l: An Evolutionary Framework for Biology

Organisms Have Changed over Billions of Years

· An early speculator of evolutionary change was Count George-Louis Leclerc de Buffon (1707–1788), who wrote Natural History of Animals.

· Buffon observed the similarity of different mammals’ limbs. (See Figure 1.2.)

· He noticed pigs had toes that were too small to be useful.

· He suggested that the limbs of mammals were inherited from a common ancestor.

· He concluded that pigs have toes, although functionless, because they inherited them from ancestors with fully formed and functional toes.

· Jean Baptist de Lamarck, a student of Buffon, suggested a mechanism of evolution.

· Some structures become larger from continued use from generation to generation; others become smaller from disuse.

· Additional investigation by other scientists revealed flaws in this mechanism of acquired structures, and this theory is currently not accepted by most scientists.

· By the middle of the nineteenth century, the theory of evolution by natural selection was proposed.

· Charles Darwin and Alfred Russel Wallace developed this idea and independently proposed it in 1858.

· The theory states that the reproductive rates of all organisms are sufficiently high that populations would be enormous if mortality rates did not balance reproductive rates.

· Differences or variations among individuals influence how well those individuals survive and reproduce in changing environments. Any traits that increase the probability that their bearers will survive and reproduce are passed on to the next generation.

· Darwin called the differential survival and reproductive success of individuals "natural selection."

· The living world has evolved over billions of years and is constantly evolving.

· Life has evolved, without direction or goals, from atoms and molecules into organisms, including those that can understand the basic laws of the universe.

Evolutionary Milestones

Life arises from nonlife

· All matter is made of chemicals.

· The smallest units are atoms.

· About four billion years ago, interactions among small molecules that stored useful information eventually resulted in the synthesis of larger molecules.

· Some of these large molecules—carbohydrates, lipids, proteins, and nucleic acids—are found in all living systems.

Cells form from molecules

· Around 3.8 billion years ago certain molecules became enclosed in compartments.

· Control over entrance, retention, and exit of molecules was possible because of compartmentalization by membranes.

· Under present conditions on Earth, cells do not arise from noncellular materials, but must come from other cells.

· For 2 billion years, cells were tiny packages of molecules, each enclosed in a membrane.

· These were prokaryotic cells, which lived separately from one another.

· These cells obtained raw materials and energy from their environment.

· The conversions of energy and matter are called metabolism.

· A major theme of evolution is the ability of organisms to develop diverse ways of capturing external energy and using it to drive biologically useful reactions.

Photosynthesis changes Earth's environment

· About 2.5 billion years ago some prokaryotes acquired the ability to photosynthesize.

· The energy of sunlight was captured, and oxygen was generated as a waste product.

· Oxygen slowly increased in concentration in the atmosphere.

· The presence of oxygen gas made much more efficient metabolism possible.

· Another effect of O₂ was O₃ (ozone) accumulation in the upper atmosphere.

· Ozone has the property of preventing excess ultraviolet light from the sun from reaching Earth.

· Around 800 million years ago, ozone accumulation shielded the landmass from radiation adequately enough to allow the movement of organisms to land.

Sex enhances adaptation

· Earliest organisms reproduced by doubling their hereditary material and then dividing into two new cells.

· This is called asexual reproduction. Essentially, the resulting organisms were clones.

· Sexual reproduction, the combining of genes from two cells, appeared early in the evolution of life.

· Sexual reproduction allows genes from surviving cells to combine to generate more variable offspring.

· Variation allows organisms to adapt to a changing environment.

· Because environments are constantly changing, organisms that produce variable offspring have an advantage over those that produce genetically identical clones.

Eukaryotes are "cells within cells"

· Some prokaryotic cells became large enough to attach, engulf, and digest smaller cells. (See Figure 1.5.)

· About 1.5 billion years ago, some cells had surviving smaller cells within them: These were early eukaryotic cells.

· In contrast to prokaryotes, genetic information in modern eukaryotes is compartmentalized in a nucleus.

Multicellularity permits specialization of cells

· Around 1 billion years ago, only single-celled organisms existed.

· Two developments made the evolution of multicellular organisms possible.

· One was cell differentiation: cells being able to alter their structure and function. Bacteria evolved the ability grow vegetatively or be dormant and become spores.

· Another was the clumping of cells. After the cells divided, they stayed together, forming a multicellular organism.

· Once organisms were composed of many cells, it became possible for certain cells to specialize.

· Some cells might photosynthesize, while others might transport essential chemicals.

· Specialization of sex cells made complex genetic transmission mechanisms possible.

· Mitosis—simple nuclear division—was supplemented by meiosis.

· Meiosis provided a means for genetic information to be combined and rearranged in an organized fashion, thus promoting variability and adaptation.

Controlling internal environments becomes more complicated

· Organisms need to keep constant internal environments despite external environmental changes and challenges.

· Maintenance of a relatively stable internal condition (such as constant human body temperature) is called homeostasis.

· Evolution of life is characterized by increasingly complicated homeostatic systems.

Multicellular organisms undergo regulated growth

· Multicellular organisms must regulate cell division and differentiation to achieve their adult shapes.

· This regulated growth process is called development.

· Some organisms undergo radical change in body form during their lives. Metamorphosis of insects is an example. The monarch butterfly goes from an egg to a larva (caterpillar) to a pupa (transform stage) to an adult (butterfly).

· Activation of gene-based information within cells causes striking changes to allow this transformation.

· Just a few genes can control processes that result in dramatic changes during the life of an organism. (See Figure 1.6 and photos of the life stages of tunicates, comet-tail moths, and katydids in Chapter 43 of the Instructor’s Resource CD-ROM.)

Speciation produces the diversity of life

· All organisms on Earth today descended from an original unicellular type formed around 4 billion years ago.

· Major evolutionary events have led to more complex organisms with larger quantities of information and more complex mechanisms for using it.

· Genetically independent groups, called species, have evolved.

· If individuals of a species are separated into isolated populations, differences between the populations may accumulate over time, and the isolated groups may evolve into different species.

· Since all organisms evolved from the same original organisms and have been evolving for the same length of time, terms like "primitive," "advanced," "lower," and "higher" are best avoided.

· All organisms alive today are the exceptional survivors of eons of generations of life, and they have survived because of their appropriate adaptation to their environments.

· See Figure 1.1 for Life’s Calendar.

The Hierarchy of Life

· Biologists study life in two ways: They study processes and structures, and they study patterns of life's evolution.

· Biologists study the hierarchy of interactions among the units of biology from cells to the biosphere and the hierarchy of evolutionary relationships among organisms.

· See Figures 1.9 and 1.10.

Biologists study life at different levels

· Biology can be visualized as a hierarchy of units that include atoms, molecules, cells, tissues, organs, organisms, populations, and communities.

· Each level of biological organization has its emergent properties.

· Molecules have properties not found in the component atoms; organelles have properties not found in the individual molecules of which they are composed.

· Human emotions result from the interactions of many nerve cells, but each cell does not explain the complexity.

· Emergent properties exist because of interactions of many parts, and because aggregations have collective properties—the resulting complexity of living things is greater than the sum of its individual cellular parts.

· Emergent properties do not violate the principles that operate at the lower levels of organization.

Biological diversity is organized hierarchically

· As many as 30 million different species may inhabit Earth.

· Organisms are grouped in ways that attempt to define their evolutionary relationships, or how recently the different members of a group shared a common ancestor.

· Fossils, physical structure, and gene similarity are used to gain an understanding of evolutionary relationships.

· Three major life domains form the hierarchical scheme: Archaea and Bacteria (prokaryotes), and Eukarya (eukaryotes).

· Eukarya are then divided into four groups: the protists, and the kingdoms of Plantae, Fungi, and Animalia.

· Some bacteria, some protists, and most members of Plantae convert light energy to chemical energy by photosynthesis; these are autotrophs (“self-feeders”).

· Fungi and Animalia are heterotrophs (“other feeders”) and must rely on energy-rich molecules made by other organisms.

· See Figure 1.10 for a simplified version of the hierarchical levels. Photos on the Instructor’s Resource CD-ROM include examples of organisms from each kingdom.

· Each species is identified by two names.

· The first, the genus name, refers to a group of species that share a recent common ancestor.

· The second name, the species name, identifies a single species with the genus.

· The scientific name of Modern humans is Homo sapiens.

Asking and Answering "How?" and "Why?"

· Two major questions of biologists are How does it work? and Why has it evolved to work that way?

· For example: "How do these animals crawl?” and "Why do they crawl?"

· The Instructor’s Resource CD-ROM includes photos of biologists at work in the field and in the lab.

Hypothesis testing guides scientific research

· Underlying all scientific research is the hypothetico-deductive (H-D) approach.

· The H-D approach allows scientists to modify and correct their beliefs as new observations and information become available.

· There are five parts to the H-D system:

· Making observations

· Asking questions

· Forming hypotheses, or tentative answers to the questions

· Making predictions based on these hypotheses

· Testing the predictions by making additional observations or conducting experiments

· Last of all, a biologist must evaluate the results of the experiments or observations. If the data support the hypothesis, biologists will subject it to further tests in an attempt to confirm it, refine it, and explore its implications. If the hypothesis is not supported, it is abandoned or modified, and further verification is required.

Applying the hypothetico-deductive method

· The H-D approach begins with an observation that generates a question.

· For example: "Why do amphipods crawl on the surface of the mud rather than stay hidden within it?"

· Assemble available information on amphipods and their predators, e.g.:

· Sandpipers assemble annually on the mudflats.

· Sandpipers feed primarily on amphipods.

· Nematodes parasitize both amphipods and sandpipers.

· Nematodes mature within sandpipers' digestive tracts, mate, and release their eggs in the birds’ feces.

· Larvae hatch and enter amphipods.

· Sandpipers are reinfected when they eat parasitized amphipods.

· Generate a hypothesis and predictions: Nematodes alter the behavior of their amphipod host in a way that increases the chance that the nematodes will be ingested by the sandpiper.

· Predict that infected amphipods increase activity at the surface during the day, when sandpipers are feeding by sight, but not at night when sandpiper feeding is less.

· Predict that only amphipods with late-stage nematode larvae, those that can infect sandpipers, have this modified behavior.

· All hypotheses have a corresponding null hypothesis, which asserts there will be no such effect:

· Predict that no differences exist in the behaviors of infected and noninfected amphipods.

· Predict that all larval stages of nematodes affect their hosts in the same manner.

· Testing predictions:

· Amphipods were collected in the mud and surface, during day and night.

· More activity was found in the amphipods on the surface of the mud during the day than at night. This agrees with the first prediction.

· No such difference was found at night.

· When collected at the surface versus in the mud, amphipods were more likely to be infected by late-stage, not early-stage, nematode larvae. This agrees with the second prediction.

· Conclusion—The null hypothesis and its predictions is denied.

· See Figures 1.11, 1.12, and 1.13.

Experiments are powerful tools

· The essential feature of experimentation is the control of most factors so that the influence of a single factor can be seen clearly.

· For the amphipod experiment, all amphipods were infected with nematodes at the same time or were left uninfected.

· Those amphipods that were infected and were in the late, infective stage were more likely to expose themselves at the surface.

· The nematodes manipulate their host to increase their likelihood of survival. (See Figure 1.14.)

· Experiments help isolate specific variables, but sometimes organisms behave differently in nature than they do in the laboratory.

· Combinations of laboratory and field experiments are needed to test hypotheses related to biological science.

· A single piece of supporting evidence rarely leads to widespread acceptance of a hypothesis. Most often, it is subject to further retesting and refinement.

Accepted scientific theories are based on many kinds of evidence

· Statements of biological "fact" are mixtures of observations, predictions, and interpretations.

· No amount of observation can substitute for experimentation, and scientists must be sensitive to the welfare of organisms during experimental manipulation.

Not all forms of inquiry are scientific

· It is important to distinguish science from nonscience. For example, creation science is not science.

· Science begins with observations and the formulation of hypotheses that can be tested and that will be rejected if significant contrary evidence is found.

· Creation science begins with assertions that Earth is just 7,000 years old, (the text says "a few thousand" years old) and that all species were created in their present forms. But creationists do not subject these assertions to objective testing.

· Evidence presented in the text supports the assertion that Earth is approximately 4 billion years old.

· Creation science is essentially “faith-based” and some elements may give important meaning and spiritual guidance to human life, but it is not science.

Biology and Public Policy

· After World War II, physical sciences were highly influential in shaping public policy.

· More recently (1960–2000), biological science had a major influence on policy.

· One reason is the importance in understanding the human impact on the biosphere.

· Another is the development of genetics, including the decoding of DNA, which provides a means to control human disease and agricultural productivity.

· Currently, biological science is positioned at the forefront of many ethical, ecological, social and medical challenges and dilemmas.

· It is important to have a basic understanding of biology to respond wisely to these confrontations.

Photos in the Instructor’s Kit include Earth from outer space (the biosphere), biologists at work, and examples of organisms from each kingdom.