Revise Diversity of organisms for IB Biology (A3.1)

The most widely used definition is the biological species concept: a species is a group of organisms that can interbreed and produce fertile offspring. Read that carefully, because both halves carry weight. It is not enough for two organisms simply to mate, and it is not enough for them to produce any offspring at all — the offspring must themselves be fertile. That fertility requirement is what keeps a gene pool connected: alleles can pass from one generation to the next and spread through the whole population, so the group evolves as a single unit. When two populations can no longer exchange fertile offspring, gene flow between them stops, their gene pools drift apart, and biologists recognise them as separate species.

The biological species concept works beautifully for most sexually reproducing animals, but it has clear limits — and examiners love to probe them. There are four classic situations you should be able to describe. First, asexually reproducing organisms: bacteria and many plants reproduce without mating, so the question 'can they interbreed?' has no meaning and the concept simply cannot be applied. Second, hybrids: closely related species can sometimes mate to produce a hybrid, such as the mule (from a female horse and a male donkey) or the liger (lion and tiger), but these hybrids are almost always sterile because the two parental chromosome sets cannot pair correctly at meiosis. Third, ring species: a chain of neighbouring populations in which each can interbreed with its neighbours, yet the two ends of the chain, where they overlap, can no longer interbreed — so there is no single line at which one species becomes another. Fourth, chronospecies: species defined from the fossil record as a lineage changes gradually over vast spans of time; the organisms are long extinct and separated by millions of years, so no interbreeding test is possible and the boundary between one chronospecies and the next has to be drawn arbitrarily.

No two members of a species are identical, and this variation within a species (intraspecific variation) — for example the range of human height or the different shell patterns in a snail population — is the raw material on which natural selection acts. Its sources are worth stating precisely: mutation is the ultimate origin of new alleles, while meiosis (through crossing over and independent assortment) and random fertilisation during sexual reproduction reshuffle those alleles into an almost limitless number of new combinations. Variation between species (interspecific variation) is generally far greater, reflecting long, separate evolutionary histories and the accumulation of different mutations under different selection pressures. Recognising the difference matters: intraspecific variation exists within one shared gene pool, whereas interspecific variation lies between gene pools that no longer exchange genes.

To refer unambiguously to any species, biologists use binomial nomenclature — a two-part Latin name devised so that scientists in every country name the same organism the same way. The first part is the genus, always written with a capital letter; the second is the species epithet, always lower case. Both parts are italicised in print, and underlined when handwritten (since you cannot italicise by hand). Thus the human species is Homo sapiens and the domestic dog is Canis familiaris. Once the full name has been given, the genus may be abbreviated to its initial — H. sapiens — but the species epithet is never used alone. These conventions are not decoration: they make the name unique and instantly recognisable, and they encode relationship, because species that share a genus name are close relatives.

The variety of life divides at the most basic level into two cell types. Prokaryotes — the bacteria and archaea — have no nucleus and no membrane-bound organelles; their DNA sits free in the cytoplasm. What they lack in cellular complexity they make up for in metabolic diversity and sheer numbers: prokaryotes photosynthesise, fix nitrogen, respire with or without oxygen, and thrive in boiling springs, deep rock and acidic pools where nothing else survives. Eukaryotes — animals, plants, fungi and the diverse protists — have a true nucleus and membrane-bound organelles, and they span an enormous range of size and form, from single-celled algae to whales and giant trees. Appreciating this breadth is the point of A3.1: life is not just abundant but astonishingly varied in structure, chemistry and lifestyle, and every one of these forms is a species that could, in principle, be named and placed.

Diversity is not random — it is generated and maintained by the environment acting through natural selection. Different habitats impose different selection pressures: temperature, water availability, predators, competitors and food supply all differ from place to place, so the individuals best suited to one environment leave the most offspring there, and populations gradually accumulate different adaptations. Because an environment offers many distinct niches — ways of making a living — many species can coexist without directly competing, and along environmental gradients (up a mountain, down a shore, across a rainforest) the community changes as conditions change. Over long timescales this same process, working on isolated populations, drives the origin of new species. The living variety we survey is therefore the accumulated product of countless environments selecting for countless different solutions.

When ecologists want to describe how diverse a community is, they combine two ideas. Species richness is simply the number of different species present — a count. Species evenness describes how equally the individuals are distributed among those species. The two are independent: a woodland with ten tree species split evenly is more diverse than one with the same ten species where a single species makes up ninety per cent of the trees, even though their richness is identical. True biodiversity takes both into account, which is why a bare species count can be misleading. In practice, richness and evenness are estimated from field sampling — you rarely count every individual — and combined into a diversity measure so that habitats can be compared or monitored over time. For A3.1 the emphasis is understanding what richness and evenness mean and why both matter, rather than performing a particular calculation.

A3.1 questions are marked analytically: each distinct, valid biological point is worth one mark, up to the total available. Method-style reasoning marks (M) credit correct biological reasoning, answer marks (A) credit a correctly stated example or conclusion, and error-carried-forward (ECF) means a slip early on does not automatically forfeit the marks that follow, as long as your reasoning is shown. Equivalent correct wording is always accepted — the engine rewards the idea, not a fixed phrase. Study how each mark below attaches to one specific, named idea.

The ideas in A3.1 underpin much of the rest of biology. The species concept and its limits set up classification and the tree of life; variation within species is the fuel for the evolution and natural selection you meet later in Unit A; and the survey of prokaryotic and eukaryotic diversity connects forward to ecology, where richness and evenness become quantitative tools for conservation. Master the habit this topic teaches — define a species by fertile interbreeding, then check the awkward cases, and name every organism precisely — and you have a framework that organises the whole of the living world.