Marine discussion 4

Evolution of Littorina obtusata

Overview

• The study reports rapid shell morphology changes in the intertidal snail Littorina obtusata from 1871 to 1984 in northern New England.

• Shells prior to 1900 were high-spired with thin walls; shells from 1982-84 are low-spired with thick walls.

• The introduction of the crab Carcinus maenas around 1900 likely drove this change in shell form.

Key Findings

• Predation Pressure: Experiments demonstrated that high-spired L. obtusata are more susceptible to predation by Carcinus compared to low-spired forms.

• Morphological Differences: High-spired shells are thinner and more exposed, making them more vulnerable to crab attacks.

• Genetic Evidence: Electrophoretic analysis confirms that the high- and low-spired forms are variations within the same species (Nei's D = 0.003), not distinct species.

• Rapid changes in morphology can occur without speciation, supporting classical Darwinian selection.

Theoretical Context

• Discussion of punctuated equilibrium in fossil records highlights the disagreement among biologists regarding the mechanisms driving morphological changes.

• Some argue that rapid changes in fossils represent speciation events, while others suggest natural selection can drive these transitions.

• Recent models indicate that genetic selection processes can potentially explain rapid morphological evolution without invoking new mechanisms.

Experimental Methodology

• Shells of L. obtusata were collected from historical and contemporary populations across various sites in northern New England.

• Measurements of spire height, shell thickness, shell width, and shell height were taken to assess morphological variations and vulnerabilities to predation.

• Field experiments were conducted to evaluate survival rates under differing crab abundances, showing lower survival rates for high-spired forms.

• Laboratory tests assessed the crabs' ability to crush the shell types, revealing higher vulnerability among high-spired shells.

Results Summary

• Changes in shell morphology were statistically significant:

  • At Nahant, relative spire height decreased significantly over time, paralleling increases in crab population density.

  • Survival rate was notably higher for low-spired individuals (57%) at sites with high crab abundance compared to only 14% for high-spired individuals.

• Genetic similarity between high and low-spired populations indicates an intraspecific evolutionary response rather than a shift to a new species.

Discussion

• Findings strongly support the hypothesis that morphological changes in L. obtusata were adaptations stemming from predation by Carcinus.

• The observed evolutionary response aligns with models predicting rapid transitions driven by environmental changes.

• Future investigations should continue exploring the balance between natural selection and morphological adaptations across different marine species.

Implications for Evolutionary Biology

• The study reinforces the notion that significant evolutionary changes can occur within short time frames under intense selection pressures.

• Understanding these dynamics enhances insights into the historical patterns of evolution and their respective ecological contexts.