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, such as South American jungles, grasslands, and the Galápagos Islands.
Darwin noted:
Species in temperate regions of South America resembled those in tropical South America more closely than European species.
Fossils closely resembled living organisms in the same area.
Unique species of Galápagos Islands resembled mainland species, suggesting descent with modification from ancestral species.
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 to different environments.
Example: Galápagos finches evolved different beak shapes suited to different foods, demonstrating adaptation-driven evolution.
Natural Selection: Darwin's Mechanism for Evolution
Darwin formulated natural selection, based on two observations and two inferences:
Observation #1: Individuals within populations vary in inherited traits.
Observation #2: Species produce more offspring than the environment can support, many of which fail to survive and reproduce.
Inference #1: Individuals with favorable inherited traits have higher survival and reproduction rates.
Inference #2: This unequal reproductive success leads to the accumulation of favorable traits over generations.
Key Features of Natural Selection
Individuals do not evolve; populations evolve.
Natural selection acts only on heritable variations.
Traits favorable in one environment may not be favorable in another; natural selection depends on environmental context.
Over time, natural selection can lead to new species adapted to different environments.
Artificial Selection as Evidence for Natural Selection
Darwin used artificial selection (human-guided breeding) as a familiar example, highlighting how rapid selection can dramatically alter species over short periods.
Example: Domestic plants (wild mustard selectively bred into broccoli, cabbage, kale, Brussels sprouts) demonstrate selection’s potential power.
Darwin’s Influence from Malthus
Darwin was influenced by economist Thomas Malthus, who argued that human populations outgrow resources, resulting in competition and struggle for survival.
Darwin recognized that this principle applied broadly to all species, influencing his understanding 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.