1. Population Genetics
1.1 Terminologies
1.1.1 Population genetics
It is the study of the genetic composition of populations. It is concerned with the frequency and distribution of gene alleles, the genotypes in natural populations and the factors that determine them.
1.1.2 Populations
It is a local community of sexually reproducing species in which the individuals share a common gene pool. Random mating occurs in the gene pool. How are populations, genes and evolution related?
Evolution is a property not of individuals but of populations, which include all the individuals of a species living within a given area. The recognition that evolution is a population-level phenomenon was one of Darwin’s key insights.
But populations are composed of individuals and it is the actions and fates of individuals that determine which characteristics will be passed to descendant populations. In this fashion, inheritance provides the link between the lives of individual orgamisms and the evolution of populations.
1.1.3 Gene Pool
It is the sum total of all the genes (i.e. all the alleles at all gene loci) of all breeding individuals (represented by their gametes) in a population at any one time.
1.1.4 Taxonomy
Every organism is given a scientific name according to an internationally agreed system of nomenclature, first devised by a Swedish botanist Carl Linneaus.
The name is always in Latin and is in 2 parts. The first name indicates the genus and is written with an initial capital letter. The second name indicates the species and is written with a small initial letter. These names are always distinguished in text by italics or underlining. When generic names have been written once, they can be subsequently abbreviated.
1.1.4.1 Taxon
It is a unit of classification. It is any named taxonomic unit or group of any rank in the hierarchical classification of organisms that show common features.
1.1.4.2 Taxonomic Ranks
The complete modern classification of an organism includes the following groups: kingdom, phylum, class, order, family, genus, species, strains.
1.2 Computing Gene (allelic) Frequencies
Gene frequency is the relative proportion of the alleles of a gene present in a population. If there are only alleles, if one is known, the other can be computed using the formula “p + q = 1”. The Hardy-Weinberg equation is “p2 + 2pq + q2 = 1”
1.3 Genes in Populations and the Hardy-Weinberg Equilibrium
The method used to calculate the expected proportion of different genotypes in a population was published in 1908 by the British mathematician Godfrey H. Hardy and independently, by the German physician Wilhelm Weinberg.
This equilibrium shows that under certain conditions, allele frequencies and genotype frequencies in a population will remain constant no matter how many generations have passed. In other words, evolution will not occur in this population. Population geneticists call this idealized, evolution-free population, which will remain in genetic equilibrium as long as several conditions are met.
The principle states that the proportion of dominant and recessive alleles of a particular gene in a population always remains the same. It is not altered by interbreeding. It is a mathematical law which depends on five conditions. No mutations arise. The population must be very large. All mating must be random, with no tendency for certain genotypes to mate with specific other genotypes. There must be no gene flow between populations (through immigration)) or out of the population (through emigration). There must be no natural selection; i.e. all genotypes must be equally adaptive and reproduce equally well.
While these conditions are probably never met in a natural population, the Hardy-Weinberg principle nonetheless forms a basis for the study of gene frequencies. Under these conditions, allele frequencies within a population will remain the same indefinitely. Hence, a population geneticist defines evolution as changes in gene frequencies that occur in a gene pool over time. Thus evolution is a change in the genetic make up of populations over generations.
2. Theories of Evolution
2.1 What Evolution is NOT
It is not a fact. It is a theory, a highly probable explanation for all biological events.
It is not something one should believe in. It is based on science, not faith.
It is not concerned with the origin of life, but the origin of species.
It is not just concerned with the origin of humans, no more than any other species.
It was not discovered or first explained by Charles Darwin.
It is not the same thing as natural selection; this deals with how evolution occurs.
It is not something that happened in the past as it is still going on.
It is not something that happens to individuals, it happens to populations
It is not an accidental or random process; there are build-in limits and constraints.
It was not developed to undermine religion; rather, it was developed to explain observations of life in a testable way.
It does not deny the existence of God; it is neutral, god is neither required nor eliminated.
It does not conflict with any religion. It cannot. Since it is only another way of trying to understand the natural world.
2.2 What is Evolution?
Evolution is the change in gene frequency that occurs in a gene pool over time. It is a change in the genetic makeup of populations over generations.
Evolution ultimately leads to the formation of a new species, developed from earlier species by accumulated changes. This is sometimes referred to as “descent with modification”. By extension, as this process of speciation proceeds with time, increasing number of species appear, becoming increasingly different. All the species we see today are like the growing tips of a branching tree: close clusters of tips have most recently branched (evolved); more distant tips must be traced to much lower (earlier) branching in the tree. What we call “genus” would be a close cluster of tips. The “family” level of classification refers to a group of several closely branched clusters.
The idea of evolution was developed from many observations of life. It has been tested and challenged many times and in many ways, and has survived in great shape. There are also many independent lines of evidence that are consistent with evolution as a real process. There is NO observed evidence against evolution. Evolution therefore holds the high status of near certainty as it is a scientific theory.
Microevolution is the evolutionary changes that have alterations in gene frequencies due to mutation and recombination. It can be noticed over a relatively short time e.g. pesticide resistance in insects. It is the evolution of populations.
Macroevolution is the evolutionary processes extending through geological times involving e.g. human evolution.
2.3 Lamarck’s Theory of Evolution
Jean-Baptist de Lamarck (1744 – 1829) was the first biologist to believe that evolution does occur and to link diversity with adaptation to the environment.
Lamarck concluded after studying the succession of life forms in strata, that more complex organisms are descended from less complex organisms. He mistakenly said that increasing complexity was the result of a natural force – a desire for perfection - that was inherent in all living things. He postulated that living things modify their bodies through the use or disuse of parts and these changes can be inherited by their offspring.
To explain the process of adaptation to the environment, Lamarck supported the idea of inheritance of acquired characteristics; that the environment can bring about inherited change.
Lamarck’s classic example is that the long neck of the giraffe developed over time because animals stretched their necks to their offspring.
2.4 Darwin-Wallace Theory of Evolution
2.4.1 Significance of the Voyage of Darwin
During the voyage, at least 3 items impressed Darwin and in a sense helped lay the foundation for his ideas on evolution.
2.4.1.1 His Large collection of Fossils
He discovered many species of extinct animals and noticed the close resemblance in design between the fossil forms and the living species of; for instance, armadillos, tapirs and anteaters.
2.4.1.2 He noticed the variety of animals and plants found within and between species
These differences could be related to differences in the environment. All these organisms showing basic similarities had a common ancestor in the past. From this ancestor, different varieties had arisen which could live successfully in various conditions to which their structure and habits were suited. Because of migration and the development of barriers, they could no longer interbreed, hence species were produced (speciation) with even more pronounced differences.
2.4.1.3 The Galapagos Islands
Here, he noticed how different the principal groups of plants and animals were from those on the mainland. He observed how the giant tortoises varied from island to island and distinctive races could be easily recognized by the form and pattern of their shells. He noticed the 13 species of finches that displayed remarkable adaptations suiting them to the different ecological niches that they occupied. He considered that originally a few finches had strayed from the mainland to these islands and, as they bred, they produced new types with differences that allowed them to fare better in the new conditions.
2.4.2 Essential Features of Darwin’s Theory
2.4.2.1 Overproduction of Offspring
All organisms produce large numbers of offspring that, if they survived, would lead to a geometric increase in the size of any population.
2.4.2.2 Constancy of Numbers
Despite the tendency to increase numbers due to overproduction of offspring, most populations actually maintain relatively constant numbers. This is due to the limited resources available e.g. food and appropriate habitat. The majority of offspring must therefore die, before they are able to reproduce.
2.4.2.3 Struggle for Existence
Darwin deduced on the basis of 2.4.2.1 and 2.4.2.2 that members of a species were constantly competing with each other in an effort to survive. In this struggle for existence, only a few would live long enough to breed.
2.4.2.4 Variation among Offspring
The sexually produced offspring of any species show individual variations so that generally, no 2 offspring are identical.
2.4.2.5 Survival of the fittest by natural selection leads to differential reproduction
Among the variety of offspring will be some better able to withstand the prevailing conditions than others. That is, some will be better-adapted (litter) to survive in the struggle for existence. These types are more likely to survive long enough to breed. Thus nature selects those more well-adapted organisms and allows them to survive and reproduce but reject and eliminate the poorly adapted one. This process is called natural selection. Natural selection is the process by which the environment selects for those individuals of a population whose traits best adapt them to that particular environment.
2.4.2.6 Like Produces Like
Those that survive to breed are likely to produce offspring similar to them. The advantageous characteristics that gave them the edge in the struggle for existence are likely to be passed on to the next generation.
2.4.2.7 Formation of a new Species
Individuals lacking favourable characteristics are less likely to survive long enough to breed. Over many generations, their numbers will decline. The individuals with favourable characteristics will breed, with consequent increase in their numbers. The inheritance of one small variation will not, by itself produce a new species. However, the development of a number of variations in a particular direction over many generations will gradually lead to the evolution of a new species.
As environmental conditions are constantly changing, the continuous operation of natural selection will cause different variations to emerge. The accumulation of these variations over a very long period of time may be great enough to develop an organism that is quite different from its ancestor. In other words, a new species is produced. It is described by Darwin as the origination of species, speciation.
2.5 Neo-Darwinism / Synthetic Evolution (modern theory of evolution)
It is the new theory of organic evolution by natural selection of genetically determined characteristics. The genetic variations that exist in natural populations provide the basis of change. Gene frequencies are not always constant in the way predicted by the Hardy-Weinberg Law. In real populations, various forces act upon the variations and may alter the allele and genotype frequencies within the population. This is an evolutionary mechanism that gives rise to new breeds, strains, varieties, races and subspecies.
Neo-Darwinism states the following. Organisms overproduce. Organisms possess intrinsic variation (a.k.a. genetic variation). Variation is controlled by genes. Natural selection keeps species adapted to changes in the environment. Natural selection changes gene frequencies. New species can only arise only be genetic isolation, either geographic isolation or sympatric speciation.
Natural selection alone is not enough to explain how new species are formed. This can only happen if populations of a species are separated so they do not interbreed. The separated populations adapt to their own particular environments and may diverge, eventually forming a new species. There are many isolating mechanisms.
2.5.1 Evidence for Organic Evolution
There is paleontology and fossil records, comparative anatomy of living organisms, embryology and cytology, comparative biochemistry, geographical distribution and finally the direct evidence.
Selection imposed by man has brought about profound changes of our cultivated crops and domestic breeds of animals. Chances in the genetic composition of population, by natural selection and by human activities, have demonstrated evolution in action.
3. Processes of Evolution
3.1 Mutation
A mutation that is transmitted in gametes immediately changes the gene pool of a population by substituting one allele for another. For example, when a recessive allele undergoes a mutation into a dominant allele, the frequency of the dominant allele would increase. Thus, mutations are a source of new alleles that are new heritable variations on which other evolutionary processes can work. It is important that mutations are not goal-directed, as they do not arise as a result of, or in anticipation of, environmental necessities.
3.2 Gene Flow / Migration
The population may gain or lose alleles by gene flow, the migration of fertile individuals or the transfer of gametes, between populations. Migration into or out of a population can breakdown genetic differences between populations i.e. gene flow tends to reduce between-population differences that have accumulated because of natural selection or genetic drift. It is extensive enough, gene flow can merge neighbouring populations into one.
Mutations developing in one populations may also be spread to other populations by migration. This serves, like mutation, to introduce new alleles into populations. Consider a red-flower population was to begin receiving wind-blown pollen from an all white-flower population in a neighbouring field. This new pollen could greatly increase the frequency of the white flower allele, thus also altering the frequency of the red-flower allele.
There are 2 significant effects. Gene flow spreads advantageous alleles throughout the species. Suppose a new allele arises in one population and that this new allele benefits the organisms that possess it. Migration can carry this new allele to other populations of the species.
The other effect is gene flow helps maintain all the organisms over a large area as one species. If migrants constantly carry genes back and forth among populations, then populations can never develop large differences in allele frequencies.
3.3 Genetic Drift
The frequency of alleles can change from generation to generation as a result of chance alone in a small gene pool. This phenomenon is known as genetic drift.
When a population is started by one or a few individuals who randomly separate from larger populations, chance may dictate that allele frequencies in the new population may be very different from those of the original population.
Genetic drift is an important agent of evolution in small populations for example, populations with 100 or less individuals. The 2 situations that lead to populations small enough for genetic drift to occur are founder effect and bottleneck effect.
3.3.1 Founder Effect
This occurs when a few individuals colonize an isolated island, lake or some other habitat new to that species. The smaller the sample size, the less the genetic makeup will represent the gene pool of the parent population. The most extreme case would be the founding of a new population by one pregnant animal or a single plant seed. If the colony is successful, random drift will affect the allelic frequencies until the population is large enough. Genetic drift in a new colony is known as founder effect.
3.3.2 Bottleneck Effect
Drastic short term reductions of population size caused by natural disasters e.g. volcanic eruptions, earthquakes, fires, disease, or predators may result in (by chance) the survivors representing only a small fraction of the original gene pool.
By chance, certain alleles will be over-represented among survivors, other alleles underrepresented and some alleles may be eliminated. Even when the population increases to its original size, a portion of its original genetic diversity remains lost.
Bottlenecking reduces the overall genetic variability. This is a problem with many endangered species ; changes in the gene pool of a small population die to chance.
3.4 Artificial Selection / non-random mating
Artificial selection is the process were allelic frequencies in animals and plants are changed by man through the breeding of only those plants and animals with certain desired characteristics. Here Man exerts a directional selection within the population. It is very rapid and effective.
So, artificial selection occurs through non-random mating (a.k.a. differential reproduction). Difference in the reproductive success of various genotypes is brought about by selection artificially i.e. certain individuals are selected to mate and they make greater contributions to the gene pool than others. By its action, alleles favourable to reproduction will increase in frequency at the expense of alternative alleles. The isolation of natural populations and the selective breeding of organisms showing characters or traits which have some usefulness to Man.
3.4.1 Inbreeding
This involves selective reproduction between genetically closely related individuals, in order to propagate particularly desirable characteristics by selfing. Livestock breeders to produce cattle, pigs, poultry and sheep which yields of milk, meat, eggs and wool respectively use it. Self-fertilization, which is common in plants, is the most extreme example of inbreeding. Intensive inbreeding reduces the variability of the genome by increasing the number of homozygous genotypes. Prolonged inbreeding increases the frequency of homozygous recessive phenotypes that may lead to reduction in fertility. This is known as inbreeding depression.
3.4.2 Outbreeding
This involves crossing individuals from genetically distinct populations. It usually takes place between members of different varieties or strains or closely related species. The progeny are known as hybrids and have phenotypes showing characteristics which are superior to either of the parental stocks; known as hybrid vigor. It is particularly useful in plant breeding to increase plant’s resistance to disease, fruit size and its early maturity
3.5 Natural Selection
Natural selection is a process by which organisms that re better adapted to their environment survive and breed, while those less well-adapted fail to do so. The better-adapted organisms are more likely to pass their characteristics to succeeding generations. Every organism is therefore subjected to a process of selection based upon its suitability for survival given the conditions that exist at that time.
There are some important facts to note about natural selection. It does not cause genetic changes in individuals. It acts on individuals whilst evolution occurs in populations. The organism’s environment exerts a selection pressure and this determines the spread of any allele within the gene pool. The intensity and direction of this pressure varies in both time and space.
3.5.1 Forms of Selection
There are 3 types of selection that operate in a population of a given species.
3.5.1.1 Stabilizing Selection
The strongest selection pressure is against the extreme variants furthest from the mean. Stabilizing selection leads to a reduction in variance without any change in mean. It is the form of selection that operates in a constant environment to maintain the best adapted genotypes within the population. Such a selection works against both extremes.
3.5.1.2 Directional Selection
It operates in a changing environment and results in a reduction in variance and progressive shift in the population mean for the character concerned until a state of adaptation is reached. It occurs in 1 direction only i.e. only one extreme is selected for. It is the main form of selection practiced by man in the improvement of domesticated plants and animals.
3.5.1.3 Disruptive Selection
It favours 2 optimum phenotypic classes at the expense of intermediate i.e. individuals at both extremes of variation are favoured. It occurs in natural populations where 2 distinct habitats, or different kinds of resources, exist. In plant and animal breeding where selection is practiced for the extremes of size and form. This gives rise to polymorphism that is the existence of 2 or more morphs or distinct phenotypes.
3.5.2 Its Occurrence in Nature
First is the industrial melanism in peppered moth (directional selection). Second is antibiotics resistance in bacteria (directional selection). Third is sickle-cell anaemia (stabilizing selection). Fourth is African finches (disruptive selection).
4. Outcomes of Evolution
4.1 Adaptation
It is the change of form, function or behaviour that makes a population better able to survive in its environment. It is any genetically controlled characteristic that aids an individual organism, or the species to which it belongs, to survive and reproduce in the environment it inhabits.
It may be genetically simple, controlled by only one or a few genes, or they may be genetically complex, controlled by large numbers of genes. It may involve individual cells or subcellular components, or they involve whole organs or organ systems. For example, cryptic colouration in animals: warning colouration and mimicry in animals.
4.2 Adaptive Radiation
It is the diversification of population of the species. A group of organisms share a homologous structure that is differentiated to perform a variety of different functions.
4.3 Speciation
It is the formation of a new species.
5. Origin of Species, Speciation
5.1 What is a Species?
A Species is an actually or potentially interbreeding population that does not interbreed with other such populations when there is an opportunity to do so, producing viable offspring.
It may be also a natural group of organisms which show a close similarity in morphological, anatomical, physiological and behavourial characters descended from a common ancestor.
It is a non-arbitrary biological unit that represents the lowest and smallest taxonomic group in the classification of organisms.
Genetically, it is a distinct group of natural populations that share a common gene pool.
In breeding aspects, it is a group of organisms capable of interbreeding and producing fertile offspring.
Ecologically, it is a group of organisms sharing the same ecological niche; no 2 species can share the same ecological niche.
In evolutionary terms, it is a group of organisms sharing a unique collection of structural and functional characteristics descended from a common ancestor.
Yet no single criterion has ever been successful in defining species across the animal, plant and microbial kingdom. An ecological niche is the sum total of an organism’s utilization of the biotic and abiotic resources of its environment.
5.2 Difficulties in Species Definition
5.2.1 Genetic Aspect
Common gene pool and common karyotypes of a species may change due to directional selection following changes in the environment. It may also change due to inter breeding between 2 different species.
5.2.2 Interbreeding Aspect
5.2.2.1 Asexually reproducing species and self-fertilizing species (hermaphrodites)
Many microorganisms seldom, if ever, show a sexual phase in their life cycles. Some plants and animals are hermaphrodites and they carry out self-fertilization. For these forms, interbreeding criterion is meaningless.
5.2.2.2 Rare hybrids between different species
Hybridization of 2 completely separate and distinct species is able to produce viable, sterile offspring. In animals, a cross between the horse and the jackass gives a mule. A cross between a lion and a tiger gives a tiglon. Both mule and tiglon are sterile hybrids.
5.2.2.3 Some same species rarely interbreed
There is interbreeding between geographically apart grass frog in North America which was unsuccessful.
5.2.3 Ecological Aspect
Species from different evolutionary branches may come to resemble one another if they have similar ecological niches. This is the result of convergent evolution. Species which are distributed over a wide geographical range or have occupied well-separated geographical habitats for a long period of time may show considerable phenotypic differences. This is the result of divergent evolution.
5.2.4 Evolutionary Aspect
Species are not really stable but are by-products of the dynamic evolutionary process and are constantly changing e.g. mutations may occur. As 2 or more species evolve from closely related ancestors, they often diverge considerably in their phenotypic characters. Direct evidence on sharing a common ancestor is always lacking. The exact range of such terns as “similarities” and “differences” is also extremely difficult to determine. Difficulties arise with polymorphism, sexual dimorphism or there is little apparent difference between species. In polymorphism, species have members that exist in many different morphological and anatomical forms. It is shown by many colonial insects.
5.3 Reproductive Isolating Mechanisms (RIMs)
The critical event in the origin of a new species is the establishment of reproductive isolation between speciating populations. Thus need to know what kinds of barriers to interbreeding occur and to understand how reproductive isolation may arise between a parental species and a newly evolving species. These are our main objectives. Before we consider these 2 aspects of speciation, we need a little background information.
A reproductive isolating mechanism is a structural, functional or behavioural characteristic that prevents successful reproduction from occurring.
5.3.1 Types of RIMs
5.3.1.1 Prezygotic Mechanisms
In ecological isolation, populations occupy different habitats in the same general region. In temporal isolation, mating or flowering times occur at different seasons. In behavioural isolation, mutual attraction between the sexes of different species is weak. In mechanical isolation, physical non-correspondence of the genitalia or flower parts prevents copulation or pollen transfer. In isolation by different pollinators, related flowering plant species may be specialized to attract different insects as pollinators. In gametic isolation, gametes may not attract each other or gametes may be inviable in the reproductive tract of the opposite sex.
5.3.1.2 Postzygotic Mechanisms
In hybrid inviability, hybrid zygotes are inviable or have reduced viability. In hybrid sterility, hybrids of one or both sexes fail to produce functional gametes. In hybrid breakdown, The F2 or backcross hybrids have reduced fertility or viability.
5.4 Modes of Speciation
5.4.1 Mechanisms
Species are reproductively isolated groups of organisms. The question of how speciation occurs is equivalent to the question of how reproductive isolation arises between groups of populations because reproductive isolation is the crucial process. Issues include the following questions. Does speciation require geographic separation? Does complete reproductive isolation evolve during geographic separation? Does isolation begin during separation with it being completed only when an incipient species comes into contact with its ancestral population?
There are 2 general modes of speciation based on how gene flow among populations is interrupted.
5.4.1.1 Allopatric Speciation
It is a geographical barrier that physically isolates populations and initially blocks gene flow. Populations segregated by a geographical barrier as known as allopatric populations. Geographical barriers include the following. A mountain range may emerge and gradually split a population of organisms that can inhabit only lowlands. A creeping glacier may gradually divide a population. A land bridge may form and separate the marine life on either side. A large lake may subside until there are several smaller lakes with their populations now isolated.
The classical model of speciation, allopatric speciation, holds that speciation may occur when some geographic barrier separates populations that show continuous geographic variation.
5.4.1.1.1 Three phases leading to allopatric speciation
5.4.1.1.1.1 First Phase: Physical Separation (Subdivision)
A single population (gene pool) is split into 2 or more spatially separated populations (gene pools) by geological or ecological process: mountain uplift, rise and fall of land, or vegetational changes. This process stops gene flow between 2 groups of populations, allowing genetic divergence to occur as the populations evolve independently.
5.4.1.1.1.2 Second Phase: Genetic divergence and development of reproductive isolating mechanisms (RIMs)
Selective pressures differ in different areas because the ecological conditions differ. Any mutation or recombination of genes which increases fitness in each environment will be favoured, resulting in genetic / phenotypic divergence. With the passage of time, new alleles will be fixed in each of the 2 populations. As the populations become more and more genetically different, RIMs may appear. Isolating mechanisms become more firmly established with continuing evolutionary divergence.
5.4.1.1.1.3 Third Phase: Fall of geographic barrier
The geographic barrier “falls” (or one or more populations disperse) and secondary contact occurs after establishment of some isolating mechanisms, with possible results, depending on the duration of separation, effectiveness of geographic barrier, degree to which RIMs have evolved and degree to which ecological divergence (ED) has evolved. When 2 formerly isolated populations come into secondary contact with each other, RIMs and ED may be complete or incomplete.
Under the allopatric theory of speciation, reproductive isolation may evolve between 2 geographically isolated populations. If it does not, the 2 populations simply merge back into 1 when they meet again. A third alternative is that partial reproductive isolation sometimes might evolve during separation and then undergo reinforcement after secondary contact, which probably operates with the strongest force soon after the populations meet again. Both scenarios involving reproductive isolation are consistent with the allopatric theory of speciation.
5.4.1.1.2 Adaptive radiation on island chains
It is the evolution of many diversely adapted species from a common ancestor.
Islands are living labs for the study of speciation. Flurries of allopatric speciation have occurred on island chains where organisms that have strayed or become passively dispersed from their parent populations have founded new populations that evolved in isolation.
The many indigenous species of the Galapagos descended from stragglers that floated, flew or were blown over the sea from the South American mainland. For example, consider the Galapagos finches. A single dispersal event may have seeded one island with a small population of ancestral finch and this peripheral isolate formed a new species.
Later, a few individuals of this island species may have reached neighbouring islands, where geographical isolation permitted additional speciation episodes. After developing on one of these other islands, a new species could recolonize the island from which its founding population emigrated and coexist there with its parent species, or form still another species. Multiple invasions of islands by peripheral isolates of species from neighbouring islands could eventually lead to the coexistence of several species on each island. The islands are far enough apart to permit populations to evolve in isolation, but close enough together for occasional dispersion events to occur.
Adaptive radiation of the Galapagos finches is evident in the many types of bills specialized for different foods.
The Hawaiian Archipelago is one of the world’s greatest showcases of evolution. The volcanic islands are about 3500 km from the nearest continent. They become progressively younger to the southeast, terminating with the youngest and largest island. Hawaii, which is less than a million years old, still has active volcanoes. Each island was born naked and was gradually populated by species derived from strays that rode the ocean currents and winds either from distant islands and continents or from older islands of the archipelago itself. The physical diversity of each island, including a range of altitudes and extensive differences in rainfall, provides many environmental opportunities for evolutionary divergence by natural selection.
Multiple invasions and allopatric speciations have ignited an explosion of adaptive radiation; most of the thousands of species of plants and animals that now inhabit the islands are found nowhere else in the world. In contrast, there are no indigenous species on the Florida Keys. Apparently, those islands are so close to the mainland that founding populations are not sufficiently sequestered for their gene pools to become isolated from the steady stream of immigrants from the parent populations on the mainland.
5.4.1.2 Sympatric Speciation
It is a result of intrinsic factors such as chromosomal changes (in plants) and non-random mating (in animals), alter gene flow. Sympatric populations become genetically isolated even though their ranges overlap.
In sympatric speciation, new species arise within the range of parent populations; genetic isolation evolves in a variety of ways without geographical isolation. For instance, in a single generation a new species (defined by the biological species concept) can be generated if a genetic change results in a reproductive barrier between the mutants and the parent population. Many plant species have their origins in accidents during cell division that result in extra sets of chromosomes, a mutant condition called polyploidy.
