Evolution above the species level (macroevolution) consists of two
distinct processes: changes within a lineage (anagenesis) and
the branching of lineages (cladogenesis). Evolutionary trends are
adaptive and are usually opportunistic, following no plan or goal but
rather taking the path of least resistance. Cladogenesis fills the biosphere
with an ever-increasing number of species, arranged into classifications
which reflect descent.
Species that fail to adapt to changing conditions
become extinct.
Lineage: An ancestor-to-descendent sequence of species.
Trend: Continued morphological change within a lineage.
Parallelism: Independent occurrence of the same or similar
trends in different lineages.
Convergence: Similar adaptations in unrelated lineages.
Cladogenesis: The branching of lineages by speciation.
Anagenesis: Evolution within a lineage, between branching points.
Evidence for the adaptiveness of trends:
- Trends often persist for a long time.
- Parallel trends often occur independently.
- Evolutionary rates vary: trends speed up or slow down; they may even
stop altogether or reverse direction.
- Trends in different characters do not always go together but occur
independently (mosaic evolution) and at different rates and times.
For this reason, transitional species like Archaeopteryx are a mosaic
of primitive and advanced features mixed together.
Opportunism: Evolution follows no plan or goal, but
instead takes the path of least resistance.
- Cladogenesis fills the biosphere with more and more species (and niches).
- Diversity among the descendents of a single species (adaptive
radiation) often results.
- Functional problems are often solved differently in different
lineages (multiple solutions, such as diversity among eyes).
- The same trend often occurs repeatedly (iterative evolution).
- Convergence (and its imperfections) show that similar adaptive
opportunities may arise independently more than once.
- Organs that change function usually serve both old and new functions
simultaneously during the transition.
Rates and modes of evolution: Evolutionary rates may measure either
anagenesis or cladogenesis or both. Rates calculated in different ways
are usually not comparable.
- Since Darwin, most evolutionists have viewed evolution as
a continuous, gradual process.
- Many scientists now view evolution as a series of steady equilibria
punctuated by infrequent episodes of very rapid change (the
punctuated equilibrium theory).
Results of evolution: The results of macroevolution can be seen in the diversity
among species that is reflected in their anatomical structure and
in our classifications. These results include:
- Adaptations: Features which help organisms cope with and exploit
their environments.
- Analogy: Similarity among species resulting from adaptations
to similar functional requirements.
- Homologies: Deep-seated resemblances reflecting common ancestry,
often despite adaptive differences.
- Evolutionary classifications: Descent with modification results in classifications
that contain groups within groups; these groups have always been considered
"natural," even by pre-evolutionary taxonomists. Whenever new technology
allows new types of variation to be studied (e.g., DNA sequences), most
variation follows the groups established by earlier methods.
Extinction: Species that cannot adapt to change become extinct.
- Extinction may come either early or late in the history of a group.
- Extinction may occur at times of either low or high diversity.
- Some paleontologists believe that rates of speciation and extinction
tend to be approximately equal for many large groups.
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