Principles of Evolution

WEB TUTORIAL 16.1 Principles of Evolution

Text Sections Section 16.1 Evolution and Its Core Principles, p. 253 Introduction Charles Darwin provided great insight to the theory of evolution based on two principles: common descent with modification and natural selection. Descent with modification explains the changes in a common ancestral species that have occurred over time which led to the development of new species. The driving force of this theory is that of natural selection. Organisms that are genetically more fit than others will pass on those genetic traits at a higher frequency in the next and subsequent generations. This leads to changes in the species over time. There are several types of biological evidence to support the theory of evolution. Learning Objectives • Summarize the principles underlying the theory of evolution. • Know what is meant by “common descent with modification.” • Understand how natural selection shapes evolution.

Narration

Principles of Evolution Charles Darwin deserves primary credit for the theory of evolution. He developed existing ideas about descent with modification, and he was the first person to per- ceive that natural selection is the primary force behind evolution.

In the theory of evolution, descent with modification describes the process by which species of living things can undergo modification over time, with such change sometimes resulting in the formation of new, separate species. All species on Earth have descended from other species and a single, common ancestor lies at the base of the evolutionary tree.

It did not take long for common descent with modification to be accepted. With evolution as a framework, many things that had previously seemed curious now made sense. For example, years before Origin was published, scientists had known that, at a certain point in their embryonic development, species as diverse as fish, chickens, and humans all have structures known as pharyngeal slits. In fish, these structures go on to become gill slits; in humans they develop into the eustachian tubes of the ear, among other things.

The question was: Why would land-dwelling and sea-dwelling organisms share a common embryonic structure? For that matter, why would organisms as different as a human and a chicken share this structure? With evolution, the answer became clear. All these organisms shared a common vertebrate ancestor, who had the slits. The vertebrate ancestral line had evolved into various species—and the slits into various structures—yet this element of the common ancestor persisted in the embryos of all these species.

Darwin understood descent with modification for several years before he compre- hended that natural selection was the most important force driving it. It was his reading of a work by Malthus that sparked his realization about natural selection.

Natural selection has been the most important force in shaping evolution. Through natural selection, the 'fit' of an organism with its environment will select those traits in a population that will be passed on with greater frequency from one gen- eration to the next. Groups of living things evolve in the direction of traits that increase survival and reproduction.

Five lines of evidence are consistent with the theory of evolution: 1) Radiometric dating indicates that the Earth is greater than 4 billion years old. 2) The placement of fossils is consistent with the theory of evolution and with radiometric dating. 3) The theory of evolution explains the common occurrence of the same physical structures in different organisms. 4) The theory of evolution is consistent with variations found in the DNA, RNA, and protein sequences of various organisms. 5) Experimental demonstrations of evolution have been carried out in the laboratory and in nature.

Many different organisms have genes that code for the enzyme cytochrome c oxi- dase. These organisms inherited cytochrome c genes from a common ancestor many millions of years ago. Over time, however, the cytochrome c genes have undergone mutations that have altered the sequence of their DNA nucleotide bases. The longer it has been since any two species shared a common ancestor, the more differences there should be in their cytochrome c nucleotides. There are 13 differences between the nucleotides found in human cytochrome c genes and those found in pigs, but there 66 differences between human and yeast genes.

You should now be able to… • Explain how one organism can be more fit than another organism in the same population. • List three lines of evidence that are consistent with the theory of evolution. • Explain how variation in the base sequences of the same gene among species can provide evidence for evolution.