Diversity of Organisms (DP IB Biology)

The biological species concept states that a species is a group of organisms that can interbreed to produce fertile offspring.

Limitations might include:

• The species concept cannot be applied to organisms that reproduce asexually, such as bacteria.

• Hybrid 'species' can be fertile on rare occasions.

• Extinct organisms are not able to breed.

Speciation is the process by which one species gives rise to two or more new species.

Isolation can lead to speciation when a population becomes separated from other populations of the same species, preventing gene flow and allowing natural selection or genetic drift to act differently on each population.

False.

Speciation occurs over very long time periods, with differences between isolated populations accumulating incrementally.

A diploid cell is a cell that contains two complete sets of chromosomes (2n).

The haploid chromosome number is half the diploid number, and is equal to the number of chromosome pairs in a diploid cell.

False.

The diploid number must always be an even number, as it must be divisible by two to produce a whole haploid number.

The number of chromosomes possessed by different species varies and is dependent upon changes that have occurred during that species' evolution.

The chromosome number of humans is 46.

The chromosome number of chimpanzees is 48.

No, the number of chromosomes a species possesses is not linked to how 'advanced' a species is in evolutionary terms.

The image shows a karyogram.

A karyogram is an image that shows all of the chromosomes in a cell, arranged by size, shape, and banding pattern, and placed with their homologous pairs.

A karyotype is the appearance of a complete set of an individual's chromosomes, including their number, size, shape, and banding.

False.

Metacentric chromosomes have their centromere located roughly in the middle.

An acrocentric chromosome is a chromosome with the centromere near the end and arms of unequal length.

Karyograms are made using the following process:

• Cells are stained and viewed under a light microscope

• Photographs are taken of the contents of the nucleus during metaphase of cell division

• The photographs of the chromosomes are cut up and arranged by size, shape, and banding pattern

Chromosomes in pairs 12 and 13 in a common ancestor fused to form the chromosomes in human pair 2.

Some examples of evidence for the formation of human chromosome 2 pair are:

• Chimpanzee chromosomes 12 and 13 match the length of human chromosome 2

• The location of the centromere of chimpanzee chromosome 12 matches that of human chromosome 2

• Human chromosome 2 contains a region of non-coding DNA known as satellite DNA that corresponds to the location of the centromere in chimpanzee chromosome 13

• The banding of the long arms of acrocentric chimpanzee chromosomes 12 and 13 corresponds to the banding of metacentric human chromosome 2

• Human chromosome 2 contains telomeric DNA in the middle of the chromosome

Telomeric DNA is a region of DNA found at the ends of chromosomes.

Satellite DNA is a region of non-coding DNA that can correspond to the location of a centromere.

A testable hypothesis is a statement that can be accepted or rejected based on available evidence and does not contain vague terms, predictions, or causal conclusions.

False.

Testable hypotheses should not contain vague statements that use terms like 'may' or 'could'.

This karyogram shows a male, as determined by the Y chromosome in homologous pair number 23.

A genome is all of the genetic information in an organism.

SNP is an abbreviation for single nucleotide polymorphism, which is a difference in a single nucleotide between DNA sequences.

False.

Humans share around 99.9% of their DNA with other humans.

DNA sequencing means determining the entire base sequence of the genes within an organism's genome.

True.

Eukaryotic genomes can vary in size, which is determined by the mass of DNA present in a nucleus.

Mitochondrial DNA is genetic material found in mitochondria, which is included in the genome of eukaryotic cells.

Plasmid DNA is genetic material found in small circular DNA molecules separate from chromosomal DNA in prokaryotic cells, which is included in the prokaryotic genome.

Databases such as GenBank and NCBI could be used to find information about genome size of different taxonomic groups.

False.

Prokaryotes tend to have smaller genomes than eukaryotes.

Mb stands for megabases, where 1 Mb = 106 bases.

False.

In some cases similar species may have very different genome sizes.

E.g. common wheat above has a genome size of around 17 000 Mb, while red wild einkorn wheat has a genome size of around 5 000 Mb.

Genome sequencing is the process of identifying and recording the base sequence of an organism's genome.

The Human Genome Project was a project that involved sequencing the entire human genome.

Genomic medicine is the use of information about genes to design medical treatments.

True.

Genetic screening can identify individuals with a high chance of developing specific diseases.

Genome sequencing can contribute to personalised medicine by allowing doctors to select treatments based on an individual's genotype and identify genetic risks for certain diseases.

Phylogenetics is the classification of species based on their evolutionary origins and relationships.

A phylogenetic tree is a diagram that shows the evolutionary relationships between species based on sequence analysis and comparison.

False.

DNA, mRNA, and amino acid sequence data are all used to investigate evolutionary relationships.

Speciation is the splitting of one species into two or more.

True.

If organisms have incompatible rituals for mating then it will prevent interbreeding which may result in the divergence of populations, over time leading to the development of new species.

A challenge associated with the biological species concept might include examples where interbreeding does not occur e.g.:

• For organisms that reproduce asexually

• Horizontal gene transfer between organisms of the same species or a different species

One example of asexual reproduction which challenges the biological species concept might include...

• parthenogenesis

• binary fission

• vegetative propagation

• budding

In asexual reproduction, individuals can reproduce without the need for interbreeding. The definition of a species can therefore not be applied to organisms that reproduce asexually.

Horizontal gene transfer describes the process by which bacteria (and sometimes viruses) transfer genes between members of the same species or another species without sexual reproduction.

A mule is not considered a biological species because it is infertile and therefore cannot reproduce to pass on its alleles to any subsequent generation.

If the diploid cell numbers are different then the haploid number of chromosomes will also be different and so when the gametes fuse, it would not be possible to form homologous pairs.

False.

The chromosomes that are circled are the sex chromosomes and show that the donkey is male and the horse is female.

These two organisms cannot interbreed to produce fertile offspring because they have different numbers of chromosomes.

The diploid number in a body cell would be 39 x 2 = 78 chromosomes.

Haploid gametes are produced in meiosis.

True.

Different numbers of chromosomes mean that gametes will have different haploid numbers and so a viable zygote would not result from gamete fusion.

A dichotomous key is used to identify a species, especially when working in the field.

False.

A dichotomous key is used to distinguish between species based on observable characteristics. A phylogenetic key is used to determine evolutionary relationships.

False.

The dichotomous key is based on observable characteristics such as colour or the number of legs.

Move through a series of paired statements choosing one option from each pair based appearance of the insect. The key would eventually direct you to the most likely possible correct species based on the answers.

A DNA barcode is a DNA sequence that can be used to quickly identify a species.

Environmental DNA (eDNA) is DNA collected from environmental samples, such as water or soil, to identify species present in a habitat.

False.

DNA barcodes are usually a few hundred base pairs in length.

DNA barcodes in eukaryotes are often taken from mitochondrial DNA.

True.

DNA in environmental samples can be amplified using PCR.

One reason environmental DNA sampling may be preferred is that some species may be difficult to capture, or some, like insect larvae, can be difficult to identify manually due to similar appearances.