Evolution

What is evolution?

It is descent with modification. It is the change in allele frequency of a population over time. In other words, current living species are descendants of ancestral species that were different from the present-day ones.

What is microevolution

Microevolution is the change in the allele frequencies within a population for a given species over generations. It refers to evolutionary changes on a small scale,

What is macroevolution

Macroevolution refers to evolutionary changes on a larger scale, beyond a single species

What is lamarck’s theory of evolution

• one key part of Lamarckian evolution is the theory of use and disuse

• lamarck proposed that organisms are driven by necessity and would gain physical attributes as a result of exercising that particular body part parts of the organisms that are not used will deteriorate

• organisms will then pass on the traits/modifications acquired during their lifetime to their offspring.

• cycle repeats itself and the species develops specific and unique adaptations.

• as a result, organisms change over time. E.g. over time, the giraffe’s neck grew longer.

• Lamarck also proposed that organisms have an innate drive to become more complex.

What is Darwin’s theory of evolution

• Darwin’s theory of evolution states that the environment exerts selection pressure on individuals of a population, and would select for individuals that are most adapted for it

• individuals are only considered ‘successful’ or ‘fit’ if they live long enough to reproduce will pass on their adaptations to their offspring, giving rise to viable and fertile offspring with the adaptations

• individuals that are unfit for survival in the current environment will be eliminated

• appearance and characteristics of a species will change over time as a result of such natural selection, organisms are said to have evolved and may be classified as new species

• he proposed that some organisms are better already adapted to survive than others in a particular environment due to variation

• this is known as descent with modification

What are the main features of Darwin’s evolution

1. population posses great reproductive capabilities

   • individuals of a population are capable of producing large numbers of offspring (more than needed to replace)

2. constancy of population size

   • most offspring die before they reach reproductive age or fail to mate to produce offspring

3. struggle for survival

   • members of a species are constantly competing with each other for resources, only a few individuals would survive long enough to reproduce.

4. variation in population

   • individuals in a population are different

5. for individuals produce similar offspring

   • individuals with the features best suited for the prevailing environment are more likely to survive,

6. accumulation of favourable traits in populations over generations

   • unequal ability of individuals to survive and reproduce will lead to certain advantageous traits becoming fixed in the population over time

What are the features of neo Darwinism

4. variation within a population

   • individuals in a population are genetically different from one another, hence there is variation within a species

5. survival of the fittest by natural selection

   • individuals with the alleles coding for traits best suited for the prevailing environment are more likely to survive, better adapted to survive to reproductive age to produce fertile, viable offspring (selective advantage)

6. for individuals produce similar offspring

   • characteristics or features similar to themselves, these offspring have inherited the alleles from both their parents, offspring are more likely to be successful if the selection pressure remains the same.

7. accumulation of favourable alleles in the population over generations

   • advantageous alleles have a higher chance of survival, hence they will produce more offspring, this unequal ability of individuals to survive and reproduce will lead to certain advantageous alleles becoming fixed in the population over time, over time, there would be changes in allele frequency in a population. The frequency of advantageous alleles increases and that of disadvantageous alleles decreases.

Describe stabilising selection

• selection favours the intermediate phenotype out of a range of phenotypes.

• it operates when phenotypic features coincide with optimal environmental conditions and competition is not severe.

• it usually occurs when the environment remains constant

• extremes in variation are selected against, resulting in a narrower range of phenotypes in the population.

• mean phenotype does not change.

• selection pressure operates to eliminate individuals with the alleles for extremely long and short fur lengths.

Describe destabilising selection

• form of selection operates in response to gradual changes in environmental conditions

• operates on the range of phenotypes existing within the population

• selection pressure tends to select phenotypes at one extreme of the range of phenotypes

• mean phenotype changes.

• individuals with allele for _ are at a selective advantage. allele is present at a higher frequency in the population. The alleles for

• other phenotype  are present in the population, but at lower frequencies.

• when selection pressure is exerted on the population, _ would be at a selective advantage. Individuals with longer fur would survive to produce fertile and viable offspring.

• more individuals with the alleles for _ will survive as they inherited the alleles from their parents.

• eventually, the allele for would predominate and exist at higher frequency while the alleles (disadvantaged allele)_ would be presented at lower frequency

Describe disruptive selection

• selection favours individuals at both extreme phenotypes out of a range of phenotypes

• rare form of selection.

• mean phenotype does not change.

• occurs when selection pressure is exerted on the middle range of variation for a particular trait

• fluctuating environmental conditions within an environment may favour this kind of selection resulting in the population being split into two distinct sub-populations.

• individuals with one extreme will be at a selective advantage i

• individuals with another extreme will be at a selective advantage

• however, individual with intermediate range are at a selective disadvantage and are eliminated from the population

• aftermany generations, two sub-populations of mice will be produced – one population with extreme and another population with other extreme The allele for for extreme would be fixed in one population and the allele_other extreme_ would be fixed in another population.

• individuals with the allele for would be at a selective disadvantage, so the allele for would be found at lower frequencies.

Why a population is the smallest unit that can evolve

• population comprises a group of individuals of the same species that live in the same area and interbreed, producing fertile offspring.

• evolution involves changes in allelic frequencies within a population over generations, changes do not occur in an individual.

• only a population of interbreeding individuals can evolve but not individual by itself

• evolution requires the individual to pass on its alleles to its offspring.

Why variation is important for natural selection to operate

• variation within a population must exist before natural selection can take place.

• members of a population are genetically different thus exhibit variation within the population.

• environmental change acts as selection pressure to select those individuals with advantageous trait or allele

• variation is important as it decreases the chances of extinction

• more variation a species has, the higher the chances of the species surviving different types of environmental change

How genetic variation arises in a population

• new genes/alleles arise from gene and chromosomal mutations

• organisms very rarely pass mutations gained in their lifetime to their offspring unless the mutation arose during gamete formation.

• meiosis can contribute to genetic variation due to recombination of alleles via

  • crossing over between non-sister chromatids of a pair of homologous chromosomes during prophase I

• independent assortment and segregation of homologous chromosomes at metaphase I and anaphase I respectively.

• independent assortment and segregation of non-identical sister chromatids at metaphase II and anaphase II respectively.

• random fusion of gametes results in genetic variation during fertilisation

• phenotypic variation can also arise as a result of environmental influence. Genes can interact with the environment to give rise to variation.

• migration into a population from another population, together with random mating, promote gene flow. which increases variation within a population by introducing new alleles to another population.

How genetic variation is preserved in a population

natural selection tends to produce genetic uniformity in a population by eliminating unfavourable alleles, tendency is opposed by a few mechanisms that preserve variation.

What is diplody

• most eukaryotes are diploid organisms (two alleles present at each gene locus), a considerable amount of genetic variation is hidden from selection pressure in the form of recessive alleles

• variation is only exposed to selection when two heterozygotes mate and produce offspring homozygous for the recessive allele.

• dominant alleles will appear more frequently in phenotypes and be selected for or selected against more rapidly

• recessive alleles on the other hand, exhibit no effect in the heterozygous state.

• recessive allele may be harmful but can persist without being eliminated from the population in heterozygote individuals.

What is neutral mutation

• many new mutations that are harmful are removed quickly by natural selection, some

• mutations are neutral and are not selected for or against, and thus being preserved in the population

• mutation that produces alleles that code for traits which are selectively neutral,

• mutation in non-coding regions,

• mutation that changes the last base of a triplet, resulting in the same amino acid being coded for.

• only mechanism that can alter the frequencies of neutral alleles is genetic drift

What is the heterozygote advantage

• disease-causing mutations and lethal alleles which confer selective disadvantage, would usually be eliminated from the population by natural selection.

• however, in certain environments, heterozygote individuals are at a selective advantage and are more likely to survive and reproduce to pass on

• the deleterious (lethal) allele to the offspring compared to homozygous individuals

• example of this is sickle cell anaemia.

  • heterozygote individuals with sickle cell trait have a higher chance of survival in regions where malaria is prevalent.

Describe natural selection

selection pressure can be seen as a means of increasing or decreasing the frequency of an allele within the gene pool and these changes in allele frequency lead to evolutionary change, where the extent of selection and time taken depend upon the nature of the mutant allele and the effect of it on the phenotype under the prevailing environment conditions.

Describe the founder effect

when a few individuals become isolated from a larger population this smaller group may establish a new population whose gene pool differs from the source population. This is called the founder effect

Describe natural selection

• sexual selection occurs when some members of a population mate more often than other members

• individuals with certain inherited characteristics are more likely than other individuals to obtain mates

• this is mating based on phenotype, which is any observable trait in an organism, including differences in appearance and behaviour

• if one male mates four times as much as the average male of his generation, his alleles stand to increase proportionately in the next generation

• differential mating success among members of one sex in a species often is based on choices made by members of the opposite sex in that species.

Describe the bottle neck effect

bottleneck effect is the situation where there is a drastic reduction in population size.

• drastic change in environmental conditions or disasters may reduce the size of a population significantly

• small surviving population may not be representative of the original population’s gene pool

• by chance alone, certain alleles will be overrepresented among the survivors, while others underrepresented or even eliminated altogether from the gene pool.

• bottleneck effect usually reduces the overall genetic variability in a population because at least some alleles are likely to be lost from the gene pool

What are the effects of genetic drift

• genetic drift is significant in small populations

  • chance events can cause allele to be disproportionately over- or under-represented in the next generation

  • although chance events occur in populations of all sizes, they tend to alter allele frequencies substantially only in small populations.

• genetic drift can cause allele frequencies to change at random

  • an allele may increase in frequency one year, then decrease the next; the change from year to year is not predictable

  • unlike natural selection, which in a given environment consistently favors some alleles over others, genetic drift causes allele frequencies to change at random over time.

• genetic drift can lead to a loss of genetic variation within populations

  • causing allele frequencies to fluctuate randomly over time, genetic drift can eliminate alleles from a population

  • evolution depends on genetic variation, such losses can influence how effectively a population can adapt to a change in the environment.

• genetic drift can cause harmful alleles to become fixed.

  • alleles that are neither harmful nor beneficial can be lost or become fixed entirely by chance through genetic drift small populations, genetic drift can also cause alleles that are slightly harmful to become fixed. When this occurs, the population’s survival can be threatened.

Describe gene flow

Gene flow is the movement of alleles into or out of a population due to movement of fertile

individuals or their gametes. Gene flow increases variation within a population by introducing

new alleles to another population

Gene flow can transfer alleles that improve the ability of populations to adapt to local conditions.

ene flow has become an increasingly important agent of evolutionary change in human

populations. Humans today move much more freely about the world than in the past. As a result,

mating is more common between members of populations that previously had very little contact,

leading to an exchange of alleles (through random mating) and fewer genetic differences

between those populations.

• Interestingly, reduced gene flow between two populations of the same species, as a result of isolations can drive the evolution of the two populations into two distinct species over

• primary mechanism for microevolution is the formation of new alleles by mutation.

• mutations can alter allele frequencies, but because mutations are rare, the change from one generation to the next is likely to be very small.

• nevertheless, mutation ultimately can have a large effect on allele frequencies when it produces new alleles that confer strong selective advantage to the organism

• mutations in gametes will be inherited by the next generation whereas mutations in somatic cells will not be inherited

• hence mutations in gametes are more significant

What is speciation

• speciation is an evolutionary process by which one species diverges into two or more species as a result of accumulated genetic differences due to changes in allele frequencies

• speciation explains not only differences between species, but also similarities between them

• when one species splits into two, the species that result share many structural similarities because they are descended from a common ancestral species

What is the biological species concept

• species is a group of populations whose members have the

• potential to interbreed in nature and produce viable and fertile offspring.

• biological species concept emphasizes on reproductive isolation

• the existence of biological barriers that impede members of two species from interbreeding and producing viable, fertile offspring

• members of a biological species are united by being reproductively compatible.

What are the limitations of the biological species concept

• cannot be applied to organisms that reproduce asexually

• does not apply to extinct species known to us by fossil records as mating can no longer be observed

• there is significant difficulty involved in observing mating in the wild, especially for microscopic organisms, plants that reproduce via pollination and marine organisms thatbrelease their gametes into water, where it is impossible to determine which organism was mating with which

• there are many pairs of species (especially in plants) that are morphologically and ecologically distinct, and yet gene flow occurs between them

• eg the grizzly bear and polar bear, whose hybrid offspring has been dubbed ‘grolar bears’

What morphological species concept

• concept defines a species in terms of a unique set of structural/physical features

• most of the species recognised by taxonomists are based on physical resemblance and differences

What are the advantages and limited of morphological species concept

• pros: it can be applied to both asexual and sexual organisms.

• cons: similarity in structure does not equate an evolutionary relationship.

• cons: concept relies on subjective criteria; researchers may disagree on which structural features distinguish a species

What is phylogenetic species concept

• concept defines a species as the smallest set of organisms that share a common

• ancestor, forming one branch on the tree of life.

• biologists trace the phylogenetic history of a species by comparing its characteristics, such as morphology and/or molecular sequences, with those of other organisms.

• it can distinguish groups of individuals that are sufficiently different to be considered separate species

What are the advantages and limitations of phylogenetic species concept

pros: it can be applied to both asexual and sexual organisms

con: the difficulty with this species concept is determining the degree of difference required to indicate separate species

What is genetic species concept

• concept defines species as a group of genetically compatible, interbreeding natural

• populations that is genetically isolated from other such groups

• focuses on genetic isolation rather than reproductive isolation which distinguishes the Genetic Species Concept from the Biological Species Concept

• genetic data from mitochondrial and nuclear genomes is used to study genetic differences in the genome of different species

What is the limitation of genetic species concept

con: there is a to determine the magnitude of genetic difference required to indicate separate species

What is ecological concept

• defines a species as a set of organisms adapted to the same ecological niche.

• ecological niche is the sum total of how members of a species interact with the biotic (living) and abiotic (non-living) part in its environment.

• defines a population of organisms which are adapted to the same set of resources in their habitat.

• forms and behaviour of organisms are adapted to the resources they exploit and habitats they occupy

• each species will evolve to exploit resources that it is better adapted to

What are the pros and cons of ecological concept

pro: accommodates asexual species, whereas the biological species concept does not

con: may be difficult to apply definition to species that are extinct as cannot be certain

about its role in the biological community

What is speciation

• speciation is the process of formation of a new species by which one species diverges from another at some point along the evolutionary timeline. Distinct species usually derive from one ancestral group.

• most commonly used for definition is the biological species concept, a new species is said to be formed when a population can no longer interbreed with another closely-related population to form fertile and viable offsprin

What is geographic separation

• geographical isolation reduces gene flow between the two groups within the population

• different mutations arise, due to natural selection and genetic drift may alter allele frequencies in different ways in the separated populations

• over time, as the isolated populations accumulate different types of advantageous alleles, the gene pool from the two populations becomes so distinct that members of the populations will no longer be able to interbreed, resulting in two populations becoming reproductively isolated from one and other

• by the biological species concept, the two populations become two new species

• when new species are formed as a result of such geographical isolation, it is known as allopatric speciation

What are examples of geographical separation

• 2 species of antelope squirrels inhabiting opposite rims of the Grand Canyon

• salamanders from the Central Valley in California

What is physiological isolation

incompatibility of the anatomical structure of the reproductive organs prevents the transfer of gametes between species

What is gametic isolation

• sperm of one species may not be able to fertilize the eggs of another species

• eg, sperm may not be able to survive in the reproductive tract of females of the other species, or biochemical mechanisms may prevent the sperm from penetrating the membrane surrounding the other species’ eggs

• gametic recognition may be based on the presence of specific molecules on the coat around the egg which may adhere only to complementary molecules on the sperm cells of the same species

eg gametic isolation separates certain closely related species of aquatic animals, such as sea urchins. Sea urchins release their sperm and eggs into the surrounding water, where they fuse and form zygotes. It is difficult for gametes of different species, such as the red and purple urchins, to fuse because proteins on the surfaces of the eggs and sperm bind very poorly to each other.

What is reduced hybrid viability

• genetic incompatibility between the two species may abort development of the hybrid zygote at some embryonic stage.

• eg some salamander subspecies of the genus Ensatina live in the same regions and habitats, where they may occasionally hybridize. But most of the hybrids do not complete development, and those that do are frail, thus they cannot become one single species.

What is reduced hybrid fertility

• even if hybrid is vigorous, they may be sterile

• if The chromosomes of the two parent species differ in number or structure, meiosis in the hybrids may fail to produce normal gametes

• infertile hybrids cannot produce offspring when they mate with either parent species, genes cannot flow freely between the species

• eg hybrid offspring of a male donkey and a female horse is a mule, which is robust but sterile. A “hinny”, the offspring of a female donkey and a male horse, is also sterile.

What is temporal isolation

• different species have different mating seasons or flowering seasons

• different species may also become sexually mature at different times of the year, hence decreasing the possibility of reproduction.

What is behavioural isolation

Behavioral isolation, a type of isolating mechanism, occurs when two populations are capable of

interbreeding but have differences in courtship rituals or other behavourial strategies that

prevents mating.

• Many insects and animals like birds have species-specific mating displays which include

visual, olfactory, auditory and tactile stimuli.

• For example, the eastern and western meadowlarks (Fig. 4.8) have similar body shapes and

colorations. However, because their courtship songs and other behaviors are different

enough, they do not interbreed should they meet in the wild. Gene flow between them is

prevented.

What is classification

• biological classification is the organisation of species based on shared characteristics.

• classification may not take into consideration evolutionary relationships between species due to the lack of information about genetics

• hierarchical classification system was first established by Linnaeus in an attempt to name and classify the diversity of organisms

• early system used easily identifiable physical features to classify organisms

• Linnaean system has two main characteristics:

  • binomial naming system (a two-part name for each species).

  • hierarchical classification into broader taxonomic categories.

What is binomial nomenclature

• common names for organisms – such as monkey, finch, and lilac – convey meaning in casual usage, but they can also cause confusion. (names refer to more than one species)

• moreover, some common names do not accurately reflect the kind of organism they signify.

• avoid ambiguity when communicating about their research, biologists refer to organisms by Latin scientific names.

• two-part format of the scientific name, commonly called a binomial, was instituted in theb18th century by Linnaeus.

• 1st part of a binomial is the name of the genus (plural, genera) to which the species belongs

• 2nd part, called the specific epithet, is unique for each species within the genus.

What is hierarchical classification

• first grouping is built into the binomial: species that appear to be closely related are grouped into the same genus

• beyond genera, taxonomists employ progressively more comprehensive categories of classification called the Linnaean taxonomy

• Linnaean taxonomy places related genera in the same family, families into orders, orders into classes, classes into phyla (singular, phylum), phyla into kingdoms, and, more recently, kingdoms into domains.

• named taxonomic unit at any level of the hierarchy is called a taxon (plural, taxa). In the

• each taxon possesses features which are diagnostic / distinguishing.

• moving from kingdom to species, number of similarities between the members of each taxon increases, number of members in each taxon decreases.

• thus a hierarchy is the grouping of organisms in ranks of increasingly smaller and more specific categories.

What is the order of hierarchical classification

What is phylogeny

• phylogeny is the organisation of species to show the evolutionary relationship between species

• not the same as Linnaean classification as not all species have been classified according to their evolutionary relationships yet.

• major role of phylogeny is to determine the ancestral relationships among known species (both living and extinct).

• evolutionary history of a group of organisms can be represented in a branching diagram called a phylogenetic tree

• groups of organisms nested within more inclusive groups.

• however, taxonomists have placed a species within a genus (or other group) to which it is not most closely related

• one reason for misclassification might be that over the course of evolution, a species has lost a key feature shared by its close relatives

• DNA or other new evidence indicates that such a mistake has occurred, the organism may be reclassified to accurately reflect its evolutionary.

• place species into groups called clades, each of which includes an ancestral species and all of its descendants

• clades are nested within larger clades.

• the chronology represented by the branching pattern of the tree is relative (earlier versus later) rather than absolute (how many millions of years ago).

• with data from molecular homology and fossil records, a tree that places branch points in

• the context of geologic time can be constructed to provide more information about the evolutionary relationship

What is divergent solution

• divergent evolution (also called adaptive radiation), whereby a group of organisms from a common ancestry share a homologous structure that is modified and specialised to perform a variety of different functions

• homologous structures is due to the different selection pressures in different environment, those individuals with selective advantageous traits or alleles were selected for and reproduce to pass on their advantageous alleles to reproduce fertile and viable offspring.

• eg need to adapt to sea condition results in the evolution of the flippers in dolphins. The need to adapt to flight condition results in the evolution of wings in bats.

• over many generations, the allele frequency changes differently within the different populations.

• sufficient accumulation of these genetic changes occurs within these different populations, the two populations can diverge into different species when they can no longer interbreed to produce fertile and viable offspring

What is convergent evolution

• different geographical areas sometimes exhibit groups of plants and animals of strikingly similar appearance, even though the organisms may be distantly related

• difficult to explain so many similarities as the result of coincidence, instead, natural selection appears to have favoured evolutionary adaptations in similar environments

• selection in these instances has tended to favour changes that made the two groups more alike, their phenotypes have converged

• form of evolutionary change is referred to as convergent evolution

• convergent evolution is the independent development of similar structures in groups of organisms with different common ancestors

often observed in organisms living in similar environments.

What is homology

• first type of evidence for evolution comes from analysing similarities among different organisms

• evolution is a process of descent with modification: characteristics present in an ancestral organism are altered in the population of descendants over time as they face different environmental conditions

• as a result, related species can have characteristics that have an underlying similarity yet function differently

• similarity resulting from common ancestry is known as homology

• more recently two species have shared a common ancestor, the more homologies they share, and the more similar these homologies are.

What are examples of anatomical homology

pentadactyl forelimbs of all mammals, including humans, cats, whales, and bats, showed a common basic plan with the same arrangement of bones from the shoulder to the tips of the digits, even though these appendages have very different functions: lifting, walking, swimming, and flying

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• previously, the finches occupied the South American mainland, but somehow managed to arrive at the Galapagos Islands, over 600 miles away

• occupied an ecological niche with little competition

• population began to flourish in these advantageous conditions, there is intra-specific (within species) competition for food on the islands

• variation (already exists) in the size of beak in the finches due to genetic differences as a result of mutations

• different environments of the different islands each exerts a different selection pressure (e.g. type of food) on the birds with different size of beak.

• advantageous allele will be selected for and survive to pass on the advantageous allele to reproduce viable and fertile offspring.

• over many generations, the accumulation of genetic differences eventually leads to the formation of distinct species from a single ancestral one where the different species can no longer interbreed to reproduce fertile and viable offspring (biological species concept).

• different species of Darwin’s finches share a homologous structure (beak) that is

• modified from that of a common ancestor and specialised to perform a variety of different functions

What are fossil records

• fossil record documents the pattern of evolution, showing that past organisms differed from present-day organisms and that many species have become extinct. Fossils also show the evolutionary changes that have occurred in various groups of organisms.

• fossil records are the preserved remains of the ancestral species of plants and animals (include bones, shells, imprints of organisms preserved in stone and preserved footprints)

• fossil record shows that over time, descent with modification produced increasingly large differences among related groups of organisms, ultimately resulting in the diversity of life today

• allow geologists to estimate the time of existence of the organism through carbon-dating or determining the geological strata in which fossil was found

• allow scientists to hypothesize how the organism looked like in the past.

• allow evolutionary biologists to study the structure of extinct species in order to discover

• possible evolutionary relationships between past and present species

• provide evidence of transitional forms along the evolutionary history – the intermediate species that link ancestral species with modern species

How is molecular homologies used to determine evolutionary relatedness

• every organism has DNA or RNA as its genetic material. This genetic material is inherited by offspring.

• when an ancestral species gives rise to two or more descendants, those descendants will initially exhibit fairly high overall similarity in their DNA. However, as the descendants evolve independently, they will accumulate more and more differences in their DNA

• organisms that are more distantly related will accumulate a greater number of differences in DNA sequence, whereas two species that are more closely related should share a greater similarity in their DNA sequence

• molecular differences between homologous molecules of related species are due to

• different selection pressures in different environments acting on existing variation.

• over many generations, changes in allele frequencies occur. In addition, mutations lead to changes in nucleotide sequences or changes in amino acid sequences in proteins, which is evidence that evolution occurred

What is DNA homology

• organisms utilize the same kind of nucleotides to synthesize their genetic material (DNA).

• more closely related two species are, the fewer nucleotide base pair differences they have.

• similarities in homologous DNA sequences imply a common ancestry.

State the comparison of nucleotide sequences through multiple sequence alignment

• homologous DNA sequences from two species are aligned and compared

• with the aid of computer programs, the exact number of nucleotide differences between two species can be accurately determined.

• more identical the DNA nucleotide sequences of the two species, the more recently they would have evolved from a common ancestor, more closely related they would be

• mitochondrial DNA (mtDNA) is commonly used to determine the phylogenetic relationship between species

• phylogenetic relationship shows how species are related to a common ancestor

What are the properties of mtDNA

• animal mtDNA is a small (15-20 kb) circular molecule, composed of 37 genes coding for 22 tRNAs, 2 rRNAs and 13 mRNAs.

• mRNAs code for proteins involved in electron transport and oxidative phosphorylation.

• mtDNA lacks introns

Why mtDNA is commonly used for phylogenetic studies

• mtDNA is present in all eukaryotes.

• mtDNA has a high level of genetic variation due to high rate of mutation in the non-coding sequences (control region) which can be used to determine phylogenetic relationships among recently diverged species

• high rate of mutation of mtDNA is due to the absence of DNA repair mechanism in mitochondria.

• mitochondrial genes encode many proteins involved in respiration (e.g. cytochrome oxidase) and genes that code for the ribosomal RNA (e.g. 16S rRNA gene). mutations to these genes are likely to be lethal in an individual, and therefore are less likely to be passed on to offspring.

• over time in a population, these genes evolve at a much slower rate (highly conserved) and are useful in determining phylogenetic relationships among species that diverged hundreds of millions of years ago.

• mtDNA lacks germline recombination i.e. there is no crossing over, independent assortment of mtDNA. Thus, variation in the mtDNA sequence is largely by mutation.

• mtDNA is inherited through the maternal line. This enables genealogical researchers to trace maternal lineage far back in time.

• probability of recovery of mtDNA from very small or degraded biological samples is higher than the one of nuclear DNA, because the mitochondrial DNA molecules exist in thousands of copies per cell, while nuclear DNA has only two copies per cell

Explain total genome comparison through DNA-DNA hybridization

• DNA-DNA hybridisation provides a way of comparing the total genome of two or more species.

• method operates on the basic principle of complementary base-pairing and the denaturation of the DNA molecule.

1. DNA obtained from different species are allowed to form DNA-DNA hybrids.

2. DNA-DNA hybrids with more complementary regions will denature at higher temperatures (due to higher number of hydrogen bonds) while DNA-DNA hybrids with fewer complementary regions will denature at lower temperatures.

3. species with more complementary regions between them are considered to be more closely related than species that exhibit fewer complementary regions.

4. this technique was applied to primate relationships,