Page 315 - ABCTE Study Guide_Neat

Since the only variations that can be passed directly to offspring are heritable, natural selection acts
upon the genetic traits. Over time, three basic patterns of selection can occur: stabilizing, directional, and
disruptive. Directional selection occurs when populations shift toward one extreme or another for any
given trait (e.g., size). Disruptive selection acts against the intermediate trait in favor
of both extremes. Stabilizing selectionacts against the extremes, making organisms more similar.

Other forms of variation come from learned behavioral adaptations to the environment. Some of these
traits can be taught to young ones, although they may not be passed down genetically. For example, a
bear cub learns much about hunting from traveling with its mother. An older, more experienced mother is
likely to be a more effective hunter and a better provider for her offspring than a new, young mother.
However, this variation is not necessarily heritable unless the behavior is somehow coded in the genes.

Origin and Evolution of Life on Earth

Evidence points to the age of Earth as being about four billion years. The surface four billion years ago
was extremely harsh and inhospitable to any type of life, but about that same time, Earth began to retain
water on its surface. The materials required for the formation of organic molecules were present in the
prototypic oceans and atmosphere. Scientists have demonstrated, under laboratory conditions, that
molecules like amino acids can be formed if the requisite ingredients are present and they are subjected
to sufficient energy. Lightning and volcanic eruptions were common on Earth at that time, and in waters of
the early surface, self-replicating molecules (nucleic acids) were formed.

The formation of these molecules was not entirely random but actually fairly probable given the physical
conditions at the time. Experiments have also shown that in heated water, amino acids will form protein
chains that when cooled can form selectively permeable spheres—a likely precursor to the plasma
membranes we recognize today.

As conditions eventually quieted on Earth’s surface, fossils of living organisms formed. Rocks as old as
3.5 billion years contain fossils of prokaryotic cells resembling bacteria found in tidal mud flats today. As
photosynthetic bacteria evolved around 2.5 billion years ago, the atmosphere began to change markedly
and it became suitable for many more life-forms.

Over subsequent years, climate changes, catastrophes, and other factors have brought about numerous
changes and the extinctions of once living organisms. The best known of these catastrophes is an
asteroid impact that occurred sixty-five million years ago. According to some theories, that impact greatly
cooled Earth, brought about the extinction of the dinosaurs, and ushered in the age of mammals. There
have been other events and changes in Earth’s climate. In fact, most species that have existed on Earth
are now extinct.

Today, as DNA analysis has become available, we have been able to identify relationships among
existing organisms, revealing common ancestry or lack thereof, and thus further filling in the gaps in our
knowledge.