Detailed Notes on Natural Selection and Related Topics

Unit Overview

Unit 7: Natural Selection

Lesson 1: Natural Selection

Lesson 2: Population Genetics

Lesson 3: Hardy Weinberg Equilibrium

Lesson 4: Evidence of Evolution & Common Ancestry

Lesson 5: Phylogeny

Lesson 6: Speciation & Extinction

Lesson 7: Origins of Life on Earth

Lesson 1: Natural Selection

Aim: What causes natural selection and how does it affect populations?

Key Concepts:

• Variation is essential in populations, providing the raw material for natural selection.
• Adaptations: Inherited characteristics that enhance survival; these traits arise from genetic variations and can lead to improved reproductive success.
• Overproduction of Offspring leads to competition for limited resources, increasing the pressure on individuals to survive based on their traits.
• Selection: Beneficial adaptations allow certain individuals to survive and reproduce, leading to changes in allele frequency over generations, which can result in evolutionary change.

Charles Darwin

Background:

• Notable research at the Galapagos Islands illustrated significant diversity in species.
• Focused on biogeography, the geographic distribution of species; observed how similar species adapted to different environments, contributing to his understanding of natural selection.
• Developed the hypothesis that species colonized new areas (e.g., Galapagos) and diversified to form new species, laying the groundwork for the theory of evolution.

True/False Statements about Natural Selection

1. Natural selection gives organisms what they need

   • FALSE: It acts on existing genetic variation, not on needs. Adaptations occur through random mutations rather than as responses to environmental demands.

2. Natural selection acts for the good of a species

   • FALSE: It works on traits beneficial for survival; traits can be harmful to a species overall, as in the case of traits that lead predators to overconsume prey, potentially driving prey species towards extinction.

3. Natural selection involves organisms trying to adapt

   • FALSE: It is based on random mutations leading to genetic variations; organisms do not consciously adapt but rather the most fit traits become more common due to reproductive success.

4. Survival of the fittest

   • FALSE: The fittest refers to reproductive success, not necessarily health or strength. A weaker individual may survive due to a favorable trait that allows it to reproduce more effectively.

Evolution

Definition:

• Change in the genetic makeup of a population over time; descent with modification highlights how species change due to environmental pressures and genetic variations.

The Theory of Natural Selection

Foundational Observations:

1. Traits are heritable; they can be passed from parent to offspring, providing the basis for evolution.
2. More offspring are produced than can survive, creating competition and a selection process where favorable traits become more common in the gene pool.

Implications for Evolution:

• Populations evolve, not individuals; evolution is a gradual change in the population's genetics over generations, as individuals do not change within their lifetimes.
• Favorable traits accumulate due to differential survival; advantageous traits can lead to greater reproductive success, influencing the population's evolution over time.

Modes of Natural Selection

1. Directional Selection:

   • Favors one phenotype; shifts frequency toward favored phenotype (e.g., peppered moths demonstrating color variations based on industrial pollution).

2. Stabilizing Selection:

   • Favors average phenotypes; extremes are selected against to maintain a balanced trait distribution (e.g., robins laying approximately four eggs to maximize offspring survival).

3. Disruptive Selection:

   • Favors extreme phenotypes over intermediate ones, which can lead to the formation of two distinct species with different adaptations.

Artificial Selection

Definition:

• Selective breeding by humans to enhance desirable traits in domesticated plants and animals, which can lead to significant changes in characteristic traits.

Impact:

• Helps in understanding natural selection's principles by showcasing how selective pressures can lead to dramatic physical and behavioral changes in short timeframes.

Population Genetics

Relevant Areas:

• Changes in allele frequencies over time due to:
  • Mutations introducing new genetic variations.
  • Genetic drift causing random changes in allele frequencies, especially in small populations.
  • Migration leading to gene flow between populations, altering allele distributions.
  • Natural selection favoring beneficial alleles.

Gene Pool:

• Genetic makeup of a population (all alleles present), providing a framework to study genetic variation within populations.

Microevolution:

• Small-scale changes resulting from these processes, leading to adaptation and potential speciation.

Hardy-Weinberg Equilibrium

Purpose:

• Determines if a population is evolving; serves as a null hypothesis for evolution, indicating a stable genetic distribution under specific conditions.

Equations:

• p + q = 1
• p^2 + 2pq + q^2 = 1

Conditions for Equilibrium:

1. No mutations.
2. Random mating.
3. No natural selection.
4. Large population size (no genetic drift).
5. No gene flow between populations.

Evidence of Evolution

Types of Evidence:

• Fossil record documenting historical species and gradual changes across geological time.
• Morphological comparisons revealing similarities and differences among species.
• Biogeography showing how geographical barriers influence the distribution of species and their evolution.

Comparative Morphology:

• Homologous structures indicate common ancestry, pointing to evolutionary relationships despite differing functions (e.g., forelimbs of mammals exhibit structural similarities).

Phylogeny

Phylogenetic Trees and Cladograms:

• Visual representations of evolutionary relationships that depict the lineage of species and their common ancestors through branching diagrams.

Use of DNA, proteins, homologous structures in constructing phylogenetic trees enhances accuracy in understanding evolutionary relationships.

Key Concepts:

• Synapomorphy (derived traits) help reveal relationships between different species; understanding monophyletic vs. paraphyletic vs. polyphyletic groups is crucial in classifying organisms correctly.

Speciation & Extinction

Speciation Types:

• Allopatric: Geographic isolation where populations evolve separately, leading to the emergence of new species.
• Sympatric: New species arise within the same geographic area through mechanisms such as niche differentiation without physical barriers.

Extinction:

• Factors leading to extinction include environmental change, habitat loss, and human impacts, which may open niches for adaptive radiation and the evolution of new species in response to selective pressures.

Origins of Life on Earth

Hypotheses:

• Research indicates life began from simple organic molecules on primordial Earth, leading to the first forms of life.
• Miller-Urey experiments showed potential for life's building blocks to form under early Earth conditions simulated in a laboratory setting.

RNA World Hypothesis:

• Suggests RNA was the first genetic material due to its ability for replication and catalyzing chemical reactions, providing a