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5.1 Evidence for evolution Cheat Sheet by

Cheat sheet for IB Biology HL Chapter 5.1 (2016 syllabus)


Evolution occurs when heritable charac­ter­istics of a species change.
Evolution is a change in the heritable charac­ter­istics of a species over time. As heritable charac­ter­istics are encoded for by genes, and may be transf­erred between genera­tions as alleles, biological evolution is a change in the allele frequency in a popula­tion's gene pool over successive genera­tions.

Selective Breeding

Selective breeding of domest­icated animals shows that artificial selection can cause evolution
Selective breeding is a form of artificial selection, whereby man intervenes in the breeding of a species to produce desired traits in the offspring. It provides evidence for evolution as targeted breeds can show signif­icant variation over a (relat­ively) short period of time.
Selective breeding of plant crops has allowed fro the generation of new types of foods from the same ancestral plant source. Examples include the plants of genus Brassica, which have produced broccoli, cabbage, kale, etc.
Selective breeding of domest­icated animals has also resulted in the generation of diverse breeds of offspring. Examples include dogs, horses and cows.

Pentad­actyl limp structures in different animals


Continuous variation across a geogra­phical range of related popula­tions matches the concept of gradual diverg­ence.
Within a population of any given species, there will be genetic variation that is typically continuous and follows a normal distri­bution curve. This is because the rate of change is gradual and cumula­tive.
If two popula­tions of species become reprod­uct­ively isolated, they will experience different ecological conditions and adapt to the different enviro­nmental conditions around them, gradually diverging from one another.
The degree of divergence between two species will depend on the time since the isolation has occurred. Popula­tions the separated recently will have less variation than those who separated for a longer period of time.
Popula­tions of a species can gradually diverge into separate species via evolution
The degree of divergence between reprod­uct­ively isolated population will gradually increase the longer they are separated. As the genetic divergence between these species increases, their genetic compat­ibility decreases, eventually resulting in them being unable to interb­reed.

Fossil Record

The fossil record provides evidence for evolution
A fossil is the preserved remains or traces of any organism from the remote past. The fossil record is the totality of all fossils, both discovered and undisc­overed.
Fossils can be dated by determ­ining the age of the strata they are found in, as sedime­ntary rock develops in a chrono­logical order. The chrono­logical sequence of complexity by which charac­ter­istics appear to develop is known as the law of fossil selection.
The fossil record is incomplete due to the specific set of circum­stances required for an organism to be preserved, as well as the fact that only the hard parts of organisms are preserved. Transi­tional fossils demons­trate the interm­ediary forms that occurred over the evolut­ionary pathway taken by a single genus.

Compar­ative anatomy

Evolution of homologous structures by adaptive radiation explains simila­rities in structure when there are differ­ences in function
Compar­ative anatomy of groups of organisms may show certain structural features that are similar, implying common ancestry. Anatomical features that are similar in basic structure despite being used in different ways are called homologous struct­ures. Homologous structures show adaptive radiation, as several species diversify to utilise a specific unoccupied niche.
Comparison of the pentad­actyl limb of mammals, birds, amphibians and reptiles with different modes of locomotion
A classical example of homologous structures is the pentad­actyl limb in a variety of different animals. Mammals, birds, amphibians and reptiles all share a similar arrang­ement of bones in their appendages based on a five-digit limb

Industrial Melanism

Peppered moths Biston betularia exist in two distinct polymo­rphic forms, a light colour­ation and a darker melanic variant.
In an unpolluted enviro­nment, the trees are covered by a pale-c­oloured lichen, which provides camouflage for the lighter moths.
In a polluted enviro­nment, sulphur dioxide kills the lichen while soot blackens the bark, providing camouflage for the darker moth.
The frequency of the two different forms of peppered moth is dependant on the enviro­nment and evolves as conditions change.
- Before the industrial revolu­tion, the enviro­nment was largely unpolluted and the lighter moth had a survival advantage
- Following the industrial revolu­tion, the enviro­nment became heavily polluted, conferring a survival advantage to the darker moth
- Recent enviro­nmental policies in Europe are reducing pollution levels, altering the frequency of the two popula­tions once again.

Peppered moth graph



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