Chapter 15 EVOLUTION
OVERVIEW OF THE EARLY EVOLUTIONARY CONCEPTS.
Heraclitus of Ephesus, 540 – 480 BC (6th BC)
Heraclitus proposed that things are in a constant state of change. Nothing remains the same except change itself. According to him, all things are but one thing. He uses opposites to explain this unity: day and night, if you remove “day”, night disappears also. He was the last proponent of change in nature until the 18th and 19th biologists (Lamarck, Wallace, Darwin) began to propose ideas about the evolution of species.
Plato, 428-348 B. C. Greek Philosopher.
Generalizations are real and everything else is a reflection of the reality. The form of a structure could be understood from its function, since the function dictated the form. Species became the initial mold for all later replicates of that species. The only way we can know is if things do not change. If things change, how can we have true knowledge? As soon as something is known, it changes into something else; therefore, we could not have true knowledge. Knowledge is only possible is things are static.
“Two ideas in Plato's Timaeus. First, God created the cosmos and everything in it because "He was good, and the good can never have any jealousy of anything. And being free from jealousy, he desired that all things should be as like himself as they could be." Second, God created a huge variety of different life forms because "without them the universe will be incomplete, for it will not contain every kind of animal which it ought to contain, if it is to be perfect" (29-30). This second idea (that God created all conceivable life forms because that was essential in a perfect creation), which was to play a very significant role in Christian theodicies and natural philosophy, is called The Principle of Plenitude.” http://www.mala.bc.ca/~johnstoi/darwin/sect3.htm
Aristotle, 384-322 B. C. Greek philosopher.
To Aristotle, the species was the union of essential characters common to all members of the group. This he called the Form, and remained identical in all members of the species. Variation between individuals was due to Matter, the material makeup of the organism. Aristotle proposed that the last stage of development, the adult form, explains the changes that occur in the immature forms (teleological explanation); the advanced stages influence the earlier stages. He also proposed that all living forms are linked in a progression from imperfect to most perfect. He called this the Scale of Nature. The Scala Natura is a hierarchical view of the world in which the Godhead possesses the plenitude of being, and organisms are more and more imperfect as they are ranked away from the creating Divinity. So the most imperfect beings are soil, following in perfection will be rocks, invertebrates, vertebrates, humans, angels or spiritual beings and finally God. Aristotle introduced the ideas of continuity and gradation in nature.
Leibniz, Gottfried Wilhelm, Baron Von. 1646-1716, German philosopher and mathematician. Leibniz proposed that the universe was not perfect, only becoming so, and that it may go through successive intermediary stages on the pathway to perfection. He even proposed that gaps in the Scale of Nature were the result of evolutionary changes.
Buffon, George Louis Leclerc, Comte de. 1707-88, French naturalist and author.
Buffon said that species are the only biological units that have a natural existence. He introduced the idea of reproductive barriers separate the species. He recognized that variation could exist within the species and give rise to new varieties: "...that man and ape could have a common origin; that in fact, all the families among plants as well as animals, have come from a single stock, and that all animals are descended from a single animal..." (Natural History, 1753). However, he thought that species remained permanently distinct from other species. Buffon did not provide a mechanism for the change of species. Buffon thought that the age of the Earth was about 75,000 years old thus challenging the common belief of the time that the Earth was 6000 years old.
Baer, Karl Ernst Von. 1792-1876, Estonian naturalist and embryologist.
One of the founders of the modern science of development and one of his era's most influential scientists. Two conflicting schools of thought had been based on this question: the preformation school maintained that the egg contains a miniature individual that develops into the adult stage in the proper environment; the epigenesis school believed that the egg is initially un-differentiated and that development occurs as a series of steps. Von Baer provided evidence against the preformation theory. His discovery of the mammalian egg and his recognition of the formation of the germ layers out of which the embryonic organs develop became the foundation of modern embryology. He proposed what is now called Von Baer's Law. Von Baer's Law states that structures that are present early in development are widely distributed among animals, while structures that are present late in development are less widely distributed. Von Baer's important works include Papers on the Origin of the Mammalian Egg and Man (1827), in which the mammalian egg is described for the first time; and Research into the Development of Fishes (1835).
Hutton, James. 1726-97, Scottish geologist.
Hutton formulated controversial theories of the origin of the earth in 1785. He was of the opinion that the earth must be very old.
Uniformitarianism: the doctrine that past geological changes in the earth were brought about by the same causes as those now taking place. It stressed the slowness and gradualness of rates of change, and that physical and chemical laws remain unchanged with time.
Cuvier, Georges Léopold Chrétien Frédéric Dagobert, Baron. 1769-1832, French zoologist and paleontologist.
Cuvier is considered to be the father of comparative anatomy. He believed that organisms were perfectly adapted to their role in nature and there was no need to move into a more perfect state, thus disagreeing with the Scala Natura concept. Cuvier was of the opinion that every part of the organism had its function and the organism itself was perfectly integrated into a harmonious being. Any change in the component parts will destroy this integration and make the organism unable to live. Cuvier originated a system of zoological classification based on structural differences of the skeleton and organs.
Catastrophism: the geological doctrine that the physical features of the earth's surface, e.g., mountains and valleys, were formed during violent worldwide cataclysms, e.g., earthquakes and floods. He argued that all living things were destroyed and replaced with wholly different forms during these cataclysmic events. Cuvier stated that fossils represent species that no longer exist and had been replaced by modern ones.
Lyell, Sir Charles. 1797-1875, English geologist.
Lyell argued that presently observable geological processes were adequate to explain geological history. He thought the action of the rain, sea, volcanoes and earthquakes explained the geological history of more ancient times. Lyell conclusively showed that the earth was very old and had changed its form slowly, mainly from conditions such as erosion. Lyell was able to date the ages of rocks by using fossils embedded in the stone as time indicators. Lyell helped win acceptance of James Hutton's theory of uniformitarianism and of Charles Darwin's theory of evolution. Evolutionists supported uniformitarianism because evolutionary processes take a long time
Lamarck, Jean Baptiste Pierre Antoine de Monet, Chevalier de, French botanist and zoologist, 1744-1829.
Lamarck was the first biologist to actively propose evolution.
LAMARCKISM
Lamarck based his theory of evolution in part on his study of the fossils of marine invertebrates. He thought that species do change over time.
He believed, furthermore, that animals evolve because unfavorable conditions produce needs that animals try to satisfy.
Species not only become extinct but also undergo a gradual modification through time.
Organisms have an inner perfecting principle that …
could sense the needs of the environment respond by developing adaptations… mostly from simple to complex.
Organisms are not passively altered by their environment.
A change in the environment causes changes in the needs of organisms living in that environment, which in turn causes changes in their behavior.
Two mechanisms are involved in evolution:
1. Principle of use and disuse. 2. Inheritance of acquired characters. Lamarck viewed evolution as a process of increasing complexity and "perfection," not driven by chance.
"Nature, in producing in succession every species of animal, and beginning with the least perfect or simplest to end her work with the most perfect, has gradually complicated their structure." Philosophie zoologique.
While the mechanism of Lamarckian evolution is quite different from that proposed by Darwin, the predicted result is the same: adaptive change in lineages, ultimately driven by environmental change, over long periods of time.
Lamarck did not believe in extinction: for him, species that disappeared did so because they evolved into different species.
Lamarck's Philosophie zoologique mentions the great variety of animal and plant forms produced under human cultivation (Lamarck even anticipated Darwin in mentioning fantail pigeons!); the presence of vestigial, non-functional structures in many animals; and the presence of embryonic structures that have no counterpart in the adult.
Lamarck believed that the Earth was very old.
Charles Darwin, 1809-1882.
Charles Robert Darwin was born in Shrewsbury, England in 1809. The son of an eminent local doctor, Dr Robert Darwin, Charles was born into a modestly wealthy family.
He was the grandson of Erasmus Darwin, English naturalist, and promoter of the idea of evolution.
Darwin studied medicine at Edinburgh University from 1825 to 1827.
He transferred to Christ's College, Cambridge in 1828 with the intention of becoming a minister in the Church of England.
He was a mediocre student and did not like classical education. His interests were in natural history, botany, geology, collecting and hunting.
In 1831, with the help of his botany professor John Henslow and his uncle Josiah Wedgewood, he took the post of naturalist on board the H.M.S. Beagle on a scientific mission to South America.
By the time of his return in 1836, he had become an authority on many forms of life.
Between 1842 and 1844 he developed his theory of natural selection, although he did not announce his work until 1858.
In 1859 he published a considerably expanded version of his researches in the controversial Origin of Species by Means of Natural Selection.
He published The Descent of Man in 1871. THE VOYAGE OF THE BEAGLE
H.M.S. Beagle, under the command of Captain Robert Fitzroy, left for Patagonia, South America, in 1831 with Charles Darwin on board.
At this time, Charles did not believe in evolution, including his grandfather’s theory, as any evidence presented so far could not convince him.
The Beagle reached South America in 1832 and Charles took care to observe the flora and fauna.
Charles went then to Buenos Aries where he saw fossils of more ancient animals, including a mastodon.
He experienced a violent earthquake in Chile that raised the land in some places between 2 and 10 feet.
Darwin was most interested in the plants and animals on the Galápagos, a group of 16 large islands (and many smaller) off the coast of Ecuador.
Giant tortoises inhabit every one of the islands, which gave the island chain its name, from the Spanish Galápagos, meaning tortoise. Each island has its own type of tortoise, distinguishable by the shape and pattern of its shell. Darwin was astonished that the islanders felt that this was due to the difference in environment on each island. Darwin also observed the finches, which varied in size and shape from island to island. Their beaks also varied depending on which food they ate and some even had extra long tongues for grabbing certain types of foods such as insects, nuts or seeds.
Island animals and plants were different to those on the mainland but a relationship could be seen. What was even stranger though, to Darwin, was the fact that organisms on different islands varied, but still seemed related.
The complete text of The Voyage of the Beagle is on line: http://www.infidels.org/library/historical/charles_darwin/voyage_of_beagle/ http://www.literature.org/authors/darwin-charles/the-voyage-of-the-beagle/
Darwin was deeply impressed by Malthus' ideas on population.
1. Populations grow geometrically while supporting resources grow arithmetically
"...I think I may fairly make two postulata. First, That food is necessary to the existence of man. Secondly, That the passion between the sexes is necessary and will remain nearly in its present state. Assuming then my postulata as granted, I say, that the power of population is indefinitely greater than the power in the earth to produce subsistence for man. Population, when unchecked, increases in a geometrical ratio. Subsistence increases only in an arithmetical ratio..." An Essay on the Principle of Population.
2. Population, if not purposefully checked (“preventative checks”), would outpace resources and lead to unplanned “positive checks” that would return population to sustainable levels
"...Famine seems to be the last, the most dreadful resource of nature. The power of population is so superior to the power in the earth to produce subsistence for man that premature death must in some shape or other visit the human race. The vices of mankind are active and able ministers of depopulation. They are the precursors in the great army of destruction; and often finish the dreadful work themselves. But should they fail in this war of extermination, sickly seasons, epidemics, pestilence, and plague, advance in terrific array, and sweep off their thousands and ten thousands. Should success be still incomplete, gigantic inevitable famine stalks in the rear, and with one mighty blow levels the population with the food of the world..." An Essay on the Principle of Population.
DARWIN'S THEORY OF NATURAL SELECTION.
Fact #1 - Without constraints, populations will grow exponentially, producing an ever more rapidly growing number of organisms.
Fact #2 - In spite of this prediction, the numbers of individuals in a population remains near equilibrium, fluctuating above and below some mean value.
Fact #3 - Resources are limited.
Conclusion: From these three facts, Darwin concluded that there was a struggle for existence. Darwin combined this with two additional facts:
Fact #4 - Individuals are unique. There is individual variation. This came from observing animal breeding.
Fact #5 - Much, but not all, of the individual variation is heritable. This observation also came from animal breeders. Some of the observed variation is environmental, some is genetic.
Conclusion: These facts led Darwin to the conclusion that some individuals are better equipped to survive and reproduce (Natural Selection) in their struggle for existence: Differential survival and reproduction.
Through many generations of time, evolution is the result. Darwin used "descent with modification."
Natural selection is often described as "survival of the fittest"; it maintains that the organisms best suited to survive in their environment are more likely to reproduce and pass their genetic material to the next generation, while those with less advantageous traits are less likely to survive long enough to reproduce.
According to Darwin, the diversity of life forms arose by descent with modification from ancestral species.
Darwin illustrated the power of selection as a force in evolution with examples from artificial selection, the breeding of domesticated animals and plants.
Humans have modified many species by the selective breeding of individuals with preferred traits. The evolution of insecticide resistant insects, antibiotic resistant bacteria, and herbicide resistant weeds illustrate the process of selection and the gradual change in a population.
Darwin's main ideas:
1. Natural selection is differential success in reproduction.
2. Natural selection occurs through the interaction between the environment and the variability found in a population.
3. The product of natural selection is the adaptation of populations of organisms to their environment.
Origin of Species http://www.infidels.org/library/historical/charles_darwin/origin_of_species/ http://www.literature.org/authors/darwin-charles/the-origin-of-species/
ALFRED RUSSEL WALLACE, 1823-1913.
8 January 1823: Alfred Russel Wallace born at Usk, Monmouthshire, England.
mid/late 1837: Joins the eldest brother William in Bedfordshire to learn the surveying trade.
25 April 1848: Wallace and Bates leave England for Amazonian South America to begin a natural history collecting expedition.
March 1854: Leaves England for the Far East to begin a natural history collecting expedition.
20 April 1854 to 20 February 1862: Collecting expedition in the Malay Archipelago.
February 1858: Writes 'On the Tendency of Varieties to Depart Indefinitely From the Original Type' and sends it off to Charles Darwin for comment.
1 July 1858: Wallace's and Darwin's writings on natural selection are presented at a meeting of the Linnaean Society.
November 1859: 'On the Zoological Geography of the Malay Archipelago,' the paper describing Wallace's Line, is read before the Linnaean Society; Darwin's On the Origin of Species is published.
Wallace independently concluded that natural selection contributed to the origin of new species.
Wallace’s famous paper: http://www.wku.edu/~smithch/index1.htm
SUPPORTING EVIDENCE Structures with a similar underlying plan can be explained by relationship through common ancestry.
1. Homologous vs. analogous.
Homologous features are variations of the same basic structural pattern, even though the structure may be used differently.
Homologous structures are considered to indicate evolutionary affinity among organisms possessing them.
Homoplastic or analogous features have similar function but lack the basic pattern of homologous structures, e.g. wings of birds and insects.
Homoplastic features show what is called convergent evolution.
2. Embryological homologies
Homologies exist between developing embryos.
Sometimes homologies are evident in the developing embryo but not in the adult organism.
There is a remarkable similarity between vertebrate embryos: embryological homologies.
3. Molecular homologies
There are molecular homologies, e.g. the structure of DNA and RNA, the process of replication, transcription and translation, pathway of cellular respiration, and the genetic code.
Lines of descent based on molecular characters closely resemble lines of descent based on structural and fossil evidence.
The universality of the genetic code is considered to be an evidence that all organisms have a common ancestor.
Amino acid sequence in common proteins reveals greater similarities in closely related species.
A greater proportion of nucleotide sequence in DNA is identical in closely related organisms.
The universal genetic code and many pathways (glycolysis, Krebs cycle, photosynthesis, etc) support a common origin.
4. Biogeography or geographical distribution
Closely related species tend to be found in the same geographic area.
Same ecological niches found in distant lands are occupied by very different, though superficially looking species, e. g. Australian marsupials and Eurasian placental mammals. Geographical barriers lead to great differences between organisms, e.g. adaptive radiation of Australian marsupials
Isolation tends to produce endemic species.
Endemic, endemism refer to species native and restricted to a geographical region.
5. Fossils
The fossil record shows a general progression from the earliest, single-celled organisms to organisms living today.
Some fossils appear to be intermediates between living groups, e. g. Archaeopteryx, evolution of the horse, a Coelacanth.
The fossil record is incomplete.
Radioactive isotopes found in fossils and rocks provide a means of measuring the age of the fossil or rock layer.
Mutations
Genes are sections of DNA.
Genes contain a specific sequence of nucleotides.
Offspring acquire genes from parents by inheriting chromosomes.
Most genes program cells to synthesize specific enzymes and other proteins whose cumulative action produces an organism’s inherited traits.
Breakage of chromosomes can lead to four types of alterations in chromosomal structure.
Deletions are loss of chromosomal material.
A chromosome breaks and fails to rejoin. Most deletions are lethal. Cri-du-chat is due to a deletion in chromosome 5.
Duplication occurs when a piece of a chromosome breaks off and becomes attached on the sister chromatid causing a duplication of genetic material in the recipient chromosome.
An inversion happens when the detached piece is reattached to its chromosome but in the reverse orientation.
Translocation is the attachment of part of a chromosome to a nonhomologous chromosome.
Non-homologous chromosomes may exchange parts, reciprocal translocation. It may result in the elimination or duplication of genes. A type of Down syndrome results from the translocation of a portion of chromosome 21 to chromosome 14. The individual has two normal chromosomes 21, one normal chromosome 14 and one abnormal chromosome 14 with a portion of chromosome 21 attached. In chronic myelogenous leukemia (CML) a piece of chromosome 22 has switched places with a fragment from the tip of chromosome 9. The production of white blood cells is affected.
Deletions and duplication are especially likely to occur during meiosis.
Duplication and translocation tend to have harmful effects because essential genes may be affected.
Inversions do not cause an imbalance in the genes but the change in location may influence the phenotype due its new location and neighboring genes.
Migration
Gene flow between populations occurs when individuals or gametes migrate from on population to another.
The size of the population and the degree of isolation of the population affect how much gene flow occurs between populations of the same species.
Populations that are isolated tend to have gene pools showing different frequencies of alleles.
Gene flow tends to decrease the amount of variation between the populations.
Genetic drift
Genes tend to remain at a constant frequency from generation to generation given certain conditions.
The following conditions have to be met for a population to remain in genetic equilibrium.
1. Random mating. Each individual of the population has equal chance of mating.
2. No net mutations. The frequencies of genes must not change due to mutations.
3. Large population size in order to avoid frequency changes due to random fluctuations.
4. No migration. There can be no exchange of genes with other populations that might have different allele frequencies.
5. No natural selection in order to avoid that some genotypes be favored over others.
Genetic drift refers to the production of random evolutionary changes in small breeding populations.
Alleles present in low frequency could be lost completely by pure chance, e.g. predators, by chance, kill individuals with the low frequency allele. Alleles are lost regardless of being beneficial, harmful or neutral.
Genetic drift decreases the genetic variation within the population but it tends to increase the differences among different populations.
Genetic drift becomes a major evolutionary force when the population goes through a genetic bottleneck.
In genetic bottlenecks, a small population survives a catastrophe and it determines the gene frequency of the succeeding generation.
As the population increases in size, the allele frequency might be very different from that of the previous generation before the catastrophe.
The founder effect occurs when a few individuals, founders, establish a new colony.
The founders bring only a small fractions of the genetic variation of the original population.
RATES OF EVOLUTION
The pace of evolution is being debated.
Punctuated equilibrium.
According to this model, there are long periods of stasis when no evolution occurs, punctuated or interrupted by short periods of rapid speciation possibly triggered by changes in the environment.
Questions the idea that the fossil record is as incomplete as it initially appeared. Allopatric and sympatric speciation can occur in very short time. It accounts for the abrupt appearance of new species in the fossil record. Transitional forms are absent for the most part. New species appears in a few hundred thousand years instead of millions of years.
Gradualism proposes that evolution occurs continually over long periods of time.
Populations gradually diverge from one another by the accumulation of adaptive characteristics in each population. It is rarely observed in the fossil record because most organisms decompose without a trace. There are transitional forms.
MACROEVOLUTION – HOW SPECIES EVOLVED
Macroevolution includes the appearance of evolutionary novelties, which are so great that the new species are assigned to new genera or higher taxonomic categories.
It refers to dramatic changes that occur over long time spans in evolution.
Usually the evolutionary novelties are derived from preexisting structures called preadaptations. Preadaptations played a role in the life of the species but changed to fulfill another role.
Slight genetic changes in regulatory genes can cause dramatic changes in the phenotype.
Regulatory mutations can have significant developmental effects and probably account for important differences among various groups.
The varied rate of growth of different parts of the body is called allometric growth.
Changes in allometric growth results in changes in the shape of an organism. Pictures: Created on 07-05-98 by Gábor Lendvai http://www.micro.utexas.edu/courses/levin/bio304/evolution/speciation.html
Reproductive isolating mechanisms prevent interbreeding between two different species whose ranges overlap.
They preserve the genetic integrity of the species by preventing the flow of genes between the different species.
Most species have two or more isolating mechanisms that block interbreeding.
A. Prezygotic barriers prevent fertilization.
1. Temporal isolation: similar species reproduce at different times.
2. Behavioral isolation: similar species have distinct courtship behavior.
3. Mechanical isolation: the reproductive organs of the species are different.
4. Genetic isolation: the gametes of similar species are chemically incompatible.
B. Postzygotic barriers reduce the viability and/or fertility of the offspring.
1. Hybrid inviability: interspecific hybrid dies at early stage of embryonic development.
2. Hybrid sterility: interspecific hybrid survives into adulthood but is sterile.
3. Hybrid breakdown: offspring of interspecific hybrid are unable to reproduce successfully. In interspecific hybrids, the chromosomes are not homologous.
Geographic isolation
Some biologists are of the opinion that geographic isolation of populations is always needed for the development of new species.
This process of speciation is called allopatric speciation.
Different environments have different selective pressures.
Natural selection acts on different aspects of the two isolated populations eventually resulting in two distinct species adapted to their particular environment.
They are likely to differentiate for two reasons:
1. Different geographic regions are likely to have different selective pressures. Temperature, rainfall, predators and competitors are likely to differ between two areas 100's or 1,000's of kilometers apart. Thus, over time, the two populations will differentiate.
2. Even if the environments are not very different, the populations may differentiate because different mutations and genetic combinations occur by chance in each. Thus, selection will have different raw material to act upon in each population.
Ecological isolation
Many of the selective pressures in different environment are due to different ecological factors, e.g. soil type, temperature.
THE ROLE OF HYBRIDIZATION IN EVOLUTION
"The role of hybridization in evolution has been one of the most controversial topics in the whole field of evolutionary study" (Stebbins 1963)
Hybridization is the production of offspring from different populations by parents that differ in one or more characteristics. .
Hybridization may provide new combinations of genetic material that can be acted upon by other process at a later time, e.g. polyploidy or chromosomal rearrangement.
Interspecific hybrids are often sterile. They are genetic dead-ends.
Polyploidy is a major factor on plant evolution.
Spontaneous doubling of chromosomes before meiosis has been documented in plants and a few animal species.
When polyploidy occurs in conjunction with hybridization is called allopolyploidy.
Hybridization followed by doubling of chromosomes (polyploidy). Allopolyploids are reproductively isolated from both parents. They have different number of chromosomes.
“Allopolyploid speciation:
1. Believed to be very common in plants--47% of angiosperms, 97% of ferns and fern allies are probably polyploid; majority of these presumably of allopolyploid origin.
2. Begins with a hybrid of two different species or "races" (subspecies or varieties).
3. Sterile hybrid undergoes chromosome doubling (both means below appear to be common events).
a) Somatic cells of the stem may generate a tetraploid branch that produces tetraploid flowers; diploid gametes produce tetraploid progeny. b) Diploid plant produces two sets of unreduced (diploid, not haploid) gametes; union produces tetraploid progeny.
4. Tetraploid is fertile--2 sets of chromosomes representing the 2 parental genomes pair up, yielding "balanced" gametes.
5. Famous examples of whole species groups include spleenwort ferns (Asplenium), henbit (Galeopsis), cotton (Gossypium), wheat (Triticum).”
Harvey E. Ballard, Jr. PLANT SPECIATION AND EVOLUTION LECTURES. Hybridization http://oak.cats.ohiou.edu/~ballardh/pbio475/Hybridization/Hybridization.htm
Check this interesting case of speciation due to hybridization: http://www.biosci.ohio-state.edu/~awolfe/Penstemon/hybridization.html
Introgression plays a role in the circumscription of species.
Typical species tend to occur in their specialized habitat. Introgression between two species usually occurs in the fringes of their habitats. Introgression creates hybrid zones between a putative parent and the daughter species.
“Infiltration of the genes of one species into the gene pool of another through repeated backcrossing of an interspecific hybrid with one of its parents.”
Dictionary definition of introgression. The American Heritage® Dictionary of the English Language, Fourth Edition Copyright © 2004, 2000 by Houghton Mifflin Company. Published by Houghton Mifflin Company. APOMIXIS
The development of seeds without fertilization.
The new generation can come from an unfertilized ovule or from a vegetative cell. A diploid cell in the ovule gives rise to an embryo.
Mechanisms of organic evolution include mutations in chromosomes or genes, hybridization, introgression polyploidy, apomixis, and reproductive isolation.