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6
Dimension 3
DISCIPLINARY CORE IDEAS—LIFE SCIENCES
T
he life sciences focus on patterns, processes, and relationships of living
organisms. Life is self-contained, self-sustaining, self-replicating, and evolv-
ing, operating according to laws of the physical world, as well as genetic
programming. Life scientists use observations, experiments, hypotheses, tests,
models, theory, and technology to explore how life works. The study of life ranges
over scales from single molecules, through organisms and ecosystems, to the
entire biosphere, that is all life on Earth. It examines processes that occur on time
scales from the blink of an eye to those that happen over billions of years. Living
systems are interconnected and interacting. Although living organisms respond
to the physical environment or geosphere, they have also fundamentally changed
Earth over evolutionary time. Rapid advances in life sciences are helping to pro-
vide biological solutions to societal problems related to food, energy, health, and
environment.
From viruses and bacteria to plants to fungi to animals, the diversity of
the millions of life forms on Earth is astonishing. Without unifying principles, it
would be difficult to make sense of the living world and apply those understand-
ings to solving problems. A core principle of the life sciences is that all organ-
isms are related by evolution and that evolutionary processes have led to the
tremendous diversity of the biosphere. There is diversity within species as well as
between species. Yet what is learned about the function of a gene or a cell or a
process in one organism is relevant to other organisms because of their ecologi-
cal interactions and evolutionary relatedness. Evolution and its underlying genetic
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mechanisms of inheritance and variability are key to understanding both the unity
and the diversity of life on Earth.
The committee developed four core ideas reflecting unifying principles in
life sciences. These core ideas are essential for a conceptual understanding of the
life sciences and will enable students to make sense of emerging research findings.
We begin at the level of organisms, delving into the many processes and struc-
tures, at scales ranging from components as small as individual atoms to organ
systems that are necessary for life to be sustained. Our focus then broadens to
consider organisms in their environment—how they interact with the environ-
ment’s living (biotic) and physical (abiotic) features. Next the chapter considers
how organisms reproduce, passing genetic information to their offspring, and
how these mechanisms lead to variability and hence diversity within species.
Finally, the core ideas in the life sciences culminate with the principle that evolu-
tion can explain how the diversity that is observed within species has led to the
diversity of life across species through a process of descent with adaptive modifi-
cation. Evolution also accounts for the remarkable similarity of the fundamental
characteristics of all species.
The first core idea, LS1: From Molecules to Organisms: Structures and
Processes, addresses how individual organisms are configured and how these
structures function to support life, growth, behavior, and reproduction. The first
core idea hinges on the unifying principle that cells are the basic unit of life.
The second core idea, LS2: Ecosystems: Interactions, Energy, and Dynamics,
explores organisms’ interactions with each other and their physical environment.
This includes how organisms obtain resources, how they change their environ-
ment, how changing environmental factors affect organisms and ecosystems, how
social interactions and group behavior play out within and between species, and
how these factors all combine to determine ecosystem functioning.
The third core idea, LS3: Heredity: Inheritance and Variation of Traits
across generations, focuses on the flow of genetic information between genera-
tions. This idea explains the mechanisms of genetic inheritance and describes
the environmental and genetic causes of gene mutation and the alteration of
gene expression.
The fourth core idea, LS4: Biological Evolution: Unity and Diversity,
explores “changes in the traits of populations of organisms over time” [1]
and the factors that account for species’ unity and diversity alike. The section
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❚ Evolution and its underlying genetic mechanisms of inheritance and
variability are key to understanding both the unity and the diversity of
❚
life on Earth.
begins with a discussion of the converging evidence for shared ancestry that has
emerged from a variety of sources (e.g., comparative anatomy and embryology,
molecular biology and genetics). It describes how variation of genetically deter-
mined traits in a population may give some members a reproductive advantage
in a given environment. This natural selection can lead to adaptation, that is, to
a distribution of traits in the population that is matched to and can change with
environmental conditions. Such adaptations can eventually lead to the develop-
ment of separate species in separated populations. Finally, the idea describes the
factors, including human activity, that affect biodiversity in an ecosystem, and
the value of biodiversity in ecosystem resilience. See Box 6-1 for a summary of
these four core ideas and their components.
These four core ideas, which represent basic life sciences fields of
investigation—structures and processes in organisms, ecology, heredity, and
evolution—have a long history and solid foundation based on the research evi-
dence established by many scientists working across multiple fields. The role
of unifying principles in advancing modern life sciences is articulated in The
Role of Theory in Advancing 21st-Century Biology and A New Biology for the
21st Century [2, 3]. In developing these core ideas, the committee also drew on
the established K-12 science education literature, including National Science
Education Standards and Benchmarks for Science Literacy [4, 5]. The ideas
also incorporate contemporary documents, such as the Science College Board
Standards for College Success [6], and the ideas are consistent with frame-
works for national and international assessments, such as those of the National
Assessment of Educational Progress (NAEP), the Programme for International
Student Assessment (PISA), and the Trends in International Mathematics and
Science Study (TIMSS) [7-9]. Furthermore, the ideas align with the core concepts
for biological literacy for undergraduates to build on as described in the American
Association for the Advancement of Science (AAAS) report Vision and Change in
Undergraduate Biology Education [10].
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BOX 6-1
CORE AND COMPONENT IDEAS IN THE LIFE SCIENCES
Core Idea LS1: From Molecules to Organisms: Structures and Processes
LS1.A: Structure and Function
LS1.B: Growth and Development of Organisms
LS1.C: Organization for Matter and Energy Flow in Organisms
LS1.D: Information Processing
Core Idea LS2: Ecosystems: Interactions, Energy, and Dynamics
LS2.A: Interdependent Relationships in Ecosystems
LS2.B: Cycles of Matter and Energy Transfer in Ecosystems
LS2.C: Ecosystem Dynamics, Functioning, and Resilience
LS2.D: Social Interactions and Group Behavior
Core Idea LS3: Heredity: Inheritance and Variation of Traits
LS3.A: Inheritance of Traits
LS3.B: Variation of Traits
Core Idea LS4: Biological Evolution: Unity and Diversity
LS4.A: Evidence of Common Ancestry and Diversity
LS4.B: Natural Selection
LS4.C: Adaptation
LS4.D: Biodiversity and Humans
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From Molecules to Organisms: Structures and Processes
Core Idea LS1
How do organisms live, grow, respond to their environment, and reproduce?
All living organisms are made of cells. Life is the quality that distinguishes living
things—composed of living cells—from nonliving objects or those that have died.
While a simple definition of life can be difficult to capture, all living things—that
is to say all organisms—can be characterized by common aspects of their structure
and functioning. Organisms are complex, organized, and built on a hierarchical
structure, with each level providing the foundation for the next, from the chemical
foundation of elements and atoms, to the cells and systems of individual organ-
isms, to species and populations living and interacting in complex ecosystems.
Organisms can be made of a single cell or millions of cells working together and
include animals, plants, algae, fungi, bacteria, and all other microorganisms.
Organisms respond to stimuli from their environment and actively maintain
their internal environment through homeostasis. They grow and reproduce, trans-
ferring their genetic information to their offspring. While individual organisms
carry the same genetic information over their lifetime, mutation and the transfer
from parent to offspring produce new combinations of genes. Over generations
natural selection can lead to changes in a species overall; hence, species evolve
over time. To maintain all of these processes and functions, organisms require
materials and energy from their environment; nearly all energy that sustains life
ultimately comes from the sun.
LS1.A: STRUCTURE AND FUNCTION
How do the structures of organisms enable life’s functions?
A central feature of life is that organisms grow, reproduce, and
die. They have characteristic structures (anatomy and morphol-
ogy), functions (molecular-scale processes to organism-level
physiology), and behaviors (neurobiology and, for some animal
species, psychology). Organisms and their parts are made of
cells, which are the structural units of life and which themselves
have molecular substructures that support their functioning.
Organisms range in composition from a single cell (unicellular
microorganisms) to multicellular organisms, in which different
groups of large numbers of cells work together to form systems
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of tissues and organs (e.g., circulatory, respiratory, nervous, musculoskeletal), that
are specialized for particular functions.
Special structures within cells are also responsible for specific cellular func-
tions. The essential functions of a cell involve chemical reactions between many
types of molecules, including water, proteins, carbohydrates, lipids, and nucleic
acids. All cells contain genetic information, in the form of DNA. Genes are specif-
ic regions within the extremely large DNA molecules that form the chromosomes.
Genes contain the instructions that code for the formation of molecules called pro-
teins, which carry out most of the work of cells to perform the essential functions
of life. That is, proteins provide structural components, serve as signaling devices,
regulate cell activities, and determine the performance of cells through their enzy-
matic actions.
Grade Band Endpoints for LS1.A
By the end of grade 2. All organisms have external parts. Different animals use
their body parts in different ways to see, hear, grasp objects, protect themselves,
move from place to place, and seek, find, and take in food, water and air. Plants
also have different parts (roots, stems, leaves, flowers, fruits) that help them sur-
vive, grow, and produce more plants.
By the end of grade 5. Plants and animals have both internal and external struc-
tures that serve various functions in growth, survival, behavior, and reproduction.
(Boundary: Stress at this grade level is on understanding the macroscale systems
and their function, not microscopic processes.)
By the end of grade 8. All living things are made up of cells, which is the smallest
unit that can be said to be alive. An organism may consist of one single cell (uni-
cellular) or many different numbers and types of cells (multicellular). Unicellular
organisms (microorganisms), like multicellular organisms, need food, water, a way
to dispose of waste, and an environment in which they can live.
Within cells, special structures are responsible for particular functions, and
the cell membrane forms the boundary that controls what enters and leaves the
cell. In multicellular organisms, the body is a system of multiple interacting sub-
systems. These subsystems are groups of cells that work together to form tissues
or organs that are specialized for particular body functions. (Boundary: At this
grade level, only a few major cell structures should be introduced.)
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By the end of grade 12. Systems of specialized cells within organisms help them
perform the essential functions of life, which involve chemical reactions that take
place between different types of molecules, such as water, proteins, carbohydrates,
lipids, and nucleic acids. All cells contain genetic information in the form of DNA
molecules. Genes are regions in the DNA that contain the instructions that code
for the formation of proteins, which carry out most of the work of cells.
Multicellular organisms have a hierarchical structural organization, in which
any one system is made up of numerous parts and is itself a component of the
next level. Feedback mechanisms maintain a living system’s internal conditions
within certain limits and mediate behaviors, allowing it to remain alive and func-
tional even as external conditions change within some range. Outside that range
(e.g., at a too high or too low external temperature, with too little food or water
available), the organism cannot survive. Feedback mechanisms can encourage
(through positive feedback) or discourage (negative feedback) what is going on
inside the living system.
LS1.B: GROWTH AND DEVELOPMENT OF ORGANISMS
How do organisms grow and develop?
The characteristic structures, functions, and behaviors of organisms change in
predictable ways as they progress from birth to old age. For example, upon reach-
ing adulthood, organisms can reproduce and transfer their genetic information to
their offspring. Animals engage in behaviors that increase their chances for repro-
duction, and plants may develop specialized structures and/or depend on animal
behavior to accomplish reproduction.
Understanding how a single cell can give rise to a complex, multicellular
organism builds on the concepts of cell division and gene expression. In multi-
cellular organisms, cell division is an essential component of growth, development,
and repair. Cell division occurs via a process called mitosis: when a cell divides in
two, it passes identical genetic material to two daughter cells. Successive divisions
produce many cells. Although the genetic material in each of the cells is identical,
small differences in the immediate environments activate or inactivate different
genes, which can cause the cells to develop slightly differently. This process of
differentiation allows the body to form specialized cells that perform diverse func-
tions, even though they are all descended from a single cell, the fertilized egg. Cell
growth and differentiation are the mechanisms by which a fertilized egg develops
into a complex organism. In sexual reproduction, a specialized type of cell division
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called meiosis occurs and results in the production of sex cells, such as gametes
(sperm and eggs) or spores, which contain only one member from each chromo-
some pair in the parent cell.
Grade Band Endpoints for LS1.B
By the end of grade 2. Plants and animals have predictable characteristics at dif-
ferent stages of development. Plants and animals grow and change. Adult plants
and animals can have young. In many kinds of animals, parents and the offspring
themselves engage in behaviors that help the offspring to survive.
By the end of grade 5. Reproduction is essential to the continued existence of
every kind of organism. Plants and animals have unique and diverse life cycles
that include being born (sprouting in plants), growing, developing into adults,
reproducing, and eventually dying.
By the end of grade 8. Organisms reproduce, either sexually or asexually, and
transfer their genetic information to their offspring. Animals engage in characteris-
tic behaviors that increase the odds of reproduction. Plants reproduce in a variety
of ways, sometimes depending on animal
behavior and specialized features (such as
attractively colored flowers) for reproduc-
tion. Plant growth can continue throughout
the plant’s life through production of plant
matter in photosynthesis. Genetic factors
as well as local conditions affect the size of
the adult plant. The growth of an animal is
controlled by genetic factors, food intake,
and interactions with other organisms, and
each species has a typical adult size range.
(Boundary: Reproduction is not treated in
any detail here; for more specifics about
grade level, see LS3.A.)
By the end of grade 12. In multicellular organisms individual cells grow and then
divide via a process called mitosis, thereby allowing the organism to grow. The
organism begins as a single cell (fertilized egg) that divides successively to produce
many cells, with each parent cell passing identical genetic material (two variants
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of each chromosome pair) to both daughter cells. As successive subdivisions of
an embryo’s cells occur, programmed genetic instructions and small differences
in their immediate environments activate or inactivate different genes, which
cause the cells to develop differently—a process called differentiation. Cellular
division and differentiation produce and maintain a complex organism, com-
posed of systems of tissues and organs that work together to meet the needs of
the whole organism. In sexual reproduction, a specialized type of cell division
called meiosis occurs that results in the production of sex cells, such as gametes
in animals (sperm and eggs), which contain only one member from each chro-
mosome pair in the parent cell.
LS1.C: ORGANIZATION FOR MATTER AND ENERGY FLOW IN ORGANISMS
How do organisms obtain and use the matter and energy they need to live and grow?
Sustaining life requires substantial energy and matter inputs. The complex struc-
tural organization of organisms accommodates the capture, transformation, trans-
port, release, and elimination of the matter and energy needed to sustain them.
As matter and energy flow through different organizational levels—cells, tissues,
organs, organisms, populations, communities, and ecosystems—of living systems,
chemical elements are recombined in different ways to form different products.
The result of these chemical reactions is that energy is transferred from one system
of interacting molecules to another.
In most cases, the energy needed for life is ultimately derived from the sun
through photosynthesis (although in some ecologically important cases, energy is
derived from reactions involving inorganic chemicals in the absence of sunlight—
e.g., chemosynthesis). Plants, algae (including phytoplankton), and other energy-
fixing microorganisms use sunlight, water, and carbon dioxide to facilitate photo-
synthesis, which stores energy, forms plant matter, releases oxygen, and maintains
plants’ activities. Plants and algae—being the resource base for animals, the ani-
mals that feed on animals, and the decomposers—are energy-fixing organisms that
sustain the rest of the food web.
Grade Band Endpoints for LS1.C
By the end of grade 2. All animals need food in order to live and grow. They
obtain their food from plants or from other animals. Plants need water and light
to live and grow.
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By the end of grade 5. Animals and plants alike generally need to take in air and
water, animals must take in food, and plants need light and minerals; anaerobic
life, such as bacteria in the gut, functions without air. Food provides animals with
the materials they need for body repair and growth and is digested to release the
energy they need to maintain body warmth and for motion. Plants acquire their
material for growth chiefly from air and water and process matter they have
formed to maintain their internal conditions (e.g., at night).
By the end of grade 8. Plants, algae (including phytoplankton), and many micro-
organisms use the energy from light to make sugars (food) from carbon dioxide
from the atmosphere and water through the process of photosynthesis, which also
releases oxygen. These sugars can be used immediately or stored for growth or
later use. Animals obtain food from eating plants or eating other animals. Within
individual organisms, food moves through a series of chemical reactions in which
it is broken down and rearranged to form new molecules, to support growth,
or to release energy. In most animals and plants, oxygen reacts with carbon-
containing molecules (sugars) to provide energy and produce carbon dioxide;
anaerobic bacteria achieve their energy needs in other chemical processes that do
not require oxygen.
By the end of grade 12. The process of photosynthesis converts light energy to
stored chemical energy by converting carbon dioxide plus water into sugars plus
released oxygen. The sugar molecules thus formed contain carbon, hydrogen,
and oxygen; their hydrocarbon backbones are used to make amino acids and
other carbon-based molecules that can be assembled into larger molecules (such
as proteins or DNA), used for example to form new cells. As matter and energy
flow through different organizational levels of living systems, chemical elements
are recombined in different ways to form different products. As a result of these
chemical reactions, energy is transferred from one system of interacting molecules
to another. For example, aerobic (in the presence of oxygen) cellular respiration
is a chemical process in which the bonds of food molecules and oxygen molecules
are broken and new compounds are formed that can transport energy to muscles.
Anaerobic (without oxygen) cellular respiration follows a different and less effi-
cient chemical pathway to provide energy in cells. Cellular respiration also releases
the energy needed to maintain body temperature despite ongoing energy loss to
the surrounding environment. Matter and energy are conserved in each change.
This is true of all biological systems, from individual cells to ecosystems.
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LS1.D: INFORMATION PROCESSING
How do organisms detect, process, and use information about the environment?
An organism’s ability to sense and respond to its environment enhances its chance
of surviving and reproducing. Animals have external and internal sensory recep-
tors that detect different kinds of information, and they use internal mechanisms
for processing and storing it. Each receptor can respond to different inputs (elec-
tromagnetic, mechanical, chemical), some receptors respond by transmitting
impulses that travel along nerve cells. In complex organisms, most such inputs
travel to the brain, which is divided into several distinct regions and circuits that
serve primary roles, in particular functions such as visual perception, auditory per-
ception, interpretation of perceptual information, guidance of motor movement,
and decision making. In addition, some of the brain’s circuits give rise to emotions
and store memories. Brain function also involves multiple interactions between the
various regions to form an integrated sense of self and the surrounding world.
Grade Band Endpoints for LS1.D
By the end of grade 2. Animals have body parts that capture and convey different
kinds of information needed for growth and survival—for example, eyes for light,
ears for sounds, and skin for temperature or touch. Animals respond to these
inputs with behaviors that help them survive (e.g., find food, run from a preda-
tor). Plants also respond to some external inputs (e.g., turn leaves toward the sun).
By the end of grade 5. Different sense receptors are specialized for particular
kinds of information, which may then be processed and integrated by an animal’s
brain, with some information stored as memories. Animals are able to use their
perceptions and memories to guide their actions. Some responses to information
are instinctive—that is, animals’ brains are organized so that they do not have to
think about how to respond to certain stimuli.
By the end of grade 8. Each sense receptor responds to different inputs (electro-
magnetic, mechanical, chemical), transmitting them as signals that travel along
nerve cells to the brain. The signals are then processed in the brain, resulting in
immediate behaviors or memories. Changes in the structure and functioning of
many millions of interconnected nerve cells allow combined inputs to be stored as
memories for long periods of time.
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rarely, mutations also cause variations, which may be harmful, neutral, or occa-
sionally advantageous for an individual. Environmental as well as genetic varia-
tion and the relative dominance of each of the genes in a pair play an important
role in how traits develop within an individual. Complex relationships between
genes and interactions of genes with the environment determine how an organism
will develop and function.
LS3.A: INHERITANCE OF TRAITS
How are the characteristics of one generation related to the previous generation?
In all organisms, the genetic instructions for forming species’ characteristics are
carried in the chromosomes. Each chromosome consists of a single very long
DNA molecule, and each gene on the chromosome is a particular segment of
that DNA. DNA molecules contain four different kinds of building blocks, called
nucleotides, linked together in a sequential chain. The sequence of nucleotides
spells out the information in a gene. Before a cell divides, the DNA sequence of its
chromosomes is replicated and each daughter cell receives a copy. DNA controls
the expression of proteins by being transcribed into a “messenger” RNA, which is
translated in turn by the cellular machinery into a protein. In effect, proteins build
an organism’s identifiable traits. When organisms reproduce, genetic informa-
tion is transferred to their offspring, with half coming from each parent in sexual
reproduction. Inheritance is the key factor causing the similarity among individu-
als in a species population.
Grade Band Endpoints for LS3.A
By the end of grade 2. Organisms have characteristics that can be similar or dif-
ferent. Young animals are very much, but not exactly, like their parents and also
resemble other animals of the same kind. Plants also are very much, but not exact-
ly, like their parents and resemble other plants of the same kind.
By the end of grade 5. Many characteristics of organisms are inherited from their
parents. Other characteristics result from individuals’ interactions with the envi-
ronment, which can range from diet to learning. Many characteristics involve both
inheritance and environment.
By the end of grade 8. Genes are located in the chromosomes of cells, with each
chromosome pair containing two variants of each of many distinct genes. Each
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❚ Complex relationships between genes and interactions of genes with
❚
the environment determine how an organism will develop and function.
distinct gene chiefly controls the production of a specific protein, which in turn
affects the traits of the individual (e.g., human skin color results from the actions
of proteins that control the production of the pigment melanin). Changes (muta-
tions) to genes can result in changes to proteins, which can affect the structures
and functions of the organism and thereby change traits.
Sexual reproduction provides for transmission of genetic information to
offspring through egg and sperm cells. These cells, which contain only one chro-
mosome of each parent’s chromosome pair, unite to form a new individual (off-
spring). Thus offspring possess one instance of each parent’s chromosome pair
(forming a new chromosome pair). Variations of inherited traits between parent
and offspring arise from genetic differences that result from the subset of chro-
mosomes (and therefore genes) inher-
ited or (more rarely) from mutations.
(Boundary: The stress here is on the
impact of gene transmission in reproduc-
tion, not the mechanism.)
By the end of grade 12. In all organ-
isms the genetic instructions for forming
species’ characteristics are carried in the
chromosomes. Each chromosome con-
sists of a single very long DNA molecule,
and each gene on the chromosome is a
particular segment of that DNA. The
instructions for forming species’ char-
acteristics are carried in DNA. All cells
in an organism have the same genetic
content, but the genes used (expressed)
by the cell may be regulated in different
ways. Not all DNA codes for a protein;
some segments of DNA are involved in
regulatory or structural functions, and
some have no as-yet known function.
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LS3.B: VARIATION OF TRAITS
Why do individuals of the same species vary in how they look, function, and behave?
Variation among individuals of the same species can be explained by both genetic
and environmental factors. Individuals within a species have similar but not identi-
cal genes. In sexual reproduction, variations in traits between parent and offspring
arise from the particular set of chromosomes (and their respective multiple genes)
inherited, with each parent contributing half of each chromosome pair. More rare-
ly, such variations result from mutations, which are changes in the information
that genes carry. Although genes control the general traits of any given organism,
other parts of the DNA and external environmental factors can modify an indi-
vidual’s specific development, appearance, behavior, and likelihood of producing
offspring. The set of variations of genes present, together with the interactions of
genes with their environment, determines the distribution of variation of traits in a
population.
Grade Band Endpoints for LS3.B
By the end of grade 2. Individuals of the same kind of plant or animal are recog-
nizable as similar but can also vary in many ways.
By the end of grade 5. Offspring acquire a mix of traits from their biological par-
ents. Different organisms vary in how they look and function because they have
different inherited information. In each kind of organism there is variation in the
traits themselves, and different kinds of organisms may have different versions of
the trait. The environment also affects the traits that an organism develops—dif-
ferences in where they grow or in the food they consume may cause organisms
that are related to end up looking or behaving differently.
By the end of grade 8. In sexually reproducing organisms, each parent contributes
half of the genes acquired (at random) by the offspring. Individuals have two of
each chromosome and hence two alleles of each gene, one acquired from each par-
ent. These versions may be identical or may differ from each other.
In addition to variations that arise from sexual reproduction, genetic infor-
mation can be altered because of mutations. Though rare, mutations may result
in changes to the structure and function of proteins. Some changes are beneficial,
others harmful, and some neutral to the organism.
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By the end of grade 12. The information passed from parents to offspring is
coded in the DNA molecules that form the chromosomes. In sexual reproduc-
tion, chromosomes can sometimes swap sections during the process of meiosis
(cell division), thereby creating new genetic combinations and thus more genetic
variation. Although DNA replication is tightly regulated and remarkably accu-
rate, errors do occur and result in mutations, which are also a source of genetic
variation. Environmental factors can also cause mutations in genes, and viable
mutations are inherited. Environmental factors also affect expression of traits,
and hence affect the probability of occurrences of traits in a population. Thus
the variation and distribution of traits observed depend on both genetic and
environmental factors.
Biological Evolution: Unity and Diversity
Core Idea LS4
How can there be so many similarities among organisms yet so many different
kinds of plants, animals, and microorganisms?
How does biodiversity affect humans?
Biological evolution explains both the unity and the diversity of species and pro-
vides a unifying principle for the history and diversity of life on Earth. Biological
evolution is supported by extensive scientific evidence ranging from the fossil
record to genetic relationships among species. Researchers continue to use new
and different techniques, including DNA and protein sequence analyses, to test
and further their understanding of evolutionary relationships. Evolution, which is
continuous and ongoing, occurs when natural selection acts on the genetic varia-
tion in a population and changes the distribution of traits in that population grad-
ually over multiple generations. Natural selection can act more rapidly after sud-
den changes in conditions, which can lead to the extinction of species. Through
natural selection, traits that provide an individual with an advantage to best meet
environmental challenges and reproduce are the ones most likely to be passed on
to the next generation. Over multiple generations, this process can lead to the
emergence of new species. Evolution thus explains both the similarities of genetic
material across all species and the multitude of species existing in diverse condi-
tions on Earth—its biodiversity—which humans depend on for natural resources
and other benefits to sustain themselves.
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LS4.A: EVIDENCE OF COMMON ANCESTRY AND DIVERSITY
What evidence shows that different species are related?
Biological evolution, the process by which all living things have evolved over
many generations from shared ancestors, explains both the unity and the diver-
sity of species. The unity is illustrated by the similarities found betwen species;
which can be explained by the inheritance of similar characteristics from related
ancestors. The diversity of species is also consistent with common ancestry; it is
explained by the branching and diversification of lineages as populations adapted,
primarily through natural selection, to local circumstances.
Evidence for common ancestry can be found in the fossil record, from
comparative anatomy and embryology, from the similarities of cellular processes
and structures, and from comparisons of DNA sequences between species. The
understanding of evolutionary relationships has recently been greatly accelerated
by using new molecular tools to study developmental biology, with researchers
dissecting the genetic basis for some of the changes seen in the fossil record, as
well as those that can be inferred to link living species (e.g., the armadillo) to their
ancestors (e.g., glyptodonts, a kind of extinct gigantic armadillo).
Grade Band Endpoints for LS4.A
By the end of grade 2. Some kinds of plants and animals that once lived on Earth
(e.g., dinosaurs) are no longer found anywhere, although others now living (e.g.,
lizards) resemble them in some ways.
By the end of grade 5. Fossils provide evidence about the types of organisms (both
visible and microscopic) that lived long ago and also about the nature of their
environments. Fossils can be compared with one another and to living organisms
according to their similarities and differences.
By the end of grade 8. Fossils are mineral replacements, preserved remains, or traces
of organisms that lived in the past. Thousands of layers of sedimentary rock not
only provide evidence of the history of Earth itself but also of changes in organisms
whose fossil remains have been found in those layers. The collection of fossils and
their placement in chronological order (e.g., through the location of the sedimen-
tary layers in which they are found or through radioactive dating) is known as the
fossil record. It documents the existence, diversity, extinction, and change of many
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life forms throughout the history of life on Earth.
Because of the conditions necessary for their pres-
ervation, not all types of organisms that existed
in the past have left fossils that can be retrieved.
Anatomical similarities and differences between
various organisms living today and between them
and organisms in the fossil record enable the recon-
struction of evolutionary history and the inference
of lines of evolutionary descent. Comparison of the
embryological development of different species also
reveals similarities that show relationships not evi-
dent in the fully formed anatomy.
By the end of grade 12. Genetic information,
like the fossil record, also provides evidence of
evolution. DNA sequences vary among species,
but there are many overlaps; in fact, the ongoing
branching that produces multiple lines of descent
can be inferred by comparing the DNA sequences
of different organisms. Such information is also
derivable from the similarities and differences in
amino acid sequences and from anatomical and
embryological evidence.
LS4.B: NATURAL SELECTION
How does genetic variation among organisms affect survival and reproduction?
Genetic variation in a species results in individuals with a range of traits. In any
particular environment individuals with particular traits may be more likely than
others to survive and produce offspring. This process is called natural selection
and may lead to the predominance of certain inherited traits in a population and
the suppression of others. Natural selection occurs only if there is variation in the
genetic information within a population that is expressed in traits that lead to dif-
ferences in survival and reproductive ability among individuals under specific envi-
ronmental conditions. If the trait differences do not affect reproductive success,
then natural selection will not favor one trait over others.
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Grade Band Endpoints for LS4.B
By the end of grade 2. [Intentionally left blank.]
By the end of grade 5. Sometimes the differences in characteristics between
individuals of the same species provide advantages in surviving, finding mates,
and reproducing.
By the end of grade 8. Genetic variations among individuals in a population give
some individuals an advantage in surviving and reproducing in their environment.
This is known as natural selection. It leads to the predominance of certain traits
in a population and the suppression of others. In artificial selection, humans have
the capacity to influence certain characteristics of organisms by selective breeding.
One can choose desired parental traits determined by genes, which are then passed
on to offspring.
By the end of grade 12. Natural selection occurs only if there is both (1) varia-
tion in the genetic information between organisms in a population and (2) varia-
tion in the expression of that genetic information—that is, trait variation—that
leads to differences in performance among individuals. The traits that positively
affect survival are more likely to be reproduced and thus are more common in
the population.
LS4.C: ADAPTATION
How does the environment influence populations of organisms over multiple
generations?
When an environment changes, there can be subsequent shifts in its supply of
resources or in the physical and biological challenges it imposes. Some individu-
als in a population may have morphological, physiological, or behavioral traits
that provide a reproductive advantage in the face of the shifts in the environment.
Natural selection provides a mechanism for species to adapt to changes in their
environment. The resulting selective pressures influence the survival and repro-
duction of organisms over many generations and can change the distribution of
traits in the population. This process is called adaptation. Adaptation can lead to
organisms that are better suited for their environment because individuals with
the traits adaptive to the environmental change pass those traits on to their off-
spring, whereas individuals with traits that are less adaptive produce fewer or no
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offspring. Over time, adaptation can lead to the formation of new species. In some
cases, however, traits that are adaptive to the changed environment do not exist in
the population and the species becomes extinct. Adaptive changes due to natural
selection, as well as the net result of speciation minus extinction, have strongly
contributed to the planet’s biodiversity.
Adaption by natural selection is ongoing. For example it is seen in the emer-
gence of antibiotic-resistant bacteria. Organisms like bacteria, in which multiple
generations occur over shorter time spans, evolve more rapidly than those for
which each generation takes multiple years.
Grade Band Endpoints for LS4.C
By the end of grade 2. Living things can survive only where their needs are met.
If some places are too hot or too cold or have too little water or food, plants and
animals may not be able to live there.
By the end of grade 5. Changes in an organism’s habitat are sometimes benefi-
cial to it and sometimes harmful. For any particular environment, some kinds of
organisms survive well, some survive less well, and some cannot survive at all.
By the end of grade 8. Adaptation by natural selection acting over generations is
one important process by which species change over time in response to changes
in environmental conditions. Traits that support successful survival and reproduc-
tion in the new environment become more common; those that do not become
less common. Thus, the distribution of traits in a population changes. In separated
populations with different conditions, the changes can be large enough that the
populations, provided they remain separated (a process called reproductive isola-
tion), evolve to become separate species.
By the end of grade 12. Natural selection is the result of four factors: (1) the
potential for a species to increase in number, (2) the genetic variation of indi-
viduals in a species due to mutation and sexual reproduction, (3) competition
for an environment’s limited supply of the resources that individuals need in
order to survive and reproduce, and (4) the ensuing proliferation of those organ-
isms that are better able to survive and reproduce in that environment. Natural
selection leads to adaptation—that is, to a population dominated by organisms
that are anatomically, behaviorally, and physiologically well suited to survive
and reproduce in a specific environment. That is, the differential survival and
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❚ Adaptive changes due to natural selection, as well as the net
result of speciation minus extinction, have strongly contributed to the
❚
planet’s biodiversity.
reproduction of organisms in a population that have an advantageous heritable
trait leads to an increase in the proportion of individuals in future generations
that have the trait and to a decrease in the proportion of individuals that do not.
Adaptation also means that the distribution of traits in a population can change
when conditions change.
Changes in the physical environment, whether naturally occurring or human
induced, have thus contributed to the expansion of some species, the emergence
of new distinct species as populations diverge under different conditions, and the
decline—and sometimes the extinction—of some species. Species become extinct
because they can no longer survive and reproduce in their altered environment. If
members cannot adjust to change that is too fast or too drastic, the opportunity
for the species’ evolution is lost.
LS4.D: BIODIVERSITY AND HUMANS
What is biodiversity, how do humans affect it, and how does it affect humans?
Human beings are part of and depend on the natural world. Biodiversity—the
multiplicity of genes, species, and ecosystems—provides humans with renewable
resources, such as food, medicines, and clean water. Humans also benefit from
“ecosystem services,” such as climate stabilization, decomposition of wastes, and
pollination that are provided by healthy (i.e., diverse and resilient) ecosystems.
The resources of biological communities can be used within sustainable limits, but
in many cases humans affect these ecosystems in ways—including habitat destruc-
tion, pollution of air and water, overexploitation of resources, introduction of
invasive species, and climate change—that prevent the sustainable use of resources
and lead to ecosystem degradation, species extinction, and the loss of valuable
ecosystem services.
Grade Band Endpoints for LS4.D
By the end of grade 2. There are many different kinds of living things in any area,
and they exist in different places on land and in water.
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By the end of grade 5. Scientists have identified and classified many plants and
animals. Populations of organisms live in a variety of habitats, and change in
those habitats affects the organisms living there. Humans, like all other organisms,
obtain living and nonliving resources from their environments.
By the end of grade 8. Biodiversity is the wide range of exist-
ing life forms that have adapted to the variety of conditions
on Earth, from terrestrial to marine ecosystems. Biodiversity
includes genetic variation within a species, in addition to spe-
cies variation in different habitats and ecosystem types (e.g.,
forests, grasslands, wetlands). Changes in biodiversity can
influence humans’ resources, such as food, energy, and medi-
cines, as well as ecosystem services that humans rely on—for
example, water purification and recycling.
By the end of grade 12. Biodiversity is increased by the for-
mation of new species (speciation) and decreased by the loss
of species (extinction). Biological extinction, being irrevers-
ible, is a critical factor in reducing the planet’s natural capital.
Humans depend on the living world for the resources
and other benefits provided by biodiversity. But human activ-
ity is also having adverse impacts on biodiversity through
overpopulation, overexploitation, habitat destruction, pol-
lution, introduction of invasive species, and climate change.
These problems have the potential to cause a major wave of
biological extinctions—as many species or populations of a
given species, unable to survive in changed environments, die
out—and the effects may be harmful to humans and other living things. Thus sus-
taining biodiversity so that ecosystem functioning and productivity are maintained
is essential to supporting and enhancing life on Earth. Sustaining biodiversity also
aids humanity by preserving landscapes of recreational or inspirational value.
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Print copies of the Next Generation Science Standards are available for pre-order now or you can view the online version at nextgenscience.org
The standards are based largely on the 2011 National Research Council report A Framework for K-12 Science Education: Practices, Crosscutting Concepts, and Core Ideas.
