Variation is a defining feature of life
pp. 102–103No two individuals are identical in all their traits. These complex patterns of variation are the basis for naming and classifying organisms.
Guiding questions
- How do we define a species?
- What patterns of genome diversity appear within and between species?
Slides
Notes
Written for this site from the class slides, with textbook page references for each statement. The chapter is on pages 102–117 of the course textbook. Image credits are on slide 16.
What is a species?
Species as groups with shared traits
pp. 103–104Linnaeus grouped organisms by what they looked like and how they were built. This is useful, but it can mislead: some species look alike, and some members of one species look quite different.
Binomial names
pp. 104–106A two-part name gives the genus, with a capital letter, and then the species, in lower case. Names are printed in italics and underlined when handwritten, as in Homo sapiens. Species in one genus have similar traits, and the names are understood worldwide.
The biological species concept
pp. 106–107A species is a group that can breed and produce fertile offspring. A horse and a zebra can produce young, but the young are normally infertile. The concept has limits: it cannot be used for asexual organisms or fossils, and other species concepts exist.
Populations, species and speciation
pp. 107–109Speciation is one species splitting into two or more. It is usually gradual, with non-interbreeding populations becoming more different over time. There is no single point at which two populations clearly become two species.
Chromosomes
Chromosome numbers vary
p. 110Humans have 46 chromosomes and chimpanzees have 48. Numbers vary widely between plants and animals, and they do not show how complex an organism is.
Karyotypes and karyograms
pp. 111–113
A karyogram shows chromosomes in pairs, sorted by length, banding pattern and centromere position. Sex chromosomes come last.
Human chromosome 2 seems to have formed by the fusion of two ancestral chromosomes. It has telomere sequences near its middle, a second inactive centromere, and bands matching two chimpanzee chromosomes.
Watch out: humans did not evolve from chimpanzees. Both evolved from a shared ancestor.
Genomes
Genomes within a species
p. 115A genome is all of an organism's genetic information. Members of a species share most of their genome, but variations such as single-nucleotide polymorphisms give some diversity.
Diversity of eukaryotic genomes
pp. 115–116Genomes vary in total size and in base sequence. Differences between species are much larger than differences within a species.
Comparing genome sizes
p. 116Data from genome databases show that size does not simply follow complexity. A lungfish has a genome many times larger than a human's, because much DNA does not code for proteins.
Current and future uses of sequencing
p. 117Sequencing has become far faster and cheaper. It is now used to research evolutionary relationships, and may in future allow personalized medicine.
Videos
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Note-taking sheet
Print this guide and complete it as you work through the slides and videos. It asks for your own explanations, so close the notes while you try each section.
Quick self-check
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Practice questions
A printable worksheet covering all eleven statements, from quick recall to data questions and a longer answer. It has 40 marks and takes about 40 minutes, or you can do one section at a time.
Your teacher has the mark scheme.
Practical
Making a karyogram
You will need: Printed image of human chromosomes from a public genetics resource, scissors, glue
- Cut out each chromosome
- Pair and arrange them by size and banding pattern
- Number the pairs and identify the sex chromosomes
- Look for any abnormal number, and write the karyotype
Safety: No practical hazards. Keep any discussion of conditions respectful.
Carry this out with your teacher's supervision and follow your school's lab rules.
Nature of science and TOK
Nature of science
Observations
For decades, textbooks said that humans have 48 chromosomes. Better techniques let Tjio and Levan count 46 in 1956.
Theory of knowledge
How can expertise make errors last?
Some researchers doubted their own counts because 48 was the accepted number. Consider how authority can affect what people report.
Syllabus statements
Summarised in our own words. Use each code to find the exact wording in your textbook or the IB subject guide.
- A3.1.1Organisms differ from one another, and this is a hallmark of lifep. 102
- A3.1.2Species as groups of organisms that share traitsp. 103
- A3.1.3Two-part scientific names: the binomial systemp. 104
- A3.1.4The biological species conceptp. 106
- A3.1.5Why it is hard to separate populations from species as non-interbreeding groups diverge during speciationp. 107
- A3.1.6How chromosome numbers vary among plant and animal speciesp. 110
- A3.1.7What karyotyping involves and how to read a karyogramp. 111
- A3.1.8How genomes are alike and how they differ within a speciesp. 115
- A3.1.9The diversity of genomes among eukaryotesp. 115
- A3.1.10Comparing the sizes of genomesp. 116
- A3.1.11What whole genome sequencing is used for now, and could be used for in futurep. 117
HL extension
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