Problem 12 - Campbell Biology Concepts & Connections

Question

Which of the following factors would not contribute to allopatric speciation? a. A population becomes geographically isolated from the parent population. b. The separated population is small, and genetic drift occurs. c. The isolated population is exposed to different selection pressures than the parent population. d. Gene flow between the two populations continues to occur.

Step-by-Step Solution

Answer

d. Gene flow between the two populations continues to occur.

Step 1: Understand Allopatric Speciation

Allopatric speciation occurs when a population is divided by a geographical barrier, leading to reproductive isolation and eventually the formation of separate species.

Step 2: Identify Key Factors

Key factors that contribute to allopatric speciation include geographical isolation, genetic drift in a small population, and different selection pressures on the separated population.

Step 3: Evaluate Each Option

a. Geographical isolation definitely contributes to allopatric speciation.b. A small population experiencing genetic drift also contributes to it.c. Exposure to different selection pressures helps drive speciation.d. Continuous gene flow between populations prevents speciation by allowing interbreeding.

Step 4: Identify the Correct Answer

Option d (Gene flow between the two populations continues to occur) would not contribute to allopatric speciation, as it prevents the formation of distinct species.

Key Concepts

Geographical Isolation

Geographical isolation is a crucial element in allopatric speciation, where a population is divided by a physical barrier such as a mountain, river, or distance. This physical separation limits or stops gene flow between groups. Over time, each isolated group experiences different evolutionary forces, like mutations and natural selection.

Without geographical isolation, populations would continue to mix, and new species would not form. The geographical barrier ensures that the populations do not interbreed, which is vital for the diversification into separate species.

Examples include island populations and separated habitats due to natural disasters or human activities, like urban development.

Genetic Drift

Genetic drift refers to changes in the frequency of alleles (variants of a gene) within a population due to random sampling. In small, isolated populations resulting from geographical isolation, genetic drift can significantly impact genetic variation.

Because the population size is small, random events can change allele frequencies more drastically than in larger populations. This unpredictability can lead to the fixation or loss of alleles over generations, contributing to the genetic divergence necessary for allopatric speciation.

For instance, if a small, geographically isolated population has a rare allele that becomes common due to chance, this can create genetic differences from the original population over time.

Selection Pressures

Selection pressures are environmental factors that influence which individuals in a population are more likely to survive and reproduce. In allopatric speciation, different isolated populations may be subjected to varying selection pressures due to differing local environments.

This results in each population adapting uniquely to their specific conditions. These adaptations can lead to significant differences in traits, behaviors, and even mating preferences between the populations.

Examples of selection pressures include climate changes, availability of food, predators, and competition for resources. Over time, these pressures can drive the evolution of unique characteristics in separated populations, further propelling the process of speciation.

Reproductive Isolation

Reproductive isolation occurs when two populations can no longer interbreed and produce fertile offspring. This is a crucial step in allopatric speciation since it ensures that the separated populations develop into distinct species.

Reproductive isolation can be prezygotic (before fertilization) or postzygotic (after fertilization). Prezygotic barriers include behavioral differences, mechanical isolation, and temporal isolation. Postzygotic barriers include hybrid inviability and hybrid sterility.

For example, if two populations of birds evolve different mating calls, they might not recognize each other as potential mates, leading to prezygotic isolation. Over time, even if geographical barriers are removed, these reproductive barriers can maintain the separation of species.