Evidence of Evolution

Introduction

Evolution is a central concept in biology, explaining the diversity of life on Earth. Scientists have gathered extensive evidence supporting evolution through multiple approaches, filling gaps in Darwin's original theory. The main methods for studying evolutionary evidence include direct observation, homology, fossil records, and biogeography.

Direct Observation of Evolutionary Change

Introduction

Direct observation allows scientists to witness evolutionary changes as they occur in populations, often in response to environmental pressures or human activities.

• Natural Selection: The process by which certain traits become more common in a population due to differential survival and reproduction. Natural selection edits existing traits rather than creating new ones.

• Rapid Evolution: Evolution can occur quickly, especially in species with short generation times or strong selective pressures.

• Example: The evolution of beak length in the soapberry bug in response to changes in food sources, and the emergence of MRSA (Methicillin-resistant Staphylococcus aureus) as a result of antibiotic selection.

Key Points:

• Local environments determine which traits are favored or disfavored.

• Natural selection is not a creative force; it acts on existing variation.

Homology

Introduction

Homology refers to similarities among organisms due to shared ancestry. These similarities can be anatomical, embryological, or molecular.

Anatomical Homology

• Definition: Structures in different species that are similar due to inheritance from a common ancestor.

• Example: The forelimbs of humans, cats, whales, and bats share the same bone arrangement (humerus, radius, ulna, carpals, metacarpals, phalanges) but are adapted for different functions.

• Vestigial Structures: Remnants of features that served important functions in ancestors but are no longer functional (e.g., human tailbone).

Embryological Homology

• Definition: Similarities in embryonic development among different species, indicating common ancestry.

• Example: The presence of pharyngeal arches and post-anal tails in both chick and human embryos.

Molecular Homology

• Definition: Similarities in DNA sequences or genes among different organisms.

• Pseudogenes: Nonfunctional segments of DNA that resemble functional genes, inherited from ancestors.

• Example: The GULO gene for vitamin C synthesis is nonfunctional in primates, making them dependent on dietary vitamin C.

Analogy vs. Homology

Introduction

Not all similarities among organisms are due to common ancestry. Some arise from similar environmental pressures, leading to analogous structures.

• Homologous Traits: Similar due to shared ancestry (e.g., forelimbs of mammals).

• Analogous Traits: Similar due to convergent evolution, not common ancestry (e.g., wings of birds and insects).

• Example: The wings of dragonflies and birds are analogous, as they evolved independently for flight.

Convergent Evolution

Introduction

Convergent evolution occurs when unrelated species evolve similar traits independently, often due to similar environmental challenges.

• Example: Marsupial sugar gliders in Australia and eutherian flying squirrels in North America both developed gliding adaptations, despite being distantly related.

Fossil Record

Introduction

The fossil record provides chronological evidence of evolutionary change, showing how species have transformed over time.

• Transitional Fossils: Fossils that show intermediate states between ancestral forms and their descendants.

• Example: Cetaceans (whales, dolphins, porpoises) evolved from even-toed ungulates, as shown by shared ankle bone structures (the astragalus) in fossils.

Biogeography

Introduction

Biogeography is the study of the geographic distribution of species. It provides evidence for evolution by showing how species are distributed in relation to historical events like continental drift.

• Endemic Species: Species found only in specific geographic areas, often due to isolation.

• Island Biogeography: Islands often have unique species closely related to those on nearby continents, suggesting colonization and subsequent adaptation.

• Example: The adaptation of mainland species that colonize islands, leading to the evolution of new endemic species.

Review Questions

• How are vestigial structures explained by evolution?

• Contrast homologous and analogous traits.

• Give an example of a human trait homologous to a lizard trait.

• Give an example of a human trait analogous to an octopus trait.

• What would happen to a population of brown mice if their environment turned green?

• Why might a trait advantageous in one habitat not be in another?