Bioscience 2_Chapter 18_JKele

Chapter 18: Evolution and Origin of Species

• Course: BIOL 122: Bioscience II

• Instructor: Joseph Kele, Ed.Dc., M.S.

• Term: Spring 2025

• Source: Adapted from Openstax Biology 2e

Section 18.1: Understanding Evolution

• Learning Outcomes:

  • Describe development of present-day theory of evolution.

  • Define adaptation.

  • Explain convergent and divergent evolution.

  • Describe homologous and vestigial structures.

  • Discuss misconceptions about evolution theory.

Charles Darwin

• Foundations of Natural Selection:

  • Developed the framework of natural selection.

  • Postulates:

    • Most characteristics are heritable (passed from parent to offspring).

    • More offspring are produced than can survive (competition for resources).

    • Characteristics of offspring vary and these variations are heritable.

    • Adaptation occurs through natural selection.

Charles Darwin’s Observations

• Observed that finch beak shapes varied among species.

  • Proposed that ancestral beaks adapted to different food sources over time.

Natural Selection vs. Artificial Selection

• Focus: Natural selection as the primary mechanism for evolution.

• Artificial Selection:

  • Humans have practiced for thousands of years through selective breeding of crops and domesticated animals.

Homologous Structures

• Similar appendage structures across different species indicate a common ancestor:

  • Examples: Human, dog, bird, whale.

Convergent vs. Divergent Evolution

• Convergent Evolution:

  • Two distinct species share traits not due to a common ancestor.

    • Examples: Birds, butterflies, bats.

• Divergent Evolution:

  • Two or more species diverge from a common ancestor.

    • Example: Wolves and dogs.

Gene Flow vs. Genetic Drift

• Gene Flow:

  • Exchange of genes between populations through migration.

  • Occurs in larger populations.

• Genetic Drift:

  • Random events or sampling that influence allele frequencies.

  • Occurs more significantly in smaller populations.

  • Causes: Bottleneck effect or founder effect.

Speciation

• Concept: Speciation refers to the process of one species splitting into two species.

  • Explains shared features among organisms reflecting common ancestry.

Microevolution & Macroevolution

• Microevolution:

  • Changes in allele frequencies within a population over time.

• Macroevolution:

  • Large-scale evolutionary changes above the species level including origin of new groups (e.g., mammals).

Learning Outcomes Section 18.2: Understanding Evolution

• Learning Outcomes:

  • Define species and methods for identifying them.

  • Describe genetic variables leading to speciation.

  • Identify prezygotic and postzygotic reproductive barriers.

  • Explain allopatric and sympatric speciation.

  • Describe adaptive radiation.

Species Definition

• A species consists of groups of populations whose members can interbreed in nature; they produce viable, fertile offspring.

• Members are reproductively compatible and do not interbreed with other groups.

  • Example: All human beings belong to the same species (Homo sapiens).

Biological Species Concept

• Based on reproductive compatibility, not physical similarity.

• Example: Eastern and western meadowlarks.

Speciation Overview

• Allopatric Speciation:

  • Involves geographic isolation causing divergence.

  • Types: Dispersal and vicariance.

• Sympatric Speciation:

  • Occurs in the same geographic area through mechanisms like polyploidy, habitat differentiation, and sexual selection.

Reproductive Isolation

• New species emerge through reproductive isolation, involving biological barriers preventing interbreeding and hybrid viability.

• Barriers limit gene flow between species.

Barriers to Reproductive Isolation

• Prezygotic Barriers:

  • Impede mating attempts, prevent successful mating, or hinder fertilization.

• Postzygotic Barriers:

  • Affect viability or reproductive success of hybrid embryos.

Types of Reproductive Isolation

• Prezygotic Barriers:

  • Temporal, habitat, and behavioral isolation.

• Postzygotic Barriers:

  • Hybrid inviability and hybrid sterility.

Alternative Species Definitions

• Morphological Species Concept (MSC):

  • Species distinguished by body shape and structural features.

• Ecological Species Concept (ESC):

  • Species defined by ecological niches and interactions.

Allopatric Speciation

• Occurs when gene flow is interrupted due to geographic isolation.

  • Examples: Divergence of species in separated lakes or river systems.

Example of Allopatric Speciation

• Kaibab Squirrel:

  • Geographical isolation in the Grand Canyon led to distinctive traits.

Sympatric Speciation

• Occurs in populations within the same area due to reduced gene flow.

  • Influenced by polyploidy, habitat differentiation, and sexual selection.

Detailed Mechanisms of Sympatric Speciation

• Polyploidy: Increased chromosome number due to cell division errors (common in plants).

• Aneuploidy: Abnormal chromosome numbers in gametes due to nondisjunction.

• Allopolyploid: Viable offspring from mating between two species.

Hybrid Zones

• Occur when species with incomplete barriers meet and can produce hybrid offspring.

Outcomes of Hybrid Zones

1. Reinforcement: Strengthening of reproductive barriers reducing hybrid formation.

2. Fusion: Weakening of barriers leading to a merging of species.

3. Stability: Hybrid individuals continue to be produced successfully.

Rates of Speciation

• Punctuated Model: Rapid changes during speciation followed by long periods of stability.

• Gradual Model: Changes occur slowly and steadily over time.