The field of biology covers an enormous range of subject matter, any aspect of which could form the basis of an informative or persuasive speech. The first step is deciding whether you want to inform or persuade or both. Knowing that will determine the angle of the speech and the kind of sources used. Research is the next step. Be sure to cite authoritative and topical research material. Lastly, any good speech requires a conversational delivery that includes rather than excludes its audience.
Biology has evolved a great deal in the past several hundred years. An interesting speech could discuss some of the early notions about life. Another speech might answer the question of who was the very first "biologist." You could take a more modern approach by writing a speech that explored one or more of the greatest discoveries in biology in the past hundred years. The discovery of DNA, the building block of life, would definitely be near the top of that list.
Cells and Chemistry
The microscopic structures of life forms and the chemical reactions that take place on that level make for good speech material. Not everyone has a detailed understanding of what makes a cell function. Similarly, most people don't know what cancer is and how it attacks healthy tissue. The variety of topics available with the fields of microbiology and molecular biology are almost limitless. A small sampling of these topics would include photosynthesis, respiration, cell death, cell division and cell structures.
DNA stands for deoxyribonucleic acid, and it is the fundamental structure and blueprint of life. Information contained in DNA is passed on to the next generation through inheritance. Scientists are today beginning to manipulate this information, and not everyone is pleased with that. You could formulate a meaty persuasive speech on a discussion of the ethics of DNA manipulation. A speculative speech might instead take an optimistic approach and discuss the possible benefits of gene therapy.
Evolution and Adaptation
The theory of evolution is the single biggest point of controversy generated by modern biological science. An informative speech might skirt this controversy and simply discuss the finer points of evolutionary theory. Some case studies of adaptation in action would liven up the subject matter. Competition and survival of the fittest are both topics that play into the concept of evolution. Avoiding highly emotional points of view and sticking with the science may be advisable.
Kinds of Life
All life falls into one of five kingdoms: monera, protista, fungi, plant or animal. Plants and animals are easy to recognise, but most people are stumped by the other three kingdoms. An informative speech could go into detail as to what defines each one of these kingdoms. An even more detailed speech could discuss all the categories underneath of the kingdom level, such as class, phylum or genus.
Some biologists specialise in population dynamics, the study of how populations change with circumstances and follow predictable patterns. Birth and death rates are two variables that this kind of biologist would be interested in. A frequently discussed term is "carrying capacity," meaning the maximum population of a species that a given environment can support. Historically, a debate around the human carrying capacity of the Earth has been taking place. A biology speech could discuss some of the human carrying capacity numbers that have been advanced by different biologists.
Ecology and Environmentalism
On April 22 of each year, people celebrate Earth Day, an event designed to raise awareness of ecological issues and solution. A biology speech topic along these lines might discuss how the typical ecosystem is structured and how delicate the balance of energy is within such an ecosystem. A persuasive speech might highlight the necessities of conservation and recycling. A good approach might be to focus on a topic of local interest, such as a lake or waterway that the audience will be familiar with.