Genetics and Evolution Notes

Evolution

  • Evolutionary thought has a relatively short history, with theories suggesting new species arise from older ones proposed in the 19th century.
  • Significant alterations to original theories have been made since then.

Natural Selection

  • Natural selection, or "survival of the fittest," proposes that certain heritable traits allow individuals to have greater reproductive success.
  • This theory was proposed by Charles Darwin in his 1859 publication On the Origin of Species.
    • Tenets of Darwin's Theory:
      • Organisms produce many offspring, but few survive to reproductive maturity.
      • Chance variations exist within individuals in a population and may be heritable.
      • Favorable variations give a survival advantage.
      • Individuals with favorable variations are more likely to survive and reproduce.
      • The result is an increase in these traits in future generations.
  • Fitness:
    • Reproductive success.
    • Related to the relative genetic contribution of an individual to the next generation.
  • Darwin's theory has been proven correct in many ways but has also been updated with modern genetics.

Modern Theories

  • Modern Synthesis Model (Neo-Darwinism):
    • Combines Darwin's original theory with knowledge of genetic inheritance and gene pool changes.
    • Inheritance occurs through genes passed from parent to child.
    • Genes change due to mutation or recombination.
    • When mutation or recombination results in a change that is favorable to the organism's reproductive success, that change is more likely to pass on to the next generation.
    • This process is termed differential reproduction.
    • Over time, traits passed on by more successful organisms become common in the gene pool.
    • Populations evolve, not individuals.

Inclusive Fitness

  • Shift in scope to inclusive fitness over individual organism fitness.
  • Inclusive Fitness:
    • A measure of an organism's success based on:
      • Number of offspring.
      • Success in supporting offspring.
      • Ability of offspring to support others.
    • Early theories focused only on the number of viable offspring.
    • Contemporary theories consider the benefit of behaviors on the population.
      • Altruism can be supported by the observation that close relatives of an individual will share many of the same genes, thus promoting the reproduction and survival of related or similar individuals can also lead to genetic success.
      • Other species show examples of inclusive fitness by protecting the offspring of the group at large. By endangering themselves to protect the young, their organisms ensure the passing of genes to future generations.
      • Inclusive fitness promotes altruistic behavior that can improve the success of a species.

Punctuated Equilibrium

  • Proposed by Niles Eldridge and Stephen J. Gould in 1972.
  • Explains the fossil record showing long periods of little evolution followed by bursts of change.
  • Suggests changes in some species occur in rapid bursts rather than evenly over time, contrasting with Darwin's theory.

Modes of Natural Selection

Stabilizing Selection

  • Keeps phenotypes within a specific range by selecting against extremes.
  • Example: Human birth weight (w) is maintained within a narrow band.
    • Fetuses weighing too little may not be healthy enough to survive (w << 1).
    • Fetuses weighing too much can experience trauma during delivery (w >> 1).
    • Larger fetus weights require more resources from the mother (birth weights within a narrow range is advantageous).

Directional Selection

  • Adaptive pressure leads to the dominance of an initially extreme phenotype.
  • Example: Antibiotic resistance in bacteria.
    • A heterogeneous plate of bacteria, treated with ampicillin, an antibiotic, exhibit resistance in only some colonies. Those colonies that exhibit resistance to this antibiotic will survive.
    • Differential survivorship leads to a new standard phenotype.
  • The emergence of mosquitoes resistant to DDT (dichlorodiphenyltrichloroethane) is attributed to directional selection.

Disruptive Selection

  • Two extreme phenotypes are selected over the norm.
  • Example: Darwin's finches on the Galapagos Islands.
    • Finches had either large or small beaks but no medium-sized beaks.
    • Seed sizes (food for finches) were either large or small, requiring large or small beaks, respectively.
    • Animals with slightly larger or smaller beaks would be selected for over time.
  • Facilitated by polymorphisms (naturally occurring differences) such as light and dark coloration in butterflies.

Adaptive Radiation

  • Rapid rise of different species from a common ancestor.
  • Allows species to occupy different niches.
  • Niche:
    • A specific environment including: habitat, available resources, and predators.
  • Favored by environmental changes or isolation of small groups of the ancestral species.

Speciation

  • Species:
    • The largest group of organisms capable of breeding to form fertile offspring.
  • Speciation:
    • The formation of a new species through evolution.
    • If two populations from the same species are geographically separated, different evolutionary pressures lead to different adaptive changes.
    • Over time, changes become sufficient to lead to reproductive isolation.

Reproductive Isolation

  • Progeny of separated populations can no longer freely interbreed.
  • Two groups are considered separate species.
    • Prezygotic Mechanisms: Prevent zygote formation.
      • Temporal isolation.
      • Ecological isolation (different niches).
      • Behavioral isolation (lack of attraction due to differences in pheromones, courtship displays, etc.).
      • Reproductive isolation (incompatibility of reproductive anatomy).
      • Gametic isolation (intercourse occurs, but fertilization does not).
    • Postzygotic Mechanisms: Allow gamete fusion but yield nonviable or sterile offspring.
      • Hybrid inviability (zygote cannot develop to term).
      • Hybrid sterility (hybrid offspring cannot reproduce).
      • Hybrid breakdown (F1 generation hybrids are viable and fertile, but F2 generation hybrids are inviable or infertile).
  • Example: Mules (horse and donkey) are an example of hybrid sterility.
    • The mule will be sterile and thus unable to establish a self-perpetuating mule lineage.

Patterns of Evolution

  • Similarities between species can be due to shared ancestry or shared environments.
  • Divergent Evolution:
    • Independent development of dissimilar characteristics in two or more lineages sharing a common ancestor.
    • Seals and cats are both in the order Carnivora but differ in appearance as they live in different environments and adapt to different selection pressures.
  • Parallel Evolution:
    • Related species evolve in similar ways in response to analogous environmental selection pressures.
  • Convergent Evolution:
    • Independent development of similar characteristics in two or more lineages not sharing a recent common ancestor.
    • Fish and dolphins resemble each other physically despite belonging to different classes of vertebrates, adapting to aquatic life.

Measuring Evolutionary Time

  • Evolution is a slow process involving changes in genotype and phenotype over time.
  • Rate of evolution is measured by the rate of change of a genotype, related to the severity of evolutionary pressures.
  • Slow evolution occurs when a species is well-suited to its habitat with no changes in conditions.
  • Faster evolution occurs in rapidly changing environments where selection actively occurs.
  • DNA sequence comparison quantifies the degree of similarity between organisms.
    • Chimpanzees share over 95% of their genome with humans.
    • Mice share about 70%.
    • As species become more taxonomically distant, the proportion of shared genome decreases.

Molecular Clock Model

  • Molecular evolutionists correlate genomic similarity with time since species diverged from a common ancestor.
  • The more similar the genomes, the more recently the species separated.

Conclusion

  • Genetics and evolution are vital in medicine due to the rise of antibiotic-resistant bacteria.
  • Antibiotic stewardship appropriately uses antibiotics only when necessary to preserve effectiveness.
  • Understanding that environmental pressures lead to directional selection in microorganisms is crucial for preserving the effectiveness of antibiotics.