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The Origin of Species
One species can give rise to a different species if there is
genetic variation within the species.
The variety of living species that has resulted from millennia of speciation is known as earth's biodiversity.
The Pace of Evolution
Biodiversity has changed not only in space, as the continents drifted apart, but also across time. How long does it take for a species to evolve? It depends.
This is the traditional Darwinian view that
large evolutionary changes happened via gradual accumulation of
many, small changes. The classic example found in many natural
history museums (in the form of a nice display) is the evolution of the horse (Equus caballus).
In 1972, Darwin's classical idea was upended when Niles Eldredge and Stephen Jay Gould proposed a new theory, Punctuated Equilibrium.
Eldredge and Gould suggested that major changes can occur relatively suddenly, and that such changes can "punctuate" long periods of relatively little change (equilibrium/stasis).
The term "sudden" is relative, geologically speaking. Punctuations in species diversity can happen over thousands of generations (quick!) instead of over millions (not so quick!)
But they also might happen very quickly, sometimes in a single generation.
, as we will see.
Punctuated equilibrium could explain how some awkward
intermediate forms needed to transform a land reptile into a flying bird
or a terrestrial
tetrapod into a marine whale
...might have been skipped.
A major genetic event could have produced a phenotype that was drastically different from the original. That new phenotype could quickly displace the old one, if it were highly adaptive.
Examples of punctuated equilibrium in action:
Genetic drift in a small island population
polyploidy resulting in relatively sudden reproductive isolation (many annual "wildflowers")
But what are the mechanisms of those changes?
The ultimate "raw material" of evolution is mutation.
But how are those mutations incorporated into evolving populations?
Macroevolution: The Genesis of Reproductively Isolated Populations from an Ancestral Population
Over generations, a population can undergo a great deal of change from its
original state. But all members of that population are still members of the same species unless some members become reproductively isolated from one another. Speciation is the separation of two previously interbreeding
populations into two populations that can no longer mate to produce
fertile, viable offpring.
Speciation is a temporal process.
Populations exist in various stages of
speciation at any given time
Extant populations are even now
undergoing microevolutionary changes that may eventually give rise to new species.
Species on the verge of becoming separated are called incipient species.
How might one species become two?
anagenesis (= phyletic evolution) - the conversion of an entire population, over time, to a recognizably different. (No net increase in species diversity)
cladogenesis (= diversifying evolution) - the divergence of two new
species from a single ancestral species. (Net increase in species
Repeated Cladogenesis and Adaptive Radiation
An ancestral species can give rise to a variety of diverse
species through repeated cladogenesis if each of its descendant species "radiates" into a new ecological niche. When this occurs, the related species are said to have undergone
This diversification is driven by one or more of the
five factors that can alter allele frequencies:
On the Hawaiian islands, a single, finchlike ancestor gave rise to about 40 different species of Honey Creepers. Each is specialized in bill shape and size, as selected by its particular microhabitat and diet.
And let's not forget our classic Galapagos finches
Colors may have evolved in response to differences in sexual selection.
How does it happen?
Modes of Speciation
ALLOPATRIC SPECIATION - A single population is divided into two by a physical, geographic barrier.
The physically separated population may not always undergo complete reproductive isolation: re-establishment of physical contact may simply result in resumed mating.
However, if the two separated populations become so differentiated during their isolation that they can no longer interbreed when they meet again, allopatric speciation has occurred.
PERIPATRIC SPECIATION - By entering a new ecological niche, a small subset of a large population becomes isolated at the periphery of the original population's range.
Over generations, the small group becomes reproductively isolated from the original population.
This is sometimes considered a special case of allopatric speciation.
PARAPATRIC SPECIATION - This occurs on a larger scale than parapatric speciation, with large numbers of a population gradually becoming differentiated (due to genetic drift and/or selection) along the range of the population.
Adjacent demes may be able to interbreed to some degree, but widely separated demes cannot. In such isolation and under different selective pressures, the more widely separated populations undergo reproductive isolation/speciation.
Ring species are products of parapatric speciation.
Their name comes from the way their geographic distribution often describes a ring around some type of physical barrier, such as a mountain range or other habitat not conducive to their colonization.
SYMPATRIC SPECIATION - speciation occurs without physical separation, within the range of the ancestral population. (This is often due to a sudden genetic event that causes very rapid reproductive isolation of a subset of the original population.)
Sympatric speciation is well known in plants, which can speciate quickly via polyploidy, either
autopolyploidy (chromosomes in the new species all from the same ancestral species)
allopolyploidy (chromosomes in the new species come from two different (but related) ancestral species)
The modes of speciation can be visualized this way.
Natural Selection Can Drive Speciation
At the start of a "selection cycle" a population may be comprised of
individuals expressing a particular trait along a continuum (bell-shaped curve).
stabilizing selection: selective forces at work on
a population favor greatest reproduction by individuals exhibiting
the average state of a particular character. In this instance,
the composition of the population doesn't change.
directional selection: the individuals at one extreme
or the other of the bell shaped curve have a reproductive advantage
over the rest.
disruptive (= diversifying) selection:
individuals at the average point on the curve are at a selective
disadvantage; individuals with either extreme have a reproductive
A classic example of diversifying selection is personified by the Rock Pocket Mouse in New Mexico's Valley of Fire.
Horizontal Gene Transfer
You already have seen the phylogeny of the three Domains of life.
Organisms in all three Domains give rise to others like themselves via vertical gene transfer: the passing of genes from parent to offspring.
As we now know, small changes can occur from generation to generation.
Over time, such changes can accumulate to produce new species.
The length of the branches between the taxa (represented as the nodes and the tips of the branches) represent the overall difference in DNA between them.
The longer the branch, the more different the DNA.
The diversity of Bacteria and Archaea is far greater than that of Eukaryota.
they have been around for billions of years longer than eukaryotes
they reproduce quickly, providing more chances for genetic change
they are amazingly proficient at getting resources, and can be either
autotrophic (perform photosynthesis to obtain energy)
heterotrophic (feed on other organisms to obtain energy)
Amazingly, individual bacteria readily take up and swap pieces of DNA with each other and with viruses, often incorporating them into their own genome.
Incorporation of foreign DNA by an individual organism is known as horizontal (or lateral) gene transfer.
Microbes (bacteria and archaea) undergo horizontal gene transfer so readily that an average of more than 80% of their DNA is a product of this process.
The phylogenetic tree sometimes looks more like a web!
So are bacteria all one big species?
Several studies have demonstrated that bacteria in a given ecosystem occur in genetic "clusters" and ecological "clusters", too. Genetic makeup determines ecological niche, and that can drive reproductive isolation.
Dr. Frederick Cohan proposes that bacterial strains (genetic variants of a species) be given the code "ecovar" (for "ecological variant") to distinguish it from genetically similar but ecologically distinct bacteria.
The ability to recognize and characterize ecological strains of bacteria will be vital for epidemiologists and other health care workers.
What About Humans? Paleoanthropology is the study of human origins and evolution. Because
humans and chimpanzees have existed as separate species for only a few
million years, this branch of science examines only a very small, recent
portion of the fossil record.
anthropoid - of or pertaining to monkeys and apes
hominoid - of or pertaining to the great apes (including humans)
hominid - member of family Hominidae (Humans and Chimpanzees)
Humans, apes, and monkeys all diverged from
a common anthropoid ancestor that shared the characteristics common to all primates. Natural selection has driven the specialization of each primate species to be what it is today.
The earliest fossil members of our genus (Homo) range in age from
about 2.5 to 1.6 million years, and are currently classified as Homo
Homo erectus shared common ancestry with H. habilis, and
shows up in the fossil record from about 1.8 million to 0.5 million years
Homo sapiens neanderthalensis, named for the Neander Valley in
Germany where its fossils were first found, may have arisen from an H.
erectus-like ancestor, as H. erectus is known to have migrated into both Europe
and Asia (timing uncertain) from Africa.
Neanderthal DNA is absent in modern humans descended from Africans who never left the continent.
There are multiple, competing hypotheses about human origins.
Homo sapiens migrated north from Africa into Europe and interbred with Neanderthals that already lived there (having evolved independently from H. erectus
Homo sapiens migrated north from Africa into Asia and interbred with resident H. erectus.
Homo erectus migrated north from Africa into Europe, Asia, and elsewhere, evolving independently into Homo sapiens several times.
As more DNA data become available, incorrect hypotheses may be rejected.
The oldest hominoid fossils are found in Africa. It is clear that the common ancestor of all modern humans originated there.
Homo sapiens evolved in each of the
regions where its fossils are now found from ancestral Homo
erectus that migrated out of Africa about 1.5 million years ago.
Advocates of this hypothesis consider H. erectus to be an early
version of H. sapiens, and not a different species.
Constant interbreeding between neighboring populations of this "archaic"
may have prevented reproductive isolation, resulting in our
present-day races of Homo
rather than multiple species of Homo.
"Out of Africa" Hypothesis:
All H. sapiens now living evolved from a second major migration out of
Africa that occurred about 100,000 years ago, and not from wandering Homo
erectus. These later migrants replaced the descendants of the
earlier H. erectus migrants.
So far, DNA analyses have supported the "Out of Africa" ("Replacement")
hypothesis. But as any hypothesis, this one is subject to further
testing as new analytic methods become available.
What makes humans different from the other great apes?
Hominoids of 6 million years ago had brains of about 400 -
450cm2. This is about the same as modern chimpanzees. Modern
humans' average brain volume is 1300cm2. This threefold
increase in volume is associated with cultural trends such as development
of complex language
Anthropoids and ancient hominoids have prognathic jaws: the
upper and lower jaws protrude beyond the nose. Recall that the human
face is paedomorphic with respect to that of other modern apes. The
face is flatter, the prognathous jaws lost. Changes in dentition
accompanied this change in jaw shape.
Ancestral anthropoids and some of the earliest hominoids walked on all
fours, though--like modern apes--they could probably walk on their hind
legs with some degree of balance and skill. Humans are different from
all other apes in that the body posture is fully upright and
locomotion is entirely bipedal. A number of skeletal and muscular
modifications make this possible, but there is still a great deal of
academic argument about why humans became bipedal.
Reduced sexually dimorphic size differences
In orangutans and gorillas, the male weighs about twice as much as the
female. In chimpanzees and Bonobos, males weigh about 1.4 times as much
as females. In humans, the difference is still less, with males
averaging 1.2 the body weight of females. (Why do you suppose this is the
case? HINT: social structure is a major selective factor here.)
Key changes in family and other social structures
Gibbons are social, with a dominant male defending a group of
females from other male rivals.
Orangutans are solitary and do not form permanent social groups
Gorillas are social, with a single "silverback" dominant male
getting all the mating opportunities with the females in his band.
Immature and subordinate males are allowed to stay in the group, but
do not get many mating opportunities.
Chimpanzees are social and promiscuous. When a female comes into
estrus, all males will attempt to mate with her, and she will mate
with multiple males.
Bonobos will have sex with anyone who holds still long enough.
Hunter-gatherer human societies may be polygamous or
polyandrous. But in most human societies, monogamy is the norm.
(But is it biologically programmed?)
Young are even more altricial than other great apes' young.
a precocial baby animal relatively well developed and able to function shortly after birth.
an altricial baby animal is relatively undeveloped and is highly dependent on its parents for food, care, and protection.
Newborn humans are exceptionally dependent on their mothers.
Parental care lasts longer after birth than in other ape species.
This extended period, coupled with the enlarged brain, enhances learning and is one
factor that contributes to the behavioral complexity of the human animal.