically unique at many loci from all neighbouring populations. Further assessment shows that this group is derived from a small group from “Chocoland” and that they do not outbreed. Which of the following likely explains their uniqueness?
If an allele becomes fixed, there is no opportunity for genetic variation to exist at that locus anymore.
What type of selection is occurring in each scenario?
a) Large cockroaches avoid predation by lizards while the medium and small kin do not.
b) Pansies (flowers) vary in colour. Bees search for the most common color and preferentially pollinate them.
It was found that guppies (live-bearing fish), when raised in predator free waters, produce larger sized offspring that mature slowly. This is in comparison to guppies raised in the presence of predators whereby the progeny are smaller, mature quicker and reproduce earlier. What phenomena is occurring?
Ecology Practice Problem 4
It was found that guppies (live-bearing fish), when raised in predator free waters, produce larger sized offspring that mature slowly. This is in comparison to guppies raised in the presence of predators whereby the progeny are smaller, mature quicker and reproduce earlier. What phenomena is occurring?
Consider the following selection scenarios. State whether it is disruptive, directional or stabilizing.
a. Population of bats subject to selection where only bats with large wings or small wings survive:
b. Population of elephants in arid climate where only those with small ears are selected against since they cannot dissipate heat fast enough:
Why is phenotypic variation essential for evolution to occur?
Which of the following is not a source of genetic variation for evolution?
Evolution cannot occur within a generation.
It was found that guppies (live-bearing fish), when raised in predator free waters, produce larger sized offspring that mature slowly.
This is in comparison to guppies raised in the presence of predators whereby the progeny are smaller, mature quicker and reproduce earlier.
What phenomena is occurring?
Characteristics of life
Some scientists argue that having genetic material (e.g. RNA and DNA) make viruses living creatures. Which characteristic(s) of life does this criteria satisfy? Select all that apply
Practice: Selection Types
Consider the following selection scenarios. State whether it is disruptive, directional or stabilizing.
a. The population of bats subject to selection where only bats with large wings or small wings survive:
b. The population of elephants in an arid climate where only those with small ears are selected against since they cannot dissipate heat fast enough:
Evolution Mechanisms
Which of the following is not a source of genetic variation for evolution?
Practice: Phenotypic Variation
Why is phenotypic variation essential for evolution to occur?
Which of the following is a phenomenon in which allele frequencies change randomly.
Practice: Frog Evolution Scenario
Consider the following scenario. Answer all questions using key terminologies and justifications.
A tropical rainforest supports a species of poison dart frog which feeds primarily on a local tree beetle. Providing the weather is constant, and food is plentiful, this species continues to breed and populate the ecosystem. Further, the ratio of males and females is equal so no competitive mating is occurring and the entire species is phenotypically identical.
a) Providing the above situation is maintained, will these species evolve in the next 1000 years? Justify your answer.
A certain community in rural Alberta was identified as being genetically unique at many loci from all neighbouring populations. Further assessment shows that this group is derived from group of immigrants from a specific region of Ukraine that arrived in the early 1900's and that no outbreeding occurred until ~1995.
1. Which of the following likely explains the uniqueness in the Albertan population?
2. This population also has an elevated number of children born with Hemophilia, a disease caused by a recessive allele. Which of the follow explains this observation?
Which of the following is an event that causes the loss of allelic variation associated with start a new population with few individuals?
During historical glacial periods when large areas of land are covered by ice sheets (ice ages), species distributions are fragmented into small disjunct areas that remain ice free. These are known as glacial refugia.
1. What evolutionary phenomena are the populations in the glacial refugia likely experiencing?
2. As the glaciers melt and species reestablish their previous distribution, what phenomena will the refugial populations experience?
The population of squirrels in a forest has increased and there are no longer enough Maple trees to nest in. This is an example of:
Carrying capacity refers to:
Are the wings of birds and flies homologous or analogous traits?
Which of the following does not tend to promote speciation?
For the following scenarios, identify the type of selection/effect that may be operating. Choose from the following terms:
directional selection
bottleneck
Types of Selection
You are a graduate student studying a population of birds that was introduced to a small tropical island. You visit the island in year 2016 and measure the beak sizes of 100 birds in the population. You then return in 2018 and again measure the beak sizes of 100 birds in the population. You obtain the following data. What type of selection is occurring?
Selection types
In the amazon in 1986, a new species of non-poisonous frog was introduced. In the region, there are two species of snakes, one very large and one very small. These two snake species feed on frogs. Upon introduction into the region, scientists measured the size of the new frog species. In year 2000, the frogs were again measured. The following data was obtained. What kind of selection is this? Why do you think this type of selection was observed?
You are observing a population of pigs where the B locus controls pig colour with two alleles B1 and B2, there are so many pigs in this population that we can ignore the effects of genetic drift.You note that the starting frequencies of B1 and B2 are 0.7 and 0.3, respectively. Under which situation would B2 reach a frequency of 1 and B1 would disappear completely in the population?
WB1B1 > WB1B2 < WB2B2
WB1B1 < WB1B2 < WB2B2
Evolutionary Theory
Which of the following statements are TRUE regarding evolutionary theory?
1. The shared amino acid code for methionine, AUG, is an example of molecular homology
2. Speciation occurs as the result of a large extinction event occurring rapidly
Which of the following statements are TRUE?
Natural selection is the only cause of evolution
Evolution is the result of individual organisms acquiring traits over their lifetime that they then pass on to their offspring
Falsifiable Statements
Which of the following statements are falsifiable?
1. All mammals have a vestigial structure relating us to reptiles
2. Mice are evolutionarily more closely related to chimpanzees than humans
Evolutionary Theory
Box turtles (Terrapene carolina) and the Gray Tree Frog (Hyla versicolor) live in North America where winters cause freezing to sub-freezing temperatures. Although these two species are not closely related to each other, they both can undergo a process called freeze avoidance where circulating sugar molecules protect their organs while more than 50% of their body water content is frozen. This process is common in other North American frog species but not in turtle species.
Which of the following is highlighted in the given information?
1. Freeze tolerance resulted from homology
How does stabilizing selection affect the genetic and phenotypic variation within a
population?
What are the major sources of genetic variation resulting from sexual reproduction?
i. Recombination during prophase I
ii. Recombination during prophase II
The Pennsylvania Amish have a high frequency of polydactyly, a genetic disorder that gives rise to more than ten digits. Why is this?
Imagine a new volcanic island is created off the coast of a mainland country. Somehow, a small population of marine lizard manages to swim to the island and start a new colony on the island. What is true about the fate of this new marine lizard population?
Under what conditions do populations evolve?
What is the relationship between evolution and natural selection?
How does stabilizing selection affect the genetic and phenotypic variation within a
population?
What is the relationship between evolution and natural selection?
Under what conditions do individuals evolve?
Imagine a new volcanic island is created off the coast of a mainland country. Somehow, a small population of marine lizard manages to swim to the island and start a new colony on the island. What is true about the fate of this new marine lizard population?
Sympatric speciation can result from:
Which of the following is a source of genetic variability?
Which of the following contribute to the reasoning that peripheral isolates are the sort of population from which new species usually develop?
Which of the following drivers of evolution is least likely to be maladaptive?
Stabilizing selection is acting on which of the following examples?
Which of the following is not a method for enhancing genetic diversity?
Which form of natural selection acts against both extreme phenotypes and favours an intermediate variant?
Which of the following changes the genetic structure of a population due to immigration/emigration?
A diploid plant species growing in Mystic Vale was is infected by a plant virus that wipes out 90% of the population. Which mechanism(s) of evolutionary change are immediately acting on these plants?
What is the difference between natural and artificial selection?
Which of the following is “theoretical” about evolution?
Which of the following is always adaptive?
Based on fossil records, over the course of centuries, the American black bear population has significantly increased in number, but the average size of each individual bear has significantly decreased. What type of selection does this indicate?
Humans hunted buffalo to near extinction in the 1800's. Which of the following is most relevant to the effect of this on buffalo genetics?
Which of the following cannot evolve?
The most simple definition of evolution is:
Which of the following does not tend to promote speciation?
The theory of uniformitarianism states that geological processes operate today as they did in the past helped us understand and interpret what?
In convergent evolution, species share characteristics that are similar because of
An organisms relative fitness is measure by