Meditaliano IMAT Prep

Part 1: The Mechanisms of Evolution

1.1 Sources of Genetic Variation

Natural selection can only act on existing variation. This variation is the raw material for evolution. The ultimate source of all new alleles is mutation, a random change in the nucleotide sequence of DNA. In sexually reproducing organisms, most of the variation that is acted upon in each generation comes from the recombination of existing alleles during meiosis (crossing over and independent assortment) and fertilization.

1.2 Darwin's Theory of Natural Selection

The core of Darwin's theory is natural selection, the process where individuals with certain inherited traits tend to survive and reproduce at higher rates than other individuals because of those traits. This leads to adaptation, the accumulation of favorable traits in a population over generations. Natural selection is the only mechanism that consistently causes adaptive evolution.

Modes of Natural Selection

Natural selection can alter the frequency distribution of heritable traits in three ways:

• Directional Selection: Favors individuals at one extreme of the phenotypic range. Common when a population's environment changes or when members of a population migrate to a new habitat.
• Stabilizing Selection: Favors intermediate variants and acts against extreme phenotypes. This mode of selection reduces variation and tends to maintain the status quo for a particular phenotypic character.
• Disruptive Selection: Favors individuals at both extremes of the phenotypic range over intermediate variants. This can lead to the formation of two distinct subgroups in a population.

1.3 Other Mechanisms of Evolution

While natural selection is a major driver of adaptive evolution, it is not the only mechanism that changes allele frequencies.

• Genetic Drift: Chance events can cause allele frequencies to fluctuate unpredictably from one generation to the next, especially in small populations. This is a random, non-adaptive process.
• Gene Flow: The transfer of alleles into or out of a population due to the movement of fertile individuals or their gametes. Gene flow can reduce genetic differences between populations and can also introduce new alleles to a population.

Part 2: Evidence for Evolution

The theory of evolution is one of the best-supported theories in science, with evidence from many different fields.

• The Fossil Record: Fossils provide a physical history of life on Earth, showing that past organisms differed from present-day organisms and documenting the emergence of new groups.
• Homology and Convergent Evolution:
  • Homologous Structures: Features shared by related species due to common ancestry. These structures may have been modified for different functions in different species (divergent evolution).
  • Analogous Structures: Features that share a similar function but not common ancestry. They arise due to convergent evolution, where similar environmental pressures and natural selection produce similar adaptations in organisms from different evolutionary lineages.
• Vestigial Structures: Remnants of features that served important functions in the organism’s ancestors.
• Biogeography: The geographic distribution of species. The presence of closely related species in a specific geographic area suggests they evolved from a common ancestor in that location.
• Molecular Biology: The universal genetic code (DNA and RNA) and the similarity of DNA sequences and proteins in related species provide powerful evidence for a common ancestor.

Part 3: The Classification of Life

3.1 Hierarchical Taxonomy and Binomial Nomenclature

Taxonomy is the science of naming and classifying organisms. The Linnaean system uses a hierarchy of increasingly inclusive categories. The major levels are:

Domain → Kingdom → Phylum → Class → Order → Family → Genus → Species

The system uses binomial nomenclature to give each species a unique, two-part scientific name. The first part is the Genus (capitalized), and the second is the specific epithet (lowercase). For example, humans are Homo sapiens.

3.2 The Three Domains and Major Kingdoms

All of life is grouped into three Domains: Bacteria, Archaea, and Eukarya. Phylogenetic trees are used to illustrate the evolutionary relationships among groups of organisms.

Part 4: Interactive Quiz