Evolution

The Hardy-Weinberg principle can be applied to work out allele frequencies and genotype frequencies for a gene in a population. The Hardy-Weinberg principle is expressed by these equations: $p^2 + 2pq + q^2 = 1$ $p + q = 1$

Feb/March 2017

The aye-aye, Daubentonia madagascariensis, is a primate that lives in Madagascar. Aye-ayes are nocturnal, meaning they are active during the night, and they build nests high in trees. Their diet includes insect larvae living inside tree trunks. Fig. 1.1 is an image of an aye-aye. The International Union for Conservation of Nature (IUCN) is the world’s largest international environmental organisation. The IUCN Red List of Threatened Species™ assesses the conservation status of plant and animal species. The aye-aye is listed as endangered on the IUCN Red List, so it is at a very high risk of becoming extinct in the wild.

Feb/March 2018

State the general theory of evolution and explain how natural selection works in evolution.

Feb/March 2019

The European eel, Anguilla anguilla, is a fish species. Although eels produce huge numbers of young, the size of eel populations usually stays fairly steady.

Feb/March 2021

Natural selection and artificial selection (selective breeding) both play major roles in evolutionary change.

Feb/March 2022

Fill in the paragraphs below with the most suitable word or words.

Feb/March 2024

The orca, Orcinus orca, has the broadest range of any aquatic mammal and occurs in almost every sea and ocean. Orca are social mammals and usually live in groups, although group size varies. Several distinct forms of orca exist, and these are classified as the same species. However, there is evidence that sympatric speciation is taking place. In the Northeast Atlantic Ocean there are two distinct forms of orca: Type 1 and Type 2. Type 1 orca feed mainly on fish, whereas Type 2 orca feed mainly on aquatic mammals such as seals. Fig. 2.2 shows where Type 1 orca and Type 2 orca have been seen in the Northeast Atlantic Ocean. Orca are not restricted to only these regions, and some groups travel very long distances.

Feb/March 2025

Anole lizards occur across the Caribbean and the nearby mainland, and the genus includes many species. With the exception of Anolis carolinensis, which is present in mainland Florida, each species is restricted to a single island or to a small cluster of islands. Fig. 5.1 illustrates where four species of anole lizards are found. A study was then carried out on the relationships among these four species by means of DNA analysis. The base sequences of a section of mitochondrial DNA from the four species were compared, and the findings are given in Table 5.1. A smaller number indicates a smaller difference between the base sequences of the two species.

May/June 2010

Drosophila, the fruitfly, includes many species. Three of them, Drosophila pseudoobscura, D. persimilis and D. miranda, are believed to be closely related. Samples of these three species were gathered from the western United States of America. Fig. 5.1 shows the places where these species naturally occur.

May/June 2010

Fruitflys of the genus Drosophila include many species. Three of these, Drosophila pseudoobscura, D. persimilis and D. miranda, are believed to be closely allied. Specimens of these three species were taken from the western United States of America. Fig. 5.1 indicates their natural distributions. The base sequences of four DNA regions from each species were determined. The divergence in the base sequences of D. pseudoobscura and D. persimilis relative to the sequences in D. miranda was calculated. Table 5.1 shows the results.

May/June 2010

Erinaceus europaeus, the hedgehog, is a small carnivorous mammal native to Northern Europe. In 1974, hedgehogs were brought to a small cluster of islands off the west coast of Scotland. Their numbers have risen so that there are now more than 5000 breeding pairs. These hedgehogs have no natural predators on these islands and their diet consists mainly of birds’ eggs. Fig. 8.1 depicts a hedgehog. Fig. 8.2 illustrates the hedgehog population density in the year 2000. Table 8.1 presents the changes in the populations of the species of birds from 1983 to 2000.

May/June 2011

The greenish warbler, Phylloscopus trochiloides, is a small bird species that first evolved in northern India, along the southern rim of the Himalayan mountain range. Many thousands of years ago, greenish warbler populations spread round the western and eastern sides of the Himalayan mountain range and became established in north-eastern Europe and Siberia. Over time, these populations showed gradual changes in characteristics, which produced different forms of the greenish warbler. One gradual change is seen in the male warbler’s song, which is highly distinctive and plays a part in mating behaviour. When greenish warblers from north-eastern Europe meet those from Siberia, no mating occurs. The greenish warblers from north-eastern Europe and Siberia are now regarded as two separate species.

May/June 2012

The greenish warbler, Phylloscopus trochiloides, is a small bird species that first evolved in northern India, along the southern edge of the Himalayan mountain range. Thousands of years ago, greenish warbler populations spread around the western and eastern sides of the Himalayan mountain range and became established in north-eastern Europe and Siberia. As time passed, their characteristics changed gradually, producing different forms of the greenish warbler. One gradual change is in the male warbler’s song, which is highly distinctive and is part of mating behaviour. When greenish warblers from north-eastern Europe meet those from Siberia, mating does not occur. The greenish warblers from north-eastern Europe and Siberia are now regarded as two separate species. Fig. 1.1 depicts a greenish warbler. Fig. 1.2 illustrates the spread of the greenish warbler.

May/June 2012

In the Death Valley area of Nevada in the USA, there was once a very large lake. Around 20 000 years ago, the lake began to dry out and today it exists only as separate pools. Four species of desert pupfish have been recorded in these pools. Evidence shows that more than 20 000 years ago there was just one pupfish species in the lake system. Fig. 8.1 shows a desert pupfish.

May/June 2013

Wheat, Triticum aestivum, originated through hybridisation between three different but closely related grass species, A, B and C. Each species possessed seven pairs of chromosomes ($2n = 14$). Figure 3.1 shows the hybridisation pathway.

May/June 2014

Mole rats, Spalax ehrenbergi, are mammals that live together in underground burrow systems, are blind, and communicate with one another by means of sound and scent. When males are trying to persuade females to mate, they produce a purring call. In Israel, the mole rats in different regions of the country all appear the same. However, they are actually four separate populations with different chromosome numbers, and each one lives in a different climatic zone. These are shown in Table 5.1. This table also gives information about the purring calls used by the males in each population. The males' calls were analysed by measuring how many sound pulses were produced per second, as well as the frequencies of the sounds they made.

May/June 2015

Mole rats, $Spalax\,ehrenbergi$, are mammals that live together in social groups in underground burrow systems, cannot see, and exchange information with one another by means of sound and scent. Males produce a purring call when they are trying to attract females for mating. In Israel, mole rats from different regions all appear identical. Even so, they actually belong to four distinct populations with different chromosome numbers, living in different climatic regions. These are shown in Table 5.1. This table also gives information about the purring calls used by the males in each population. The males’ calls were studied by counting the number of sound pulses per second, and also by measuring the frequencies of the sounds they produced.

May/June 2015

Adult Drosophila fruit flies consume yeasts on the skins of ripe fruit. The females place their eggs within the fruit, and the larvae feed there before becoming adults. The tree Morinda citrifolia bears fruits known as noni fruits. Noni fruits contain a toxin and are rejected by almost all Drosophila species, including Drosophila yakuba in mainland Africa. One population of D. yakuba lives on Mayotte, an island off the African coast. The D. yakuba population on Mayotte has adapted to feed on noni fruit. Flies in this population produce enzymes that make them resistant to the toxin in noni fruits. The adults can feed on yeasts on the fruit surface, and their eggs and larvae can develop and grow inside the fruit. Scientists have used bioinformatics to estimate that the distinctive D. yakuba population on Mayotte arose within the last 30 000 years.

May/June 2018

Section B. Answer one question.

May/June 2019

State what the general theory of evolution is.

May/June 2019

Regressive evolution is a population change over time in which some phenotypic characteristics are lost. It is thought to arise through either genetic drift or natural selection. One example of regressive evolution is the disappearance of eyes in one form of the Mexican cavefish, Astyanax mexicanus. These eyeless cavefish live in caves with complete darkness. Three theories are offered to explain how the cavefish lost their eyes. Theory A In a cave with total darkness and scarce energy sources, eyes give no benefit. Having eyes may even be a disadvantage because they may require energy. Theory B A mutation has occurred in a single gene. This mutation has two effects: • no eye development • an increase in the number of chemoreceptors on the skin. Theory C Several mutations happened in the genes responsible for eye development over time. By chance, these mutations became more frequent in small isolated populations. In the end this produced a population of eyeless cavefish.

May/June 2019

Cats belong to the Felidae family. Leopardus and Panthera are two genera within Felidae. Leopardus contains small wild cat species with spotted coats. In 2013, biologists studied the evolution of Leopardus tigrinus in South America. Fig. 3.1 shows where two South American populations of L. tigrinus, population A and population B, are found. Population A lives in grassland and desert habitats, whereas population B lives in forest habitats. The cats in population A have a lighter coat colour and a spot pattern that differs from the cats in population B. Genetic analysis indicates that population A is genetically distinct from population B. Population B has therefore been reclassified as a new species, L. guttulus.

May/June 2020

A subspecies is a genetically distinct population within a species that shows some phenotypic differences from the rest of the species, although it is not yet reproductively isolated. Nine subspecies of the tiger, Panthera tigris, have been identified. Six of these subspecies occur on mainland Asia. The remaining three subspecies come from the Sunda Islands, which include Bali, Java and the much larger island of Sumatra. Fig. 3.1 shows these three islands.

May/June 2020

The collared flycatcher, Ficedula albicollis, and the pied flycatcher, F. hypoleuca, are two bird species that are very closely related. DNA analysis indicates that speciation from a shared ancestor took place about 1 million years ago. The investigation was done on Öland island in Sweden. On Öland, the breeding ranges of the two species overlap, and only a small number of hybrid flycatchers are formed.

May/June 2020

Explain how speciation may happen as a result of geographical separation.

May/June 2021

Two subspecies of reindeer, Rangifer tarandus, are found in North America. They are members of the same species, but they show some differences in morphology and occur in different geographical regions. Fig. 5.1 shows a reindeer. Table 5.1 compares the characteristics of the two North American reindeer subspecies.

May/June 2022

Bison are a kind of large wild cattle. Ancestors of modern bison first appeared in Asia 2.5 million years ago. This ancestral bison species expanded into Europe and North America. Although the ancestral species is now extinct, its descendants are Bison bonasus, the European bison, and Bison bison, the American bison. Fig. 5.1 depicts an American bison.

May/June 2022

The puma, Puma concolor, is native to North and South America. Fig. 5.1 shows a puma. Fig. 5.2 shows the puma species' distribution.

May/June 2022

Variation within populations of a species can create the opportunity for evolution to take place.

May/June 2024

As organisms reproduce, they pass their alleles to the next generation. Many different factors can influence how allele frequencies alter over time in a population.

May/June 2025

The Grand Canyon lies in Arizona, USA. It is thought to have been formed more than five million years ago, when the Colorado River started cutting a deep channel (canyon) into the surrounding rocks. Before the canyon came into being, an ancestral species of antelope squirrel lived in the area. An antelope squirrel is a rodent and a member of the squirrel family, Sciuridae. It is thought that about 3.6 million years ago, this ancestral species split into the two species found today: • Harris’s antelope squirrel, $A. harrisii$, has a habitat range that extends from the south rim of the canyon. • The white-tailed antelope squirrel, $A. leucurus$, has a habitat range that extends from the north rim of the canyon. Fig. 4.1 shows where the Colorado River is in the Grand Canyon and where these antelope squirrel species are found.

May/June 2025

The Ethiopian wolf, *Canis simensis*, belongs to the carnivorous Canidae family. Fig. 1.1 depicts an Ethiopian wolf.

Oct/Nov 2011

Asthma is a disease of the respiratory system.

Oct/Nov 2011

The Ethiopian wolf, Canis simensis, belongs to the Canidae family of carnivores. Fig. 1.1 shows one Ethiopian wolf.

Oct/Nov 2011

Mammoths are mammals that are now extinct and are closely related to elephants. Roughly three million years ago, mammoth ancestors moved out of Africa and spread into Europe and Asia. After that, around $1.7$ million years ago, the steppe mammoth evolved and became adapted to cooler environments. Later, about $700\,000$ years ago, when the climate altered and the Arctic turned much colder, the woolly mammoth evolved. Woolly mammoths had several clear adaptations that reduced heat loss, such as dense fur, relatively small ears and short tails.

Oct/Nov 2013

The evolutionary origin of four-legged amphibians, including frogs and toads, has been debated for a long time. Of the living fish, the elusive coelacanth and the lungfish are regarded as the closest relatives of these amphibians. Blood samples were collected from two coelacanths that had been caught recently near Comoros. The amino acid sequences of the $\alpha$ and $\beta$ chains of coelacanth and lungfish haemoglobin were compared with the established sequences of adult amphibians and their aquatic larvae (tadpoles). Organisms with more amino acid matches in a polypeptide chain have a more recent common ancestor than organisms with fewer matches. The comparisons with three amphibian species, $\textit{Xenopus laevis}$ ($Xl$), $\textit{X. tropicala}$ ($Xt$) and $\textit{Rana catesbeiana}$ ($Rc$) are shown in Table 2.1.

Oct/Nov 2013

Fig. 3.1 depicts a red squirrel, $Sciurus\ vulgaris$. This species is native to the British Isles, so it has been present there for at least 10 000 years. In the 1800s, a related but slightly larger species, the grey squirrel, $Sciurus\ carolinensis$, was brought in from North America. A record of the interaction between red squirrels and grey squirrels includes the following points: - Grey squirrels, which occupy a similar niche to red squirrels, spread quickly after being introduced. - Grey squirrels carry a virus that does not affect them, but that kills red squirrels. - Red squirrels died out in many parts of the British Isles. - More recently, some areas have shown the opposite pattern. Grey squirrel numbers have fallen. Red squirrels have returned to their former habitat. - This has occurred where safeguarding a native predator species, the pine marten, $Martes\ martes$, has led to an increase in the number of these predators. - Examination of pine marten faeces shows that they catch and eat far more grey squirrels than red squirrels.

Oct/Nov 2017

The Hawaiian archipelago is a chain of volcanic islands in the Pacific Ocean. Fig. 4.1 shows where five of these islands are situated relative to one another. Table 4.1 gives the size and age of these five islands, together with the total number of $Mecyclothorax ground beetle species and their species density on each island.

Oct/Nov 2018

The Hawaiian archipelago comprises a chain of volcanic islands in the Pacific Ocean. Fig. 4.1 displays the relative positions of five of these islands. Table 4.1 gives the area and age of these five islands, together with the total number of \textit{Mecyclothorax} ground beetle species and their species density on each island. For Maui, the data are presented as two separate regions, West and Haleakalā, because they formed at different times from two different volcanoes. Fig. 4.2 shows a ground beetle from the genus \textit{Mecyclothorax}.

Oct/Nov 2018

The genus Heliconius includes over 40 species of butterflies with vivid patterns. Scientists have used laboratory studies to examine how one species, Heliconius heurippa, may have arisen as a distinct species. The phenotype of H. heurippa is intermediate between that of two other species, H. cydno and H. melpomene. Laboratory breeding investigations found that: • crosses between H. cydno and H. melpomene (parent species) produce fertile hybrid offspring • controlled crosses of the hybrids produce individuals that are visually identical to H. heurippa within three generations • hybrid butterflies prefer to mate with one another, instead of with individuals from either parent species. The researchers concluded that the H. heurippa species may include DNA from the two parent species as a consequence of hybridisation.

Oct/Nov 2019

The genus Heliconius includes over 40 species of butterflies with vivid patterns. In laboratory studies, scientists examined how one species, Heliconius heurippa, might have arisen as a distinct species. The phenotype of H. heurippa is intermediate between that of H. cydno and H. melpomene. Breeding tests in the laboratory found that crosses between H. cydno and H. melpomene give fertile hybrid offspring; when these hybrids are crossed under controlled conditions, offspring matching the appearance of H. heurippa are produced within three generations; and the hybrid butterflies are more likely to mate with one another than with members of either parental species. The researchers therefore concluded that the H. heurippa species may contain DNA from both parent species as a result of hybridisation.

Oct/Nov 2019

Mimulus is a plant genus made up of a wide variety of species with bright flowers that attract pollinators such as bees and hummingbirds. Pollinators move pollen from one flower to another during plant sexual reproduction. Table 5.1 sets out a comparison of some characteristics of two closely related Mimulus species that both occur in the same part of North America. The differences between them are the altitude at which each species grows; their flower traits, including petal colour and the distance from the flower opening to the nectar that pollinators feed on; and the proportions of pollinator visits they receive from bees or from hummingbirds.

Oct/Nov 2019

DNA barcoding is used to identify species. To make a DNA barcode, a chosen section of DNA is sequenced and then matched against an online reference database of DNA. A region commonly used is a 648 base pairs long section within a mitochondrial gene that codes for the enzyme cytochrome c oxidase I.

Oct/Nov 2020

A subspecies is a population within a species that is genetically different, shows some differences in phenotype, but has not yet become reproductively isolated. About 500 000 years ago, the European house mouse, $\textit{Mus musculus}$, split into two subspecies, $\textit{Mus musculus domesticus}$ and $\textit{Mus musculus musculus}$.

Oct/Nov 2020

DNA barcoding is used to identify species. To produce a DNA barcode, a chosen section of DNA is sequenced so that it can be matched with an online database of reference DNA. One section of DNA that is often selected is 648 base pairs long within a mitochondrial gene, which codes for the enzyme cytochrome c oxidase I.

Oct/Nov 2020

California salamanders are terrestrial amphibians. An ancestral population of California salamanders lived in a forest region in northern California. Around 10 million years ago, that original population split, with two groups, A and B, moving away. Population A moved down the California coast on the western side of the Great Central Valley. Population B moved to the east of the Great Central Valley. Today, the two populations are found near one another in southern California. Fig. 8.1 shows the migration routes followed by populations A and B.

Oct/Nov 2021

Twenty million years ago, the area that is now Panama lay beneath an ocean. A gap existed between North America and South America, and the Atlantic and Pacific Oceans flowed through it without obstruction. The porkfish, Anisotremus sp., lived in this zone between North America and South America. Fig. 8.1 shows a porkfish. Around 3 million years ago, volcanic activity and sedimentation created a narrow land strip, Panama, that linked North America and South America. Fig. 8.2 shows the region 20 million years ago and at the present day.

Oct/Nov 2021

California salamanders belong to the group of terrestrial amphibians. The original population of California salamanders lived in a forested region in northern California. About 10 million years ago, two separate populations, A and B, began moving away from the original population. • Population A moved along the Californian coast, to the west of the Great Central Valley. • Population B moved to the east of the Great Central Valley. These two populations now occur close to one another in southern California. Fig. 8.1 shows the migratory routes followed by populations A and B.

Oct/Nov 2021

A broad range of alpine plant species grows in the mountains of New Zealand’s South Island. Alpine plants are plants that live above the treeline, meaning the height above which trees are unable to grow. For most of South Island, the present treeline is $1200\,\text{m}$.

Oct/Nov 2022

Many alpine plant species are found in the mountains of New Zealand’s South Island. Alpine plants are plants that live above the treeline, meaning the altitude beyond which trees are unable to grow. For most of South Island, the present treeline is $1200\,\text{m}$.

Oct/Nov 2022