(a) What is isolation ? Describe the major isolating mechanisms which lead to speciation. 20 (b) Enlist theories of origin of…

(a) Isolation and the isolating mechanisms that lead to speciation

Isolation is any barrier that prevents gene flow between populations. Once populations are isolated, mutation, selection and drift act separately on them, so they diverge until they can no longer interbreed, which is speciation. Isolating mechanisms are classed as pre-zygotic (before a zygote forms) and post-zygotic (after it).

Pre-zygotic mechanisms:

• Habitat (ecological) isolation: populations living in different habitats in the same area rarely meet.
• Temporal isolation: breeding at different seasons or times of day, as in two species of toad that breed in different weeks.
• Behavioural (ethological) isolation: species-specific courtship signals, such as the flash patterns of fireflies and the songs of crickets and birds.
• Mechanical isolation: differences in genital structure (lock and key) or flower structure prevent copulation or pollination.
• Gametic isolation: sperm and egg do not recognise each other, as in sympatric sea urchins.

Post-zygotic mechanisms:

• Hybrid inviability: the hybrid zygote or embryo dies, as in some frog crosses.
• Hybrid sterility: the hybrid is viable but sterile, as the mule (mare × donkey) whose chromosomes cannot pair.
• Hybrid breakdown: the F₁ is fertile but the F₂ or backcross is weak or sterile, as in some cotton species crosses.

Geographical barriers (allopatric speciation) usually start the process, and the reproductive isolating mechanisms then complete and reinforce it.

(b) Theories of origin of life and the Oparin–Haldane theory

Theories: special creation; spontaneous generation (abiogenesis, refuted by Redi and Pasteur); biogenesis; cosmozoic theory or panspermia (Arrhenius); steady-state theory; and the chemical (biochemical) evolution theory of Oparin and Haldane.

Oparin (1924) and Haldane (1929) proposed that life arose gradually from non-living matter through chemical evolution:

• The early Earth's atmosphere was reducing, with methane, ammonia, hydrogen and water vapour but no free oxygen.
• Energy from ultraviolet rays, lightning and volcanic heat produced simple organic molecules (amino acids, sugars, purines and pyrimidines), which collected in the oceans as a "hot dilute soup" (Haldane).
• These polymerised to proteins and nucleic acids, which, in the absence of oxygen and of living decomposers, were not degraded.
• The polymers aggregated into membrane-bound droplets called coacervates (Oparin), or protobionts, with a simple metabolism.
• With genetic material that could replicate, the first living cells appeared, anaerobic heterotrophs. Autotrophs then evolved and added oxygen to the atmosphere.

The theory was supported experimentally by Miller and Urey (1953), who sparked a mixture of methane, ammonia, hydrogen and water vapour and obtained amino acids.

(c) Fossil data and the chronological order of human evolution

Fossil data are the preserved remains of ancient organisms, such as bones, teeth and footprints, along with tools made by them. They are dated by stratigraphy and radiometric methods (potassium–argon, argon–argon, and carbon-14 for the last 50,000 years) and show the sequence of change in time.

The order of the main hominid fossils, from oldest:

• Dryopithecus (Miocene, about 12 million years ago), an ape-like ancestor with a Y-5 molar pattern.
• Sivapithecus and Ramapithecus (late Miocene, about 14–8 million years ago; Siwalik Hills, India), with thick enamel; Ramapithecus is now regarded as a female Sivapithecus, not a human ancestor.
• Australopithecus (about 4–2 million years ago, Africa): bipedal, cranial capacity 400–500 cc, as in A. afarensis ("Lucy").
• Homo habilis (about 2.4–1.4 million years ago): cranial capacity about 650–800 cc; made Oldowan stone tools.
• Homo erectus (about 1.9 million to 0.1 million years ago; Java and Peking man): about 900–1100 cc, upright, used fire and Acheulean hand-axes. The Narmada skull from Hathnora, India, is assigned to H. erectus or archaic H. sapiens.
• Neanderthal man (about 400,000–40,000 years ago): about 1400–1500 cc, with heavy brow ridges and Mousterian tools, and buried the dead.
• Homo sapiens (about 300,000 years ago to the present; Cro-Magnon about 40,000 years ago): about 1350 cc, with a chin, a high forehead, blade tools and cave art.

The trends are upright posture, a larger brain, smaller jaws and face, and tool use.