MACROEVOLUTION


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.
The geological time scale divides the last ~530 million years into 12 periods. Fossils differ in the degree to which smaller structural details are preserved and in the degree of chemical alteration of the original material. The fossil record allows us to test various evolutionary theories against the actual long-range history of life on Earth.

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.
    Illustrations
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.

Lessons learned from studying the fossil record: The fossil record can be used to test various theories against the actual record of life on Earth. No proposed theory or evolutionary mechanism is acceptable if it conflicts with this historical record.
  • The fossil record shows a historical process of branching and diversification, not just a single linear sequence. "Evolution is a bush, not a ladder."
  • Cope's rule: Size increases frequently and decreases far less often.
  • Williston's rule: Repeated parts (like multiple legs or segments) often become less numerous and more different from one another.
  • Dollo's law: Like other historical processes, evolution never repeats itself exactly. Because of probability considerations, small, simple changes may reverse, but larger and more complex changes never do. Evolution is thus constrained (limited) by its own history.