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Chapter 3

Chapter 3-Exploring the Diversity of Life

Biological Sciences
Course Code
Zachariah Campbell

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Chapter 3: Selection, Biodiversity, and Biosphere
3.1 Biodiversity
Grouping Organisms : How organisms obtain carbon
oAutotrophs (self-nourishment) : most plants are autotrophs which synthesize organic carbon molecules
using inorganic carbon.
oHeterotrophs : all animals are heterotrophs, meaning that they obtain carbon from organic molecules,
either from living hosts or from organic molecules in the products, wastes or remains of dead organisms.
oOrganisms are also divided according to the source of the energy they use to drive biological activities.
oChemotrophs (chemical-nourishment): obtain energy by oxidizing inorganic or organic substances
oPhototrophs: obtain energy from light.
3.2 Selection
Selection occurs when…
oSome force or phenomenon affects the survival of individual organisms. (Ex. an unexpected spring frost
can kill many plants in your garden. Only the cold-resistant plants survive the selective force
oA large population of individuals is exposed to a lethal factor and only resistant individuals survive to
reproduce. If resistance is inherited, then the offspring of survivors will be resistant.
The key factors behind selection are a selective force (pressure) and the capacity for explosive
population growth. When these factors overlap, we can be overrun by pests, such as antibiotic-resistant bacteria.
When this happens, the consequences for humans can be deadly.
Selection is the major force responsible for evolution and biodiversity.
3.3 Evolution
Evolution: a gradual change in the characteristics of a population of organisms over time can be the result of
Evolution is a central key to understanding the diversity of life on earth.
We consider evolution to be an organizing force in life and the foundation of modern biology.
The theory of evolution explains both the unity and the diversity of all life
oIt tells us that all organisms alive today descended from a common ancestor
This explains why all organisms shape features, such as the use of ATP as a cellular energy
source, DNA as genetic material and plasma membranes composed of lipid bilayers.
Evolution also tells us that species change over time as a result of natural selection.
The central ideas of Darwin’s theory of evolution by natural selection can be summarized as
oIndividual organisms in a population vary in many heritable traits.
oAny population has the potential to produce far more offspring than the environment can
support. Competition for limited resources means that only some individuals survive.
oSome individuals in the population have traits that give them an advantage in their local
environment; these organisms are more likely to survive and reproduce.
oThese organisms pass on favorable traits to their offspring. Over time, the incidence of
the traits will change in the population.
Different environments favor different traits
oEven though all organisms share a common ancestor, they have diverged over
evolutionary time in response to the selection pressures of different environments.
oThe process of adaptive radiation is an example of such divergence.
Adaptive Radiation: Diversification of Lineages of Life
oIn the history of life on earth, organisms have had to overcome fundamental barriers that,
once crossed, opened many new opportunities for diversification.
oEx. The development of photosynthesis. Organisms with the ability to convert solar
energy into usable chemical energy survived and thrived as they exploited the ‘new’ energy source.
oAdaptive radiations, occur when an evolutionary breakthrough allows diversification of
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oAdaptive radiations are a recurring theme in the development of biodiversity.
An organism may move into a new adaptive zone after a chance innovation (advance,
improvement) allows it to use the environment in a unique way.
oThe ability of plants to move onto land opened new opportunities
for animals.
oThey dehydration-resistant eggs of early reptiles enabled them to
complete their life cycle on land, opening terrestrial habitats to them.
oThe evolution of flowers that attract insect pollinators was a key
innovation in the history of flowering plants.
oAn adaptive zone may open up after the demise of a successful
Land: Organisms Conquer a New Frontier
oThe movement of organisms onto land presented many
challenges and many ‘terrestrial’ organisms actually live in films of water, so they have not abandoned
an aquatic existence.
oIncluded on the list of challenges are matters of support,
conservation of water, reproduction and disposal of wastes.
oDifferences between water and air include density and
viscosity which in turn affect rates of diffusion and availability of oxygen.
oAnimals operating in water extract dissolved oxygen,
although some photoautotrophs actually break water molecules in the process of photosynthesis.
oAnimals that breathe air have more ready access to oxygen
and spend less of their overall energy budget acquiring it than aquatic animals.
oTruly terrestrial plants and animals have more complex
body designs than many of their aquatic counterparts.
oBy the end of the Devonian, terrestrial plants had
developed specialized sexual organs, stems with mechanisms for fluid transport, structural elements
such as wood to provide support, roots for anchorage, leaves as sites of photosynthesis, stomata in the
leaves to allow passage of CO2 and O2, and seeds.
oTerrestrial animals had skeletons for support and anchoring
muscles (allowing locomotion), organs for gaseous exchange (breathing atmospheric oxygen), and
systems for circulating materials within the body.
oTerrestrial animals and plants also had waterproof
coverings to minimize the chances of desiccation. Terrestrial animals used nontoxic excretory products
(urea and uric acid), whereas aquatic ones still relied heavily on ammonia.
Tree of Life
oEvolution is the idea behind representation of ‘trees of life’.
oTraditional trees of life are designed to illustrate the
relationships between organisms over time and may be presented in the context of a geologic time
oThe diversity of life is overwhelming because of the broad
coverage that can be achieved with genetic data.
3.4 The Biosphere
The biosphere is the area occupied by life on earth, from
the depths of the ocean to the sky above.
The various physical environments of earth and their
different abiotic factors such as sunlight, temperature, humidity, wind speed, cloud cover and rainfall, influence
the evolution and diversity of organisms.
These abiotic factors contribute to a region’s climate, the
weather conditions prevailing over an extended period of time.
Climates vary on global, regional and local scales and
undergo seasonal changes almost everywhere.
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