As ringo points out, ancient common ancestors are inferred from observations. Such inferences should be regarded as testable scientific theory rather than facts.
Indeed. It is important to differentiate between the various processes of evolution, which are real, observed, facts, and the theory of evolution, which has been tested thousands of times and validated by the absence of negative data. The theory of evolution is accepted as the best explanation of life as we know it, from the world around us, from historical information, genetic information and fossil information.
This is a work in progress I have written which is currently in limbo in proposed topics:
Introduction to Evolution:
An Introduction to Evolution
(1) The process of evolution involves changes in the composition of hereditary traits, and changes to the frequency of their distributions within breeding populations from generation to generation, in response to ecological challenges and opportunities for growth, development, survival and reproductive success in changing or different habitats.
This is sometimes called microevolution, however this is the process through which all species evolve and all evolution occurs at the breeding population level.
Mutations to existing hereditary traits (ie for eyes and ears) can cause changes in the composition of hereditary traits for individuals in a breeding population, but not all mutations are expressed (and many are in non-hereditary areas). In addition there are many different kinds of mutations and they have different effects (from small to large), especially if they affect the developmental process of an organism.
Natural Selection and Neutral Drift can cause changes in the frequency distribution of hereditary traits within a breeding population, but they are not the only mechanisms known that does so. Selection processes act on the expressed genes of individual organisms, so bundles of genetic mutations are selected rather than individual genes, and this means that non-lethal mutations can be preserved. The more an individual organism reproduces the more it is likely to pass on bundles of genes and mutations to the next generation, increasing the selection of those genes.
The ecological challenges and opportunities change when the environment changes, when the breeding population evolves, when other organisms within the ecology evolve, when migrations change the mixture of organisms within the ecology, and when a breeding population immigrates into a new ecology. These changes can result in different survival and reproductive challenges and opportunities, affecting selection pressure, perhaps causing speciation, perhaps causing extinction.
This is a two-step feedback response system that is repeated in each generation:
Like walking on first one foot and then the next.
Mutations of hereditary traits have been observed to occur, and thus this aspect of evolution is an observed, known objective fact, rather than an untested hypothesis.
Different mixing of existing hereditary traits (ie Mendelian inheritance patterns) have been observed to occur, and thus this aspect of evolution is an observed, known objective fact, rather than an untested hypothesis.
Natural selection has been observed to occur, along with the observed alteration in the distribution of hereditary traits within breeding populations, and thus this aspect of evolution is an observed, known objective fact, and not an untested hypothesis
Neutral drift has been observed to occur, along with the observed alteration in the distribution of hereditary traits within breeding populations, and thus this aspect of evolution is an observed, known objective fact, and not an untested hypothesis.
Thus many processes of evolution are observed, known objective facts, and not untested hypothesies.
If we look at the continued effects of evolution over many generations, the accumulation of changes from generation to generation may become sufficient for individuals to develop combinations of traits that are observably different from the ancestral parent population.
(2) The process of lineal change within species is sometimes called phyletic speciation, or anagenesis.
This is also sometimes called arbitrary speciation in that the place to draw the line between linearly evolved genealogical populations is subjective, and because the definition of species in general is tentative and sometimes arbitrary.
If anagenesis was all that occurred, then all life would be one species, readily sharing DNA via horizontal transfer (asexual) and interbreeding (sexual) and various combinations. This is not the case, however, because there is a second process that results in multiple species and increases the diversity of life.
(3) The process of divergent speciation, or cladogenesis, involves the division of a parent population into two or more reproductively isolated daughter populations, which then are free to (micro) evolve independently of each other.
The reduction or loss of interbreeding (gene flow, sharing of mutations) between the sub-populations results in different evolutionary responses within the separated sub-populations, each then responds independently to their different ecological challenges and opportunities, and this leads to divergence of hereditary traits between the subpopulations and the frequency of their distributions within the sub-populations.
Over generations phyletic change occurs in these populations, the responses to different ecologies accumulate into differences between the hereditary traits available within each of the daughter populations, and when these differences have reached a critical level, such that interbreeding no longer occurs, then the formation of new species is deemed to have occurred. After this has occurred each daughter population microevolves independently of the other/s. These are often called speciation events because the development of species is not arbitrary in this process.
If we looked at each branch linearly, while ignoring the sister population, they would show anagenesis (accumulation of evolutionary changes over many generations), and this shows that the same basic processes of evolution within breeding populations are involved in each branch.
An additional observable result of speciation events, however, is a branching of the genealogical history for the species involved, where two or more offspring daughter species are each independently descended from the same common pool of the ancestor parent species. At this point a clade has been formed, consisting of the common ancestor species and all of their descendants.
With multiple speciation events, a pattern is formed that looks like a branching bush or tree: the tree of descent from common ancestor populations. Each branching point is a node for a clade of the parent species at the node point and all their descendants, and with multiple speciation events we see a pattern form of clades branching from parent ancestor species and nesting within larger clades branching from older parent ancestor species.
Where A, B, C and G represent speciation events and the common ancestor populations of a clade that includes the common ancestor species and all their descendants: C and below form a clade that is part of the B clade, B and below form a clade that is also part of the A clade; G and below also form a clade that is also part of the A clade, but the G clade is not part of the B clade.
The process of forming a nested hierarchy by descent of new species from common ancestor populations, via the combination of anagenesis and cladogenesis, and resulting in an increase in the diversity of life, is sometimes called macroevolution. This is often confusing, because there is no additional mechanism of evolution involved, rather this is just the result of looking at evolution over many generations and different ecologies.
The process of anagenesis, with the accumulation of changes over many generations, is an observed, known objective fact, and not an untested hypothesis.
The process of cladogenesis, with the subsequent formation of a branching nested genealogy of descent from common ancestor populations is an observed, known objective fact, and not an untested hypothesis.
This means that the basic processes of "macroevolution" are observed, known objective facts, and not untested hypothesies, even if major groups of species are not observed forming (which would take many many generations).
(4) The Theory of Evolution (ToE), stated in simple terms, is that the process of anagensis, and the process of cladogenesis, are sufficient to explain the diversity of life as we know it, from the fossil record, from the genetic record, from the historic record, and from everyday record of the life we observe in the world all around us.
This theory is tested by experiments and field observations carried out as part of the science of evolution.
If a species is observed to change over generations (anagenesis), we can predict that it will be due to (a) changes in the expressed hereditary traits (genes, morphology, development), (b) that the changes were either neutral or improved the survival and reproductive success of individuals in response to their ecological challenges and opportunities and (c) that if they improved the fitness of the carriers that it will spread within the breeding population in following generations.
If a clade is observed to form (cladogenesis), we can predict that it will be due to (a) reproductive isolation between daughter populations and (b) independent evolution (anagenesis) within each daughter population. We can also predict the formation of the clade will fall within a nested hierarchy pattern.
These predictions can be tested against the fossil record, the genetic record, the historical record, and the everyday record of life we observe in the world all around us. Biologists have been testing this theory for 150 plus years, and thus far they have confirmed that the process of evolution, and the process of speciation, are sufficient to explain the diversity of life as we know it.
General references and further study resources
- Berkeley U. and U. of California Museum of Paleontology Teachers Guide
- U. of Michigan on-line course material
- Talk Origins Introduction to Evolutionary Biology
- Overview of cladistics by Wikipedia