AAC - 07 - Concept 2 Notes - Natural Selection PPT

Principles of Natural Selection

• It’s not Rocket Science ®

Evolution

• Definition: The process of biological change in populations over time that makes descendants genetically distinct from their ancestors.

• Types of Evolution:

  • Microevolution: Small scale changes affecting a single population.

  • Macroevolution: Large scale changes affecting species across populations.

Historical Beliefs About Evolution

• Creationism

• Catastrophism (Cuvier)

• Gradualism (Hutton)

• Struggle for Existence (Malthus)

• Uniformitarianism (Lyell)

• Inheritance of Acquired Traits (Lamarck)

• Intelligent Design

Charles Darwin

• Background: English naturalist who voyaged to the Galapagos Islands.

• Observations: Noticed different species of finches, tortoises, etc., had specific traits adapted to their environment.

• Theory Developed: Natural selection as a mechanism for evolution.

Natural Selection

• Definition: Organisms with the best traits (adaptations) survive longer and reproduce more, leading to population changes over time.

• Concepts:

  • Survival of the Fittest: Fitness = measure of an organism's ability to survive in its environment.

  • Influenced by:

    • Overproduction of offspring

    • Variation among individuals

    • Adaptation to the environment

    • Descent with modification.

Principles of Natural Selection

Overproduction of Offspring

• High offspring numbers lead to competition for limited resources.

Variation

• Definition: Differences in physical traits among organisms.

• Sources of Variation:

  • Random mutations (ultimate source)

  • Genetic recombination during meiosis (crossing over)

  • Migration (gene flow)

Adaptation

• Definition: A beneficial feature for survival in an environment.

• Impact: Beneficial traits become common over time as organisms reproduce more, altering the gene pool (combined alleles in a population).

Change in Gene Frequency

• Natural selection causes populations to develop new phenotypes suited to changing situations.

• Over time, beneficial traits should dominate the population.

• Key Note: Individuals do not evolve, only populations.

Modes of Selection

1. Directional Selection

• Definition: Increases the expression of an extreme trait in a population.

• Example: Moths with dark wings gain higher survival rates in dark environments.

2. Disruptive Selection

• Definition: Splits a population into two groups, favoring extreme traits.

• Example: Snakes on rocks are grey and those on grass are green; intermediate coloration is at risk from predators.

3. Stabilizing Selection

• Definition: Eliminates extreme traits, promoting average traits for higher fitness.

• Example: Human birth weights show higher survival at average weights.

Gene Pool and Allele Frequencies

• Allele Frequency: Rate at which an allele appears in a population. Higher frequency implies more common traits.

• Formula: p + q = 1, where p = frequency of dominant alleles and q = frequency of recessive alleles.

Allele Frequencies Example #1

• In red/white flower population (500 plants):

  • 20 white (rr), 320 homozygous red (RR), 160 heterozygous red (Rr).

  • Total alleles = 1000.

  • Count of recessive alleles = 200, red alleles = 800.

Allele Frequencies Example #2

• In tall/short pea plant population (400 plants):

  • 50 short (tt), 200 homozygous tall (TT), 150 heterozygous tall (Tt).

Mechanisms of Microevolution

• Key Mechanisms:

  1. Mutations

  2. Natural Selection

  3. Genetic Drift

  4. Gene Flow

  5. Non-random Mating (Sexual Selection)

• Microevolution: Occurs on a small scale, affecting individual populations.

Genetic Equilibrium

• Definition: Hardy-Weinberg equilibrium where allele frequencies remain unchanged.

• Conditions for Non-Evolution:

  1. Large population

  2. Random mating

  3. No migration

  4. No mutations

  5. No natural selection

• If one condition is unmet, evolution occurs.

Hardy-Weinberg Equilibrium (HWE)

• Equations:

  • p + q = 1

  • p² + 2pq + q² = 1

• Interpretation:

  • p² = frequency of homozygous dominant individuals

  • 2pq = frequency of heterozygous individuals

  • q² = frequency of homozygous recessive individuals.

Example Calculations in HWE

Example #3

• Given a population with round and cone heads:

  • 51% display round heads.

  • Calculate homozygous frequency from overall dominance.

  • Use HWE formulas to determine genotypes in a balanced population.

Example #4

• Population of turtles with red/yellow traits:

  • 34 yellow out of 241 total.

  • Determine allele frequencies and genotype proportions in a population under HWE.

Final Problem Solving Strategies for HWE Issues

• Assess actual genotype frequencies against those predicted by HWE.

• If discrepancies arise, the population is evolving, identifying selection modes at play.