Chapter 15, Problem 36A

Summary Introduction

To explain:

The non- external mechanism that Darwin proposed in the way modern scientists would explain

Introduction: 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.

Answer to Problem 36A

Natural selection is a non- external mechanism that Darwin proposed as a cause of variation. Modern scientists can explain this phenomenon in terms of genes and amino acid sequences. They can explain how genes are responsible for variety of phenotypes, how genes are inherited from parents to offspring and how alterations in amino acid sequences can produce new species.

Explanation of Solution

Darwin compiled evidence for evolution and explained how natural selection might provide a mechanism for development of new species. He considered natural selection as the non- external mechanism for variation.

Modern scientists would describe the non- external mechanism in terms of genetics. Today scientists know that the genes of an organism are responsible for their phenotypes. So they would describe variations in terms of the genetic makeup of an organism. Different organisms have different genetic makeup. The genes code for proteins which are responsible for the variety of phenotypes that appear in a population of organisms. Darwin said that the variations are inheritable. Modern scientists would describe these in terms of genes. They would describe that genes are passed down from parents to offspring and would even identify the particular DNA sequence that is responsible for a particular trait in the organism.

Modern scientists also compared the biochemical patterns in DNA and RNA of many organisms and found that similarities in these molecules across species. This suggested that the molecules were passed down from ancient life forms to modern day life forms.

Scientists today can explain how new species can evolve through mutations, genetic drift, gene flow and nonrandom mating.