地球上的生物成千上萬 ，為了方便研究，科學家都會把各種物種分類、命名。物種以不同級別分類，順序為：域、界、門、綱、目、科、屬、種，物種的學名則是取其屬名及種名合併而成。然而，還有亞種、變種、栽培品種等，它們又是甚麼呢？

在介紹之前，我們必須為「物種」下定義。一般對「物種」的定義，是指一類形態和遺傳組成相似、在自然狀態下能產生正常後代的生物。

 

亞種

「亞種」在動植物界的分類都適用。當同一物種的個體擁有交配並產生正常後代的潛力，在自然中卻因為不同因素（例如地理分隔）而沒有這樣做時，我們便可把這兩群生物分類為兩個亞種。因此，一個物種要不沒有亞種，要不有兩個或以上亞種，而不會只有一個亞種。

在表達動物的亞種名稱時，直接把亞種名加在物種名後面。有時候，分類學家無法取得共識，不確定動物應該分為物種還是亞種時，則會在種名加上括號。

不少動物都有亞種。例如銀腳帶有兩個亞種，Bungarus multicinctus multicinctus 和 Bungarus multicinctus wanghaotingi。彩龜則有三個亞種，巴西龜（Trachemys scripta elegans）是其中一個，另外兩個分別是 Trachemys scripta scripta 和 Trachemys scripta troostii。 牠們都可以互相交配誕下正常、具生育能力的後代，但因為居住地點和形態結構等差異，在自然程況下不會互相生育。

至於植物，命名時需在種名後先加上subsp.（不需斜體），然後才加亞種名。例如慈姑 Sagittaria trifolia subsp. leucopetala，以及白千層 Melaleuca cajuputi subsp. cumingiana。

 

變種

「變種」用於植物分類，是在「物種」之下的級別。一般對變種的定義，是兩種物種相同、形態稍有差異，但仍可以互相繁殖產生雜交種的植物，而差異多是環境、地理位置等造成。

在命名變種時，會在物種名後加上var.（不需斜體），然後是變種名。例如朝天椒 Capsicum annuum var. conoides ，跟它同種的還有 Capsicum annuum var. glabriusculum 。

 

栽培品種

「栽培品種」亦是用於植物分類 。它指的是一組為了保留某些特徵而人工培殖出來的植物。「栽培品種」的英文 cultivar，便是源自cultigen（經人工選擇的植物）和 variety （變種）。

栽培品種的命名受《國際栽培植物命名法規》約束。表達名稱時，先寫植物的學名，然後加上栽培品種名，每字開頭皆要大寫，並置在單引號內，不用斜體。1959年以前的栽培品種名都是拉丁文，但因容易與學名混淆，改成使用本地語言或現代英文。

現今大部分的園藝植物及農作物都是栽培品種，例如日本柳杉的栽培品種Cryptomeria japonica 'Elegans Compacta'（拉丁文），美國扁柏的 Chamaecyparis lawsoniana  'Golden Wonder' （英文）及 赤松的 Pinus densiflora 'Akebono' （日文）。

 

其實，如何分類和命名物種，常常因不同學者有不同見解而有異，而且亦會隨著新的發現而改變。

 

There are millions of living beings on earth. To better study them, scientists classify and name different organisms with a series of taxonomic hierarchy. From broad to narrow they are: Domain, kingdom, phylum, class, order, family, genus and species. A species’ name is given by combining the genus name and the species name. But there are also subspecies, variety and cultivar. What are they exactly?

Before we begin our introduction, it is necessary to define the term ‘species’.  The generally accepted definition is ‘a group of organisms with similar morphology and genetic materials that can, under natural circumstances, produce normal offspring’.

 

Subspecies

‘Subspecies’ is used in both animal and plant classification. When individuals of the same species have the potential to produce normal offspring with each other, but do not naturally do so due to factors such as geographic isolation, they can be seen as two subspecies. Hence, a species either have no subspecies or more than one subspecies, never just one.

To express the name of an animal subspecies, the subspecies name is added behind the species name. There are times when scientists cannot reach an agreement on whether to perceive an animal as a species or a subspecies. In this case, the species name is enclosed in parenthesis.

Quite a number of animals have subspecies. For example, the Many-banded Krait has two, Bungarus multicinctus multicinctus and Bungarus multicinctus wanghaotingi. The Pond Slider, on the other hand, has three. One of them is the Red-eared Slider (Trachemys scripta elegans), while the other two are named Trachemys scripta scripta and Trachemys scripta troostii respectively. They can interbreed to produce normal fertile offspring but do not due to geographic and morphological differences.

As for plants, the abbreviation ‘subsp.’ (non-italicized) has to be added before adding the subspecies name. Examples are the Chinese Arrow-head (Sagittaria trifolia subsp. leucopetala) and the Paper-bark Tree (Melaleuca cajuputi subsp. cumingiana).

 

Variety

‘Variety’ is used in plant classification. It is a category under ‘species’.  Generally it is defined as plants of the same species which have certain differences but can still reproduce with each other to produce hybrids. The differences are commonly caused by the location of the plants.

When naming varieties, the abbreviation ‘var.’  (non-italicized) is added after the species name, followed by the varietal name. An example is the Long Pepper (Capsicum annuum var. conoides), which is under the same species as another variety Capsicum annuum var. glabriusculum.

 

Cultivar

‘Cultivar’ is also used for plant classification. It refers to a group of plants artificially cultivated to retain certain characteristics. The term ‘cultivar’ is a combination of ‘cultigen’ (meaning plants that have undergone artificial selection) and ‘variety’.

The International Code of Nomenclature for Cultivated Plants regulates the naming of cultivars. When naming a cultivar, the scientific name of the plant is first written.  Then the cultivar epithet is added, enclosed in single quotation marks with every word capitalized. Unlike the scientific name, it is not italicized. Before 1959, all cultivars were given Latin names. However, since it was easy to confuse it with the scientific name, modern English and local languages are used later.

Today many garden plants and crops are cultivars, such as that of the Japanese Cedar Cryptomeria japonica ‘Elegans Compacta’ (Latin), that of the Lawson’s Cypress Chamarcyparis lawsoniana ‘Golden Wonder’ (modern English), and that of the Japanese Red Pine Pinus densiflora ‘Akebono’ (Japanese).

 

In reality, the classification and naming of organisms are often disputed, and change as new discoveries are made.