Chapter 15, Problem 35A

Summary Introduction

To explain:

The non- external mechanism that Darwin proposed as a cause of variation.

Introduction: In Darwin’s time, most naturalists considered only external conditions as causes of variation but Darwin believed that there are some other causes as well that might be causing variation in organisms. Darwin gave a new theory about the origin of species.

Answer to Problem 35A

Natural selection is a mechanism that Darwin proposed as a cause of variation.

Explanation of Solution

Many contemporaries of Charles Darwin believed that external conditions of environment such as climate, food etc played a major role in causing variation in organisms. Darwin believed that while this may be true to some extent but there are some non- external factors also which cause variations.

Darwin proposed that there are some genetic variations in organisms and these variations are heritable. Because of these variations organisms have different phenotypes but they also resemble their parents. These variations can be transferred to the offspring.

Charles Darwin gave a theory of natural selection. He argued that if enough time is given, natural selection could modify a population to produce new species. There are four principles of this theory.

1. Individual in a population show differences called variations.
2. Variations can be inherited; i.e they can be passed down from parent to offspring.
3. Organisms produce more offspring than can survive on available resources.
4. Variations that increase reproductive success will have a greater chance of being passed on than those that do not increase reproductive success.

Later Darwin compiled evidence for evolution and explained how natural selection might provide a mechanism for development of new species.