Rucete ✏ Campbell Biology In a Nutshell
Unit 4 MECHANISMS OF EVOLUTION — Concept 22.2 Descent with Modification by Natural Selection Explains the Adaptations of Organisms and the Unity and Diversity of Life
Darwin's theory of evolution through natural selection offers a powerful explanation for how organisms adapt, why they share certain characteristics (unity), and why they differ from one another (diversity). It underscores that species descend from common ancestors and adapt to their environments over generations.
Darwin’s Observations and the Origin of His Theory
- During the voyage of HMS Beagle (1831–1836), Darwin observed diverse adaptations of plants and animals in different environments.
- Darwin noted:
- Species in temperate South America resembled those in tropical South America more than European species.
- Fossils resembled living organisms in the same region.
- Galápagos species resembled mainland species, suggesting descent with modification.
Darwin’s Focus on Adaptation
- Adaptations: inherited traits that enhance survival and reproduction in specific environments.
- Darwin concluded that new species could arise through gradual accumulation of adaptations.
- Example: Galápagos finches evolved different beak shapes suited to different foods.
Natural Selection: Darwin's Mechanism for Evolution
Darwin formulated natural selection based on two observations and two inferences:
- Observation #1: Individuals vary in inherited traits.
- Observation #2: Species produce more offspring than the environment can support.
- Inference #1: Individuals with advantageous traits tend to survive and reproduce more.
- Inference #2: Favorable traits accumulate in the population over generations.
Key Features of Natural Selection
- Individuals do not evolve; populations evolve.
- Natural selection acts only on heritable traits.
- Environmental context determines which traits are favorable.
- Over time, this can lead to new species adapted to different niches.
Artificial Selection as Evidence for Natural Selection
- Darwin used artificial selection (selective breeding) to illustrate how traits can change over time.
- Example: Wild mustard bred into cabbage, broccoli, kale, and Brussels sprouts.
Darwin’s Influence from Malthus
- Thomas Malthus argued that populations grow faster than resources, leading to struggle and competition.
- Darwin applied this principle to all species, helping develop the idea of natural selection.
In a Nutshell
Darwin's theory explains life's unity and diversity through "descent with modification" by natural selection. Natural selection acts on heritable variation within populations, leading to adaptations that increase organisms' survival and reproductive success. This theory, supported by evidence from artificial selection, fossils, and biogeography, transformed biological understanding profoundly.