Introduction to Evolution
Introduction to Evolution: Evidence and Mechanisms
The Centrality of Evolution in Biology
Theodosius Dobzhansky's Quote: "Nothing in biology makes sense except in the light of evolution."
This highlights evolution as the unifying principle that explains the diversity and unity of life.
Evidence for Evolution
1) Universally Shared Features
DNA Replication: All known life forms utilize DNA as their genetic material and replicate it through a highly conserved process.
The process involves an original DNA template, which is unwound by helicase at the replication fork.
DNA polymerase adds free nucleotides (Adenine, Thymine, Cytosine, Guanine) to synthesize new leading and lagging strands.
(A)denine pairs with (T)hymine, and (C)ytosine pairs with (G)uanine, forming the double helix with a phosphate backbone.
Cell Cycle: The fundamental process of cell division is conserved across eukaryotes.
Interphase: A period of growth and DNA replication.
G Phase: Cell grows and performs normal functions.
S Phase: DNA replication occurs (DNA synthesis).
G Phase: Cell prepares for mitosis.
M Phase (Cell Division):
Prophase: Chromosomes condense and the spindle forms.
Metaphase: Chromosomes align at the cell equator.
Anaphase: Sister chromatids separate and move to opposite poles.
Telophase: Chromosomes de-condense, nuclear envelopes reform.
Cytokinesis: Cytoplasm divides, forming two daughter cells.
G Phase (Resting Phase): Cells perform specialized functions without dividing.
"Frozen Accident" of the Genetic Code: The shared biochemistry of life, particularly the genetic code, is largely arbitrary.
Any code that maps the possible triplet codons onto the amino acids could work and could be implemented just as easily by an appropriate set of transfer RNAs.
The universality of this arbitrary code suggests a single origin that became "frozen" or fixed early in life's history.
2) Hierarchical Classification
Organisms are classified into a nested hierarchy of groups within groups (e.g., Species within Genera, Genera within Families, etc.).
This arrangement is explained by "descent with modification", reflecting shared ancestry.
Taxonomic Ranks (from most inclusive to least inclusive):
Kingdom: Animals (organisms able to move on their own).
Phylum: Chordates (animals with a backbone).
Class: Mammals (chordates with fur or hair and milk glands).
Order: Primates (mammals with collar bones and grasping fingers).
Family: Hominids (primates with relatively flat faces and three-dimensional vision).
Genus: Homo (hominids with upright posture and large brains).
Species: Homo sapiens (member of the genus Homo with a high forehead and thin skull bones).
Homology: Biological similarity due to common ancestry.
Example: Vertebrate Forelimbs: Despite different functions, the skeletal structure of forelimbs in humans, cats, whales, and bats shows a conserved pattern (single bone Humerus, two bones Radius and Ulna, followed by Carpals, Metacarpals, and Phalanges).
Embryonic Similarity: Early embryonic stages of diverse organisms often exhibit remarkable similarities, which diverge as development proceeds.
Arthropod Examples: Early embryos of Insecta, Arachnida, and Myriapoda share common developmental patterns.
Vertebrate Examples: Early tailbud embryos of lamprey, dogfish, gar, salmon, lungfish, axolotl, hellbender, snake, chicken, possum, cat, bat, and human show striking resemblances, which become distinct in intermediate and late stages.
Molecular Homology (Homeobox Genes):
Homeobox (Hox) genes are a group of related genes involved in the developmental patterning of the body axis (anterior-posterior).
They dictate the identity of body segments (e.g., Head, Thorax, Abdomen).
Specific genes (e.g., lab, pb, Dfd, Scr, Antp, Ubx, abd-A, Abd-B in Drosophila; Hox to Hox in humans) are homologous across vast evolutionary distances.
The most recent common ancestor of a fly and a human existed approximately Million years ago (Mya), yet these genes and their organizational clusters are conserved.
Analogy: The result of convergent evolution, where similar traits evolve independently in different lineages due to similar environmental pressures, not common ancestry.
3) Geographic Distribution
Principle: Geographic proximity, not ecological similarity, is the strongest predictor of relatedness.
Example 2: Wallace's Line: This biogeographical boundary separates the Indomalayan and Australasian Realms.
It marks a deep-water channel that maintained a barrier to species dispersal even during periods of lower sea levels when landmasses like Sunda, Wallacea, and Sahul were larger.
Consequently, organisms on either side of the line, despite similar climates (e.g., tropical rainforests and savannas), have vastly different evolutionary histories and fauna (e.g., placental mammals vs. marsupials).
4) Direct Observation of Evolution
Artificial Selection: Humans selectively breed organisms for desired traits, rapidly driving evolutionary change.
Natural Selection in Action (HIV Resistance to Ritonavir):
Step 1: Treatment with the antiviral drug ritonavir begins.
Step 2: The viral load of drug-sensitive HIV falls rapidly as these viruses are suppressed or killed.
Step 3: However, drug-resistant HIV strains (e.g., exhibiting mutations in the HIV protease enzyme: PQITLWQRPLVTVKIGGQLREALLDTGADDTVLEDINLPGKWKPKMIGGIGGFIKVKQYEQVLIEICG KKAIGTVLVGPTPVNIIGRNMLTQIGCTLNF) continue to grow despite the presence of the treatment. Over time, the growth of these resistant viruses can cause the viral load to rise again in the host.
Polyploidy: A mechanism of rapid speciation, particularly in plants.
Process: A hybrid (F) formed from Species 1 (with X chromosomes) and Species 2 (with Y chromosomes) may undergo genome duplication, resulting in a new species with or chromosomes.
Outcome: This new polyploid species cannot successfully interbreed with either of the parental species, thus establishing reproductive isolation and creating a new species almost instantly.
5) Fossil Record
Provides a historical sequence of life, demonstrating continuous evolutionary change over vast periods.
Geological Time Scale: Groups that have diverged more recently should appear later in the fossil record.
Archean Eon: Earth forms billion years ago.
Proterozoic Eon: billion years ago.
Paleozoic Era:
Mesozoic Era:
Cenozoic Era:
6) Ancient DNA (aDNA)
Genetic material recovered from ancient remains (fossils, bones, mummified tissues) provides direct molecular evidence of past evolutionary events.
Example: Human Ancestry: aDNA studies have revealed complex admixture events between modern humans, Neanderthals, and Denisovans.
Two distinct waves of Denisovan admixture have been detected in East Asian genomes.
This genetic exchange likely contributed to adaptations, such as the ability to thrive at high altitudes.