Gene Flow, Speciation, and Rates of Evolution
Gene Flow and Speciation
Definition of Species: A species consists of multiple populations, and almost all species are divided into distinct populations to some extent.
Role of Gene Flow: Gene flow acts as a unifying force, holding these populations together within a single species.
Mechanism of Speciation: New species are formed when gene flow between populations is interrupted, regardless of the specific process causing the interruption.
Divergence Factors: Once gene flow is interrupted, each isolated population is subjected to its own unique:
Random fluctuations in survival and reproduction.
Natural selection pressures.
Sexual selection pressures, including the evolution of female mate choice preferences.
Diane Dodd's Fruit Fly Experiment (A Classic)
Objective: To experimentally test the hypothesis that isolating populations and allowing divergent evolutionary pressures will lead to reproductive isolation and speciation.
Experimental Design:
A single population of fruit flies was split into two separate groups.
One group was fed a starch-based food source.
The other group was fed a maltose-based food source.
These food sources represented novel environmental pressures, driving different natural and sexual selection.
Duration: The experiment ran for generations.
This is a very short duration in biological terms (e.g., the Earth is billion years old, and life has existed for billion years).
Short enough to be completed by graduate students.
Results:
Within just generations, a clear pattern of separation emerged.
Reduced Gene Flow: Starch-fed females began to preferentially mate with starch-fed males, and maltose-fed females preferred maltose-fed males.
This reduction was significant, leading to a decrease in gene flow between the two groups.
Conclusion:
The experiment demonstrated that natural selection, genetic drift, and sexual selection can radically decrease gene flow, pushing populations toward reproductive isolation.
While the two groups had not yet evolved into distinct species, the experiment showed the crucial initial steps of macroevolution in a controlled setting.
Allopatric Speciation
Definition: Speciation that occurs when populations are geographically separated by a physical barrier, preventing gene flow.
Traditional Model: This is often considered the typical and most easily understood model of speciation.
Often involves geological events (e.g., formation of mountain ranges, valleys, or separation of landmasses into islands).
Example: Snapping Shrimp (Alpheus) of the Isthmus of Panama
Observation: There are pairs of sister species (each other's closest relatives) of snapping shrimp, with one species from each pair living on the Atlantic side and the other on the Pacific side of the Isthmus of Panama.
Genetic Analysis: Studies comparing morphological and genetic traits (e.g., genetic fingerprinting) revealed that these sister species are very closely related genetically.
Divergence Timing: By analyzing the genetic divergence and known mutation rates, scientists inferred that all these species pairs diverged at roughly the same time, approximately tens of millions of years ago.
Geological Correlation: This timing perfectly matches the geological data for the formation of the Isthmus of Panama.
Process: An ancestral population of shrimp was separated by the rising Isthmus.
The two separated populations then experienced different natural selection, sexual selection, patterns of mutation, and genetic drift.
Over millions of years, they diverged to the point where they are now reproductively isolated and cannot interbreed.
Significance: This is a classic illustration of allopatric speciation, demonstrating how a physical barrier leads to the blocking of gene flow and subsequent speciation into distinct lineages.
Sympatric Speciation
Definition: Speciation that occurs between populations inhabiting the same geographical area, without a physical barrier to gene flow.
Historical View: Previously thought to be a rare event during the lecturer's undergraduate years (a