Ch 21: The Evolution of Populations

Question 1

Define the characteristics of life

Answer 1

• Organized in hierarchical levels
• Need for external material and energy sources
• Homeostatic (”staying the same”)
• Growth and development
• Reproduction: process by which an organism makes more of itself
• Responding to Stimuli
• Capacity to Adapt

Question 2

Define briefly the concepts of biological evolution and descent with modifications

Answer 2

Evolution → Different traits may be naturally selected for/against under different environmental conditions

Descent with modification → Darwin defined evolution as “descent with modification,”the idea that species change over time, give rise to new species, and share a common ancestor. The mechanism that Darwin proposed for evolution is natural selection.

Question 3

Define microevolution, macroevolution, allele, allele frequency, gene pool, population.

Answer 3

• Macroevolution: evolution occurring above the species level, including the origination, diversification, and extinction of species over long periods of evolutionary time.
• Microevolution: evolution occurring within populations; changes in allele frequency from one generation to the next.
• Alleles: Different forms/versions of a gene (ex blood type in humans) [the existence of different allelic form of a gene in a population is called polymorphism]
• Allele Frequency: number of a particular allele in a population divided by the total number of alleles in the population.
• Gene Pool: all of the alleles present in all individuals of a population or species, at an instant.
• Population → Interbreeding members of the same species living in the same geographic area.

Question 4

Describe the ultimate source of genetic variation within a population, and explain why evolution cannot occur without variation

Answer 4

• Mutation is the ultimate source of all genetic variation, and is essential for evolution by natural selection
• Variations in the environment drive evolution; Without genetic variation,a population cannot evolve in response to changing environmental variablesand, as a result, may face an increased risk of extinction.

Question 5

Describe what conditions must be met for Hardy-Weinberg to apply and what it means if a population is not in Hardy Weinberg equilibrium

Answer 5

• A population in Hardy-Weinberg equilibrium is not evolving; A population not in Hardy-Weinberg equilibrium could evolve

Conditions for Hardy-Weinberg equilibrium:

• There can be no differences in the survival and reproductive success of individuals (i.e. no natural selection)
• Populations must not be added or subtracted from by migration
• There can be no mutation
• The population must be sufficiently large to prevent sampling errors
• Individuals must mate at random

Question 6

Define the conditions that can cause changes in allele and genotype frequencies within a population

Answer 6

• Mutations
• Genetic Drift: Change in allele frequencies of a gene pool due to random meeting of gametes
  
  • Bottleneck effect: Change in environment→ reduction in population
    
    • It results in a gene pool that is not the same compared to the original population
  • Founder effect: Type of bottleneck in which a new population is established by a small number of colonists or founders
    
    • Can lead to allopatric (geographic) speciation
• Gene Flow: Transfer of alleles from one population to another
• Nonrandom mating: Selection for traits that increase access to mates and reproductive success
• Natural Selection

Question 7

Explain how natural selection can affect allele and genotype frequencies

Answer 7

Natural selection also affects allele frequency. If an allele confers a phenotype that enables an individual to better survive or have more offspring, the frequency of that allele will increase.

Question 8

Define adaptation and explain why natural selection is the only evolutionary mechanism that leads to adaptation

Answer 8

Adaptation: The process by which a species becomes fitted to its environment; it is the result of natural selection’s acting upon heritable variation over several generations.

• Natural selection leads to adaptation by increasing the frequency of advantageous alleles (positive selection) in the population

Question 9

Compare natural selection and artificial selection to understand how different strengths of selection can affect rates of evolution

Answer 9

• Natural selection leads to adaptation by increasing the frequency of advantageous alleles (positive selection) in the population
• Artificial selection: Breeders select desired phenotypes/ genotypes for mating; Efficient at generating genetic change; Can lead to increase in proportion of “negative traits”

Question 10

Describe the limitations of natural selection

Answer 10

• Limits of Natural Selection:
  
  • Limited by variations already present in a population
  • N.S has no goal: It is based on random mutations in genes that provide (or not) advantageous traits in the current environment
  • Limited by developmental constraints
  • Acts on the entire organism
  • Limited by trade-offs

Question 11

Define sexual selection and contrast intrasexual and intersexual selection

Answer 11

• Selection for traits that increase access to mates and reproductive success
• Two mechanisms:
  
  • Intrasexual (competition)- occurs between members of the same sex
  • Intersexual- mate choice by members of the opposite sex

Question 12

Describe and contrast the founder effect and the bottleneck effect.

Answer 12

• Bottleneck effect: Change in environment→ reduction in population
  
  • It results in a gene pool that is not the same compared to the original population
• Founder effect: Type of bottleneck in which a new population is established by a small number of colonists or founders
  
  • Can lead to allopatric (geographic) speciation

Question 13

Explain how genetic drift can lead to character fixation

Answer 13

Over time, genetic drift can lead to fixation or loss of genetic variants, thereby systematically eliminating diversity from a population.

Question 14

“Types” of Natural Selection

Answer 14

• Disruptive selection favors individuals at both ends of the phenotypic spectrum.
• Stabilizing selection acts against both extreme phenotypes and favors intermediate variants. This mode of selection reduces variation and tends to maintain the status quo for a particular phenotypic character.
• Directional selection occurs when conditions favor individuals exhibiting one extreme of a phenotypic range, thereby shifting a population’s frequency curve for the phenotypic character in one direction or the other.
• Balancing selection maintains stable frequencies of two or more phenotypic forms in a population.