Evolution: Evidence and Mechanisms in General Biology

Evolution

Introduction to Evolution

Evolution is the central unifying theory in biology, explaining both the diversity and unity of life on Earth. The concept was popularized by Charles Darwin in his 1859 publication On the Origin of Species by Natural Selection, and independently developed by Alfred Russel Wallace. Evolution describes how organisms change over time through descent with modification from common ancestors.

• Descent with modification: The process by which species change over generations, giving rise to new species while retaining traits from their ancestors.

• Common ancestor: All living organisms share distant ancestors, accounting for both diversity and unity among species.

• Accumulated changes: Evolution occurs through gradual accumulation of genetic and phenotypic changes over time.

Example: The diversity of mammals, birds, and reptiles can be traced back to common vertebrate ancestors.

Fossil Evidence

Definition and Formation of Fossils

Fossils are the preserved remains or traces of ancient organisms, typically older than 10,000 years. They provide direct evidence of past life and evolutionary changes.

• Petrification: The process where organic tissue is replaced by minerals, commonly seen in plants (e.g., petrified wood).

• Mold fossils: Formed when an organism is buried in sediment, decays, and leaves an impression that is later filled with minerals.

• Covered in sediments: Organisms buried by sediment are compacted over time, aiding fossilization.

Example: Ammonite fossils and petrified wood are classic examples of fossilized remains.

Importance of Fossil Record

The fossil record documents the appearance, evolution, and extinction of species over geological time. It allows scientists to reconstruct evolutionary relationships and timelines.

• Relative dating: Fossils are dated based on the sedimentary rock layers in which they are found; older layers are deeper.

• Transitional fossils: Fossils showing intermediate forms between groups, such as whales evolving from terrestrial mammals.

• Historical record: Fossils provide evidence for gradual changes and adaptation in lineages.

Example: Fossil whales show vestigial hind limb bones, indicating their terrestrial ancestry.

Other Sources of Evidence for Evolution

Biogeography

Biogeography studies the geographic distribution of species and how it relates to evolutionary history.

• Pangaea: About 200 million years ago, all continents formed a single landmass called Pangaea, which later split into Laurasia and Gondwana, and eventually into present-day continents.

• Distribution of species: Species that evolved before continental drift are found on multiple continents (e.g., lungfish species in Africa, South America, and Australia).

Example: Fossil lungfish are found on all continents, supporting their ancient origin before continental separation.

Comparative Anatomy

Comparative anatomy examines structural similarities among organisms, providing evidence for common descent.

• Homologous structures: Anatomical features with similar structure due to shared ancestry, but possibly different functions (e.g., mammal forelimbs in humans, cats, and whales).

• Analogous structures: Features with similar function but different evolutionary origins.

Example: The basic skeletal elements of mammal forelimbs are similar, despite adaptations for walking, swimming, or flying.

Comparative Embryology

Comparative embryology reveals similarities in early developmental stages among related organisms, indicating common ancestry.

• Pharyngeal slits: Present in vertebrate embryos, develop into gill arches in fish or throat structures in terrestrial vertebrates.

• Post-anal tail: Found in embryos of all vertebrates, though it may be lost or modified in adults.

Example: Vertebrate embryos (fish, birds, mammals) show similar early development, diverging as they mature.

Molecular Biology

Molecular Evidence for Evolution

Molecular biology provides strong evidence for evolution through genetic similarities and universal biological mechanisms.

• Universality of the genetic code: All organisms use the same four nucleotides (A, T, G, C) to encode genetic information, forming 64 codons ($4^3 = 64$), with 61 coding for amino acids.

• Protein sequence comparison: Similarities in amino acid sequences of proteins (e.g., hemoglobin) reflect evolutionary relationships.

• Master control genes (homeotic genes): Highly conserved genes that regulate development, found across diverse eukaryotes.

Example: Human and gorilla hemoglobin differ by only 8 amino acids, while lamprey hemoglobin differs by 125, indicating closer evolutionary relationship between humans and gorillas.

The Genetic Code Table

The genetic code is a set of rules by which information encoded in genetic material (DNA or RNA) is translated into proteins by living cells.

Codon	Amino Acid	Function
AUG	Methionine	Start codon
UAA, UAG, UGA	---	Stop codons
UUU, UUC	Phenylalanine	Amino acid
GAA, GAG	Glutamic acid	Amino acid
AAA, AAG	Lysine	Amino acid
UGG	Tryptophan	Amino acid
Additional info: ...	Other codons	Encode other amino acids

Protein Sequence Comparison Table

Comparing protein sequences, such as hemoglobin, helps determine evolutionary relationships.

Species	Hemoglobin AA Difference from Human	Percentage Difference
Gorilla	8	5%
Lamprey	125	86%
Human	0	0%

Summary

Multiple lines of evidence—including fossils, biogeography, comparative anatomy and embryology, and molecular biology—support the theory of evolution. These sources collectively demonstrate how life has diversified and adapted over time, providing a framework for understanding biological unity and diversity.