D4.1 Natural Selection [IB Biology SL/HL]

Natural Selection: Understanding the Basics

Introduction to Natural Selection

• The video discusses standard level content on natural selection, highlighting a significant shift in biological understanding as proposed by Darwin.

• Natural selection is described as a continuous process responsible for Earth's biodiversity.

Requirements for Natural Selection

Variation

• Variation is essential for natural selection; it arises from three main sources: mutations, meiosis, and sexual reproduction.

• Mutations create new alleles, while meiosis generates genetic diversity through processes like crossing over and independent assortment.

Overproduction of Offspring

• Overproduction occurs when more offspring are produced than can survive, leading to competition for resources.

• Intraspecific competition (competition within a species) arises due to limited resources such as space and mating opportunities.

Competition and Fitness

Abiotic Factors as Selection Pressures

• Abiotic factors (non-living elements), classified into density-independent (e.g., temperature, natural disasters) and density-dependent factors (e.g., resource availability), influence natural selection.

• Density-independent factors affect populations regardless of their size, while density-dependent factors become more critical with increased population density.

Defining Fitness

• Fitness refers to heritable traits that enhance survival or reproduction within an ecological niche. It does not equate to physical strength but rather genetic advantages.

• Higher fitness increases the likelihood of survival and reproduction, contributing more genes to the gene pool.

Heritability vs. Acquired Traits

• Only heritable traits impact evolutionary change; acquired traits do not alter DNA sequences in gametes and thus cannot be passed down.

• For example, tattoos may provide individual advantages but do not affect offspring genetics.

Sexual Selection as a Biotic Factor

Understanding Sexual Selection in Birds and Guppies

Heritable Traits and Courtship Behaviors

• The discussion centers on heritable traits that provide advantages in sexual selection, particularly focusing on courtship behaviors observed in birds.

• An example is given of a bird of paradise, which showcases exaggerated plumage as a genetic trait that enhances its mating success during courtship.

• These traits become increasingly pronounced over generations as individuals with advantageous characteristics are more likely to attract mates and reproduce.

Case Study: John Endler's Guppy Experiments

• John Endler conducted experiments on guppies, revealing that female guppies prefer males with vibrant coloration, which plays a significant role in sexual selection.

• However, high coloration also increases vulnerability to predators; thus, the environment influences whether coloration or lack thereof is favored. In predator-rich areas, less colorful males were selected for survival.