Evolution and Natural Selection

Lamarckian vs. Modern Model of Evolution

• Lamarckian Model: Differences arise from an individual's experience within their environment. For example, if a parent lifts weights, their offspring would be born more muscular due to the parent's experience.
• Modern Model: Differences originate from random mutations.
  • Mutations are generally harmful, and the body has DNA repair enzymes to prevent them. However, mutations still occur and are the source of genetic diversity.

Random Mutations

• Random mutations introduce diversity into the population.
• Although mutations are generally harmful and the body tries to prevent them, they are constantly introduced into the population.

Natural Selection

• Individuals breed more than the environment can support, leading to competition for resources.
• Reproductive success depends on the alleles an individual possesses.
  • Some alleles will allow the individual to reproduce more effectively.
• Survival of the fittest is an often used term. However, it is more accurate to think of it as individuals with certain genotypes having slightly more offspring over generations.
  • Fitness refers to the ability to copy oneself slightly more due to genotype, not necessarily better adaptation to the environment.

Example

• Consider a gene with a dominant and a recessive allele.
• Individuals with the dominant phenotype have 2.1 offspring on average, while those with the homozygous recessive phenotype have 2.2 offspring on average.
• Over time, the recessive allele will become more prevalent in the population, leading to adaptation to the environment.
• Fitness is about greater reproductive success, which can be achieved through various means (e.g., being bigger, smaller, smarter, tougher).

Pace of Evolution

• Evolution occurs at different paces depending on the organism.
  • For organisms with slow reproduction rates (e.g., humans), evolution is slower and less perceptible.
  • For organisms with fast reproduction rates (e.g., bacteria, viruses), evolution is faster and can be observed within a human lifetime.

Clinical Significance

• For bacteria, mutations that confer antibiotic resistance can quickly spread in environments with antibiotics.

Thought Experiment: Eye Color on an Island

• Consider an island that can hold 10,000 people, initially populated with 5,000 homozygous dominant (brown eyes) and 5,000 homozygous recessive (blue eyes) individuals.
• Assumptions:
  • No fitness advantage to either allele.
  • Random mating.

Allele Frequency

• Allele frequency is the proportion of dominant and recessive alleles in the population.
• In this case, there are 10,000 copies of the dominant allele and 10,000 copies of the recessive allele out of 20,000 total copies of the gene.
• Initial allele frequency: 50% dominant, 50% recessive.

Next Generation

• To determine the genetic makeup of the next generation, consider random mating.
• Analogy: Sea urchins releasing gametes into the seawater, where they randomly meet.
• Probability of being homozygous dominant: 0.5 * 0.5 = 0.25 (25%)
• Probability of being heterozygous: 2 * (0.5 * 0.5) = 0.5 (50%)
• Probability of being homozygous recessive: 0.5 * 0.5 = 0.25 (25%)

Subsequent Generations

• In the next generation, 2,500 people are homozygous dominant, 5,000 are heterozygous, and 2,500 are homozygous recessive.
• Calculate the new allele frequencies:
• Frequency of dominant allele: ((2500 * 2) + 5000) / 20000 = 0.5 (50%)
• Frequency of recessive allele: ((2500 * 2) + 5000) / 20000 = 0.5 (50%)
• The underlying allele frequency did not change.
• The population will remain stable over time, with the same genotype frequencies.

Hardy-Weinberg Equilibrium

• The Hardy-Weinberg equilibrium describes the stable genotype frequencies in a population when certain conditions are met.

• If p is the frequency of the dominant allele and q is the frequency of the recessive allele, then the Hardy-Weinberg equilibrium can be expressed as: (p + q)^2 = p^2 + 2pq + q^2

  • p^2 = the frequency of the homozygous dominant genotype
  • 2pq = the frequency of the heterozygous genotype
  • q^2 = the frequency of the homozygous recessive allele
    This will give you the genotype frequency of the population.

• This equilibrium is maintained if:

  • There is no natural selection (i.e., no fitness advantage for any allele).
  • There is a large population.
  • There is random mating relative to this allele.

Deviations from Hardy-Weinberg Equilibrium

• When populations are not in Hardy-Weinberg equilibrium, it indicates:
  • The population is not large.
  • There is natural selection occurring.

Bottleneck Effect

• A small number of individuals populate a new area, this is called a bottleneck effect. Genotype frequency can change radically.

Example: Disease Allele

• In a country with 350 million people, suppose 99% of a gene is the dominant allele and 1% is the recessive allele.
• If the gene is in Hardy-Weinberg equilibrium, the expected percentage of homozygous recessive individuals is 0. 01 * 0.01 = 0.0001 (0.01%).
• This means there should be approximately 35,000 homozygous recessive individuals.
• If no homozygous recessive individuals are found, it suggests that there is natural selection against the recessive allele, possibly due to a catastrophic disease state.
• In this scenario, all recessive alleles are in heterozygotes, and the likelihood of two heterozygotes meeting and having affected offspring is small unless inbreeding occurs.

Forms of Natural Selection

Directional Selection

• One extreme of a continuously variable trait has a fitness advantage.
• Over time, the population shifts towards that extreme.

Stabilizing Selection

• The extremes of a continuously variable trait are disfavored, and the median is favored.
• Over time, the population clusters around the average.

Disruptive Selection

• The extremes of a continuously variable trait are favored, and the median is disfavored.
• Example: Biological sex, where gametes are either few and large (eggs) or many and small (sperm), with nothing in between being advantageous.

Sexual Selection

• Sexual selection can lead to extreme and ridiculous features in organisms.
• One animal (typically the female) has a mutation that compels them to evaluate only one trait when choosing a mate.
• If that trait is correlated with better overall health, it is advantageous for the female.
• Once this preference is established, the size of the trait becomes the most important feature, leading to a positive feedback loop and the evolution of exaggerated features.
• Examples: Elk antlers (which are resource-intensive to build) and peacock tails (which hinder survival).