NCEA Level 3 Biology - Speciation

Evolution

The permanent genetic change (change in allele frequencies) in a population of individuals over time

Species and populations

A group of individuals that normally interbreed in nature to produce fertile offspring. Members of a species that live in the same geographical area are called a population and share a common gene pool, the total number of alleles that occur in a population. Change in allele frequencies in the gene pool of a population indicate that evolution is occurring.

Deme

A deme is a local population that has no or limited gene flow with members of other populations. Local natural selection pressures mean that the phenotypic features of the members of one deme may differ from those of members of other demes.

Cline

When a species has an extended geographical range, variation may occur along a gradient from one point of the range to another point. This gradient of variation is called a cline. Clinal variation is linked to an environmental gradient. There are 2 different mechanisms by how this could occur:

- It could be due to environmental differences and the cline would be purely phenotypic

- It could be due to genetic differences resulting from selection

Gene flow

When individuals migrate (move) between populations and interbreed, they introduce their alleles into other populations. This movement of alleles carried by individuals between populations is called gene flow and can be an important source of genetic variation upon which natural selection can then act. Individuals can move into a population (immigration) of out of a population (emigration). Gene flow can:

- Increase the genetic differences between populations by introducing new alleles into populations: this is particularly significant if a barrier isolates populations once migration has happened so that no further gene flow can occur

- Decrease the genetic differences between populations: if migration is regular and involves large numbers of individuals, accumulated genetic differences between populations can be reduced by gene flow

While gene flow may advantage one population by introducing new beneficial alleles, it may harm another due to problems associated with hybridisation.

Genetic drift

The random change in allele frequencies of a population due to chance events. Genetic drift occurs in all populations, but it has a much greater effect on the gene pool when the population is small. It can result in alleles being lost from the gene pool (0% frequency) or fixed as the only allele present for a particular gene (100% frequency).

There are 2 types of genetic drift:

- Bottleneck effect

- Founder effect

Bottleneck effect

Catastrophic environmental events (fire, drought, floods, earthquakes etc) or human impact (habitat destruction, introduction of predators or competitors) can severely reduce a population size to just a small number of survivors. The bottleneck may randomly alter allele frequencies and/or remove alleles so that when the population recovers, allele frequencies may not be representative of the original population and genetic diversity is likely to be reduced.

Founder effect

Occurs when only a few individuals from a population move into a new area. They may not possess a representative sample of the available genes from that species' gene pool. Isolated populations that have descended from these 'founder ancestors' often have very different genes from the same species elsewhere. It is largely a matter of chance which alleles are present in the gene pool of the founders.

Inbreeding

Inbreeding is breeding between closely related individuals. It is commonly used in selective breeding (or artificial selection) and increases the chance of favourable alleles coming together, resulting in favourable phenotypes. In naturally occurring small populations such as founder populations, inbreeding may have significant effects on the gene pool and genetic diversity of the population, which may be compounded by genetic drift.

Effects of inbreeding on a population

- Increases the proportion of homozygotes in a population similar to self-fertilisations

- In a species that normally outbreeds, inbreeding results in rare recessive alleles being expressed. Parents that are related have a much higher chance of sharing the same recessive alleles. Rare and harmful alleles normally protected by dominance therefore can be exposed to selection

- Over many generations, selection against disadvantageous alleles results in a genetically less variable population. Though disadvantageous in present conditions, many of the alleles eliminated by selection could be advantageous in different circumstances. Repeated inbreeding therefore forces all alleles to be subject to selection, so only those of immediate benefit remain in the gene pool, therefore greatly reducing the genetic diversity of the gene pool

Outbreeding

Outbreeding is breeding between non-related individuals. It tends to increase the number of heterozygotes in a population and reduce the chances of harmful or unfavourable recessive alleles coming together. The offspring produced from outbreeding display hybrid vigour when they contain a variety of alleles from their genetically different parents; such organisms tend to have a greater evolutionary fitness than the offspring from inbreeding.

Natural selection

The process by which beneficial phenotypes in a population are preserved because they are selected for while unfavourable phenotypes tend to be lost because they are selected against. Natural selection provides selection pressures which act on a population. Individuals that have alleles that give them a survival advantage are more likely to survive, reproduce and pass on successful alleles to the next generation. Individuals that have alleles that don't give them a survival advantage are more likely to die and their alleles are lost from the population as they will be unable to reproduce.

Types of natural selection

- Stabilising natural selection

- Directional natural selection

- Disruptive natural selection

Stabilising natural selection

Favours the average phenotype and selects against both extremes (e.g. birth weight in babies - remains in 3-4 kg range as anything below are weaker and more likely to die, and above have difficulty being born and are also more likely to die).

Directional natural selection

Favours one extreme phenotype over the others (e.g. peppered moths and long necks in giraffes).

Disruptive natural selection

Favours both extreme phenotypes at the expense of the average. It is associated with a fluctuating environment and results in the development of quite different forms of a trait in a population, and can even lead to the development of 2 or more different species from the one original species.

Molecular biology and evolutionary relationships

Molecular biology can provide proof of evolutionary relationships as since DNA is common to all life forms, comparisons of the base sequence of the DNA of the genome of organisms can be used as an indicator of how closely related organisms are - the more DNA there is in common, the more closely related organisms are. This is good for identifying closely related organisms e.g. humans and chimps.

Proteins and evolutionary relationships

Proteins are made up of 22 different amino acids. The more similar their proteins (the more amino acids they have in common), the more closely related 2 organisms are. This is used for identifying between more distantly related species.

mtDNA

Mitochondria contain DNA. Mitochondria are not subject to meiosis and their DNA does not undergo any recombination events - any changes in mtDNA are due to gene mutation. The ovum (egg) cell is large and has mitochondria in its cytoplasm, whereas sperm only have a few mitochondria that are destroyed after fertilisation. Therefore mitochondria are sourced from the egg only, resulting in mtDNA being inherited from the mother only. mtDNA can be used as a molecular "clock" as gene mutations occur at a steady, fairly rapid rate. Comparison of mtDNA base sequences from different organisms can be used to determine relationships of organisms, producing phylogenetic trees to find where related species likely diverged from a common ancestor. It is often used to determine differences between closely related organisms, even within a species.

Mutation

Mutations are a permanent change in the DNA in a gene or chromosome. They are the ultimate source of variation because they can create new alleles. Most mutations occur in somatic cells and are not inherited, only mutations in gametes can be passed onto the next generation if they are fertilised. The fitness of a mutation describes its value to the survival and reproductive success of the organism.

Harmful mutations

These mutations are harmful because, by altering the DNA sequence, they upset the structure and function of the protein they code for, producing a disorder in the organism that significantly lowers its survival and reproductive capabilities.

Neutral (or silent) mutations

Some mutations are neither harmful nor beneficial to the organism in which they occur. These are called neutral or silent mutations. A mutation may have no adaptive value when it occurs, but this may change in the future. Neutral mutations may therefore be very important in an evolutionary sense.

Beneficial Mutations

A mutation in an organism's genome that has a beneficial effect. It affects the organism in some way as to increase its chances of reproductive success, and therefore the chance of the mutation in question being passed along.

Gene mutations

Gene mutations result from mistakes in the base pairing process during DNA replication or from the effect of mutagens. DNA bases may be incorrectly paired, bases may get deleted or extra bases inserted. This produces changes in the DNA code, and may result in a faulty protein or enzyme being formed when the gene is copied during protein synthesis. The protein may not be able to carry out its normal function in the body.

Base substitution

When one base is replaced by another, the result is a same-sense, mis-sense or non-sense mutation. The effect will depend on which base is replaced.

- Silent mutation: If the third base in a triplet is changed, there may be no change in the amino acid coded for due to the degeneracy of the genetic code

- Mis-sense mutation: If the substitution results in a different amino acid being coded for, it will either have similar properties to the original and have little effect on the protein or have a severe effect on the protein so that it cannot carry out its normal function in the body

- Nonsense mutation: If a STOP codon is coded for, the polypeptide production will be stopped and the protein is likely to be non-functional

Base insertion or deletion

When an extra base is inserted or a base is deleted it causes a reading frame shift. It changes the way the triplets are read from that point onwards in the gene. Every triplet is affected and different amino acids are coded for. This is most likely to result in a non-functional protein. If a STOP codon is coded for the polypeptide production will be cut short, resulting in a non-sense mutation.

Chromosome mutations

Aneuploidy

Variation in chromosome number involving only part of the chromosome set. The simplest type is when only one kind of chromosome is involved. Bearing in mind the normal diploid set is 2n:

- Monosomy: only 1 chromosome is present instead of the usual 2 (2n-1)

- Trisomy: 3 copies are present instead of the usual 2 (2n+1)

- Tetrasomy: 4 copies are present instead of the usual 2 (2n+2)

Aneuploidy results from the failure of chromosomes to separate during cell division. This is called non-disjunction and can happen during meiosis or mitosis. Instead of moving to opposite poles of the spindle, a pair of chromosomes (or chromatids) moves to the same pole, where one daughter cell receives both chromatids (n+1) whereas the other receives none (n-1). When a normal gamete (n) joins with an (n+1) gamete, the result is a trisomic. If a normal gamete joins with an (n-1), the result is a monosomic.

Polyploidy

In polyploidy, every chromosome is represented 3 or more times so that instead of being diploid (2n), the individual is triploid (3n), tetraploid (4n), hexaploid (6n) etc

Autopolyploidy

Involves the multiplication of the entire genome within a single species. Therefore an autotriploid has 3 sets of chromosomes and an autotetraploid has 4 sets. Autopolyploidy results from the failure of chromosomes to separate during cell division because the spindle does not function properly. As a result, all the chromosomes finish up in the same nucleus, which has twice as many chromosomes as it should. This can occur in meiosis or mitosis.

Autoploidy resulting from meiosis

This can happen in the first or second division of meiosis, and results in diploid gametes. If a diploid gamete fuses with a normal haploid gamete, a triploid zygote is formed. If it fuses with another diploid gamete, a tetraploid zygote is produced.

Autoploidy resulting from mitosis

This results in a tetraploid cell. Each replicated chromosome separates, but the cell itself does not divide. The descendants of this cell will also be tetraploid. This is called somatic doubling, and if it occurs in a shoot tip it may give rise to a tetraploid shoot.

Reduced fertility in autopolyploids

Although autopolyploids are often vigorous and may be larger, they are usually infertile, especially autotriploids (produced by fertilisation between a diploid and a haploid gamete). Triploids are highly infertile, because of the very low probability that meiosis will produce cells with balanced chromosome sets. Such plants can only reproduce asexually, therefore the only source of genetic variation in them is from mutation. Autotetraploids are usually more fertile than autotriploids, as it is more likely that the 4 homologues will be able to form viable pairs during meiosis (each daughter nucleus can receive 2 chromosomes).

Allopolyploidy

Results from hybridisation between species. Chromosome doubling must occur for viable offspring to be produced.

Ecological significance of polyploidy in plants

The effects of polyploidy depends on the environment. The frequency of polyploid species increases markedly from the equator towards the poles, and also with altitude. The lower the temperature, the greater the advantage of polyploidy. This could be because as a result of the advances and retreats of ice caps during successive glaciations, a more variable environment existed at higher altitudes and latitudes than in warmer regions. Under these conditions, the greater genetic variability of polyploids could have been advantageous. In warmer climates, the advantage of having more genes may be offset by the disadvantage of the increased energy needed for their replication and transcription. Also, polyploidy is important in terms of evolution because it can lead to instant speciation through reproductive isolation.

Speciation

Speciation is the formation of new species. It results from populations becoming reproductively isolated so gene flow can no longer occur. Speciation may occur:

- gradually by the slow accumulation of small changes

- instantly by changes in chromosome number through polyploidy

Sympatric species

Closely related species that live in the same geographic location

Allopatric species

Closely related species that live in different geographic locations

Reproductive isolating mechanism

Any factor that stops members of populations of the same species or members of different species from breeding together; it acts as a barrier to gene flow. RIMs may be:

- prezygotic: act before the egg is fertilised

- postzygotic: act after the egg is fertilised

Prezygotic isolating mechanisms

These include:

- Geographical isolation

- Structural (morphological) isolation

- Ecological isolation

- Temporal isolation

- Behavioural isolation

- Gametic isolation

Geographical isolation

Geographical isolation results from physical barriers (e.g. rivers, mountain ranges, oceans) separating populations. If the populations come together again at a later stage, they are sufficiently different (as a result of different selection pressures in different environments, and/or possibly genetic drift) that they do not/are unable to breed.

Structural (morphological) isolation

Structural (morphological) isolation results from differences between the structure of external reproductive organs (genitalia) that do not allow for transfer of sperm between the sexes. This is common in insects that have species-specific shaped genitalia which prevents hybridisation between species.

Ecological isolation

Ecological isolation results from differences in habitat within the same geographical area, so that the populations rarely come into contact with each other.

Temporal isolation

Temporal isolation results from breeding behaviour resulting at different, non-overlapping times. There can be daily or seasonal differences in timing of mating, or differences in the season in which flowering or pollination occurs.

Behavioural isolation

Behavioural isolation results from differences in behaviour, typically mating behaviour such as courtship. Females only recognise and respond to the courtship behaviour of males of their own species. Behavioural isolation may also result from different activity patterns, such as:

- although 2 species are sympatric, one is diurnal and the other in nocturnal in feeding habits

- many flowers secrete nectar at a particular time of day, resulting from insects tending to specialise by feeding from any one species at a given time

Gametic isolation

Gametic isolation results from incompatibility of gametes. Sperm may not be able to fertilise an egg of another species because:

- the egg's surface does not have the correct chemical receptors

- sperm cannot penetrate the surface of the egg

- sperm cannot survive in the chemical environment of the female reproductive system

Postzygotic isolating mechanisms

Despite prezygotic isolating mechanisms, sperm may fertilise eggs of different species to form a hybrid. Hybrids are usually prevented from producing viable offspring by postzygotic isolating mechanisms:

- hybrid inviability

- hybrid sterility

- hybrid breakdown

Hybrid inviability

The zygote, embryo or offspring is inviable (i.e. unable to survive or develop normally), so dies early in its development.

Hybrid sterility

The hybrid reaches maturity but is infertile so cannot breed.

Hybrid breakdown

Occurs when the hybrid reaches maturity and is able to breed but the next or subsequent generations are infertile or have reduced reproductive capability.

Allopatric speciation

A population becomes divided into 2 or more populations by a geographical barrier, causing the populations to become geographically isolated. Selection pressures on the two populations differ, and since there is no gene flow between them, the gene pools begin to diverge. If the barrier to gene flow disappears, the populations can merge, forming 2 sympatric populations. If any hybridisation occurs, selection against interbreeding results in evolutionary divergence of reproductive biology, i.e. the evolution of prezygotic isolating mechanisms which prevent hybridisation.

Sympatric speciation

One species gives rise to 2 or more species without prior geographical separation. The most common form is instant speciation, involving the formation of a polyploid which results in a new species being formed in a single generation due to the consequent formation of the postzygotic reproductive barrier of hybrid sterility. This is much more common in plants due to the formation of allopolyploids.

Biogeography

Biogeography is the study of the geographic distribution of species; evidence of evolution is provided by the natural geographic distribution of related species, both past and present. Evidence of biogeographical relatedness is supported by fossil analysis and modern molecular analysis.

Rates of evolution

- Gradualism

- Punctuated equilibrium

Gradualism

Initially after the acceptance of the theory evolution by natural selection, it was thought that the only way new species could evolve was over a very long time (millions of years) with a gradual transition from one form to another. This gradualism sees transitional forms in the fossil record in response to natural selection.

Punctuated equilibrium

As knowledge of the fossil record progressed, it became apparent that for many groups the rate of evolution had not been steady. Long periods of time elapsed during which there was little, if any, change - known as stasis (perhaps brought about by stabilising selection) followed by sudden bursts of rapid speciation (perhaps over thousands rather than millions of years). This punctuated equilibrium often involves the formation of many different species that are no longer closely related. There are few transitional fossils, because of the rapid pace of change.

Types of evolution

- Divergent evolution

- Convergent evolution

- Parallel evolution

- Co-evolution

Divergent evolution

Divergence occurs when two or more species result from a common ancestor - typically, this is by allopatric speciation when two populations become geographically isolated. Different selection pressures in the different environments result in the two populations diverging as different phenotypes get selected for to meet the demands of the different environments. The populations may diverge sufficiently (accumulation of different alleles in the gene pool) that reproductive isolation, hence speciation, results. Divergent evolution has occurred.

Adaptive radiation

Adaptive radiation is a type of divergent evolution. It involves the rapid evolution of a large number of species from an ancestral group to occupy a variety of different ecological niches that may have become available suddenly. When members of an ancestral species move into new environments, this may involve out-competing and displacing other species, or habitats may become available through a catastrophic event (e.g. mass dinosaur extinctions left vacant niches for mammals to occupy). Populations are subject to new selection pressures, and new adaptations evolve. Geographical isolation may occur. Accumulation of new alleles in the isolated gene pools result in reproductive isolation and therefore the formation of new species. Adaptive radiation may result from the sudden relaxation of competition, which can arise in two different ways:

- evolution of some new structure, physiology or behaviour enabling exploitation of a different aspect of a habitat

- geographical isolation from competitors or predators

Homologous structures

An important source of evidence for divergence and adaptive radiation involves homologous structures - features similar in structure and origin but different in function. The similarity in structure indicates common ancestry, while difference in function indicates adaptation to different selection pressures in different environments. Homology can be clearly seen in the limbs of different vertebrate groups.

Convergent evolution

Convergent evolution occurs when two or more unrelated species evolve to resemble each other (convergence) as a result of being subject to similar selection pressures. Convergent evolution results in analogous structures - structures with different evolutionary origins that appear very similar because they carry out the same or similar functions. Resemblances due to convergence are always superficial. Convergence is thus purely phenotypic, being confined only to certain bodily features linked to adaptation to a particular niche. At the genetic level, all evolutionary pathways diverge.

Parallel evolution

Whereas convergence involves unrelated or distantly related starting points, parallel evolution occurs when 2 or more related groups evolve along similar lines independently.

Co-evolution

Co-evolution is a reciprocal evolutionary change in which a change in a feature of a species acts as a selection pressure for a change in the feature of another (unrelated species); both species have a strong ecological relationship with each other. Co-evolution can often be seen in the relationship between predator and prey, parasite and host, mimic and model, pollinators and flowering plants, mutualistic species and between interspecific competitors.