(a) What is fossil? Describe the origin and evolution of horse based on the fossil records. 20 (b) Define variation. Describe…

(a) Fossils and the origin and evolution of the horse

A fossil is the preserved remains or trace of an organism of a past geological age, found in rock. Fossils occur as petrified or mineralised hard parts, moulds and casts, impressions, footprints and coprolites, or whole bodies in amber or ice.

The horse family (Equidae) arose in North America. The fossils form a branching bush with many coexisting forms, not the straight line of Marsh's 1879 series, but the main stages are:

• Eohippus (Hyracotherium), early Eocene, about 52 million years ago: fox-sized (25–45 cm tall), a forest browser with four toes on the forefoot, three on the hind foot, and 44 low-crowned teeth with bumpy molars.
• Mesohippus, early Oligocene, about 37–32 million years ago: about 60 cm at the shoulder, with three toes on every foot, the middle one largest, and longer limbs.
• Merychippus, Miocene: about 100 cm tall, with three toes, of which the central toe formed the hoof and the small side toes touched the ground only when running; its high-crowned (hypsodont), cement-covered teeth made it the first grazer.
• Pliohippus, about 12 million years ago: one functional toe with small side splints; it is now thought to be a side branch, and Dinohippus is the probable ancestor of Equus.
• Equus, evolved about 4–7 million years ago (oldest fossils about 3.5 million years old): a single hoof, long legs, large body, and high-crowned teeth with complex enamel ridges. It spread from North America to Asia, Africa, Europe and South America, and died out in North America about 10,000 years ago. The Siwalik beds of India have yielded the three-toed Hipparion and Equus sivalensis.

Trends: larger body and skull, longer limbs, reduction of the side toes until the weight is carried on the third toe, and taller teeth. They accompany the drying climate and the spread of grassland over woodland, which favoured running and grazing.

(b) Variation, its types and its role in evolution

Variation is the difference in form, physiology, behaviour or genes among individuals of a species, including offspring and their parents.

Types:

• Somatic (acquired) variation arises in body cells through environment, use or disuse, as in a tanned skin or a trained muscle, and is not inherited. Germinal (genetic) variation arises in the germ line and is inherited (Weismann).
• Continuous (fluctuating, quantitative) variation shows all grades between the extremes, is controlled by many genes together with the environment, and gives a bell-shaped distribution, as in height, weight and skin colour. Discontinuous (qualitative) variation shows sharp classes controlled by one or a few genes, as in ABO blood groups, albinism or the round and wrinkled seeds of pea; large sudden changes are mutations or "sports".
• Its sources are mutation, recombination (crossing over, independent assortment and random fertilisation), chromosomal changes and gene flow.

Role in evolution:

• Heritable variation is the raw material of natural selection (Darwin): individuals with favourable variants leave more offspring, so the variant spreads. Examples are dark moths in industrial areas and resistance to insecticides and antibiotics. Acquired variations play no part, which refutes Lamarck's view.
• Variation lets a population adapt to a changed environment and, with isolation, diverge into new species.
• Where heterozygotes are fitter, variation is kept as balanced polymorphism, as with sickle-cell trait and malaria.
• Without heritable variation, selection has nothing to act on and evolution stops.

(c) Biodiversity and its major types

Biodiversity is the variety of life at all levels. The Convention on Biological Diversity defines it as the variability among living organisms from all sources, including terrestrial, marine and other aquatic ecosystems and the ecological complexes of which they are part; this includes diversity within species, between species and of ecosystems. Over 1.5 million species have been described, and the true number is estimated at several million.

Major types:

• Genetic diversity: variation in the genes within a species. India is credited with over 50,000 strains of rice and about 1,000 varieties of mango. It gives a species the ability to adapt and is the basis of breeding.
• Species diversity: the variety of species in a region, in richness (the number of species) and evenness (their relative abundance). The Western Ghats have a greater amphibian species diversity than the Eastern Ghats, and diversity declines from the equator towards the poles. India holds about 8% of the world's recorded species on 2.4% of its land area.
• Ecosystem diversity: the variety of habitats, communities and ecological processes. India's deserts, rain forests, mangroves, coral reefs, wetlands, estuaries and alpine meadows give it greater ecosystem diversity than a country such as Norway.

On the scale of measurement (Whittaker), α-diversity is the diversity within one habitat, β-diversity is the change in species composition between habitats, and γ-diversity is the total diversity of a whole region.