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A4.1 Evolution and speciation
Unity and diversity—Ecosystems
Standard level and higher level: 4 hours
Additional higher level: 1 hour
Guiding questions
- What is the evidence for evolution?
- How do analogous and homologous structures exemplify commonality and diversity?
SL and HL
A4.1.1—Evolution as change in the heritable characteristics of a population
This definition helps to distinguish Darwinian evolution from Lamarckism. Acquired changes that
are not genetic in origin are not regarded as evolution.
Evolution is the change in the heritable characteristics of a population across time. The theory of evolution attempts to scientifically explain how evolution occurs through rigorous evidence. The mechanism for most but not all of evolutionary change is natural selection. Other equally important but non-deterministic (random) mechanisms include genetic drift , mutations , and gene flow (migration). While natural selection acts on existing variation to favor beneficial traits, other mechanisms may introduce new variation, redistribute existing variation, and can either promote or hinder the process of adaptation. Darwin and Wallace independently developed the theory of evolution based on natural selection, which replaced Lamarckism (a theory that falsely postulated that acquired characteristics throughout an individual’s lifetime are inherited by offspring). Darwinian theory was revised as new evidence (especially genetic) was uncovered, leading to the now current modern synthesis theory of evolution.
A4.1.2—Evidence for evolution from base sequences in DNA or RNA and amino acid sequences
in proteins
Sequence data gives powerful evidence of common ancestry.
Common ancestry/descent is a concept in modern evolutionary theory which states that all organisms on Earth are descendants from a single ancestral species. Biochemical evidence, which includes the fact that all organisms follow the same universal genetic code to synthesize proteins from DNA and RNA, supports universal common descent. DNA, RNA, and amino acid bases can be sequenced and analyzed in order to make comparisons between species. The more similar the sequences of two species are, the more likely that they recently branched off from a common ancestor.
A4.1.3—Evidence for evolution from selective breeding of domesticated animals and crop plants
Variation between different domesticated animal breeds and varieties of crop plant, and between
them and the original wild species, shows how rapidly evolutionary changes can occur.
Selective breeding is a form of artificial selection whereby humans intervene with the breeding of species to produce desired traits in offspring (i.e. breeding the fastest horse or crops with the most starch). This shows how desired alleles increase in frequency in the gene pool of species over time, which is useful as it enables us to observe evolution at a faster pace than what normally occurs in nature.
A4.1.4—Evidence for evolution from homologous structures
Include the example of pentadactyl limbs.
Homologous structures are structurally similar body parts derived from a common ancestor with different functions. They are present in organisms that faced differential selective pressures but are descendent from a common ancestor (i.e. pentadactyl limbs function as arms for humans but fins for whales). Analogous structures are structurally different body parts not derived from a common ancestor that share similar functions. They are present in organisms that do not share a common ancestor but faced similar selective pressures (i.e. birds and insects are unrelated but have wings for flight).
NOS : The theory of evolution by natural selection predicts and explains a broad range of
observations and is unlikely ever to be falsified. However, the nature of science makes it
impossible to formally prove that it is true by correspondence. It is a pragmatic truth and is
therefore referred to as a theory, despite all the supporting evidence.
The evidence for evolution we have is but a fraction of what is available to us, since we have millions of undiscovered fossils and genomes waiting to be sequenced. This makes it impossible to explicitly say “we have all the possible evidence for evolution”, as it is beyond human capability to uncover all the evidence. However, it is still a very useful ( pragmatic ) theory, so it is treated like a truth in science as it is unlikely to ever be falsified.
A4.1.5—Convergent evolution as the origin of analogous structures
Students should understand that analogous structures have the same function but different
evolutionary origins. Students should know at least one example of analogous features.
The context in which heritable changes occur in organisms (evolution) can be classified as divergent or convergent:
- Divergent evolution occurs when heritable traits from a common ancestor evolve to perform different (divergent) functions.
- Convergent evolution occurs when heritable traits in species from unrelated lineages evolve to perform similar (convergent) functions). Divergent or convergent evolution both occur through the same mechanisms of change. It is important to distinguish between the two in order to correctly evaluate ancestral and evolutionary relationships (i.e. understanding divergence or convergence helps in tracing diseases and vaccine development).
A4.1.6—Speciation by splitting of pre-existing species
Students should appreciate that this is the only way in which new species have appeared.
Students should also understand that speciation increases the total number of species on Earth,
and extinction decreases it. Students should also understand that gradual evolutionary change in
a species is not speciation.
The theory of evolution does not predict that species will constantly be evolving, or how fast they’ll change when they do; that depends on the evolutionary pressures they experience. Evolution is also without direction or purpose; adaptations are not induced and they do not arise with an intention. Speciation is the process by which two populations of a single ancestor develop and evolve sufficient genetic differences that prevent interbreeding and the production of fertile offspring. Species do not necessarily have to split, some just go extinct and others do not split for many years.
A4.1.10—Barriers to hybridization and sterility of interspecific hybrids as mechanisms for of
preventing the mixing of alleles between species
Courtship behaviour often prevents hybridization in animal species. A mule is an example of a
sterile hybrid.
Hybridization occurs when two different species crossbreed (i.e. a mule is produced when a horse and donkey crossbreed). Due to chromosomal differences in the two parents, the hybridized offspring, while containing useful traits from both parents ( hybrid vigor ), is sterile and unable to produce offspring. Breeding to produce infertile offspring is a waste of energy and resources, so species have developed barriers to hybridization through distinct courtship behavior to ensure their mate is of the same species. Thus, if mating occurs, the likelihood of producing a fertile offspring upon mating is high.
A4.1.11—Abrupt speciation in plants by hybridization and polyploidy
Use knotweed or smartweed (genus Persicaria ) as an example because it contains many species
that have been formed by these processes.
Note: When students are referring to organisms in an examination, either the common name or
the scientific name is acceptable.
Polyploidy results from non-disjunction during anaphase I (the homologous pairs do not separate), leading to an organism with a greater number of homologous chromosomes than the parent. In the case where the resultant diploid nucleus contains an even number of chromosomes, the species will usually be fertile, but if it is an odd number then the offspring may be able to survive, but it will be sterile. There are two types of polyploidies:
- Autopolyploidy occurs when the extra homologous chromosome(s) are derived from the same species (both parents belong to the same species).
- Allopolyploidy occurs when the extra homologous chromosome(s) are derived from different species (each parent is a different species). Plants belonging to the genus Persicaria commonly hybridize due to hybrid vigor. Linking questions
- How does the theory of evolution by natural selection predict and explain the unity and diversity of
life on Earth?
- What counts as strong evidence in biology? Review questions SL and HL
- Define evolution. [1]
- Distinguish between evolution and speciation. [1]
- Define differential selection. [1]
- Outline geographical isolation using an example. [2]
- Explain why the theory of evolution is considered a pragmatic truth in science. [2]
- Explain the significance of selective breeding as evidence for evolution. [3]
- Explain the implications of divergent and convergent evolution in determining evolutionary
relationships. [4]
- Discuss the evidence for evolution. [ 7 ]
Additional Higher Level
- Outline how adaptive radiation can be a source of biodiversity. [2]
- Explain how speciation occurs. [3]
- Distinguish between allopatric and sympatric speciation. [3]
- Explain the role of polyploidy in speciation. [3]
- Explain why there are barriers to hybridization even though hybrid vigor exists. [ 3 ]
- Explain the advantages and disadvantages of hybridization. [4]
- Using an example for each, distinguish between divergent and convergent evolution. [4]
- Explain how the theory of evolution provides evidence for both unity and diversity within
ecosystems. [5]
