Encyclopedia of Evolution.pdf - Online Reading Center

iology
D. Response to environment. Both plants and animals
have mechanisms that detect environmental stimuli, and processes
that allow a response to them. In plants stimuli are usually
detected by molecules such as pigments, and hormones
allow the responses, such as bending toward light. Animals
have sensory organs and a central nervous system that figures
out the appropriate response to the stimuli.
V. Sexual reproduction. Nearly all organisms have sexual life
cycles. Certain organs produce cells with only half the genetic
information that normal cells contain (see meiosis). In many
plants and animals, these reproductive cells come in two sizes:
The large female ones are either megaspores or eggs, while
the small male ones are either microspores or sperm. The
male cells of one individual may fuse with the female cells of
another, a process called fertilization. Fertilization produces a
zygote, which may be sheltered inside of a seed, an eggshell,
or a womb (see sex, evolution of; life history, evolution
of). Sexual reproduction is not necessary for survival
but generates genetic diversity.
VI. Inheritance patterns. Because the traits of different
parents can be shuffled into new combinations by sexual
reproduction, new combinations of traits can occur in each
generation (see Mendelian genetics).
VII. Populations and evolution. All the interacting organisms
of one species in one location constitute a population.
Populations can grow rapidly, but limited resources prevent
them from doing so forever. Because some individuals in a
population reproduce more successfully than others, natural
selection occurs and the population evolves (see natural
selection). If the population separates into two populations,
they can evolve into two species (see speciation).
VIII. Diversity. Evolution has produced millions of species.
Biologists have attempted to classify organisms on the basis
of their evolutionary diversification rather than merely on the
differences in their appearance. One such classification is:
A. Domain Archaea: bacteria-like cells that, today, survive
in extreme environments (see archaebacteria).
B. Domain Eubacteria: bacteria-like cells that may use
sunlight (photosynthetic bacteria) or inorganic chemicals
(chemosynthetic bacteria) as energy sources for producing
organic molecules or may obtain energy from organic molecules
in living or dead organisms (heterotrophic bacteria)
(see bacteria, evolution of). Early in evolutionary history,
some heterotrophic bacteria invaded larger cells, and today
they are the mitochondria that release energy from sugar in
nearly all larger cells. Early in evolutionary history, some
photosynthetic bacteria invaded larger cells, and today they
are the chloroplasts that carry out photosynthesis in some
larger cells (see symbiogenesis).
C. Domain Eukarya: organisms composed of complex
cells with DNA in nuclei (see eukaryotes, evolution of).
The evolutionary linages of the eukaryotes are still being
worked out. From within these lineages evolved:
1. The plant kingdom. Land plants are descendants
of green algae (see seedless plants, evolution of; gymnosperms,
evolution of; angiosperms, evolution of).
2. The fungus kingdom consists of decomposers and
pathogens that absorb food molecules.
3. The animal kingdom. Animals, descendants of one
of the lineages of protozoa (see invertebrates, evolution
of; fishes, evolution of; amphibians, evolution of; reptiles,
evolution of; birds, evolution of; mammals, evolution
of).
IX. Synecological context. This is the ecological interaction
of organisms with one another (syn- comes from the Greek
for together).
A. General ecological interactions. Evolution has refined
the interactions that broad groups of organisms have with
one another (see coevolution): for example, herbivores that
eat plants and plants that defend themselves from herbivores;
predators that eat prey, and prey that defend themselves
from predators; pollinators and flowering plants. Many complex
animal behavior patterns have evolved, particularly in
response to sexual selection.
B. Symbiotic ecological interactions. Symbiosis results
from the very close interaction of two species, in which at
least one of the species depends upon the other. These interactions
have resulted from coevolution.
1. Parasitism occurs when a parasite harms the host.
2. Commensalism occurs when a commensal has no
effect on the host.
3. Mutualism occurs when both species benefit.
Mutualism is so widespread that, in some cases, different species
of organisms have actually fused together and formed
new kinds of organisms.
4. Natural selection favors the evolution of hosts
that resist parasites and sometimes favors the evolution of
parasites that have only mild effects on their hosts. Natural
selection can favor the evolution of parasitism into commensalism,
and commensalism into mutualism.
C. Ecological communities are all of the interacting species
in a location. They generally form clusters, based upon
temperature and moisture conditions in different parts of the
world: for example, tundra, forests, grasslands, deserts, lakes,
shallow seas, deep oceans. Each community contains microhabitats
with smaller communities within them.
D. Ecological communities continually undergo change.
On a large scale, continents drift (see continental drift),
mass extinctions occur, and climates fluctuate (see ice
ages; Snowball Earth). On a small scale, disturbances
such as fires are followed by stages of regrowth called
ecological succession. Billions of years of these changes
have produced an entire evolutionary history, as described
throughout this book, and the world as humans know it
today.
The Earth is filled with living organisms. But is the Earth
itself alive? It does not match all of the characteristics listed
above to qualify as an organism, but it does have some processes
that produce a semblance of homeostasis, which is not
readily understandable by the operations of the organisms on