Bio 1070 Notes Topic Test TWO:

Natural Selection is the process by which individuals with traits that enhance their survival and reproductive success are more likely to pass those traits on to the next generation. Over time, this leads to changes in the genetic makeup of a population, favoring traits that improve fitness in a given environment.

Natural Selection

• The only evolutionary mechanism that leads to adaptations.

Adaptations:

• Heritable traits that provide a “fit” between an organism and its environment.

• These traits help individuals survive and reproduce.

Key Principles

1. Variation – Individuals within a population have differences in traits.

2. Heritability – Traits must be passed from parents to offspring.

3. Differential Survival and Reproduction – Individuals with advantageous traits are more likely to survive and reproduce.

4. Adaptation – Over generations, beneficial traits become more common in the population.

Natural selection is not goal-oriented or forward-thinking; it simply favors traits that provide an advantage at the time, which may change as the environment changes.

Natural Selection and Finch Beak Depth Adaptation

Factors That Could Prevent Beak Depth Adaptation via Natural Selection:

• Natural selection is not "need-based" or forward-thinking.

• Adaptations appear to align with "needs," but populations do not evolve because they "need" to.

• Natural selection acts on existing traits and lags behind environmental changes.

• Traits that are adaptive today may not be beneficial in the future.

• Mean beak depth decreased when rains returned to the islands, demonstrating fluctuating selection pressures.

• Some species may go extinct if they cannot adapt quickly enough.

A classic example of natural selection in action, demonstrating how changes in the environment led to adaptations in beak size and shape, influencing survival and reproduction in finches.

Defining Fitness in Evolutionary Terms:

• Fitness is measured by an organism’s reproductive success, not by strength, intelligence, or speed alone.

• Traits that enhance reproduction contribute to fitness and become adaptations.

• Example: Four fictional lizards with different characteristics.

  Lizard Fitness Analysis:

  • Lizard A: Strong and clever, 19 offspring survived, died at 4 years.

  • Lizard B: Missing toes, 28 offspring survived, died at 5 years.

  • Lizard C: Quick and dark-colored, 22 offspring survived, died at 4 years.

  • Lizard D: Largest territory, 26 offspring survived, died at 6 years.

  • Most Fit: Lizard B, as it produced the most surviving offspring despite its missing toes.

Natural Selection in Stable Environments:

• Natural selection can maintain traits over time.

• Individuals with extreme traits may be less likely to survive and reproduce.

• This heritable difference in fitness leads to stabilizing selection.

Types of Selection:

1. Stabilizing Selection:

   • Intermediate phenotypes have the highest fitness.

   • Reduces the prevalence of extreme traits.

   • Maintains trait stability in populations over time.

2. Directional Selection:

   • One extreme phenotype has higher fitness.

   • Leads to a shift in trait distribution.

   • Example: The 1976 drought, which favored finches with deeper beaks.

3. Disruptive Selection:

   • Average phenotypes become less common.

   • Favors individuals with extreme traits.

   • Can occur when specialization is beneficial.

Slide: 2


Evolution

The process that results in changes in the proportion of heritable traits within populations from one generation to the next.

“Evolution is defined as a change in allele frequencies in a population.”

Simultaneous Forces of Evolution

• All four forces are simultaneously and continuously acting on every population of living organisms.

Mechanisms of Evolution

Evolution includes four forces/mechanisms that drive these changes:

1. Natural Selection

2. Mutation

3. Genetic Drift

4. Gene Flow

Survival of Deeper Beaked Finches During the Drought

• Compared to finches with shallower beaks, finches with deeper beaks were more likely to survive the drought.

7. Key Takeaway of the Graph

• Beak depth is heritable, and the offspring beak depths resemble those of their parents.

8. Importance of This Information for Evolution

• This information shows that the finch population’s beak depth is heritable, and as the surviving finches with deeper beaks reproduce, their offspring are more likely to have similar beak sizes. This trait could influence the evolutionary trajectory of the ground finch population on Daphne Island.

Key Observations

• Variation in Finch Population:

  • Variation existed in beak depth.

  • Beak depth is a heritable trait.

  • Deeper beaked finches were more likely to survive and reproduce.

Darwin's Postulates of Natural Selection

1. Phenotypic variation exists within a population.

2. That variation is genetically heritable.

3. Differential reproduction/survival occurs based on phenotypic variation.

When these postulates are true, natural selection is occurring.

The Grants and Natural Selection in Action

• In this case, natural selection resulted in an average beak size 10% larger after the drought.

Why Beak Depth Increased in the Finch Population

• Answer D: Finches with shallower beaks were less likely to survive compared to finches with deeper beaks – so the proportion of deep-beaked finches increased.

Why Natural Selection is Not 'Need-Based'

• Natural selection is not forward-thinking. Populations do not adapt because they “need” to.

• Natural selection increases the frequency of currently adaptive traits, but what is adaptive today may not be adaptive in the future.

• For example, mean beak depth lowered when rains returned to the islands.

Potential Issues for Beak Depth Adaptation in the Finch Population

Select all that apply:

• A. If there was no variation in beak depth ahead of the drought (e.g., all finches had the same sized beak).

• B. If beak depth was completely determined by environmental factors, like a finch’s diet.

• C. If over 100 genes were involved in determining beak depth.

• D. If beak depth was determined by only 1 gene.

• E. If none of the finches had beaks large enough to crack open large seeds.

Fitness and Evolutionary Success of Lizards

• Fitness refers to the evolutionary success of an individual, particularly in terms of reproductive success.

Lizard Comparison

• Based on the offspring surviving to adulthood, Lizard B (28 surviving offspring) could be considered the "most fit," though factors like territory size or health could also influence this assessment.

Slide 3

Understanding Fitness in Evolution

• Fitness is a measure of how many surviving offspring an organism produces, not directly a measure of traits like ‘strongest’ or ‘fastest’.

• Traits that increase reproduction are considered adaptations.

• If traits such as strength or speed help a lizard survive and/or reproduce, they may be adaptations that contribute to fitness.

Natural Selection and Stable Seed Distribution

In stretches of years where seed distribution remains relatively stable – is natural selection still acting on this finch population’s beak depths?

Possible Answers:
A.
B.
C.
D.
E. Natural selection often keeps traits the same.

Heritable Differences in Fitness

• Natural selection often acts to maintain traits that are beneficial for survival and reproduction, which is why distinguishing traits are seen in species.

• Individuals born at the extremes of a trait (due to new mutations, genetic recombination, or chance) are less likelyto survive and reproduce.

• This is a heritable difference in fitness caused by the heritable trait, which leads to natural selection acting on the population.

Directional Selection

• Selection where individuals with phenotypes on one end of a spectrum are more likely to survive and reproduce.

• Leads to a directional change in the distribution of phenotypes for a specific trait within a population.

• Example: The 1976 drought is an example of directional selection.

Disruptive Selection

• Genetic variants that lead to average beak depth become less common in the population over time – selection favors genetic variants that lead to the ‘extremes’.

• This can happen when it is beneficial to be a specialist.

Challenge Questions to Think About:

1. If evolution refers to the process that results in changes in the proportion of heritable traits within populations from one generation to the next...

   • How could selection that helps to keep a population’s trait stable over time be considered an evolutionary process?

2. In 2003, another drought hit Daphne Island. However, by 2003, the large ground finch (a different species of finch) had been introduced to the island.

   • Large ground finches have bigger/deeper beaks than medium ground finches.

   • What do you think happened during the 2003 drought?

Phenotypic Frequency and Natural Selection

A fire breaks out and kills most of the frogs in this population.

• Only 3 pink frogs remain.

Before the Fire vs. After the Fire

• Did the phenotypic frequency of ‘pink’ increase due to natural selection?

Slide 4:

Mutations: Changes in DNA

• Arise through errors during DNA replication and from exposure to mutagens.

• Two main types of mutations:

  1. Genetic mutations – change small numbers of nucleotides.

  2. Chromosomal mutations – change the number or structure of chromosomes.

• As an evolutionary mechanism, the term ‘mutation’ refers to a process that occurs.

  • This differs from how you may use the term “mutant” as a way to distinguish from “wild type”.

Types of Genetic Mutations

1. Sequence of DNA nucleotides in a gene...

   • Determines sequence of nucleotides in mRNA, which...

   • Determines sequence of amino acids in a protein, which...

   • Determines the structure of that protein, which determines its function.

📌 The Central Dogma helps explain the phenotypic significance of mutations.

Mutations and Fitness

• Mutations can be beneficial, deleterious, or neutral in regard to fitness.

• Many mutations do not affect phenotype (they are silent) and are neutral with regards to fitness.

• Once in a while, mutations impact phenotype:

  • These can still be neutral with regards to fitness.

  • They can also be deleterious or beneficial.

Antibiotic Resistance and Mutations

• Consider the different concentrations of antibiotics in the video shown earlier in class.

• Where are mutations that increase antibiotic resistance most likely to occur?

Mutations Are Random with Regards to Fitness

• A population or an individual’s “need” to adapt does not make mutations any more likely to happen.

• Beneficial mutations are not more likely to occur just because an organism needs them.

• When the drought occurred on Daphne Island, natural selection acted on variation that already existed.

• No new mutations occurred as a result of the environmental change caused by the drought.

If Mutations Are Random with Regards to Fitness, Why Does It Seem Like They Are More Likely to Occur When They Are Helpful?

• A new mutation that causes a bacteria to be antibiotic resistant can only increase in frequency if that bacteria is in the presence of antibiotics.

📌 True or False?
A. True
B. False

Thought Experiment

💡 Imagine that I snapped my fingers and mutations no longer existed. Which of the following would likely be true?

📌 Possible Answers:
A.
B.
C.
D.
E.

Mutations: The Ultimate Source of Genetic Variation

• Every allele on Earth originated from mutations.

• Without mutations, no new genetic information would ever arise.

• This is the big and profound way that mutations impact evolution.

Mutations – TL;DR (Too Long; Didn’t Read)

• Mutations are a shot-in-the-dark mechanism that randomly creates new genetic variation.

• However:

  • With a large enough population,

  • Enough time,

  • Luck,

  • And natural selection,

  • The new variation created by mutations leads to all the variation we see on Earth today.

Gene Flow

• The flow of alleles from one population to another

• Can go both ways

• Gene flow makes populations more genetically similar to one another

• Can have negative, positive, or neutral effects on the fitness of the populations involved

• Brings new variation that is well adapted or poorly adapted to a new population

Gene Flow Prevents Speciation

• When there is no gene flow, populations evolve independently from one another.

• This allows populations of the same species to diverge from one another (genetically/phenotypically).

• This can eventually lead to new species.

Gene Flow Example: Frog Migration

• A land bridge forms, connecting a mainland population of frogs with an island population of the same species.

• Yellow frogs from the island population begin migrating to the mainland and start reproducing with mainland frogs.

📌 Phenotype & Genotype Representation:
A₃ A₃
A₁ A₁
A₁ A₂
A₂ A₂

Slide 5

Evolution

The process that results in changes in the proportion of heritable traits within populations from one generation to the next.

Four Forces/Mechanisms Driving Evolution:

1. Natural Selection

2. Mutation

3. Genetic Drift

4. Gene Flow

So far, we have established that...

• Evolution means changes in heritable variation in a population over generational time.

• Four mechanisms create changes in heritable variation:

  • Natural Selection

  • Genetic Drift

  • Gene Flow

  • Mutations

Our next question is: How do we know if a population is undergoing evolution?

Null Hypotheses

• As scientists, we build our knowledge by forming hypotheses and predictions that are testable... and then testing them!

• We often do this by:

  1. Defining the outcome(s) we would expect to see if the effect we are interested in does not exist/isn’t happening.

  2. Comparing these predicted null outcomes with the actual observed data from our study system.

Put another way...

If you want to test if something is happening – figure out what things would look like if that thing wasn’t happening – and find a fair and unbiased way to test if that prediction is wrong.

What is a ‘model’?

• A model is a simplified representation of something.

  • “All models are wrong, but some are useful.” – George Box

Examples of Models

• This is not an animal cell. It is a graphical model of an animal cell.

• This is not the Earth. It is a model of the Earth and its geopolitical borders.

• This is not an atom. It is a graphical model of an atom.

The general premise of an evolutionary ‘model’

• Not as simple as a static image or object – because evolution is not a static process.

• Instead: translate how allelic and genotypic frequencies change over time into math.

To calculate the observed allelic frequency of the 4R allele, follow these steps:

1. Identify the total number of alleles in the population:

   • Each individual has two alleles, so the total number of alleles in the population is twice the number of individuals.

   • From the given genotypes, we can calculate the total number of individuals:

     • 4R 4R: 436 individuals (each has 2 4R alleles)

     • 4R 7R: 138 individuals (each has 1 4R allele and 1 7R allele)

     • 7R 7R: 41 individuals (each has 2 7R alleles)

   Total individuals = 436 + 138 + 41 = 615 individuals

   Since each individual has 2 alleles, the total number of alleles in the population is:

   • 615 individuals × 2 alleles = 1230 alleles.

2. Count the total number of 4R alleles:

   • The 4R 4R individuals contribute 2 4R alleles per individual.

     • 436 * 2 = 872 4R alleles.

   • The 4R 7R individuals contribute 1 4R allele per individual.

     • 138 * 1 = 138 4R alleles.

   Total number of 4R alleles = 872 + 138 = 1010 4R alleles.

3. Calculate the frequency of the 4R allele: The frequency of the 4R allele is the ratio of the number of 4R alleles to the total number of alleles in the population:

   Allelic frequency of 4R=Number of 4R allelesTotal number of alleles=10101230≈0.8211Allelic frequency of 4R=Total number of allelesNumber of 4R alleles​=12301010​≈0.8211

So, the observed allelic frequency of the 4R allele in the population is approximately 0.821 (or 82.1%).The Hardy-Weinberg Principle is a null model
used to test if evolution is occurring
• HWE makes five assumptions:
Observed Frequency of the 7R Allele

You are given the following genotypes:

• 4R 4R: 436

• 4R 7R: 138

• 7R 7R: 41

Total number of individuals = 436 + 138 + 41 = 615.

• Total 7R alleles:

  • 138 1 (from 4R 7R) + 41 2 (from 7R 7R) = 138 + 82 = 220.

• Total alleles (2 per individual):

  • 615 * 2 = 1230 alleles.

• Frequency of 7R allele = 22012301230220​ ≈ 0.179 or 17.9%.

2. Calculating Expected Genotypic Frequencies Under Hardy-Weinberg Equilibrium (HWE)

• Given Allele Frequencies:

  • p (4R) = 0.821

  • q (7R) = 0.179

The Hardy-Weinberg equations to calculate the expected frequencies of each genotype are:

• 4R 4R (homozygous): p2p2

• 4R 7R (heterozygous): 2pq2pq

• 7R 7R (homozygous): q2q2

Let's compute them:

• Expected frequency of 4R 4R (p²) = 0.8212=0.6730.8212=0.673

• Expected frequency of 4R 7R (2pq) = 2(0.821)(0.179)=0.2942(0.821)(0.179)=0.294

• Expected frequency of 7R 7R (q²) = 0.1792=0.0320.1792=0.032

3. Comparing Observed and Expected Genotypic Frequencies

Now, compare the observed frequencies to the expected ones under Hardy-Weinberg equilibrium (HWE).

• Observed Frequencies:

  • 4R 4R: 0.71

  • 4R 7R: 0.22

  • 7R 7R: 0.07

• Expected Frequencies (under HWE):

  • 4R 4R: 0.67

  • 4R 7R: 0.29

  • 7R 7R: 0.03

4. Interpreting the Results and Answering the Question

The 7R 7R genotype is more common than expected under Hardy-Weinberg equilibrium (observed 0.07 vs expected 0.03). This suggests that some evolutionary force is acting on the population.

Here are the answer choices:

A. The 7R 7R genotype must be giving some type of reproductive advantage.

• Possible answer: If the 7R 7R genotype is more common, it could suggest that there is some selection favoring this genotype, meaning it's beneficial for reproduction. This could be a plausible explanation.

B. This observation must be the result of genetic drift.

• Not likely: Genetic drift typically has more of an effect in smaller populations. Since the population size is large (615 individuals), genetic drift is less likely to explain the results.

C. There must be mutations for 7R occurring.

• Not likely: Mutations typically happen at a very low rate. Since you are comparing observed and expected frequencies, this difference is more likely due to selection or another evolutionary force, not mutations.

D. It is not possible to determine the reason for the difference between our null expectations and the observed data based on these data alone – but it does look like some evolutionary force has led to a greater number of 7R 7R genotypes than would be expected if evolution were not acting on this gene (the DRD4 gene).

• Best answer: This answer acknowledges that while we cannot definitively say which evolutionary force is acting (without further data), there is clear evidence that evolution is at work here, particularly favoring the 7R 7R genotype.

Summary

To summarize how to approach each part:

1. Calculate the allelic frequency for 7R using the genotypic counts.

2. Compute the expected genotypic frequencies using Hardy-Weinberg equations (p², 2pq, q²).

3. Compare observed vs. expected genotypic frequencies to see if evolution is occurring.

4. Interpret the difference based on evolutionary mechanisms like natural selection. The most likely explanation here is selection for the 7R 7R genotype, so D is the best answer.1.
   2.
   3.
   4.
   Hardy-Weinberg Equilibrium (HWE)

   Key Assumptions of HWE:

   1. No mutation.

   2. No migration (no gene flow).

   3. Random mating.

   4. Large population size (no genetic drift).

   5. No natural selection.

   Allele Frequencies:

   • p: Frequency of the 4R allele.

   • q: Frequency of the 7R allele.

   Note: p+q=1p+q=1

   Genotypic Frequencies Under HWE:

   1. 4R 4R (homozygous): p2p2

   2. 4R 7R (heterozygous): 2pq2pq

   3. 7R 7R (homozygous): q2q2

   How to Calculate Allele Frequencies:

   • Observed Allele Frequency of 4R (p):

     p=(2×4R 4R individuals)+(4R 7R individuals)2×Total populationp=2×Total population(2×4R 4R individuals)+(4R 7R individuals)​

   • Observed Allele Frequency of 7R (q):

     q=1−pq=1−p

   Expected Genotypic Frequencies Under HWE:

   • Expected 4R 4R frequency: p2p2

   • Expected 4R 7R frequency: 2pq2pq

   • Expected 7R 7R frequency: q2q2

   ---

   Steps for Hardy-Weinberg Calculations:

   1. Determine Allelic Frequencies:

      • Calculate the frequency of each allele based on the genotypic counts.

      • p=(2×4R 4R)+(4R 7R)2×Total individualsp=2×Total individuals(2×4R 4R)+(4R 7R)​

      • q=1−pq=1−p

   2. Calculate Expected Genotypic Frequencies:

      • Use p2p2, 2pq2pq, and q2q2 to calculate the expected frequencies of each genotype.

   3. Compare Observed vs. Expected Frequencies:

      • Check if there are deviations from expected frequencies (this could indicate evolution is occurring).

   ---

   Example Calculations:

   • Given Genotypic Frequencies:

     • 4R 4R: 436

     • 4R 7R: 138

     • 7R 7R: 41

     • Total population: 615

   • Allele Frequencies:

     • Total alleles: 615×2=1230615×2=1230

     • Total 7R alleles: (138×1)+(41×2)=220(138×1)+(41×2)=220

     • Frequency of 7R allele: q=2201230≈0.179q=1230220​≈0.179

     • Frequency of 4R allele: p=1−0.179=0.821p=1−0.179=0.821

   • Expected Frequencies (Under HWE):

     • p2=0.8212=0.673p2=0.8212=0.673

     • 2pq=2(0.821)(0.179)=0.2942pq=2(0.821)(0.179)=0.294

     • q2=0.1792=0.032q2=0.1792=0.032

   ---

   Conclusion:

   • Compare the observed genotypic frequencies with the expected frequencies.

   • If the observed frequencies deviate significantly from the expected ones, it suggests that evolution is occurring in the population (e.g., through natural selection, genetic drift, or gene flow).5.
     • We also simplify this model by:
     • Only tracking one gene and assume it only has two allele

     • Assume there is a clear F 0 generation who all reproduce at the same time
     • Slide 6 including different sexes (males or females) – all gametes are capable
     Hardy-Weinberg Model Explanation

     To calculate the expected genotypic frequencies under Hardy-Weinberg equilibrium if evolution is not occurring, we use the following:

     • p: The allelic frequency of the 4R allele.

     • q: The allelic frequency of the 7R allele.

     • p²: The expected genotypic frequency of 4R 4R (homozygous for 4R).

     • 2pq: The expected genotypic frequency of 4R 7R (heterozygous).

     • q²: The expected genotypic frequency of 7R 7R (homozygous for 7R).

     These equations assume that evolution is not happening in the population, meaning that allele frequencies remain stable over generations.

     ---

     How to Answer Questions:

     1. Identify Allelic Frequencies (p and q):

     • First, check the problem for either allelic frequencies (p and q) or genotypic frequencies (the number of 4R 4R, 4R 7R, and 7R 7R genotypes).

     • If you're given genotypic frequencies, use them to calculate allelic frequencies first.

       • For example:

         p=2 × frequency of 4R4R+frequency of 4R7R2p=22 × frequency of 4R4R+frequency of 4R7R​

         q=2 × frequency of 7R7R+frequency of 4R7R2q=22 × frequency of 7R7R+frequency of 4R7R​

     2. Apply Hardy-Weinberg Equations:

     • Once you have p (the frequency of 4R) and q (the frequency of 7R), use the Hardy-Weinberg formulas to calculate the expected genotypic frequencies:

       • 4R 4R frequency: p2p2

       • 4R 7R frequency: 2pq2pq

       • 7R 7R frequency: q2q2

     3. Compare with Observed Data (if applicable):

     • If you're given observed genotypic frequencies, compare them to the expected genotypic frequencies. A difference between the two suggests that evolution might be occurring, meaning the population is not in Hardy-Weinberg equilibrium.

     ---

     Example Question:

     Given: Allelic frequencies are:

     • p=0.82p=0.82 (for the 4R allele)

     • q=0.18q=0.18 (for the 7R allele)

     What are the expected genotypic frequencies?

     Steps:

     1. Expected frequency of 4R 4R (p²):

        • p2=0.822=0.6724p2=0.822=0.6724

     2. Expected frequency of 4R 7R (2pq):

        • 2pq=2(0.82)(0.18)=0.29522pq=2(0.82)(0.18)=0.2952

     3. Expected frequency of 7R 7R (q²):

        • q2=0.182=0.0324q2=0.182=0.0324

     These are the expected genotypic frequencies if evolution is not occurring in this population.

     ---

     How to Answer:

     • Step 1: Identify whether you’re given allele frequencies or genotypic frequencies. If given genotypic frequencies, calculate the allele frequencies first.

     • Step 2: Use the Hardy-Weinberg equations to calculate the expected genotypic frequencies: p2p2, 2pq2pq, and q2q2.

     • Step 3: Compare the expected and observed genotypic frequencies to determine if evolution is occurring. If there’s a significant difference, evolution may be happening in the population.of ‘fertilizing’ one another
       These are absurd assumptions (by design)!
       How to Calculate the Expected Frequency of 4R 4R Individuals:

       1. Allelic frequencies are given:

          • p=0.821p=0.821 (for the 4R allele)

          • q=0.179q=0.179 (for the 7R allele)

       2. Using the Hardy-Weinberg equation:

          • The expected frequency of 4R 4R (homozygous for 4R) is p2p2.

          p2=(0.821)2=0.674p2=(0.821)2=0.674

       3. Answer: The expected frequency of 4R 4R individuals under Hardy-Weinberg equilibrium is 0.674.

       Correct Answer: B. 0.674

       ---

       Interpreting the Observed vs. Expected Frequencies:

       • Observed frequencies:

         • 4R4R: 0.71

         • 4R7R: 0.22

         • 7R7R: 0.07

       • Expected frequencies (calculated earlier):

         • 4R4R: 0.674

         • 4R7R: 0.295

         • 7R7R: 0.032

       Since the observed frequency of 7R7R is higher than the expected frequency, this suggests that something other than Hardy-Weinberg equilibrium is affecting this population (e.g., evolution is occurring).

       ---

       Extinction Vortex and Fragmentation:

       • Scenario: A highway splits a snake population into two isolated populations (200 snakes on the East side, 600 on the West side), which can no longer safely reach each other.

       • True Statements:

         1. A. The two populations may experience genetic drift because of smaller population sizes and isolation.

         2. B. Reduced genetic diversity in each population can lead to inbreeding.

         3. C. Over time, these smaller populations may have reduced fitness and increased vulnerability to extinction due to genetic bottlenecks.

         4. D. This scenario can lead to an extinction vortex, where the lack of gene flow and genetic diversity can cause the population to spiral towards extinction.Breaking any of these would mean evolution
            is occurring – and we want a null Key Concepts in Population Genetics and Inbreeding

            1. Rare Recessive Deleterious Trait:

            2. A recessive trait means that two copies of the allele (aa) are required for the phenotype to be expressed.

               In a large population with a low frequency of the deleterious 'a' allele, individuals with the genotype aa will be rare.

               • For example:

                 ¼ of Aa X Aa matings will result in aa (homozygous recessive) offspring.

                 ½ of AA X Aa matings will result in aa offspring (though these matings are less common).

            3. Effects of a Highway (Population Fragmentation):

               • Smaller populations lead to increased inbreeding (mating between closely related individuals).

               • Inbreeding can increase the chances of homozygosity for deleterious alleles.

                 In a small population, the probability of two carriers of the a allele (heterozygotes, Aa) mating and producing aa offspring increases because there are fewer potential mates.

                 Inbreeding increases the expression of deleterious recessive traits, leading to higher rates of diseasesor harmful phenotypes.

            4. Inbreeding vs. Outbreeding:

               • Inbreeding:

                 Leads to higher homozygosity (more identical alleles at a given locus).

                 Increases the chance of recessive deleterious traits being expressed (e.g., the rare aa phenotype).

               • Outbreeding:

                 Reduces homozygosity and promotes genetic diversity.

                 Can reduce the chances of expressing deleterious recessive traits because heterozygotes (Aa) are more common.

            Summary of Effects:

            • In a large population with a low frequency of the a allele, the chances of homozygous recessive aa individuals are low.

            • Fragmenting a population (e.g., building a highway) leads to inbreeding and higher chances of recessive deleterious traits becoming more common in the population due to increased homozygosity.

            • Outbreeding (breeding between individuals from different populations) would reduce inbreeding and help maintain genetic diversity.

            Thus, inbreeding depression can cause harmful recessive traits to become more frequent, leading to population declineand increased risk of extinction.m

            Why Reduced Variation Makes a Population Less Able to Adapt to New Changes:

            • Reduced genetic variation means that a population has fewer different alleles or traits to work with. In other words, if a population has low genetic diversity, it has fewer potential options for adapting to environmental changes, like the development of resistance to new threats (e.g., diseases or antibiotics).

            • Adaptation to new conditions relies on having genetic diversity within a population. For example, if some individuals have mutations that confer resistance to an antibiotic, those individuals are more likely to survive and reproduce, passing on those resistant traits. However, if the population has reduced variation, it might lack individuals who have the beneficial traits needed to survive new environmental pressures, making it more vulnerable to extinction or disease outbreaks.

            Example: Antibiotic Resistance in E. coli

            1. Penicillin and Antibiotics:

               • Antibiotics, like penicillin, are designed to kill bacteria by disrupting their ability to construct a cell wall.

               • Many antibiotics contain a beta-lactam ring, which is essential for their activity.

            2. Beta-lactamase Enzymes:

               • Some E. coli populations produce enzymes called beta-lactamases, which can break down the beta-lactam ring, rendering the antibiotic ineffective.

            3. Two Forms of Beta-lactamase:

               • TEMwt beta-lactamase is ineffective against penicillin.

               • TEM beta-lactamase* is a mutated version that can break down penicillin 100,000 times more effectivelythan TEMwt, making E. coli highly resistant to penicillin.

            Importance of Variation:

            • If the population has low genetic variation, it may not have individuals with the TEM mutation* that provides resistance to penicillin.

            • On the other hand, if there is high genetic variation, some individuals might already have the TEM mutation*, enabling them to survive in environments with penicillin, allowing the population to adapt to new threats.

            In Summary:

            • Reduced variation means fewer options for adaptation, making it harder for the population to survive when exposed to new environmental changes (like the introduction of antibiotics or disease).

            • Increased variation allows for greater adaptability and survival in changing environments, as individuals with beneficial mutations (like TEM beta-lactamase*) are more likely to survive and reproduce.odel that
              represents evolution NOT occurring

Given a knoExplanation:

• Point mutations are typically rare events, and it is extremely unlikely for all five point mutations to occur simultaneously in a single DNA replication event. Instead, mutations occur over many generations through a process called stepwise mutation, where individual mutations accumulate gradually over time.

• In this case, the TEM allele* differs from the TEMwt allele by five point mutations. The process of mutation is random, so it is more likely that each mutation occurred independently at different points in time, rather than all five occurring at once.

• After each mutation, the bacteria with the beneficial mutation (in this case, TEM* providing penicillin resistance) would have a survival advantage, and these bacteria would pass on the mutation to their offspring. Over time, the population would accumulate these mutations, and TEM* could emerge.

Conclusion:

• The five mutations leading to TEM* would most likely occur step by step, not all at once, as bacteria replicate and pass on mutations to subsequent generations. Therefore, the statement is false.

  wn allele frequency – we can figure
  out what the genotypic frequencies would be if
  To understand how we could get from TEMwt to TEM*, we need to consider how mutations accumulate in bacteria and how they impact the ability of the bacteria to survive under selective pressure, such as exposure to antibiotics.

  Explanation:

  1. Accumulation of Mutations:

     • The TEMwt allele has a sequence that differs slightly from the TEM* allele. In your example, TEMwtundergoes a series of point mutations that gradually change its structure to form TEM*, the variant with antibiotic resistance.

     • The five mutations in question occur at different positions, and each mutation might contribute incrementally to a change in the enzyme's ability to resist the effects of the beta-lactam antibiotics.

     • These mutations likely occur one at a time over multiple generations.

  2. No Intermediate Versions Providing Resistance:

     • If no intermediate versions of TEM offer significant antibiotic resistance, it means that a single mutation may not provide a detectable level of resistance. For instance, mutation 1 may not significantly change the enzyme's ability to degrade penicillin, but later mutations might combine to produce a more substantial resistance effect.

     • Therefore, TEMwt would evolve step by step to TEM* through multiple mutations, with each mutation providing a small increase in resistance. However, none of the intermediate forms (like TEMwt1, TEMwt2, etc.) would be highly resistant on their own.

  3. How Can the TEM Allele Evolve from TEMwt?*

     • A realistic path for TEMwt to evolve into TEM* would involve the accumulation of mutations over time in individual bacterial cells. Bacteria with each mutation would be exposed to antibiotic pressure. Those that survive would pass the new mutation(s) to their offspring.

     • For example:

       • Mutation 1 might change the enzyme's structure slightly, but it's not enough to provide resistance.

       • Mutation 2 adds more resistance, but again, it may not be sufficient on its own.

       • Eventually, the combination of all five mutations creates a fully resistant allele (TEM*).

     • The order in which the mutations happen is important because there are 120 different possible ways these mutations could accumulate (as described in your example). Each of these pathways could result in different intermediate stages of resistance, but in some cases, no intermediate might be significantly resistant.

  Answer to the Question:

  • If each individual mutation doesn't result in a significant improvement in resistance, the bacteria will accumulate mutations over many generations, eventually resulting in the TEM* allele, which provides full resistance.

  • What would be accurate?

    • The most accurate conclusion would be that TEM* evolves gradually, with mutations accumulating over time. The intermediate mutations alone might not give detectable resistance, but collectively they contribute to the development of full resistance.

  • When there are fitness advantages to intermediate phenotypes – mutation and natural selection can ramp somewhat quickly to complex adaptations.

    4o mini

  was not occurring at that locus.
  This “If the observed genotypic frequencies differ
  Slide 7

If there were no intermediate versions of TEM that gave any amount of antibiotic resistance - what would be accurate?

B. Getting all five TEM mutations would mostly involve mutation and natural selection.*

In this case, the mutations would provide a selective advantage by increasing antibiotic resistance, so natural selection would favor individuals with these mutations over time. Genetic drift would be less of a factor because the mutations confer an advantage, and gene flow is unlikely to play a role if we are considering only the evolution of these mutations within a population without external genetic input. Additionally, these mutations would arise through mutation, which is the source of new genetic variation.

Common House Mosquito (Culex pipiens)

• Found in both urban and suburban temperate and tropical regions across the world

• Bites mostly birds

• Hibernates in winter

• Tolerates cold

London Underground (in the UK)

• Relatively stable temperature

• Relatively high humidity

• Lots of people

• Also rats and mice

London Underground Mosquito

• Culex molestus

• Found in the London Underground railway system and other underground systems

• Bites rats, mice, birds, and humans

• Breeds all-year round

• Cannot tolerate cold

Two populations of mosquitoes likely descended from one ancestral population.

• Ancestral population

• London Underground population

• Aboveground population

A population is a group of individuals from the same geographic region that regularly mate together.

Which of the following are required for these populations to become separate species?

a) Genetic isolation
b) Reduced gene flow
c) Differences in selection pressures
d) Reproductive isolation
e) All of the above

Explanation:
For two populations to become separate species, they must be reproductively isolated, meaning they can no longer interbreed successfully. This typically involves genetic isolation, reduced gene flow, and the development of different selection pressures or environments that lead to divergence.

Speciation is the process responsible for the creation of new species.

Explanation:
Speciation occurs when populations of the same species become isolated from each other (genetically or geographically) and accumulate differences over time, leading to the formation of distinct species.

Which factors may have created genetic isolation between these two populations of mosquitoes?

a) Geographic isolation (e.g., underground vs aboveground environments)
b) Behavioral isolation (e.g., different mating times or locations)
c) Ecological isolation (e.g., different habitats or environmental conditions)
d) Reduced gene flow (e.g., physical barriers or mating preferences)
e) All of the above

Explanation:
Genetic isolation can occur through various mechanisms, including geographic, behavioral, ecological isolation, or simply through reduced gene flow due to physical or temporal barriers. In this case, the underground environment of the London Underground mosquitoes could create geographical isolation, while different habitats and mating preferences may also reduce gene flow.

Genetic isolation can result from geographic barriers

• Allopatric speciation

  • "Different place"

  • Gene flow is cut off by a geographic barrier or because some individuals move to a separate place.

• Sympatric speciation

  • "Same place"

  • Split by reproductive separation even though they are in the same place.

Genetic isolation can also happen without geographic barriers

• Example: If new variation arises that causes temporal variation in activity – this can lead to assortative mating, which cuts off gene flow.

Imagine a patch of flowers in a single field. They are pollinated by a species of bee. Over time, random mutations accumulate in a lineage of these plants leading to a new flower that no longer attracts bees; instead, it attracts hummingbirds.

Once gene flow is cut off...what causes genetic divergence to happen?

A) Mutation
B) Natural selection
C) Genetic drift
D) All of the above
E) None of the above

Explanation:
Once gene flow is cut off, populations can diverge through different mechanisms. Mutations introduce new genetic variation, natural selection acts on that variation by favoring beneficial traits, and genetic drift can cause random fluctuations in allele frequencies. All of these factors contribute to genetic divergence when gene flow is interrupted.

In this cheese pizza metaphor...what do the strings of cheese represent?

• Explanation: The strings of cheese in this metaphor likely represent gene flow, which connects different populations or groups within a species. Just like the strings bind the cheese together, gene flow connects different individuals in a population, allowing genetic material to spread throughout the group.

So...are these the same species or different species? How would we know?

• London Underground Mosquito Culex molestus

• Common House Mosquito Culex pipiens

Explanation:
Whether these two populations are the same species or different species depends on reproductive isolation. According to the biological species concept, species are defined as groups of actually or potentially interbreeding populations that are reproductively isolated from other such groups. If these two populations can interbreed and produce fertile offspring, they are the same species. If they cannot or if their offspring are sterile, they are considered different species.

A species is a fundamental unit of biological classification.

• Named using binomial nomenclature:

  • Genus species

    • Example: Vulpes vulpes, Culex pipiens

Species concepts

• A species is:

  • An independent evolutionary unit—meaning a population or group of populations that is genetically isolated from others and is evolving independently.

  • A very common way to conceptualize a species is the biological species concept, which states:

    • "Groups of actually or potentially interbreeding natural populations that are reproductively isolated from other such groups."

The biological species concept:

• "Groups of actually or potentially interbreeding natural populations that are reproductively isolated from other such groups."

Sometimes, two individuals can produce offspring, but those offspring are not fertile.

• Infertile offspring: Genes don't get passed on.

'Species' is a complex concept.

• But... what about:

  • Asexual species?

  • Hybrids?

  • Fossils?

  • Connected extremes?

There are many other species concepts based on:

• Physical characteristics

• Ecological roles and interactions

• Evolutionary relationships

• Genetic data

Which of the following would be good evidence that these two mosquito populations are different species? Select all that apply.

• Ancestral population

• London Underground population

• Aboveground population

Options:

• a) When the two varieties were crossbred, the eggs were infertile.

  • Explanation: This is strong evidence for reproductive isolation, which is a key criterion for defining different species. If the offspring are infertile, this suggests that the populations do not interbreed successfully in nature.

• b) DNA testing showed numerous differences.

  • Explanation: Significant genetic differences between the two populations suggest that there is little to no gene flow between them. This supports the idea of genetic isolation, a major factor in speciation.

• c) The two populations have different behavioral patterns.

  • Explanation: Different behaviors, such as mating rituals or habitat preferences, can contribute to reproductive isolation and thus suggest the populations may be different species.

• d) The populations are found in different environments (London Underground vs. aboveground).

  • Explanation: Geographic isolation (e.g., living in different habitats) can prevent gene flow, leading to speciation. However, in this case, behavioral isolation (such as different mating times or habits) might be more directly responsible.

• e) The populations are reproductively isolated but live in overlapping areas.

  • Explanation: This scenario would suggest sympatric speciation, where reproductive isolation occurs in the same geographic location, making it likely that they are separate species despite living in close proximity.

In 2012, the London Underground Mosquito was renamed a special form of the Common House Mosquito: 'Culex pipiens f. molestus'.

• But...biologists have found evidence that the London Underground Mosquito is a different species than the Common House Mosquito.

  • When the two varieties were crossbred, the eggs were infertile. This suggests reproductive isolation, a defining characteristic of different species.

  • DNA testing showed numerous differences, which supports the hypothesis of little to no gene flow, further suggesting they are distinct species.

“The separation between the two behavioral and physiological forms... is probably quite recent.”

This statement hints at a relatively recent divergence, which could be a case of sympatric speciation or allopatric speciation depending on the specific geographic or behavioral barriers involved.