Chapter 15, Problem 1PDT

Summary Introduction

To describe: The comparison and differences between Haeckel’s and Von Baer’s ways of thinking about changes in development over the course of animal evolution.

Introduction: Biologists from the nineteenth and twentieth centuries compared and described embryonic development of a diverse group of animals. They also described the ways in which the various morphogenetic processes in different groups of organisms result in different adult forms.

Explanation of Solution

Among the many first things, the biologists who were studying developmental biology learned that the embryos of many species are more often similar than the adults. Both Karl Von Baer and Ernst Haeckel studied embryo development to know more about the developmental pattern of organisms. In 1882, Karl von Baer noted that those features that are common to higher taxon appear early than those characters of lower tax such as families or order. According to Baer’s law, embryo development passes through the developmental stages of their adults. Haeckel also explained that the development of an organism, during the course of evolution, pass through all its adult form’s developmental stages.

Following are some differences in the way of thinking of these two biologists about changes in an organism’s development over the course of evolution:

Summary Introduction

To explain: The learning about phylogeny from development, even though Haeckel’s dictum seldom holds.

Introduction: Biologists from the nineteenth and twentieth century compared and described embryonic development of a diverse group of animals. They also described the ways in which the various morphogenetic processes in different groups of organisms result in different adult forms.

Explanation of Solution

Ernst Haeckel, a German biologist, interpreted developmental patterns to mean that an individual organism’s development repeats the evolutionary history of adult forms of their ancestors. It is known as “ontogeny recapitulates phylogeny”.

However, by the end of the 19^th century, it was clear that Haeckel’s dictum seldom holds. For example, embryos of mammal and reptile go through a development stage where they have pharyngeal clefts and branchial arches identical to fish supporting that they share a common ancestor with fish. However, mammals and reptiles never acquire the characters of typical adult fish.

Nevertheless, these developmental studies provided information about the evolutionary relationship between different organisms. By studying embryo development, biologists identified many common developmental difference patterns among species.