01 Biology and the Tree of Life

© 2011 Pearson Education, Inc.

Lectures by Stephanie Scher Pandolfi

BIOLOGICAL SCIENCE

FOURTH EDITION

SCOTT FREEMAN

1Biology and the Tree of Life


Key Concepts

All living organisms share five fundamental characteristics – energy utilization, cellularity, information processing, replication, and evolution.

The cell theory states that all organisms are comprised of cells and all cells come from preexisting cells.

The theory of evolution by natural selection describes how species change through time.


Key Concepts

A phylogenetic tree is a graphical representation of the evolutionary relationships among species. Phylogenies can be established by analyzing similarities and differences in traits.

Biologists ask questions, generate hypotheses to answer them, and design experiments that test the predictions made by competing hypotheses.


What Does It Mean to Say That Something Is Alive?

All living organisms share five fundamental characteristics

– Energy – All organisms acquire and use energy.

– Cells– All organisms are made up of membrane-bound cells.

– Information – All organisms process hereditary information encoded in

genes as well as information from the environment.

– Replication– All organisms are capable of reproduction.

– Evolution– Populations of organisms are continually evolving.


Theories

• A theory is an explanation for a very general class of phenomena or observations.

• Theories have two components.

– Pattern

– Something that occurs in the natural world

– Process

– Responsible for creating the pattern

• Two theories form the framework for modern biological science.

– The cell theory

– The theory of evolution by natural selection


The Cell Theory

• In the late 1660s, Robert Hooke and Anton van Leeuwenhoek were the first to observe cells.

• A cell is a highly organized compartment bounded by a plasma membrane that contains concentrated chemicals in an aqueous solution.

The cell theory states that

– All organisms are made of cells (pattern).

– All cells come from preexisting cells (process).


Louis Pasteur’s Experiment

• A hypothesis is a proposed explanation.

• A prediction is something that can be measured and must be correct if a hypothesis is valid.

• Louis Pasteur proved that cells arise from cells and not by spontaneous generation.


Implications of the Cell Theory

• Because all cells come from preexisting cells, all individuals in a population of single-celled organisms are related by common ancestry.

• All of the cells present in a multicellular organism have descended from preexisting cells and are connected by common ancestry.


The Theory of Evolution by Natural Selection

• In 1858, Charles Darwin and Alfred Russel Wallace made two claims regarding the natural world:

– All species are related by common ancestry (pattern).

– Characteristics of species can be modified from generation to generation.

– Descent with modification (process)


Evolution and Natural Selection

• Evolution is a change in the characteristics of a population over time. It means that species are related to one another and can change through time.

• Natural selection explains how evolution occurs.


Natural Selection and Populations

• A group of individuals of the same species living in the same area at the same time constitute a population.

• Two conditions must be met for natural selection to occur in a population:

1. Individuals in the population vary in characteristics that are heritable.

2. In a particular environment, certain versions of these heritable traits help individuals survive better or reproduce more than do other versions.


Evolutionary Change

If certain heritable traits lead to increased success in producing offspring, these traits become more common in the population over time. In this way, the population’s characteristics change as a result of natural selection acting on individuals.

– Natural selection acts on individuals, but evolutionary change occurs in populations.


Artificial Selection

• In artificial selection, changes in populations occur when humans select which individuals will produce the most offspring.

• Repeating this process over generations results in changes in the characteristics of a domesticated population over time.


Artificial Selection

Web Activity: Artificial Selection


Differential Reproductive Success

• Evolution occurs when heritable variation leads to differential success in reproduction.

• This can occur via:

– Artificial selection – humans select desirable traits within a domestic population

– Natural selection – traits beneficial to the current environment are “selected” within a natural population


Fitness and Adaptation

• Fitness is the ability of an individual to produce offspring.

– Individuals with high fitness produce many surviving offspring.

• Adaptation is a trait that increases the fitness of an individual in a particular environment.


The Tree of Life

• The cell theory and the theory of evolution by natural selection imply that all species come from preexisting species and that all species, past and present, trace their ancestry back to a single common ancestor.

• Speciation is a divergence process in which natural selection has caused populations of one species to diverge to form new species.

• The tree of life is a family tree of organisms that describes the genealogical relationships among species with a single ancestral species at its base.

• Phylogeny is the actual genealogical relationships among all organisms.


Using Molecules to Understand the Tree of Life

• Carl Woese and colleagues studied small subunit ribosomal RNA (rRNA), a molecule found in all organisms, to understand evolutionary relationships.

• rRNA is comprised of four chemical units called ribonucleotides.– Symbolized by letters A, U, C, & G

• The sequence of ribonucleotides can change during evolution.

• Based on the theory of evolution, rRNA sequences should be very similar in closely related organisms but less similar in less closely related organisms.


The Phylogenic Tree of Life

A phylogenetic tree reflects relationships between species. Branches that share a recent common ancestor represent species that are closely related; branches that don’t share recent common ancestors represent species that are more distantly related.


Interpreting the Tree of Life

• The tree of life indicates three major groups of organisms: the eukaryotes – Eukarya – and two groups of prokaryotes – Bacteria and Archaea.

• Fungi and animals are more closely related to each other than either is to plants.

• Traditional classification schemes were often inaccurate.

• The location of certain branches on the tree is hotly debated and the shape of the tree will continue to change as databases expand.


Taxonomy

• Taxonomy is the effort to name and classify organisms.

– A taxon is a named group.

• To reflect the tree of life, Woese created a new taxonomic level called the domain, which consists of three taxa: Bacteria, Archaea, and Eukarya.

• A phylum is a major lineage within a domain.


Linnaeus’ Taxonomic System of Classification

• In 1735 Carolus Linnaeus established the classification system still in use today.

• Each organism is given a unique two-part scientific name consisting of the genus and the species.

– A genus is made up of a closely related group of species.

– A species is made up of individuals that regularly breed together or have characteristics that are distinct from those of other species.


Rules of Nomenclature

• An organism’s genus and species designation is called its scientific name or Latin name.

– Scientific names are always italicized.

– Genus names are always capitalized, but species names are

not—for instance, Homo sapiens.


Doing Biology: The Nature of Science

• All scientists ask questions that can be answered by measuring things – by collecting data.

• Science is about formulating hypotheses and finding evidence that supports or conflicts with those hypotheses.– For example, using carefully designed experiments,

biologists test ideas about the way the natural world works by testing the predictions made by alternative hypotheses.

• On the other hand, religious faith addresses questions that cannot be answered by data but instead focus on why we exist and how we should live.


Hypothesis Testing

• Hypothesis testing is a two-step process:

1. State the hypothesis as precisely as possible and list the predictions it makes.

2. Design an observational or experimental study that is capable of testing those predictions.


Why Do Giraffes Have Long Necks?

• The food competition hypothesis argues that long necks evolved because those with long necks can reach food unavailable to other mammals. – Predictions:

– Neck length is variable among giraffes.– Neck length in giraffes is heritable.– Giraffes feed high in trees.

• Simmons and Scheepers tested the food competition hypothesis and found that the third prediction does not hold true.– Thus, there may be better alternative hypotheses to

explain neck length in giraffes.


The Sexual Competition Hypothesis

• An alternative hypothesis is that giraffes evolved long necks because longer-necked males win more fights than shorter-necked giraffes, and can then father more offspring.

• Data support this hypothesis.


Experimental Design – How Do Ants Navigate?

• Experiments are a powerful scientific tool because they allow researchers to test the effect of a single, well-defined factor on a particular phenomenon.

• Wittlinger and colleagues questioned how ants find their way back to their nest after foraging for food.

– The pedometer hypothesis states that ants always know how far they are from the nest because they track the number of steps taken and length of their stride.


Experimental Setup

• Wittlinger’s group manipulated the ants into three groups after walking from the nest to a feeder:

1. Stumps – legs were cut to form shorter-than-normal legs

2. Normal – individuals were left alone with normal legs

3. Stilts – bristles glued on legs to form longer-than-normal legs

• Measured the distance the ants traveled back to the nest via a different route


Results and Conclusion

• A null hypothesis specifies what we should observe if the hypothesis being tested doesn’t hold.

• Results:– “Stumps” stopped short of the nest.– “Normal” ants returned to the nest.– “Stilts” walked beyond the nest.

• Conclusion: – Desert ants use information on stride length and

number to calculate how far they are from the nest.


Elements of a Well-Designed Experiment

The experiment just described is well-designed:

1. It included a control group (the “normal” ants) to check for other factors that might influence the outcome.

2. Experimental conditions were controlled to eliminate other variables.

3. The test was repeated to reduce the effects of distortion due to small sample size.


Introduction to Experimental Design

Web Activity: Introduction to Experimental Design, Part 1

Web Activity : Introduction to Experimental Design, Part 2


The Principles of Experimental Design

• Biologists practice evidence-based decision making. They ask questions about how organisms work, pose hypotheses to answer those questions, and use experimental or observational evidence to decide which hypotheses are correct.

01 Biology and the Tree of Life

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