Darwin's theory posits that evolution occurs through survival of the fittest, where individuals possessing advantageous, heritable traits are more likely to survive and reproduce. Furthermore, Darwin suggested that all life shares a common descent, tracing back to a Last Universal Common Ancestor (LUCA).
Defining a Species: Three Major Concepts
As evolutionary theory matured, biologists faced a challenge: how do we define a "species"? Because organisms vary and evolve, scientists use different frameworks depending on the available data.
- Biological Species Concept: This is the most common definition, stating that a species is a group of populations that can interbreed to produce fertile offspring and are reproductively isolated from other groups.
- Morphological Species Concept: This defines species based on shared physical traits, such as shape, size, or skeletal structure. This is particularly vital in paleontology, where scientists cannot observe the reproductive behavior of fossils.
- Phylogenetic Species Concept: This defines a species as the smallest identifiable evolutionary lineage that shares a common ancestor.
Exam Tip: If a question asks which species concept is most useful for studying dinosaur fossils, the answer is the Morphological Species Concept, because genetic and reproductive data are rarely preserved in stone.
The Engines of Change: Evolutionary Mechanisms
Evolution is not a single process but the result of several distinct evolutionary mechanisms that cause genetic changes in populations over generations:
- Mutation: A fundamental change in the DNA sequence of a gene, creating new genetic variation.
- Natural Selection: The process where advantageous traits become more common in a population because they aid survival and reproduction.
- Genetic Drift: Random fluctuations in the frequency of certain traits (alleles) within a population, often due to chance events.
- Gene Flow: The movement of genes between different populations, typically occurring when individuals migrate and breed in a new area.
Systematics: Organizing the Tree of Life
To make sense of the vast diversity of life, scientists use Systematics, the study of organismal diversity and evolutionary relationships. Systematics is divided into two main branches:
- Taxonomy: The science of naming, describing, and classifying organisms.
- Phylogeny: The study of the evolutionary history and relationships among species.
Using Characters to Determine Relationships
To reconstruct the "tree of life," systematists compare various characters (observable or measurable features). These can include:
- Morphological characters: Physical form, such as tooth shape or body size.
- Anatomical characters: Internal structures, like bone arrangement or organ placement.
- Molecular characters: DNA, RNA, or protein sequences.
- Physiological characters: How an organism functions, such as metabolism or temperature regulation.
- Behavioural characters: Traits like mating rituals or feeding habits.
When comparing these characters, it is critical to distinguish between homologous characters, which are derived from a common ancestor, and analogous characters, which appear similar due to independent evolution (convergent evolution) rather than shared ancestry.
The End of Lineages: Extinction
Evolution is not just about the emergence of new life, but also the disappearance of the old. Extinction occurs when a species or taxon totally disappears from Earth.
Extinction can be categorized by its scale:
- Local Extinction: A species disappears from a specific area but survives elsewhere.
- Ecological Extinction: A species survives but its numbers are so low it no longer plays a significant role in its ecosystem.
- Biological Extinction: The total disappearance of a species from the entire planet (e.g., Dinosaurs or Mammoths).
On a geological timescale, scientists distinguish between Background Extinction (the normal, continuous rate of species loss) and Mass Extinction (the global loss of a large number of species in a short period). Earth has experienced several "Big Five" mass extinction events, such as the End-Permian (the largest in history, killing 96% of marine species) and the Cretaceous-Paleogene event (linked to an asteroid impact that ended the reign of the dinosaurs).
