Variations in a population can be: heritable (genetic variations due to mutations and recombination — passed to offspring and subject to natural selection) or non-heritable (phenotypic variations due to environment — not passed to offspring).

Frog tadpoles (larvae) have gills for aquatic respiration — an ancestral vertebrate characteristic reflecting the aquatic origins of amphibians. During metamorphosis, gills are absorbed and lungs develop for terrestrial life.

By the biological species concept, a species is defined as a group of organisms that can interbreed with each other and produce fertile offspring, but are reproductively isolated from other groups. This concept was formalised by Ernst Mayr in 1942.

Gene flow is the transfer of alleles between populations through migration (movement of individuals) and subsequent interbreeding. Gene flow reduces genetic differences between populations, potentially preventing speciation.

Fossils provide the most direct and strongest evidence of evolution — they are actual remains of organisms that lived in the past, showing the progression of life forms over geological time and documenting transitional forms between major groups.

Karl Ernst von Baer observed that embryos of different vertebrate groups (fish, amphibians, reptiles, birds, mammals) are remarkably similar in their early developmental stages — all have gill slits, a tail, and similar body plans. This embryological evidence supports common ancestry.

Vestigial organs are the clearest examples of ancient homologous structures — they are evolutionary remnants that persist from ancestors but have lost most or all of their original function. They provide strong evidence for common ancestry.

Evolution is the change in heritable characteristics of populations over successive generations. It means that organisms living today have descended with modification from ancestors that lived in the past — they have changed over geological time.

Jean-Baptiste Lamarck (1744-1829) proposed that organisms evolve through: (1) use and disuse — organs that are used develop; those unused shrink; and (2) inheritance of acquired characteristics — these changes are passed to offspring.

Darwin's voyage on HMS Beagle (1831-1836) took him to South America (including the Galapagos Islands), where he collected thousands of specimens of plants, animals, and fossils that provided crucial evidence for his theory of evolution.

A threatened species is one that is likely to become endangered in the foreseeable future. The IUCN Red List categorises species as: Least Concern, Near Threatened, Vulnerable, Endangered, Critically Endangered, Extinct in Wild, and Extinct.

Endangered species can be protected by: ex situ conservation (zoos, botanical gardens, seed banks — preservation outside natural habitat) and in situ conservation (national parks, wildlife reserves, protected landscapes — preservation in natural habitat).

If q² = 0.16 (recessive homozygotes), then q = 0.4 (frequency of recessive allele). Since p + q = 1, the frequency of the dominant allele p = 1 - 0.4 = 0.6. Wait — p = 0.6, not 0.84. Answer A (0.84) may be a typo for q²+2pq=0.84.

Mutation is the ultimate source of all genetic variation and new alleles. Without mutation, there would be no raw material for evolution. All other evolutionary forces (selection, drift, gene flow) act on variation created by mutation.

Genetic drift is the random change in allele frequencies due to chance sampling events, especially significant in small populations. The founder effect and bottleneck effect are special cases of genetic drift.

An endangered species faces a very high risk of extinction in the wild. Threatened species are likely to become endangered. Vulnerable species face a high risk. Extinct species no longer exist anywhere on Earth.

Natural selection acts directly on the phenotype (the observable characteristics) — organisms survive or die based on their physical and physiological traits. Selection indirectly affects the genotype by favouring alleles that produce adaptive phenotypes.

Hardy-Weinberg equilibrium requires the absence of: mutation (creates new alleles), migration/gene flow (introduces/removes alleles), genetic drift (random allele frequency changes in small populations), natural selection, and non-random mating.

An allele is 'fixed' when its frequency reaches 1.0 (100%) in a population — all individuals are homozygous for that allele. Fixation eliminates all other alleles at that locus and can result from genetic drift or strong selection.

A biological species is a group of actually or potentially interbreeding natural populations that are reproductively isolated from other such groups (Ernst Mayr's biological species concept, 1942). Reproductive isolation defines species boundaries.

Vestigial organs are the oldest homologous structures — they are evolutionary remnants of organs that were functional in ancestors but have reduced or lost their function in descendants. They are evidence of evolutionary change over time.

Evolutionary relationships are reflected in both genes (DNA sequences) and gene products (proteins/RNA sequences). Closely related species share more similar DNA and protein sequences. Molecular phylogenetics uses these similarities to reconstruct evolutionary trees.

Homology is similarity in structure (and often function) due to descent from a common ancestor. Homologous structures may differ in function (e.g., human arm, whale flipper, bat wing — all forelimbs) but share the same underlying anatomy.

The Modern Synthesis (Neo-Darwinism) united Darwin's natural selection with Mendelian genetics and population genetics to create a comprehensive theory of evolution. It explains how genetic variation arises and how natural selection acts on it.

Fossils are preserved remains (bones, shells, teeth) or traces (footprints, burrows, impressions) of organisms that lived in the geological past. They provide the most direct evidence for evolution and are found in sedimentary rocks.

Population genetics (founded by Fisher, Haldane, and Wright in the 1920s-30s) mathematically modelled natural selection using Mendelian genetics, emphasising quantitative traits and allele frequency changes in populations — forming the Modern Synthesis.

Natural selection is the primary mechanism of evolutionary change proposed by Darwin. It describes how heritable traits that improve survival and reproduction become more common in a population over successive generations.

Lynn Margulis formally proposed the endosymbiotic theory in 1967 — that mitochondria and chloroplasts evolved from free-living bacteria that were engulfed by ancestral eukaryotic cells. Evidence includes their own DNA, ribosomes, and double membranes.

The first photosynthetic organisms were anaerobic bacteria (like green sulphur bacteria) that used hydrogen sulphide (H₂S) as the electron donor to reduce CO₂ — not water. This is called anoxygenic photosynthesis and does not produce oxygen.

Charles Darwin is most associated with the theory of evolution. His 1859 book 'On the Origin of Species' established evolution by natural selection as the mechanism of evolutionary change, making him the central figure in evolutionary biology.

Jean-Baptiste Lamarck (1744-1829) proposed the theory of inheritance of acquired characteristics — that traits acquired during an individual's life (through use/disuse) could be passed to offspring. Modern genetics has disproven this theory.

Thomas Robert Malthus's 'Essay on the Principle of Population' (1798) greatly influenced Darwin. Malthus argued that populations grow faster than food supply, causing competition. Darwin extended this to nature — only the fittest survive this competition.

Mutations are abrupt, heritable changes in the DNA sequence — including point mutations (base substitutions, insertions, deletions) and chromosomal mutations. Most mutations are harmful or neutral; rarely they are beneficial.

Adaptations are heritable characters (not acquired during an individual's lifetime) that improve an organism's fitness in its environment. They arise by natural selection acting on heritable variation over many generations.

'Survival of the fittest' was a phrase coined by Herbert Spencer in 1864 in 'Principles of Biology', after reading Darwin's 'Origin of Species'. Darwin later adopted this phrase in the 5th edition of his book.

'On the Origin of Species by Means of Natural Selection' was published on 24 November 1859 by Charles Darwin. The first edition of 1,250 copies sold out on the first day. It is the most important book in the history of biology.

Mutations are the ultimate source of all heritable variation — they create new alleles in the gene pool. Sexual reproduction reshuffles existing variation but cannot create new alleles. Natural selection acts on variation but doesn't create it.

Darwin's voyage on HMS Beagle (1831-1836) to the Galapagos Islands was pivotal for his theory. The variation in finch beaks across different islands inspired his ideas about natural selection and adaptation to local environments.

Evolution results in: (1) formation of new species (speciation) through accumulated heritable changes over generations, and (2) adaptation — organisms become better suited to their environments through natural selection.

'Survival of the fittest' (coined by Herbert Spencer, adopted by Darwin) means that organisms best adapted to their environment survive and reproduce — not necessarily the fastest or largest, but those with traits best suited to their particular environment.

Georges Cuvier (1769-1832) formulated the theory of catastrophism — proposing that Earth's history was marked by sudden, violent catastrophes (floods, earthquakes) that wiped out species, followed by repopulation from other areas.

Charles Darwin (1809-1882) presented the theory of natural selection in 'On the Origin of Species by Means of Natural Selection' (1859). It remains the most important unifying principle in biology.

Charles Darwin proposed the theory of evolution by natural selection in 1859 in 'On the Origin of Species'. Alfred Russel Wallace independently conceived the same idea. Together they presented the theory to the Linnean Society in 1858.

The wings of kiwi and ostrich are vestigial — they are evolutionary remnants of functional wings in their ancestors. They retain the basic structure of wings but are too small to perform the function of flight. They serve no useful aerodynamic function.

Analogous organs perform similar functions but have different evolutionary origins (different structure). Bat wings and insect wings are analogous — both used for flight but evolved independently from different ancestral structures (convergent evolution).

Natural selection is Darwin's proposed mechanism of evolution — organisms with heritable traits better suited to their environment survive and reproduce more successfully, gradually changing the characteristics of populations over generations.

Weismann's germ plasm theory proposed that only hereditary (germinal) characters are inherited — traits encoded in germ cells (sperm/egg). Acquired characters (developed during an individual's life) are NOT inherited, refuting Lamarck's theory.

Vestigial organs in humans include: plica semilunaris (remnant of nictitating membrane), wisdom teeth, body hair, coccyx (tail bone), palmaris longus muscle, and auricular muscles. Skin hair is a vestigial remnant of fur for insulation.

According to evolutionary theory, accumulated heritable changes over many generations can lead to the formation of new species. This is speciation — the process by which populations diverge and become reproductively isolated.

This describes Lamarck's theory of use and inheritance — he proposed that the giraffe's neck became longer because ancestors stretched to reach higher leaves (use) and this acquired trait was passed to offspring. Lamarck was wrong; Darwin's natural selection correctly explains this.