There is a simple grandeur in the view of life with its powers of growth, assimilation and reproduction, being originally breathed into matter under
one or a few forms, and that whilst this our planet has gone circling on according to fixed laws, and land and water, in a cycle of change, have gone on
replacing each other, that from so simple an origin, through the process of gradual selection of infinitesimal changes, endless forms most beautiful and
most wonderful have been evolved, Charles Darwin, 1842 sketch, later used in 1859 (with some rewording) as the concluding lines of
The Origin of Species.
"These complex affinities and the rules for classification, receive a rational explanation on the theory of descent, combined with the principle of
natural selection, which entails divergence of character and the extinction of intermediate forms. How inexplicable is the similar pattern of the hand of
a man, the foot of a dog, the wing of a bat, the flipper of a seal, on the doctrine of independent acts of creation! how simply explained on the principle
of the natural selection of successive slight variations in the diverging descendants from a single progenitor!" -- Charles Darwin, 1868,
Variation of Animals and Plants Under Domestication, Introduction
"A plausible argument can be made that evolution is the control of development by ecology." -- Leigh Van Valen, 1973
"Why then is not every geological formation and every stratum full of such intermediate links? Geology assuredly does not reveal any such finely-graduated organic chain; and this, perhaps, is the most obvious and serious objection which can be urged against the theory." -- Chapter 9 "On the Imperfection of the Geological Record", On the Origin of Species by Means of Natural Selection (1859), Charles Darwin
"Since we proposed punctuated equilibria to explain trends, it is infuriating to be quoted again and again by creationists -- whether through design or stupidity, I do not know -- as admitting that the fossil record includes no transitional forms. The punctuations occur at the level of species; directional trends (on the staircase model) are rife at the higher level of transitions within major groups." --The Panda's Thumb (1980), Stephen Jay Gould
BIG QUESTION:What is evolution? And where do species come from?
What is "Evolution?"
Literally "unfolding" or "unravelling"
Pre-1860s, term used for development of an embryo
Generally used for "change through time":
Sometimes for predetermined set of changes, such as stellar evolution or evolution of
a magma
Also for the general process of change, as in "evolution of the automobile"
More specifically, organic evolution, or the change of groups of living things
through time
Often summed up in terms of genetics: "changes of gene frequency through time" (literally
true, if a bit boring...)
Darwin himself used the phrase "DESCENT WITH MODIFICATION" rather than "evolution"
In other words, evolution in the broadest sense is no more than the observation that
"none of us looks exactly like our parents."
Species and Life's Diversity
Were species fixed: that is, unchanging? Many people thought so:
"Species have a real existence in nature, and a transition from one to another does not exist" -- History of the Inductive Sciences (1837), Rev. William Whewell (incidentally, the man who coined the word "scientist")
However, several sets of observations showed that this was not so:
Geographic variations ("varieties" or "subspecies"): were the end members of the variation the same species or different ones?
Geologic variations: that is, fossils were often not the same as modern species
Natural hybrids: offspring of two distinct species that (despite the common definition otherwise) are sometimes fertile themselves
Fixity vs. Transmutation
Traditionally, people accepted the fixity of species just as they accepted that the world today is pretty much the same now as in the past.
Theological argument for fixity under the Biblical concept of the Plenum ("fullness"):
Ecclesiastes 1:9 and 3:14-15, if you want to look it up
"Nothing new under the sun": nothing has been taken from Creation, nor added to it
Many early naturalists accepted the Plenum, but evidence of extinction (man-made, as in the dodo, and natural, as in fossils) showed that things could be removed from Creation. What about adding to it?
The discoveries of the early (18th and 19th Century) geologists put paid to the idea that the surface of the Earth was unchanging:
"These facts, unknown to the vulgar, but well known to all who observe nature, force the physical scientist to recognize that all the surface of our globe has changed; that it has had other seas, other continents, another geography." --Nicolas Boulanger (1722-1759)
and
"Life, therefore, has been often disturbed on this earth by terrible events - calamities which, at their commencement, have perhaps moved and overturned to a great depth the entire outer crust of the globe, but which, since these first commotions, have uniformly acted at a less depth and less generally. Numberless living beings have been the victims of these catastrophes; some have been destroyed by sudden inundations, others have been laid dry in consequence of the bottom of the seas being instantaneously elevated. Their races even have become extinct, and have left no memorial of them except some small fragments which the naturalist can scarcely recognize." --'Preliminary discourse', to Recherches sur les Ossemens Fossiles (1812), trans. R. Kerr Essay on the Theory of the Earth (1813),
Baron Georges Leopold Chretien Frederic Dagobert Cuvier
While some thinkers once thought that life as we see it now is the way it has always been, the discovery of the fossil
record showed that strange creatures once roamed the Earth that are no longer there. Naturalist John Herschel (in an 1836 letter to Charles Lyell)
wrote:
"I allude to that mystery of mysteries, the replacement of extinct species by others. Many will doubtless think your speculations too bold, but it is as well to face the difficulty at once. For my own part, I cannot but think it an inadequate conception of the Creator, to assume it as granted that his combinations are exhausted upon any one of the theatres of their former exercise, though in this, as in all his other works, we are led, by all analogy, to suppose that he operates through a series of intermediate causes, and that in consequence the origination of fresh species, could it ever come under our cognizance, would be found to be a natural in contradistinction to a miraculous process -- although we perceive no indications of any process actually in progress which is likely to issue in such a result."
How to explain these observations? Two main possibilities:
The successive appearance and disappearance of different forms through time, without genetic connection (as
supported by Owen, Cuvier, Herschel, and others)
Transmutationism: direct lineal relationships between ancestor and descendant species. So living species are descendants of earlier distinct species, which themselves were the descendants of even earlier ones. "Transmutationism" became known as "evolution" after the work of Darwin and Wallace.
Transmutationism, a set of early evolutionary models, accepted by several prominent scientists by the late 1700s. Among them were Jean Baptiste Pierre Antoine de Monet, Chevalier de Lamarck (normally known as Jean Baptiste de Lamarck) and Erasmus Darwin (doctor, scientist, surgeon, abolitionist, and INCREDIBLY rich).
Fossils demonstrated that the living component of the Earth changed through time; shared homologies showed connections between groups; adaptations showed organisms "fit" to their environment. Already accepted the central tenets of Evolutionary Theory:
The Diversity of Living Things is the Product of Descent with Modification
New species are the modified descendants of previously existing species
But what caused the modifications?
Transmutationist models:
Spontaneous generation of new lineages of organisms throughout time; thus, many living things represent separate origins at different points in Earth History
Within each lineage, "driving forces" impel organisms towards improvement (i.e., simple forms become complex) down predetermined pathways
Inheritance is from use and disuses: characters acquired during the lifetime of an individual are passed onto descendants
Problems with these ideas, however:
Spontaneous generation doesn't work
"Driving forces" never identified, and are more metaphysical than naturalistic
Continuity of lineages through long periods of Earth history, rather than appearance, transformation, and reappearance:
Also, fossils documented linkages between groups rather than separation
Inheritance doesn't happen by use & disuse; transformations to adult are not passed onto offspring
Darwin did not discover evolution, nor did its study stop with his work. At least some of the evidence for evolution was long known before his time (although we've added a LOT, even to these lines!)
Initial Evidence for Evolution
Homologies: the same anatomical structures ("body parts") are repeated in different organisms. This allows us to recognize how they differ from each other, and how they resemble each
other.
Living things can be grouped using a nested hierarchy based on shared presence of homologous structures of similar form
System of classification codified by Carolus Linnaeus (18th Century Swedish botanist)
Many of his principles, such as Latin names for organisms, and the use of genus and species still used today
However, species are not fixed entities. They vary across their range, and they can often hybridize with closely related forms
Adaptations: any structure or behavior which allows an organism to interact with its environment in certain specific ways
Analogous structures: non-homologous structures found two or more organisms that are adapted for the same function
Vestigial structures: anatomical features which have some significant adaptive function in some forms, but are reduced and non-functional (or nearly non-functional) in a related form
Transitional fossils: extinct species intermediate in morphology between now-distinct groups
Both studied natural history, including geology, in the UK
Thus, both were familiar with fossil organisms and with the (then-new) ideas of geologic time
Both traveled to distant lands (Darwin to South America, the Galápagos Islands, and various other localities in the Pacific Ocean; Wallace to Amazonia and Indonesia)
Both made collections of organisms, and so had direct experience with the varieties of nature
The two made the same sets of important observations independently, and independently came up with the same mechanism
to explain evolution. Darwin (older than Wallace) had developed his ideas earlier, but kept them secret. In 1858 when Wallace asked Darwin for advice about his ideas, Darwin went to other scientists to present both his and Wallace's ideas at the same time, so that they both got credit for their independent discovery. (However, Darwin's book On the Origin of Species by Means of Natural Selection sold extremely well, so more people then and now know Darwin's name.)
Their model was called Natural Selection, and was analogous to "artificial selection" (e.g., domestication). Darwin and Wallace's observations:
Variability: There is variation in all populations.
No two members of a population are totally identical.
Some sources of variation include age and sexual differences; the results of factors that happened during the lifetime (differences of nutrition, disease, accident, etc.); individual difference in inherited traits; etc.
The idea that individual variation was significant was a blow to previous models of Nature. Most earlier natural historians believed in perfect types, and thought variation was degeneration from those types. Darwin and Wallace documented that the variation is the reality, and the "perfect types" were just myths.
Heritability: Some (but not all) variation is inherited.
Causal mechanism of inheritance unknown in Darwin's time.
Discovery by Gregor Mendel of genetics came later, and discovery of DNA came later still
Heritable traits are coded in DNA and passed on to descendants
Note that DNA is NOT a "blueprint" as commonly thought: it is a set of instructions for putting bodies together and maintaining them after they've been built
Each little instruction is called a gene: a piece of code that helps the cell to build a protein
Most genes have slightly different versions called alleles that produce different end products
It is these alleles (one copy for each gene per parent) that is passed on to offspring
Different combinations of alleles result in different traits being expressed (that is, different phenotypes). Depending on the particular combination of alleles an offspring gets, they might have the same trait as their mother, their father, or something different than either.
This was the major source of individual variation that Darwin & Wallace never knew about!
Mutations are new variations in heritable traits, caused by miscopied DNA (duplication of parts of genes; miswritten code; etc.)
Some mutations may be deleterious (they result in harm to the organism)
Many mutations may be neutral (they don't benefit the organism in an obvious way, nor hurt it)
A small number of mutations may wind up being beneficial (the variation they produce allow it to do better somehow
in the world)
Superfecundity: Organisms produced far more offspring than can possibly survive
Application of demographer Thomas Malthus' reproductive excess concept to Nature
Violated another previously-held belief: that Nature was perfect and everything had its place
Thus, IF some variation gives the individual a slight advantage (bigger, stronger, smaller, smarter, less tasty, whatever) at surviving; and IF that variation is heritable; THEN there is a somewhat better than average chance that organisms with that variation will survive to bear the next generation. Over the long expanse of geologic time, the
accumulation of these variations will change the population from one form to another: the origin of species.
"As many more individuals of each species are born than can possibly survive; and as, consequently, there is a frequently recurring struggle for existence, it follows that any being, if it vary however slightly in any manner profitable to itself, under the complex and sometimes varying conditions of life, will have a better chance of surviving, and thus be naturally selected. From the strong principle of inheritance, any selected variety will tend to propagate its new and modified form." -- From "Introduction", The Origin of Species, Charles Darwin (1859)
Hence,
Natural Selection is the differential survival and reproduction of variants in a population resulting in a net change in phenotype of the descendants.
(Short form: "Natural selection is the differential survival and reproduction of variants in a population.")
(Even shorter form with a 20th Century slant: MUTATION PROPOSES, SELECTION DISPOSES)
(Or, a different emphasis, from a 1973 book review by paleontologist Leigh Van Valen: Evolution is the Control of Ecology on Development)
If Evolution can be summarized as "no one is identical to their parents", then Natural Selection can be summarized as
"no one is identical to their siblings, either; plus, life's hard!"
Key points of Natural Selection:
Does NOT happen to individuals, only to populations (lineages)
Analogous to "artificial selection" (domestication), but operates:
On all traits rather than a few (humans can keep alive crops, farm animals, or pets that might otherwise die in the wild; obviously, wild plants and animals don't have that help!)
Over vast amounts of geologic time, rather than just a few generations
Does NOT require simple things evolving into complex: sometimes a simplified mutation of a structure might be advantageous
than the ancestral complex one (hence, vestigial organs)
Cannot evolve towards something with a goal in mind; only favors variations that are advantageous at the time of selection
"Survival of the Fittest"?: Not as such. Phrase not in the earlier editions of the Origin, nor was it coined by Darwin. Comes from economist/philosopher Herbert Spencer:
Unlike popular idea, evolutionary fitness is NOT being the biggest, strongest, fastest, etc.
So a great grandmother with dozens of children, grandchildren, and great grandchildren is far more "fit" (in evolutionary terms) than all the childless Nobel prize winners and Olympic athletes put together!
From Darwin and Wallace, we get the beginnings of modern evolutionary theory. It has five major components:
Evolution is descent with modification: that is, the anatomical traits and other features of populations change over
time from generation to generation
These modifications occur relatively slowly on average: small incremental changes added up over many generations
Populations may diverge into two or more distinct lineages (which may or may not produce their own descendant branches)
All species share a common ancestry: thus, the shape of the history of lineages can be seen as a Tree of Life
Much (although not all) evolutionary change is due to natural selection, which is the sole process for producing adaptations
Some of the things that come out of Darwin's work:
The Importance of Time: "No one but a practiced geologist can really comprehend how old the world is, as the measurements refer not to the revolutions of the sun & our lives." -- Notebook E, late 1838 to 1839
The Importance of Geologic Change & Isolation: "Change of external conditions, and isolation either by chance landing of a form on an island, or subsidence dividing a continent, or great chain of mountains, and the number of individuals not being numerous will best favour variation and selection... Barrier would further act in preventing species formed in one part migrating to another part." -- Sketch, 1842
Some things Evolution and Natural Selection are NOT:
An argument for atheism per se (at least any more than geologic time, the germ theory of disease,
Newtonian/Einsteinian physics, etc., are)
Although clearly, like geologic time, it required a world that operated on a different time scale than a literal reading of Genesis!
An account of the origin of Life (that's abiogenesis: a different field of research)
A prescription for social behavior or policy (after all, Natural Selection only operates by the mass death of many individuals: hardly a good society!)
A relevant video:
Here is Carl Sagan's summary of Darwinian evolution and the Tree of Life, from the TV series Cosmos:
And here is another summary of evolution and how it works (and how it ISN'T like the parody-version of evolution which Creationists claim scientists believe):
And yet another:
And its sequels:
Speciation: The Pattern of the Origin of Species
As with many things, we run into problem with typological thinking: the idea that there are ideal types of things, and that we judge a specimens membership in a group by how well it conforms from that type. Instead, we find that variation is the reality. So we need to use population-based thinking. (Next lecture we will add tree-based thinking.)
Darwin's species concept is worth revisiting:
"I look at the term species as one arbitrarily given, for the sake of convenience, to a set of individuals closely resembling each other, and that it does not essentially differ from the term variety, which is given to less distinct and more fluctuating forms. The term variety, again, in comparison with mere individual differences, is also applied arbitrarily, for convenience sake." Chap. 2 of The Origin
"Species are only well-marked varieties" Chap. 4 of The Origin
"The only distinction between species and well-marked varieties is, that the later are known, or believed, to be connected at the present day by intermediate gradations, whereas species were formerly thus connected" Chap. 14 of The Origin
An important issue which is commonly forgotten comes out here: descendants are descendants of only a small part of any ancestral group! That is, entire species do not evolve into entire other species. Instead, only some small subset of any given species population is the ancestral group leading to a particular descendant. This points to several different aspects:
It is entirely possible for the ancestral species to still be around even though some subpopulation might give rise to a later species
There is no reason why for a given ancestral group that only a single subpopulation gives rise to descendants. Thus, we have a reason for divergence of multiple descendants from a single ancestral species
Physical geographic isolation of a subgroup and a change of environmental condition favoring evolution of distinct traits would greatly increase the chance in some small subgroup on the fringes (a peripheral isolate) to be most easily split off as a potential ancestor of a new species
Note: this relates to a common anti-evolutionary rant, which goes a long the lines of "if people evolved from monkeys, how come there are still monkeys". Ignoring lots of other problems with this statement (such as the fact humans didn't evolve from any living monkey species; that "monkeys" aren't one thing, but are a vast number of species; etc.), this misses important aspects of how evolution works! Just because some monkeys evolved into apes which evolved into humans does not require that ALL monkeys evolved into apes and ALL apes evolved into humans. Plus, it doesn't mean that monkeys were TRYING to evolve into humans, or DESTINED to do so.
(Here's a way to restate an analogy to this anti-evolution argument: "If [for example] your ancestor came from Ireland (or Norway, or India, or whatever), why are there still Irish/Norwegians/Indians/etc.?")
Adaptive (or Fitness) Landscapes
One metaphorical device that is sometimes useful to use when talking about evolution is the idea of an "adaptive landscape" or "fitness landscape". Imagine all the variables in an organism's circumstances reduced to a surface with peaks and valleys. The height of the peak reflects the fitness of the organism. Evolution will always favor populations moving "uphill" (increasing fitness). But they can only move to the nearest peaks through ordinary selection, even if there are higher peaks elsewhere. That is because moving downhill would mean a decrease in the fitness of the descendant populations. So selection only moves towards "local" optima in normal situations.
However, large mutational "jumps" might place a descendant on a point on the landscape far from its ancestor, allowing it to move to adaptive peaks not accessible to the earlier forms. And the landscape itself "shifts", because the environment in which the organisms exist change, meaning what controls "fitness" will be different over time.
Speciation is the process of the origin of a species. It doesn't happen immediately or instantaneously: it is indeed a process rather than an instantaneous event. (In fact, except in rare cases, it is unlikely that it you there during it that you would recognize it as such.)
Some aspects of the origin of species to consider:
Area of origin: Did the new species arise within the main range of the ancestral species (sympatric, "same homeland"); alongside the ancestral range, with no major barriers to gene flow (parapatric, "parallel homeland"); on the edges of the species range, with some substantial (but not necessarily total) barrier to gene flow (peripatric, "edge of the homeland"); or by either subdividing the original population or by isolating a part of it (allopatric, "other homeland")?
Trends or branches: Did the the new species arise by the main population itself shifting as a group (anagenesis, "no origin") or by splitting/subdivision of the lineage (cladogenesis, "branching origin")
During the 20th Century (especially during the first half), evolutionary biologists assumed the dominant trends were sympatry and anagenesis. However, as a better understanding of genetics was developed, some (including Mayr) argued that allopatry, peripatry, and parapatry (which all require cladogenesis) were actually more common.
The problem, of course, is that speciation takes time, and field biologists are unlikely to observe it. If only there were some sort of record of changes over time. Say, for example, a fossil record...
Creeps or Jerks: Phyletic Gradualism vs. Punctuated Equilibrium
Prior to the 1970s most paleontologists considered a model which became known as phyletic gradualism:
Speciation is mostly sympatric: change occurs mostly within the ancestral range
Speciation is mostly anagenetic: the whole population changes together
Evolutionary change is largely continuous
As a consequence, fossil species were all simply chronospecies: arbitrary sections of a lineage with no particular justification for dividing the lineage up one way or the other
In 1972 paleontologists Niles Eldredge and Stephen Jay Gould proposed an alternative, which they called punctuated equilibrium:
Speciation is mostly para/peri/allopatric: change occurs mostly by peripheral isolation
Speciation is very dominantly cladogenic: branching (either subdivision of the whole population, or the origin of a peripheral isolate) is the most common
Evolutionary change is not continuous: instead, during most of the history of a species it remains relatively stable (no major directional change) (the "equilibrium" part), and most of the change is restricted to a small portion of its history (the punctuation event)
As a consequence, fossil species have a fairly distinct beginning (the punctuation)
The punctuated equilibrium model helped explain some aspects of paleontology. As Darwin noted (see quote at top), we do not see an endless series of slight gradations, each stratum with a slightly different version. Instead, species remain largely unchanged for most of their duration, with new closely-related species appearing suddenly in the fossil record. In fact, if it weren't like this, biostratigraphy would not really work! As the punctuated equilibrium supporters argued, "stasis is data".
During the 1970s and 1980s (and continuing today, but at with much less rancor), the debate over "evolution by creeps" vs. "evolution by jerks" continued. At least in the fossil record, punctuated equilibrium seems
How long are punctuation events? In a rare case, Smithsonian paleontologists Gene Hunt, Michael Bell, and Matthew Travis found that a population of the stickleback species Gasterosteus doryssus got isolated in a lake in Nevada in the Miocene Epoch. In this particular case, there were annual layers, allowing them to measure an excellent sample over time and document its change. They found the period of transition from the ancestral form to the descendent took only about 2000 generations (about 4000 years), after which the population was mostly stable. Events on the 103 year scale are unlikely to show up in the fossil record except in such situations (high sample size, restricted location, annual record), as the fossil record is much better at picking up events at the scale of 104, 105, or greater.
Supporters of the punctuated equilibrium model had to wonder how equilibrium was maintained. Evolutionary stable scenarios seem to be at least part of the reason.
Why the punctuations? A likely cause is that environmental changes are rather quick on the geologic time scale, with stable conditions in between. Rapid shifts in climates will result in shifting population ranges, shifts in habitat availability, etc. This leads to the prediction that we should see evolutionary shifts (speciations, extinctions, etc.) concentrated at moments of environment change: the so-called "Turnover-Pulse Model".
In summary, punctuated equilibrium may well be due to the following combination of aspects:
Abrupt environmental change, resulting in shifting selective conditions
Evo-devo-type evolution, allowing for a few genetic changes to result in more significant changes of form
Evolutionary stable scenarios maintaining equilibrium between punctuation events
A historical note: a close read of The Origin shows that Darwin did consider cladogenesis and parapatry/peripatry as critically important in most speciation, and that anagenesis of the main part of the ancestral population was almost never the case.
"Missing Links" As An Outdated Concept
One of the most problematic terms in popular discussion of evolution is "missing link". Coined by Lyell, it was used in the 19th Century for a potential but as-of-then undiscovered link between humans and other primates. Subsequently is has been used as the idea of an as-yet undiscovered intermediate form between two particular species.
The term is problematic for a couple reasons:
Once you actually find the specimen, it isn't missing any more!
More importantly, we are unlikely to find any individual that is a direct ancestor of the later forms
The chances that any individual making it in to the fossil record is very, very small. And even the chance that every population, or even species, will show up in the fossil record is small.
An ancestor is a very specific thing: not merely an earlier relative, but specifically an individual who was in direct line of descent with the later form. (Your great great great great uncle is NOT your ancestor, while your great great grandfather is.)
So it has a very small chance that given fossils are the ancestors of the species you are interested in. However, it might well be a more general early relative, and that could be useful indeed.
Transitional Forms
Finding a direct ancestor might be hard, but finding an early relative is easier. And since relatives share many of their same traits, finding early relatives can help establish the anatomical (and ecological, and so forth) transitions involved in that part of the tree of life.
Darwin pointed out that there is not the continuous series of transitions that might expected from a gradualistic model of evolution in the fossil record, but noted that the fossil record was great for higher-level transitions. And this record is vastly better now than in the 1850s!
Here are a handful of interesting transitions recorded in the fossil record:
The various taxa in there might not be direct ancestors of the later branches, but they do show the set of traits associated with a given phase of that evolutionary transition
You don't always have to look at the whole organism: you can concentrate on the transitional states of a particular part of the body
Although not shown here, you can actually use contemporary organisms to look at the transitional phases of a particular organ system or behavior or whatever, with the understanding that you are not looking at the actual ancestors.
"Intermediate forms" is another related term used in the field. Basically, however, every taxon is intermediate between its closest relative and the groups more distantly related.
An important thing to revisit before we move on: the specter of typological thinking. Our minds like to think of discrete types of things. However, when dealing with evolution, there is a continuum of form from one to another. Remember: at no time did a mother of one species give birth to a daughter of another species! It is only from a distance in time do we see the accumulation of changes.
This applies for groups above the species as well. At not moment in the history of life would you witness a population of one major group giving rise to a population belonging to another major group. It would always look just like ordinary speciation. It is only from a distance that we see the Tree of Life.
Macroevolutionary Patterns
Macroevolution is the term for evolution above the level of the species. In a real sense, it is the summed effect of multiple rounds of microevolutionary (below the species level) changes. But some patterns only become apparent on the grander scale.
We have already seen correlated progression (the summed affect of adaptions consistent with a particular mode of life) and divergence (the splitting of one ancestral group into two or more distinct descendant lineages) as examples of macroevolution. Here are a few more:
Adaptive radiations: when many diverse lineages with many modes of life descend from a common ancestral population in a geologically short period of time. This might happen when an ancestral population colonizes a region previously uninhabited (and thus there is no competition), or when a competitor group goes extinct, freeing up ecological niches (ways of life), or when a new adaptation allows an entirely new mode of existence. In nearly all cases adaptive radiations occur when there is a reduction or lack of competition; thus, many different variants among the descendants can be successful.
Convergence: the independent evolution of similar adaptations from dissimilar ancestors. This is almost always the result of the different groups evolving into a similar niche. For instance, tunas, fast-swimming sharks, dolphins, and the extinct ichthyosaurus have a shared torpedo-like body shape, due to independent selection for fast-swimming motion due to their common mode of life (rapid predators).
Exaptation: a very common pattern. The word means "to be fit out of" (just like adaptation means "to be fit towards"). Evolution only rarely works by the addition of brand-new body parts. Instead, normally a part already existing begins to be employed in a new fashion. For some time it might be doing multiple functions, but may eventually the new fashion might take over entirely. For instance, our arms and legs are homologous to the fins of fish. There were early fish that used their fins both to swim and to push themselves through vegetation and over ground (we still have some fish today that do this, like mudskippers and snakeheads). These ancestors of terrestrial vertebrates used these modified flippers for swimming and for walking, but their descendants (including us) use them only for walking. (Well, or flying, or digging, or picking stuff up, or [ironically] in swimming again in whales and the like!)
Heterochrony: literally "different timing". Evolution can happen by shifting the timing of development in ontogeny, so traits that normally developed earlier might develop later, or where development might continue beyond the ancestral version. This is divided up into to major patterns: peramorphosis, where the descendant species evolves into a "hyperadult" relative to the ancestor; and paedomorphosis, where the descendant is a "juvenilized" version of the ancestor.
You can have both patterns showing up in different aspects of the same species. For instance, human heads are paedomorphic compared to our relatives: we have round skulls and flat faces, like juvenile apes but not like adults. On the other hands, our legs are peramorphic: while most apes have legs that stop growing at around the toddler phase, ours keep on growing to become very long.