Lizards in an Evolutionary Tree

Topic Overview

• Goal: Develop an understanding of both the diversity and unity of life on Earth (Genes, Evolution & Ecology).

• Emphasis on integrating evolutionary concepts with ecological context to explain how biodiversity is generated and maintained.

• External resource provided: Origin of species and lizards evolutionary tree activities (e.g., biointeractive classroom resources).

Case Study: Lizards in an Evolutionary Tree

• Focus organism: Caribbean anole lizards (genus Anolis).

• Visuals/films referenced: Lizards in an Evolutionary Tree (Losos, UC Press, 2011).

• Concept explored: How different body plans (ecomorphs) evolve repeatedly in separate islands, illustrating convergent evolution and phylogenetic relationships.

Key Concepts: Ecomorphs and Ecological Niches

• What is an ecomorph?

  • A group defined by the combination of body form and habitat use that is suited to a particular ecological niche.

  • In the Caribbean anoles, six distinct ecomorphs have been documented across islands.

• Relevance: Demonstrates how similar ecological pressures can lead to similar adaptations in separate lineages (convergent evolution).

• Ecological niches: Anoles occupy diverse microhabitats (tree trunks, branches, leaves, twigs, ground, grasses), each associated with characteristic body plans.

The Six Anole Ecomorphs: Details and Table

• Purpose of Table 1: Summarize morphological traits and habitats that distinguish ecomorphs.

• Note: The same six ecomorphs are listed across multiple pages, demonstrating consistency in data.

• Ecomorphs and traits:

  • Crown-giant

  • Body length: $130-191\text{ mm}$

  • Limb length: short

  • Toe pad lamellae: Large

  • Tail length: Long

  • Color: Usually green

  • Habitat: High trunks and branches

  • Trunk-crown

  • Body length: $44-84\text{ mm}$

  • Limb length: short

  • Toe pad lamellae: Very Large

  • Tail length: Long

  • Color: Green

  • Habitat: Trunks, branches, leaves

  • Trunk

  • Body length: $40-58\text{ mm}$

  • Limb length: Intermediate

  • Toe pad lamellae: Intermediate

  • Tail length: Short

  • Color: Gray

  • Habitat: Trunks

  • Twig

  • Body length: $41-80\text{ mm}$

  • Limb length: Very short

  • Toe pad lamellae: Small

  • Tail length: Short

  • Color: Gray

  • Habitat: Narrow twigs

  • Trunk-ground

  • Body length: $55-79\text{ mm}$

  • Limb length: Long

  • Toe pad lamellae: Intermediate

  • Tail length: Long

  • Color: Brown

  • Habitat: Lower trunk and ground

  • Grass-bush

  • Body length: $33-51\text{ mm}$

  • Limb length: Long

  • Toe pad lamellae: Intermediate

  • Tail length: Very Long

  • Color: Brown

  • Habitat: Bush and grasses

Ecological Niches and Adaptive Variation

• Figure 3 illustrates that anoles occupy a variety of ecological niches and have evolved adaptations that enable success in different parts of trees, grasses, and bushes.

• Key implication: Divergent selection across microhabitats drives morphological diversification (ecomorph formation).

Observations of Two Anole Species (Hypothesis-Driven Discussion)

• Habitat differences:

  • Species A: High trunks and branches; Body length $130-191\text{ mm}$; Limb length: short; Toe-pad size: Large; Tail length: Long; Color: Green.

  • Species B: Lower trunk and ground; Body length $55-79\text{ mm}$; Limb length: Long; Toe-pad size: Intermediate; Tail length: Long; Color: Brown.

• Discussion prompts:
  1) Describe the differences between the two species of anoles.
  2) Formulate hypotheses to explain why each difference may have evolved (e.g., microhabitat use, predator pressures, foraging strategy).
  3) Describe an experiment that would test one of these hypotheses.

• This exercise demonstrates how comparisons across species and habitats can reveal patterns of adaptation and potential selective pressures.

The Caribbean Island Experiment: A Natural Laboratory

• Experimental setup described in the film and accompanying notes:

  • Nearby Caribbean islands used as laboratory settings.

  • Hurricanes had previously removed Anoles from these islands, leaving only a few individuals behind.

  • Researchers re-visited the islands after about one year to observe changes.

• Purpose: Investigate how ecomorphs arise and diversify under real ecological conditions and limited initial populations.

The Results (Overview)

• The activity includes multiple site data collections (e.g., Twig, Canopy, Grass habitats across Puerto Rico, Cuba, Hispaniola, Jamaica).

  • Examples noted: Twig-Puerto Rico; Canopy-Puerto Rico; Grass-Puerto Rico; Twig-Cuba; Twig-Hispaniola; Twig-Jamaica; Canopy-Cuba; Grass-Cuba; Canopy-Hispaniola; Grass-Hispaniola; Canopy-Jamaica; Twig-Jamaica; Grass-Jamaica; Grass-Puerto Rico; etc.

• Interpretation emphasis: The twig lizard on Puerto Rico is described as the ancestral form, with canopy and grass lizards evolving subsequently. This supports a model where body types evolved repeatedly and independently on each island, rather than all sharing a single, common recent ancestor for each body type across islands.

• Important note: The figure presents several hypotheses about how body types arose and dispersed across islands; students are asked to identify the correct description of the phylogenetic relationships.

• In addition, there is a discussion prompt asking students to select which statements best describe the phylogenetic trees shown (A–D) and why.

Phylogenetic Interpretations: Exercises from the Figure (Page 16)

• Prompt: The figure presents two trees illustrating possible evolutionary histories for Caribbean anoles; students are asked to select the pair of statements that accurately describe the phylogenetic trees.

• Statements (summarized):

  • A) The twig lizard on Puerto Rico evolved first and is the ancestor of all other lizards; the twig lizard evolved first on all islands, followed by canopy and grass lizards.

  • B) Body types evolved repeatedly and independently on each island; different body types evolved once, then spread to different islands.

  • C) Different body types evolved only once, and then populations with those body types ended up on different islands.

  • D) Puerto Rico is the origin of all three body types.

• Students are asked to select the pair that accurately describes the trees in the figure (exact correct options depend on the figure). The exercise emphasizes understanding of parallel evolution, shared ancestry, and island-specific diversification.

Most Likely Evolutionary Hypothesis and Explanation (Question)

• Question: Which tree in the figure most plausibly describes how the different Anolis species evolved on the Caribbean Islands?

• Task: Explain the reasoning behind choosing the particular tree, linking observed morphological diversity to island-specific selective pressures and historical contingency.

• Pedagogical goal: Reinforce concepts of convergent evolution, phylogenetic inference, and the role of geographic isolation in diversification.

Conclusions and Core Takeaways

• Key conclusion: Lizards on each island are more closely related to each other than to similar lizards on other islands.

• Implication: The same general ecomorph types evolved independently on each island (convergent evolution), driven by similar ecological niches and selective pressures.

• Broader message: Biodiversity results from layered, interconnected processes and interactions over evolutionary time; Darwin’s principles continue to underpin modern understanding of these processes.

• Course framing: BIOL 214 will review and expand on Darwin’s work to develop a deeper understanding of the processes that generate and unify life on Earth.

Connections to Foundational Principles and Real-World Relevance

• Foundational principles: Natural selection, adaptive radiation, convergent evolution, phylogenetics, ecological niche differentiation, island biogeography.

• Real-world relevance: Island studies offer clear, tractable systems to observe evolution in action; insights apply to biodiversity, conservation, and understanding how species adapt to changing environments.

• Ethical/philosophical considerations: Using natural disturbances (e.g., hurricanes) as experimental analogs raises questions about reliance on natural events as sources of data; nonetheless, such natural experiments can reveal fundamental evolutionary processes without direct manipulation of ecosystems at scale.

Mathematical and Analytical Notes

• Evolutionary patterns are often described in terms of phylogenetic trees and character evolution (e.g., body type, habitat usage).

• Although exact numerical models are not provided in this transcript, the study underlines the use of comparative morphology across lineages to infer evolutionary pathways and timings.

• Key quantitative data presented (examples):

  • Crown-giant body length: $130-191\,\text{mm}$

  • Trunk-crown body length: $44-84\,\text{mm}$

  • Trunk body length: $40-58\,\text{mm}$

  • Twig body length: $41-80\,\text{mm}$

  • Trunk-ground body length: $55-79\,\text{mm}$

  • Grass-bush body length: $33-51\,\text{mm}$

• Other qualitative descriptors are not numeric but are critical for distinguishing ecomorphs (e.g., limb length categories: short, intermediate, long).

Connections to Previous and Later Content

• This module builds on Darwinian thinking (natural selection, common descent) and connects to broader topics in evolution and ecology.

• The activity sets up a framework for discussing how similar selective pressures across islands yield recurrent evolutionary outcomes, reinforcing the unity of life’s principles despite diversity of form and habitat.

Practical Takeaways for Exam Preparation

• Be able to define and explain ecomorphs and give examples from the six Caribbean anole ecomorphs with their key traits and habitats.

• Understand how island biogeography and hurricane disturbance can create natural experimental conditions for studying adaptive radiation.

• Explain convergent evolution using the anole examples: similar body plans arise in different islands due to similar ecological niches, not due to identical ancestry.

• Interpret phylogenetic scenarios: recognize that multiple plausible tree configurations can describe observed patterns and know how to evaluate which configuration best explains the data (based on shared ancestry vs. repeated evolution).

• Be prepared to discuss how the study’s findings support or refine Darwinian principles and what this implies for biodiversity and conservation in island ecosystems.

Quick Reference: Terminology

• Ecomorph: A morphological and ecological category defined by habitat use and corresponding body form.

• Convergent evolution: Independent evolution of similar traits in distinct lineages due to similar selective pressures.

• Anolis genus: Group of lizards used as a model system for studying adaptive radiation and phylogenetics in the Caribbean.

• Phylogenetic tree: A diagram showing evolutionary relationships among species or lineages.

Summary of Core Points

• The Caribbean anoles demonstrate how different ecological niches lead to distinct ecomorphs with predictable morphological traits and habitat associations.

• Islands provide a natural laboratory for observing repeated evolution in response to isolation and ecological opportunity.

• The observed patterns support the idea that similar selective pressures yield similar adaptations across independent lineages, reinforcing key evolutionary principles.