19darwin-2023

DARWINIAN EVOLUTION

Introduction

Darwinian evolution refers to the theory proposed by Charles Darwin that fundamentally changed the way we understand biological life and its development over time.

Learning Objectives

By the end of this study, you should be able to:

• Define Darwinian Evolution.

• Understand the concept of selection.

• Explain natural selection, sexual selection, and evolutionary fitness.

• Describe different types of selections and their impacts on populations.

What Did Darwin Do?

Charles Darwin's contributions to evolutionary biology created a significant shift in perspective about the life forms populating Earth. His key contributions include:

• Formulating a unifying principle of biology and illustrating that species are not immutable but rather are subject to change over time.

• Providing natural selection as the mechanism of change and presenting robust evidence supporting evolution.

• Integrating various forms of evidence including fossil records and anatomical homologies to show evolutionary processes.

Evolution: Theory & Process

Theory: Descent with Modification

The theory of evolution encompasses several foundational ideas:

• Biological populations undergo changes over successive generations.

• All life evolved from a common ancestor, suggesting a interconnected history.

• The history of life can be visualized as a branching tree representing the diversity of organisms that have existed.

Homology

Homologous traits are similar features arising from a shared common ancestor, supporting the concept of descent with modification.

Developmental Evidence

During early developmental stages, organisms (particularly vertebrates) exhibit similarities that reflect their common ancestry, further validating the theory of evolution.

Vestigial Organs

Vestigial structures are remnants of organs that have lost most or all of their ancestral function, providing evidence of evolutionary change.

The Fossil Record

The fossil record serves as a critical source of historical biological data, showcasing life forms throughout geological time.

The Process or Mechanism: Natural Selection

Natural selection is a vital mechanism of evolution:

• It is a gradual and non-random process through which biological traits become more or less common based on differential reproductive success (fitness).

Propositions Underlying Darwin's Theory

Darwin's theory of natural selection includes several key propositions:

1. More individuals are produced than can survive.

2. This results in competition for resources (a struggle for existence).

3. Variation exists among individuals within a species.

4. Individuals with advantageous traits have a higher survival and reproductive success.

5. Favorable traits accumulate over generations.

6. Environmental changes can make new traits advantageous, leading to evolution.

Evolution through Natural Selection: Case Study

The Peppered Moth

This case illustrates natural selection:

• The population of moths showed a shift in coloration from light to dark due to predation on the lighter-colored individuals against darker backgrounds, demonstrating natural selection in action.

Modes of Selection

Stabilizing Selection

Maintains the status quo by favoring average phenotypes.

Directional Selection

Shifts the population toward one extreme phenotype.

Diversifying Selection

Encourages variation by favoring multiple extremes.

Other Types of Selection

Sexual Selection

Sexual selection involves the development of traits that improve reproductive success, leading to:

• Intersexual Selection: Traits that make individuals attractive to the opposite sex.

• Intrasexual Selection: Traits that help in competition with same-sex individuals.

Artificial Selection

Artificial selection is the intentional breeding for specific traits:

• Used as an analogy for natural selection.

• Key in domestication and veterinary practices (e.g., fancier pigeons, horses).

What Do We Know About Selection?

Examples of Natural Selection

1. Sickle Cell Anemia: In regions where malaria is prevalent, individuals with sickle cell anemia have a selective advantage because the malaria parasites cannot survive in their altered blood cells.

2. Deer Mice: Coloration in Nebraska Sand Hills evolved due to changes in the environment influencing camouflage against predation.

3. Snowflake Yeast: Laboratory experiments showed significant increases in size through environmental selection over thousands of generations.

4. COVID-19: Adaptive genetic changes were observed in a study of individuals related to infection responses, indicating natural selection in human populations.

5. Cichlid Fishes: Rapid evolution of multiple species in Lake Victoria within 15,000 years illustrates the effects of natural selection on adaptive radiation.

Individual Reflection Questions

1. What are the significant conflicts between teleology and Darwinian evolution?

2. How does artificial selection support Darwin's overall thesis?

3. Is natural selection inherently random? Discuss.

4. Does evolution have a specific objective or goal? Why or why not?

5. How does the mechanism of natural selection challenge the idea of perfection in evolution?