Diversity Of Organisms

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Apuntes de la lección

Variation Between Organisms

  • Variation refers to the differences that exist between organisms.
  • Variation can be seen in visual appearance, behaviour, and biochemistry.
  • Variation arises from a combination of genetic and environmental factors.
  • Genetic variation is generated by mutation and by the combination of alleles during sexual reproduction.
  • The environment can affect gene expression, e.g. temperature, nutrient availability, and oxygen concentration.
  • Variation exists both between species and within a species; no two individuals are identical when all characteristics are compared.
  • Variation can be discontinuous (distinct categories, e.g. blood type) or continuous (measured on a scale, e.g. height).

Species Classification and the Binomial System

  • Classification is the process of putting organisms into groups; the science of classification is taxonomy.
  • The smallest taxonomic group is the species.
  • Historically, species were classified by morphology (observable characteristics), known as the morphological species concept.
  • Carl Linnaeus developed the binomial system of naming species, giving each species a two-part Latin name.
  • The first part of the binomial name is the genus (capitalised), and the second is the species name (lower-case), e.g.×Canis lupus×.
  • Binomial names are written in italics when typed or underlined when handwritten.
  • After first use, the genus name can be abbreviated, e.g.×Triticum aestivum×becomes×T. aestivum×.
  • Classification based on morphology alone can be misleading because species with similar appearance are not always closely related.

The taxonomic hierarchy

The taxonomic hierarchy

Biological Species Concept

  • The biological species concept defines a species as a group of organisms that can interbreed to produce fertile offspring.
  • This concept has limitations: it cannot be applied to organisms that reproduce asexually, such as bacteria.
  • It is difficult to apply when fertile hybrids are produced, e.g. the 'wholphin' (a cross between a melon-headed whale and a bottlenose dolphin).
  • It cannot be used for extinct species because they cannot be bred to test fertility.
  • Other characteristics used to determine species include morphology, DNA sequences, biochemistry, ecology, and evolutionary lineage.
  • The characteristics used depend on the organism; e.g. bacteria may be classified by biochemistry or ecology, while extinct species may be classified by evolutionary lineage.

Speciation and Distinguishing Populations from Species

  • Speciation is the process by which one species gives rise to two or more new species.
  • Speciation can occur when populations become isolated (e.g. geographically), preventing gene flow between them.
  • Different environmental conditions can lead to different natural selection pressures, and genetic drift can also contribute to speciation.
  • Once speciation has occurred, the two species are reproductively isolated and can no longer interbreed to produce fertile offspring.
  • Speciation occurs gradually over long time periods, so it can be difficult to pinpoint when two populations become separate species.
  • Assigning separate species status can be subjective; e.g. killer whales show variation between populations and some scientists believe there may be more than one species.

Chromosome Number

  • A diploid cell contains two complete sets of chromosomes (2n); haploid cells contain one set (n).
  • In animals, haploid cells are gametes (egg and sperm).
  • During fertilisation, haploid gametes fuse to form a diploid zygote; both gametes must have the same number of chromosomes for the zygote to be viable.
  • Every body cell arising from the zygote has the same chromosome number, except red blood cells which have no nucleus.
  • Different species have different chromosome numbers; e.g. humans have 46 (23 pairs) and chimpanzees have 48 (24 pairs).
  • The diploid number is always even because it must be divisible by two to produce a whole haploid number.
  • Differences in chromosome number can prevent organisms from different species from breeding successfully.
  • Chromosome number is not linked to how 'advanced' a species is in evolutionary terms.

Karyograms: Skills

  • A karyogram is an image showing all chromosomes in a cell arranged by size, shape, and banding pattern, placed with their homologous pairs.
  • A karyotype is the appearance of a complete set of an individual's chromosomes, including number, size, shape, and banding.
  • To make a karyogram, cells are stained and viewed under a light microscope; photographs are taken during metaphase of cell division.
  • Chromosomes are arranged by size, shape, and banding pattern; this can be done with paper and scissors or on a computer.
  • Chromosomes with a central centromere are metacentric; those with a near-terminal centromere are acrocentric.
  • Chromosome pair 23 (sex chromosomes) often does not fit the size-order pattern because the X chromosome is very large.
  • Karyograms can provide evidence for chromosome fusion; e.g. chimpanzee chromosomes 12 and 13 may have fused to form human chromosome 2.
  • Evidence for this fusion includes matching banding patterns, centromere location, and the presence of satellite DNA and telomeric DNA in human chromosome 2.

Genomes and Genome Size

  • The genome is all of the genetic information in an organism, including coding and non-coding DNA, mitochondrial DNA, chloroplast DNA, and plasmid DNA (in prokaryotes).
  • DNA sequencing determines the base sequence of DNA; genome-wide comparisons can be made between individuals and species.
  • There is a high level of genome similarity within and between species; humans share about 99.9% of their DNA with other humans and about 99% with chimpanzees.
  • Differences between individuals are due to different alleles; single base changes are called single nucleotide polymorphisms (SNPs).
  • Genome size varies widely: viruses and bacteria have small genomes, prokaryotes generally have smaller genomes than eukaryotes, and plant genomes vary greatly.
  • Genome size is measured in million base pairs or C-value (haploid nuclear DNA content); 1 pg = 978 Mb.
  • Genome size does not always correlate with organism complexity; e.g. humans have 3,100 Mb, hagfish 4,200 Mb, and common wheat 17,000 Mb.
  • Factors to consider include non-coding DNA, polyploidy in plants, and that 'complexity' can be defined in different ways.

Uses of Genome Sequencing

  • Genome sequencing allows the base sequence of an organism's genome to be identified; methods are becoming faster and cheaper, with next-generation sequencing (NGS) techniques.
  • Sequence data can be used to determine evolutionary relationships (phylogenetics), personalised medicine, and more.
  • Phylogenetics classifies species based on evolutionary origins and shared common ancestry.
  • DNA, mRNA, and amino acid sequences can be compared; the more similar the sequences, the more closely related the species are.
  • Species separated for longer have accumulated more mutations, so their sequences differ more.
  • Cytochrome c is often used for comparison because it is present in many organisms and shows sufficient variation.
  • Sequence data can be used to create phylogenetic trees showing evolutionary relationships.
  • Genome sequencing can be used in personalised medicine: identifying disease risks, developing targeted drugs, and selecting treatments based on an individual's genotype.

Diapositivas

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Preguntas de práctica

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  1. 1.Which is the correct definition for a species according to the biological species concept?

    Easy
    • AA group of organisms which can interbreed to produce fertile offspring.
    • BA group of organisms which look similar.
    • CA group of organisms with similar characteristics.
    • DA group of living things.

  2. 2.Which of the following statements about genomes is not correct?

    Easy
    • AHumans have the most genes of any organism.
    • BHumans have around 20 000 genes.
    • CSpecies vary in the number of genes they have.
    • DThe number of genes of an organism is not proportional to genome size.

  3. 3.The image shows a karyogram. Which of the statements about the karyogram is correct?

    Medium
    • AThe source of DNA for the karyogram was a human cell.
    • BThe source of DNA for the karyogram was a haploid cell.
    • CAll 23 pairs of chromosomes are arranged in descending order of size.
    • DChromosome banding is visible.

  4. 4.What is an appropriate unit for comparing the length of genomes?

    Medium
    • AMillion base pairs
    • BMicrometers
    • CNanometers
    • DPicometer

  5. 5.What is the most efficient way to compare the genome sizes of different species?

    Medium
    • AExtract information from an online database.
    • BRead scientific articles and extract relevant information.
    • CCompare the chromosome number of the different species.
    • DCompare the proteome size of the different species.

  6. 6.Which of the following statements about genome sequencing are correct? (select all that apply)

    Medium
    • AScientists can now look for correlations between changes in a gene and particular human traits.
    • BThe genomes of many species have now been sequenced.
    • CAdvancing computer technology means that the rate at which a genome can be sequenced continues to increase.
    • DScientists now know the location and role of every human gene.
    • EGenome sequencing technology is now so advanced that further developments are not expected.

  7. 7.The two karyotypes belong to a donkey (Q) and a horse (R). Breeding these two individuals resulted in the production of a sterile mule (S). Why would S be sterile?

    Hard
    • AThe process of meiosis will not be possible in S as homologous pairs cannot form.
    • BThe sex of the mule (S) will be undetermined at birth due to the unnatural combination of chromosomes.
    • CDifferent combinations of alleles formed as a result of crossbreeding Q and R which leads to disadvantageous characteristics in S.
    • DThere are different numbers of chromosomes in the gametes of Q and R so S would not live long enough to breed.

  8. 8.What is the smallest taxonomic group?

    Easy
    • ASpecies
    • BGenus
    • CFamily
    • DKingdom

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