Rucete ✏ Campbell Biology In a Nutshell
Unit 5 THE EVOLUTIONARY HISTORY OF BIOLOGICAL DIVERSITY — Concept 26.4 An Organism’s Evolutionary History Is Documented in Its Genome
The genome of an organism—its complete set of DNA—preserves a detailed record of its evolutionary history. By comparing genomes and gene sequences across species, scientists can uncover deep evolutionary relationships, trace gene duplications, and better understand how life has diversified.
Molecular Data and Evolutionary Relationships
Comparisons of DNA and protein sequences allow scientists to reconstruct phylogenies, especially when fossil evidence is limited or absent.
Molecular analysis can reveal relationships that morphological data alone cannot detect—for example, between animals and fungi.
Genome comparisons help clarify relationships among living species by identifying orthologous genes (same gene in different species) and paralogous genes (duplicated genes within a species).
Orthologous vs. Paralogous Genes
Orthologous genes:
Result from speciation events.
Found in different species but retain the same function.
Example: cytochrome c gene in humans and dogs.
Useful for inferring phylogenies since their divergence reflects speciation history.
Paralogous genes:
Result from gene duplication within the same species.
Can evolve new functions or expression patterns over time.
Example: olfactory receptor genes in humans (380 functional) vs. mice (1,200), providing a wide range of smell detection.
Genome Evolution
Two major patterns emerge when comparing whole genomes:
Shared Orthologous Genes:
Even distantly related species share many orthologous genes.
Example: Humans and mice share ~99% of their genes; ~50% of human genes are shared with yeast.
This explains why diverse organisms share similar biochemical and developmental pathways.
Gene Number and Organismal Complexity:
More complex organisms don’t necessarily have more genes.
Humans have only ~4× more genes than yeast, despite far greater complexity.
Versatility of human genes explains this: many human genes encode multiple proteins through mechanisms like alternative splicing and differential regulation in different tissues.
Functional Implications
Shared genes across species enable researchers to study human gene functions using simpler model organisms like yeast or mice.
Investigating gene families reveals how evolution repurposes genetic material to generate new traits and adapt to new environments.
In a Nutshell
An organism’s genome holds a molecular record of its evolutionary past. By comparing genetic sequences, scientists can reconstruct phylogenies, distinguish orthologous and paralogous genes, and uncover surprising connections between species. Genome analysis reveals both deep ancestry and mechanisms behind biological complexity.