Teaming up with Hinrich Arnoldt, who conducted research at the Max Planck Institute for Dynamics and Self-Organization, and Steven Strogatz, a scientist at Cornell University, Marc Timme has now presented a simple mathematical model showing that the collective state with highly mixed genetic material could have coexisted with the second, far less mixed state.
Even at the beginning when the highly mixed state was predominant, the evolutionary dynamics repeatedly reverted into the less mixed state in which many cells shared similar genomes. In the genetically unmixed state of greater biological fitness, the competence of unicellular organisms to engage in horizontal gene exchange with other individuals would have decreased.
That is because, due to their slightly more developed biochemical apparatus, it would have been more difficult for these cells to randomly incorporate the components imposed on them during the horizontal gene exchange. It is more likely that in genetically unmixed phases, the life forms instead passed on more or less unchanged versions of their genomes to daughter cells to following generations. Initially, however, the genetically unmixed episodes did not last very long.
Time and again the advantage for survival was lost in what was still a very undefined genetic blur. Horizontal gene transfer once again took over the reins — but no longer to the same extent.
Many species are similar enough that hybrid offspring are possible and may often occur in nature, but for the majority of species this rule generally holds. In fact, the presence in nature of hybrids between similar species suggests that they may have descended from a single interbreeding species, and the speciation process may not yet be completed.
Given the extraordinary diversity of life on the planet there must be mechanisms for speciation: the formation of two species from one original species.
Darwin envisioned this process as a branching event and diagrammed the process in the only illustration found in On the Origin of Species Figure 3a. Compare this illustration to the diagram of elephant evolution Figure 3b , which shows that as one species changes over time, it branches to form more than one new species, repeatedly, as long as the population survives or until the organism becomes extinct.
Figure 3. The diagram shows similarities to phylogenetic charts that are drawn today to illustrate the relationships of species. For speciation to occur, two new populations must be formed from one original population and they must evolve in such a way that it becomes impossible for individuals from the two new populations to interbreed.
Biologists have proposed mechanisms by which this could occur that fall into two broad categories. Biologists think of speciation events as the splitting of one ancestral species into two descendant species. There is no reason why there might not be more than two species formed at one time except that it is less likely and multiple events can be conceptualized as single splits occurring close in time.
A geographically continuous population has a gene pool that is relatively homogeneous. Gene flow, the movement of alleles across the range of the species, is relatively free because individuals can move and then mate with individuals in their new location. Thus, the frequency of an allele at one end of a distribution will be similar to the frequency of the allele at the other end. When populations become geographically discontinuous, that free-flow of alleles is prevented.
When that separation lasts for a period of time, the two populations are able to evolve along different trajectories.
Thus, their allele frequencies at numerous genetic loci gradually become more and more different as new alleles independently arise by mutation in each population. Typically, environmental conditions, such as climate, resources, predators, and competitors for the two populations will differ causing natural selection to favor divergent adaptations in each group.
Figure 4. The northern spotted owl and the Mexican spotted owl inhabit geographically separate locations with different climates and ecosystems. The owl is an example of allopatric speciation. Isolation of populations leading to allopatric speciation can occur in a variety of ways: a river forming a new branch, erosion forming a new valley, a group of organisms traveling to a new location without the ability to return, or seeds floating over the ocean to an island.
The nature of the geographic separation necessary to isolate populations depends entirely on the biology of the organism and its potential for dispersal. If two flying insect populations took up residence in separate nearby valleys, chances are, individuals from each population would fly back and forth continuing gene flow. However, if two rodent populations became divided by the formation of a new lake, continued gene flow would be unlikely; therefore, speciation would be more likely.
Biologists group allopatric processes into two categories: dispersal and vicariance. Dispersal is when a few members of a species move to a new geographical area, and vicariance is when a natural situation arises to physically divide organisms.
Scientists have documented numerous cases of allopatric speciation taking place. For example, along the west coast of the United States, two separate sub-species of spotted owls exist. The northern spotted owl has genetic and phenotypic differences from its close relative: the Mexican spotted owl, which lives in the south Figure 4.
Additionally, scientists have found that the further the distance between two groups that once were the same species, the more likely it is that speciation will occur. This seems logical because as the distance increases, the various environmental factors would likely have less in common than locations in close proximity. Consider the two owls: in the north, the climate is cooler than in the south; the types of organisms in each ecosystem differ, as do their behaviors and habits; also, the hunting habits and prey choices of the southern owls vary from the northern owls.
These variances can lead to evolved differences in the owls, and speciation likely will occur. Figure 5. The honeycreeper birds illustrate adaptive radiation. From one original species of bird, multiple others evolved, each with its own distinctive characteristics. In some cases, a population of one species disperses throughout an area, and each finds a distinct niche or isolated habitat.
Over time, the varied demands of their new lifestyles lead to multiple speciation events originating from a single species. This is called adaptive radiation because many adaptations evolve from a single point of origin; thus, causing the species to radiate into several new ones. Island archipelagos like the Hawaiian Islands provide an ideal context for adaptive radiation events because water surrounds each island which leads to geographical isolation for many organisms.
The Hawaiian honeycreeper illustrates one example of adaptive radiation. From a single species, called the founder species, numerous species have evolved, including the six shown in Figure 5. Evolution in response to natural selection based on specific food sources in each new habitat led to evolution of a different beak suited to the specific food source. The seed-eating bird has a thicker, stronger beak which is suited to break hard nuts. The nectar-eating birds have long beaks to dip into flowers to reach the nectar.
The insect-eating birds have beaks like swords, appropriate for stabbing and impaling insects. The taxonomy of European crows had been a problem for centuries.
Carl Linnaeus, the Swedish botanist who invented the modern system of taxonomy, first classified carrion and hooded crows as separate species in Since then, many taxonomists had argued that they should be considered subspecies instead. On the one hand, the crows were visually and geographically distinct; on the other, they were ecologically identical and produced fertile offspring.
Even inside the hybrid zone, however, hooded and carrion crows tended to mate with their own kind. Only occasionally would they form a hybrid pair. Hybrid crows blended the appearances of their parents: grayer than carrion crows, darker than hooded ones. The pattern on their breasts sometimes looked like herringbone. In , Wolf won a grant to study the crow problem.
The two looked for genetic differences between the crows. Technology limited them to sequencing just a few hundred base pairs at a time. A year went by. But Wolf had also started to tinker with a new technology called high-throughput sequencing. The first human genome had taken thirteen years and billions of dollars to sequence; now advances in computing power and chemistry were making it possible to sequence a full genome for a few thousand dollars in a matter of days.
Wolf became one of the first evolutionary biologists to deploy high-throughput sequencing in the field. He obtained the full genomes of sixty individual crows in an attempt to identify every last genetic difference between the hooded and carrion populations.
Wolf and Poelstra, along with a growing team of students, spent years at their computers, poring over terabytes of data and aligning the genomes for each crow. When they first saw the results, in , Wolf wondered if they had made an error. The genomes of hooded and carrion crows mapped almost perfectly onto one another. There were, of course, variations among individual crows. But only 0. In fact, German carrion crows shared more of their DNA with German hooded crows than they did with other carrion crows from Spain.
There were only eighty-two base pairs that never matched—less than one ten-millionth of a per cent of the total genome, a fraction so tiny that it should be statistically irrelevant. By comparison, human beings have millions of fixed genetic differences with chimpanzees, their closest living relatives. Even the taxonomists who wanted to lump the crows together as a single species would have predicted magnitudes more difference. Almost as surprising as the small number of fixed differences was their distribution in the genome.
Eighty-one of them were clustered on the eighteenth chromosome, in a region involving the production of melanin, the pigment that makes a feather black. Some selective pressure was acting on this tiny region alone. See text for details. Divergent natural selection : Selection that acts in contrasting directions between two populations, usually with reference to ecological differences between their environments e.
Ecological speciation : A speciation process in which divergent natural selection drives the evolution of reproductive incompatibility i. Mutation-order speciation : A speciation process in which different and incompatible mutations alleles fix in separate populations that are experiencing similar selective regimes.
Dobzhansky-Muller Incompatibility : Hybrid dysfunction arising from negative interactions epistasis between alleles at two or more loci: an allelic substitution at a locus causes no reduction in fitness on its own genetic background, but leads to reduced fitness when placed on the alternative background.
Genomic Island : A region of the genome where differentiation between populations is stronger than expected in the absence of divergent selection stronger than occurs via purely neutral processes such as genetic drift alone. Natural selection : Differential survival of classes of entities such as alleles which differ in some characteristic s.
Sexual selection : Differential reproductive success of classes of entities such as alleles which differ in some characteristic s. Reproductive Isolation : Genetically-based differences between populations which reduce or prevent genetic exchange between them i.
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