Biology: quotes, facts and reflections - Page 3

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WonderOcean

Along some coasts, winds push surface water out to sea and cold, nutrient-rich water rises from the depths to take its place. This upwelling feeds plankton and, through them, some of the richest fishing grounds on Earth.

Five coastal upwelling zones stand out: the Humboldt Current off Peru and Chile, the California Current, the Canary Current off north-west Africa, the Benguela Current off south-western Africa and the Somali Current off Somalia and Oman. Together they cover only about 5 per cent of the ocean, yet they yield roughly a quarter of the world's marine fish catch, much of it small fish such as anchovies and sardines. Off Peru, the anchoveta supports one of the largest single-species fisheries in the world. That richness is fragile. During an El Niño, warm water spreads along the South American coast and weakens the upwelling, so fewer nutrients reach the surface and anchovy numbers crash, along with the seabirds and sea lions that feed on them. The El Niño of 1972 to 1973, on top of heavy fishing, helped bring Peru's anchovy fishery to collapse.

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DriveClimate

A marine heatwave is a spell of unusually warm sea water lasting from days to months. It can kill kelp forests and corals, drive fish and other animals into new waters and upset fisheries.

Under a definition widely used since 2016, an event counts as a marine heatwave when the water stays warmer than 90 per cent of past readings for that place and time of year for at least five days. The term itself was coined after an unprecedented event off Western Australia in the summer of 2011, when warm water killed kelp forests, underwater forests of large brown seaweed, along hundreds of kilometres of coast. In many places the kelp did not come back: low, turf-like seaweeds and fish more typical of warmer waters took over, and the reef became a different kind of ecosystem. Another well-known case was the Blob, a mass of warm water that sat in the north-east Pacific from 2014 to 2016 and disrupted marine life and fisheries along the west coast of North America.

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WonderEvolution

Three-spined sticklebacks, small fish of northern seas, have moved into lakes and rivers again and again. Each time they tend to evolve lighter armour, often using a gene variant their sea-going ancestors already carried.

Sea-going sticklebacks carry 30 to 40 bony plates along each side, protection against predatory fish; most freshwater populations have between none and a dozen. A single gene called Eda (ectodysplasin) largely decides how many plates grow, and the low-armour version survives at low frequency in sea populations, ready whenever fish settle in fresh water. The change can be fast. In 1982, Loberg Lake in Alaska was poisoned to clear it for trout and salmon, and sea-going sticklebacks recolonised it. In the twelve years from 1990, the fully armoured form fell from the whole population to about one fish in ten, while a low-plate form rose to three in four. Because thousands of lakes were colonised separately after the last ice age, the same change has happened independently again and again.

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WonderEvolution

In Hawaii, silverswords, shrubs, cushion plants, vines and trees all descend from a single tarweed, a sticky plant of the daisy family from North America's Pacific coast, that reached the islands millions of years ago.

The ancestor belonged to the tarweeds, sticky, resinous relatives of sunflowers that grow along the Pacific coast of North America. DNA studies suggest it reached Kauaʻi about five million years ago, perhaps as a seed caught in a bird's feathers. With few competitors, its descendants spread into dry scrub, wet forest, bogs and bare lava fields, and today about 34 species in three genera make up what botanists call the silversword alliance. The best known, the silverswords themselves, form globes of silvery, sword-shaped leaves on the high volcanic slopes of Maui and Hawaiʻi Island; some species flower only once, sending up a tall stalk of flower heads before dying. Many members are now endangered, chiefly by grazing animals and invasive plants brought by people.

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WonderEvolution

Lake Victoria's cichlids, small and often colourful fish, split into more than 500 species with different jaws, colours and diets in only about 15,000 years. Much of the raw genetic material came from an ancient cross between two long-separated lineages.

Lake Victoria dried out almost completely about 17,000 years ago and refilled about 14,700 years ago, as the climate turned wetter. Its cichlids belong to a larger family group of more than 700 closely related species in the region's lakes, which scientists call the Lake Victoria superflock. A genetic study led by Joana Meier, published in 2017, found that the group descends from hybrids between two lineages that had long been apart, one from the upper Nile and one from the Congo basin. The mix gave natural selection a large stock of gene variants to work with as fish specialised in scraping algae, crushing snails, catching insects or hunting other fish. Since Nile perch were introduced to the lake in the 1950s, at least 200 of its cichlid species are thought to have died out.

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WonderEvolution

The Mexican tetra, a small river fish, has colonised caves several times, and cave after cave its populations have evolved tiny or missing eyes and lost their colour, giving biologists natural repeats of the same experiment.

About 30 cave populations are known in a limestone region of San Luis Potosí and southern Tamaulipas in north-eastern Mexico, while their eyed relatives still live in rivers nearby. Genetic studies suggest the caves were entered at least five separate times, and that the cave forms are recent, less than about 20,000 years old. When fish from two different caves are crossed, some offspring can see, a sign that eye loss happened partly through different genes. Cave life brought other changes too: taste buds all over the head, a very sensitive lateral line (the row of pressure sensors along a fish's side), and bodies that store about four times more energy as fat, which helps them survive long gaps between meals.

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WonderEvolution

Bread wheat carries six sets of chromosomes, two from each of three wild ancestral grasses. Organisms with more than two complete sets of chromosomes are called polyploid, and they are common among plants.

Most animals, including humans, have two sets of chromosomes, one from each parent. Wheat gained its extra sets in two steps. Long before farming, two wild grasses, Triticum urartu and a relative of Aegilops speltoides, crossed to produce wild emmer, with four sets. After farming began in the Near East, cultivated emmer crossed with a third wild grass, Aegilops tauschii, adding two more sets and creating bread wheat. The oldest known bread wheat, identified from its DNA, comes from Çatalhöyük in Turkey and dates to about 6400 to 6200 BC. Extra gene copies can take on new roles, one reason polyploid crops are often large and hardy. Potatoes, cotton, Arabica coffee and cultivated strawberries are polyploid too.

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WonderArchaeology

Dental calculus, the hard tartar that builds up on teeth, can trap DNA, proteins, microbes and tiny food remains for thousands of years, giving archaeologists evidence of ancient diets and health.

Plaque hardens into calculus while a person is alive, sealing in traces of whatever passes through the mouth, and it survives in the ground far better than most soft remains. In 2017 a team studying DNA in the calculus of Neanderthals from El Sidrón cave in Spain found, in a young individual with a painful dental abscess, traces of poplar, a natural source of salicylic acid, the painkiller related to aspirin, and of Penicillium mould. The researchers suggested he may have been treating himself, though that remains a hypothesis. In 2014 a team led by Christina Warinner identified a milk protein preserved in calculus from ancient teeth, direct evidence of who drank milk as dairying spread through prehistoric populations.

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WonderArt

Maria Sibylla Merian, a German-born artist and naturalist, painted each stage of an insect's life, from egg to caterpillar to butterfly, on the plant it fed on. Her 1705 book on the insects of Suriname joined close observation with art.

Merian was born in Frankfurt in 1647 into a family of engravers and was raising silkworms by the age of 13. In 1679 she published the first volume of a book showing how caterpillars transform. Many people then believed insects were born from mud or rotting matter, and illustrators usually drew dead specimens in tidy rows; her careful drawings of each life stage helped show that insects follow a fixed life cycle. In 1699, aged 52, she sold 255 of her own paintings to fund a voyage to the Dutch colony of Suriname with her daughter Dorothea. For nearly two years she collected and raised insects there, relying on enslaved Africans and Indigenous people for help and knowledge, until illness, possibly malaria, sent her home. Her Metamorphosis Insectorum Surinamensium (1705) became a landmark of natural history.

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WonderBiology

The Indian botanist Janaki Ammal studied plant chromosomes and used that knowledge to help breed sweeter sugarcane suited to India's climate. In 1945 she co-wrote the Chromosome Atlas of Cultivated Plants, a reference to the chromosome counts of crops and garden plants.

Chromosomes are the thread-like structures in a cell that carry its genes, and their number helps explain which plants can be crossed. Janaki Ammal earned her PhD at the University of Michigan in 1931, rare for an Indian woman at the time. India then imported its sweetest sugarcane from Southeast Asia; at the Sugarcane Breeding Station in Coimbatore she crossed cane with related grasses to produce sweet varieties that grew well in India. In England she worked with the geneticist C. D. Darlington at the John Innes Institute, her co-author on the atlas, and then at the Royal Horticultural Society's garden at Wisley, where she treated magnolias with the chemical colchicine. A white-flowered magnolia there is named 'Janaki Ammal'. The Indian government later invited her home to reorganise the Botanical Survey of India, and she received the Padma Shri in 1977.

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WonderBiography

American biologist Ernest Everett Just showed that an egg cell's surface changes rapidly at fertilisation and helps stop extra sperm from getting in. Working with sea urchins and other marine animals, he insisted that cells be studied in conditions as close to natural as possible.

Just taught at Howard University in Washington from 1907 until his death and spent some twenty summers at the Marine Biological Laboratory in Woods Hole, Massachusetts, where his careful handling of marine eggs made him a sought-after expert. He described what is now called the fast block to polyspermy, the rapid change that keeps a second sperm out of the egg. In 1915 he became the first person to receive the Spingarn Medal from the US civil rights organisation NAACP. Racism kept him out of posts at major American research universities, so from 1929 he worked more and more in Europe: at the Stazione Zoologica in Naples, the Kaiser Wilhelm Institute in Berlin and later Roscoff in France. His 1939 book The Biology of the Cell Surface argued that the cell's outer layer, not only the nucleus, drives development. He died in 1941.

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WonderScience

Electron microscopes use beams of electrons, focused by magnets instead of glass lenses, to show details far smaller than any light microscope can, down to viruses and even rows of atoms. Ernst Ruska and Max Knoll built the first one in Berlin in 1931.

A light microscope cannot separate details much smaller than about half the wavelength of light, roughly 200 nanometres. Electrons also behave as waves, and when accelerated their wavelength becomes many thousands of times shorter. Ruska and Knoll used magnetic coils to focus them in 1931, and by 1933 Ruska's instrument could show finer detail than any light microscope. Siemens sold the first commercial model in 1939. Transmission electron microscopes send electrons through very thin slices; scanning electron microscopes sweep a beam across a surface and build a three-dimensional-looking picture from the signals that come back. Samples must sit in a vacuum and be carefully prepared, but the images opened up the inner structure of cells, viruses and materials. Ruska shared the 1986 Nobel Prize in Physics.

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WonderBiology

Environmental DNA, or eDNA, is the genetic material that animals and plants shed into water, soil and air. By collecting and testing it, scientists can find out which species live in a place without seeing or catching any of them.

Animals and plants constantly shed skin cells, scales, mucus, pollen and waste. Researchers filter a water sample or collect soil, extract the DNA and test it, either for one target species or for short, telltale DNA sequences that identify many species at once. The method finds rare, shy or invasive species with little disturbance. In June 2018 a team led by the geneticist Neil Gemmell sampled the water of Loch Ness in Scotland; they found no reptile DNA at all, but a great deal of eel DNA. A positive result is not a headcount, though: DNA drifts with currents, breaks down over time and can contaminate samples, so results still need careful interpretation by ecologists.

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