D4.1  Natural Selection

Theme:  Continuity and Change

Adaptive traits are differentially selected, maintaining the stability of a population in a consistent environment.

• Natural selection depends on a trait to heritable, meaning it is coded in DNA and passed from parent to offspring. 
  
• Stabilizing selection favors intermediate phenotypes and acts against extreme variations, effectively keeping the population the same over long periods. It preserves the status quo when the environment is stable.
  
• By producing more offspring than can survive, a species ensures the continuity of the lineage. Even if many individuals die due to competition or predation, the best adapted individuals carry the species forward.
  

Natural selection will cause the frequency of alleles to shift within a gene pool, leading to the evolution of new characteristics.

• Genetic variation is due to random mutations and the shuffling of alleles in a population.
• ​Change occurs because individuals with adaptive variations are more likely to survive and reproduce. This is not a change in an individual, but a change in the population over time. 
• Antibiotic resistance in bacteria is an example of rapid population change due to selection. 
  

Guiding Questions:  
Guiding questions help students view the content of the syllabus through the conceptual lenses of both the themes and the levels of biological organization.​

• What processes can cause changes in allele frequencies within a population?
• What is the role of reproduction in the process of natural selection?

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​Linking Questions:  
Linking questions strengthen students’ understanding by making connections between topics.  The ideal outcome of the linking questions is networked knowledge.

• How do intraspecific interactions differ from interspecific interactions? 
• What mechanisms minimize competition?

Key Terms to Know: * higher level only

Abiotic
Adaptation
Allele
Allele Frequency*
Antibiotic Resistance*
Acquired Trait
Artificial Selection*
Behavioral Trait
Biodiversity
Carrying Capacity
Competition
Crop Plant*
Darwin
Density-Dependent
Density-Independent

Directional Selection*
Disruptive Selection*
Domestic Animal*
Environmental Resources
Evolution
Fitness
Gene Pool*
Genetic Equilibrium*
Genotype
Geographically Isolated Population*
Hardy-Weinberg Equation for Allele Frequency*
Hardy-Weinberg Equation for Genotype Frequency*
Heritable Trait

Intraspecific Competition
Lamarck
Mutation
Natural Selection
Neo-Darwinism*
Overproduction of Offspring
Paradigm Shift
Physical Trait
Reproduction
Selection Pressure
Sexual Reproduction
Sexual Selection
Stabilizing Selection*
Survival
Variation

D4.1.1— Natural selection as the mechanism driving evolutionary change.

• Define natural selection.
• Outline the observations and inferences that lead to the development of the theory of evolution by natural selection. 
• Outline the theory of evolution by natural selection as an example of inductive reasoning.
• Outline the theory of evolution by natural selection as an example of the correspondence, coherence and pragmatic theories of truth.
• State that natural selection has operated continuously over billions of years, resulting in the biodiversity of life.​

D4.1.2— Roles of mutation and sexual reproduction in generating the variation on which natural selection acts.

• Explain why natural selection can only function if there is variation in a species.
• Outline sources of genetic variation (mutation, meiosis and sexual reproduction). 
• Compare variation that results from mutation to that generated from sexual reproduction.

D4.1.3-- Overproduction of offspring and competition for resources as factors that promote natural selection.

• State that species have the ability to produce more offspring than the environment can support.
• Use an example to illustrate the potential for overproduction of offspring in a population.
• State two evolutionary benefits of overproduction of offspring.
• Describe competition for resources as a consequence of overproduction of offspring.
• Define carrying capacity
• List examples of resources that may limit population size.
• Compare direct and indirect competition.

D4.1.4--  Abiotic factors as selection pressures.

• Define selective pressure and density-independent.
• State example biotic and abiotic selective pressures.
• Outline how a selective pressure acts on the variation in a population.​

D4.1.5— Differences between individuals in adaptation, survival and reproduction as the basis for natural selection.  

• Define adaptation and fitness.
• Explain the effect of the selective pressure on the more and less adapted individuals in a population.
• Explain adaptation as a consequence of natural selection. ​

D4.1.6- Requirement that traits are heritable for evolutionary change to occur.

• Distinguish between heritable and acquired characteristics. 
• Explain why only heritable characteristics can be acted upon by natural selection.

 ​​​​D4.1.7- Sexual selection as a selection pressure in animal species.

• Outline the two major mechanisms of sexual selection in evolution of courtship behavior and anatomical features. 
• Describe examples of sexual selection, including for color, size, and courtship behaviors.

D4.1.8- Modelling of sexual and natural selection based on experimental control of selection pressures.  

• Outline the selective pressures for and against coloration in guppies.
• Summarize John Endler’s experiments with guppies which demonstrate selection for and against coloration in different habitats.
• Explain what models are and their purposes in science.
  

AHL ​​​​​​​D4.1.9- Concept of the gene pool.

• Define gene pool, gene, allele and gene flow.
• Describe how it is possible for multiple gene pools to exist in a single species.

AHL ​​​​​​​D4.1.10- Allele frequencies of geographically isolated populations.

• Define allele frequency.
• Calculate allele frequency from gene pool data. 
• Outline reasons when allele frequencies may be different in geographically isolated populations of the same species.
  
• Use a database to search for allele frequency of a human gene.​

AHL ​​​​​​​D4.1.11- Changes in allele frequency in the gene pool as a consequence of natural selection between individuals according to differences in their heritable traits.

• Outline “neo-Darwinism” as the integration of genetic inheritance and the mechanism of natural selection.
  
• State that change in the allele frequencies of a gene is evidence of evolution.
• List processes that can change allele frequency in a population.
  
• Explain how natural selection can lead to change in allele frequency in a gene pool. ​

AHL ​​​​​​​D4.1.12- Differences between directional, disruptive and stabilizing selection.

• Outline the change in allele frequencies associated with stabilizing, disruptive and directional selection.
• Use graphs to illustrate or identify stabilizing, disruptive and directional selection.
• Outline an example of stabilizing, disruptive and directional selection.​

AHL ​​​​​​D4.1.13- Hardy–Weinberg equation and calculations of allele or genotype frequencies.

• State the Hardy-Weinberg equations.
• Given data, calculate allele frequencies of a gene in a gene pool.
• Given data, calculate genotype frequencies of a gene in a gene pool.​

AHL ​​​​​​D4.1.14- Hardy–Weinberg conditions that must be maintained for a population to be in genetic equilibrium.

• List the conditions under which populations maintain Hardy-Weinberg equilibrium.
• Explain how comparison of allele frequencies between two isolated populations of the same species can serve as evidence that divergence is (or is not) occurring.
• Explain how comparison of allele frequencies between one population at two points in time can serve as evidence that evolution is (or is not) occurring.​

AHL ​​​​​​D4.1.15- Artificial selection by deliberate choice of traits. 

• Define artificial selection.
• Outline the mechanism of artificial selection in evolution of crop plants and domestic animals.
• Describe an example of artificial selection of a crop plant or domestic animal.​