REVIEW ARTICLE

What good is a long neck for a giraffe?

Evolutionary insights revisited

S. Sreekumar

Department of Zoology, University College, Thiruvananthapuram, Kerala, India

Corresponding author: S. Sreekumar, Email: ssreekumar53@gmail.com

Journal of Experimental Biology and Zoological Studies. 1(2): p 57-70, Jul-Dec 2025.

Received: 30/05/2025; Revised: 15/06/2025; Accepted: 17/06/2025; Published: 01/07/2025

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Abstract

The evolution of the giraffe's neck is a complex process involving several anatomical, physiological,

and behavioural modifications. However, the reasons behind the evolution of neck in giraffe and the

associated evolutionary events remain unresolved. Various theories explaining evolution in general and

giraffe’s neck evolution in particular are discussed in the context of present-day evolutionary genetics.

This review also critically evaluates the scientific validation and logic of these theories.

Keywords: Giraffa camelopardalis, Darwinism, Lamarckism, variations, genetic drift, founder effect,

bottle neck phenomenon, mutation theory, neural theory, population genetics.

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Introduction

The giraffe, with its long neck, legs, and coat pattern of irregular patches, is a fascinating animal.

Giraffes are the tallest land animals, with males (bulls) typically growing taller and heavier than females

(cows). Males can reach heights of over 5.5 m and weigh up to 1,930 kg, while females are

approximately 4.5 m tall and weigh up to 1,180 kg.[1] It has also the largest eyes among land animals

and these are placed posteriorly to ensue more field vision.[2] Unlike most other mammals, it is capable

of colour vision. The tail is about 1 m in length. Giraffes possess a pair of horns. The coat patches

provide excellent camouflage, offering some protection from predators. Giraffes do not sweat or pant;

instead, thermoregulation is facilitated by the coat patches, which may function as 'thermal windows'

due to the presence of vascular plexuses beneath them. [2,3]

Giraffes reach their full size by the age of four. They are capable of running at speeds of up to 50 km/h.

They live in non-territorial groups of up to 20 individuals. Their gregarious nature enables them to

remain vigilant against predators. With excellent eyesight, they can spot an enemy from one kilometre

away and they have a lifespan of up to 26 years. The gestation period lasts 15 months. The newborns

are about two metres tall, weighing approximately 100 kg.[1]

Giraffes are primarily found in the grasslands and open woodlands of East Africa. They are also

protected in certain reserves in southern Africa. The West African subspecies has confronted a

significant population decline and is now confined to a small range in Niger.[1] Traditionally, giraffes

have been classified as a single species, Giraffa camelopardalis, which is further divided into nine

subspecies (Figure 1). However, mitochondrial DNA studies have revealed genetic uniqueness in four

or even six groups, making each group distinct from the others, likely due to reproductive isolation.[4,5]

According to a survey, giraffe populations declined by 36 - 40% between 1985 and 2015 due to habitat

loss and illegal hunting.[4] Recently, the International Union for Conservation of Nature (IUCN)

classified the species as "vulnerable."[5]

The long neck of the giraffe: A case study in evolution

The long neck of the giraffe is often cited as an example supporting both Lamarckian and Darwinian

concepts of evolution.[6]

Lamarckian theory and explanations

According to Lamarck, giraffes inhabited environments where surface vegetation was insufficient to

sustain large populations. Over generations, they stretched their necks to feed on foliage from taller

Figure 1. Giraffa camelopardalis: This giraffe subspecies inhabits the grasslands of South Africa. In the image on the right,

the giraffe's neck is held upright, while in the image on the left, the neck is positioned at shoulder level.

trees. This repeated stretching became a habit, and over time, the elongated neck developed as an

acquired trait, which was then inherited by subsequent generations. Lamarck's theory suggests that

acquired characteristics become heritable. This is known as the theory of the inheritance of acquired

characteristics. A supplementary proposition to this is the theory of use and disuse, which states that the

evolutionary development of an organ depends on its use. With constant use, an organ becomes more

developed, whereas disuse leads to its degeneration or even disappearance; often cited examples being

the vestigial organs. In the case of the giraffe, the argument was that neck elongation occurred because

of its constant use.[6]

Darwinian theory of natural selection with explanations

According to Darwin, nature poses several challenges to organisms in the form of harsh climatic

conditions, food scarcity, and natural disasters such as floods and droughts. In such situations, traits that

enable organisms to survive these challenges better than others in the same environment are said to be

selected by nature. Thus, nature favours traits that are better suited to overcome environmental

challenges, and this process is referred to as natural selection.[6]

Darwinian theory explained that giraffes evolved from horse-like ancestors with shorter necks. In

habitats dominated by tall trees and sparse ground vegetation, giraffes with longer necks had a

competitive advantage, as they could access food unavailable to shorter-necked individuals. Over time,

long-necked giraffes thrived, while shorter-necked ones were outcompeted and eventually eliminated.

Thus, long-necked giraffes were naturally selected.[6]

Alternative explanations

The explanations offered by Lamarck and Darwin are considered speculative and lack direct scientific

evidence. Several such speculative theories can be proposed for explaining the evolution of giraffe’s

long neck. For example, let us consider another possibility. In a population of deer-like animals, a

dominant mutation caused neck elongation in some individuals, allowing them to feed on tall foliage.

These mutated individuals became reproductively isolated, eventually forming a new species.[7-9]

Advantages and disadvantages of a long neck

The elongated neck of a giraffe provides distinct advantages, such as access to food that is unreachable

for other animals. However, it also presents challenges and causes several disadvantages some of which

are discussed below:

Anatomical adaptations

Giraffes' neck consists of seven cervical vertebrae as in other mammals, each measuring about 30 cm,

compared to 5 cm in similar-sized even-toed ungulates like buffalo. Elongation of the seven cervical

vertebrae alone is not the single event that happened during the evolution of giraffe. It necessitates

several anatomical and physiological modifications during evolution. To support its massive 2.5 meter-

long, 250-kg neck and head, the muscles and ligaments must be strong.[7] Giraffes have large ligaments

known as nuchal ligaments (ligamentum nuchae) that run from the back of the neck to the base of the

tail. These ligaments function like a giant elastic band, pulling the neck back over the front legs to

maintain an upright position. [1,2] The sturdy forelegs further aid in supporting the neck. Limb bones are

strengthened through increased mineralization, thereby achieving higher bone density, rather than an

increase in diameter of the bones. In fact, 80% of skeletal calcium is deposited in the leg bones.[2] The

tongue, which is about 50 cm long, along with the prehensile upper lip, allows the giraffe to browse

leaves from tall trees.

Postural challenges during drinking

Giraffes must lower their heads to drink, which presents several challenges. While their long necks are

advantageous for reaching high foliage, they are not long enough to reach ground-level water sources.

To drink, giraffes splay their front legs sideways and bend down, lowering their bodies to access the

water. This awkward posture makes them vulnerable to predators such as lions and crocodiles. As a

result, they tend to drink quickly, taking large gulps, up to approximately 54 litres in one go. Despite

their size, giraffes can survive for up to three weeks without drinking. Lowering their heads also causes

a rapid change in blood pressure, which is managed through specialized physiological and anatomical

adaptations.[10]

Blood flow

The blood pressure must be maintained high for supplying blood to the head i.e., up to a height of 2.5

to three metres (8-12 feet) and this is achieved by increasing the heart rate. The heart is very large,

weighing roughly 10-11 kg and could fill four litres of blood. The heart beats at 150 bpm to pump blood

up to the head.[10] Furthermore, the blood vessels in the neck are made extra thick to withstand the high

blood pressure and to prevent rupturing of blood vessels. The blood vessels expand when the giraffe

lowers its head and contract when head is raised again. These changes allow a controlled flow of blood

during upward and downward movements of the head and prevents fainting. A maze of blood vessels

known as rete mirabile present at the base of the brain acts like a sponge and soaks up blood for

controlling blood volume and regulates blood flow to the brain when the animal lowers its head.

Similarly, special ‘control valves’ are present in the jugular veins.[8] These valves prevent blood from

draining too quickly when the head is raised.[11]

Another challenge is managing the high blood pressure in the legs, which could force blood out of the

capillaries. This is addressed by filling the intercellular spaces with fluid under high pressure, a

mechanism further supported by the giraffe’s highly impermeable and tough skin. The fibrous

connective tissue, known as the inner fascia which is associated with the skin also helps prevent blood

pooling. Additional adaptation to prevent profuse bleeding is the positioning of all arteries and veins

deep within the giraffe’s legs, with only extremely small capillaries distributed superficially.

Furthermore, the red blood cells are small, only about one-third the size of human RBCs, which help

them manage the high blood pressure and facilitate easier passage through the narrow capillaries,

particularly in the brain and extremities. The smaller red blood cells also provide a greater surface area,

allowing for a higher and faster rate of oxygen absorption into the blood, ensuring an adequate oxygen

supply to all parts of the body, including the head. These adaptations appear to be interactive and

interdependent with the giraffe’s long neck.[12]

Regurgitation

As ruminants, giraffes regurgitate cud for further digestion which requires strong muscular contractions

to move food back to the mouth through the lengthy oesophagus. The oesophagus is, therefore, very

much muscular.[10,12]

Respiration

The long trachea increases the volume of dead air, i.e., the air that does not participate in gas exchange.

To compensate for this, giraffes have larger lungs and slower breathing rates. When a giraffe takes a

new breath, the oxygen-depleted air from the previous breath is not fully expelled. The larger lungs help

mitigate this issue by ensuring enough lung volume so that the dead air is accommodated as a small

percentage of the total air inhaled.[10,12]

Mother giraffes also face challenges during childbirth, as the calf's neck is proportionally as long as that

of an adult.

Food-based explanation for long necks

The question why giraffes developed long necks still remain unresolved. The explanation that it evolved

to help giraffes reach tall trees has been refuted by many biologists, even during Darwin’s lifetime.

Notably, Darwin himself did not use the giraffe as an example in the first edition of On the Origin of

Species. He later included it in the sixth edition, responding to the criticisms raised by Mivart who

accepted evolution, nevertheless expressed disagreement with the concept of natural selection. [11,12]

Darwin was well aware of the problems associated with the extraordinary evolution of the neck of the

giraffe that required modifications of several parts simultaneously and acknowledged the difficulties in

explaining this through natural selection. He himself conceded at one point that ‘on the principle of

natural selection this is scarcely possible’.[13]

Both Lamarckian and Darwinian theories suggest that the giraffe’s long neck evolved as an adaptation

for feeding on twigs and leaves from tall trees, though they differ in the mechanisms proposed for this

evolutionary process. Critics of the food-based explanation pointed out that female giraffes, being

shorter by about two feet, would face disadvantages during droughts.[11,14] Despite this, they survive

quite well. Similarly, as per this view, young giraffes up to four years of age would also struggle to

gather food due to their shorter necks.[15]

It is known that, although adult giraffes prefer Acacia leaves during the wet season, they also browse

on many other plants, including bushes, shrubs, and low-growing grasses, during the dry season.

Generally, giraffes prefer to feed at shoulder height, which is about 60% of their maximum height.

Young and Isbell have observed that the preferred feeding heights vary depending on the male-female

composition of the group.[16] Females in female-only groups feed at a height of 1.5 m, while those in

male groups feed at 2.5 m. Meanwhile, males in male groups feed at 3.0 m. This indicates that a height

of 3 m is sufficient for giraffes to outcompete all other ungulate browsers. If so, there seems to be no

reason for giraffes to evolve to a height of 5 m. [2,5] Likewise, the observation that both male and female

giraffes frequently bend their necks to browse on plants at lower levels suggest that longer necks are

not specifically evolved for feeding on leaves from tall trees.[17] Gould asks “if such a habit is so

beneficial why many other animals such as antelopes also evolved the same ability?”[18]. A

counterargument is that the various species of giraffids (e.g., sivatheres) evolved when they coexisted

with large herbivore competitors such as mastodonts, deinotheres and baluchitheres, that later became

extinct. [5,19]

The "Neck-for-Sex" hypothesis

Male giraffes engage in neck-based dominance contests, a behaviour known as "necking". It involves

fights between adult males that may potentially influence reproductive success. This theory suggests

that longer necks provide an advantage during necking.[20] If the long neck evolved in response to

intersexual competition, it can be reasonably assumed that necking behaviour evolved first, and neck

elongation followed it as a result of selection. Conversely, necking would be ineffective until giraffes

had sufficiently long necks. This implies that there is no direct relationship between the evolution of

necking behaviour and neck length. Mitchell et al., observed that the differences in neck morphology

between male and female giraffes are minimal. The longer necks in males can be attributed to their

greater body mass. According to their findings, sexual selection is unlikely to be the driving force behind

the evolution of long necks in male giraffes.[21]

Other explanations

Increased height increases the reach of vision and vigilance. According to some evolutionists, the

giraffe’s long neck, coupled with its excellent vision, may enable it to spot predators far away. [1] While

this is true, it must be noted that giraffes have virtually no enemies except lions. However, lions

typically attack giraffes only when they are desperate, and the giraffe is in a vulnerable position, such

as when drinking water with its legs spread sideways. Giraffes defend themselves primarily by

kicking.[9] A kick from their heavy, hooved, long legs can be deadly to predators. According to Hitching,

a lion is no match for a 900-kilogram giraffe, as a single blow from the giraffe’s hoof can kill a lion.[8]

He states, “This explains why they supposedly evolved long legs… but not why they evolved long

necks.”

Other reasons cited for the giraffe's long neck include thermoregulation and facilitating forward travel

at high speeds.[9] Gould concludes that “the giraffe’s neck cannot provide evidence for any specific

adaptive scenario, Darwinian or otherwise. Truth be told, the giraffe’s neck serves better as an example

of the many challenges in explaining evolution through Darwinism”.[9,22]

How did neck elongation happen in giraffes?

The giraffe (Giraffa camelopardalis) is the only living member of the genus Giraffa, and there is no

evidence that any animal similar to it ever existed. Nine subspecies of Giraffa camelopardalis are

recognized. Neck elongation in giraffes is believed to have begun around 14 million years ago, with the

ancestors of modern giraffe emerging approximately five million years ago. [23,24] The fossils of Giraffa

camelopardalis recovered from East Africa revealed that they were one million years old.[2]

Regarding the evolution of giraffes, although a significant number of giraffe bone fossils have been

recovered, the individual bones are more or less of the same shape and size. Studies involving the dating

of existing fossils suggest that the giraffe has remained largely unchanged for about at least one million

years. In giraffes, the cervical vertebrae and leg bones are greatly elongated. However, no fossils have

yet been recovered to provide insights into the step-by-step evolution of the giraffe’s neck and leg

bones.[9] Additionally, the origins of all three major lineages of pecorans which include giraffes, deer,

and cattle remain unclear due to significant gaps in the fossil record. It is believed that at the beginning

of the Pleistocene, giraffes inhabited large parts of Eurasia and Africa. Some evolutionists speculate

that the ancestor of the giraffe was an elk-sized animal called Palaeotragus, whose fossils were

recovered near Athens. Palaeotragus is thought to be an early giraffid that gave rise to two groups of

descendants during the Pleistocene. One of these groups included the heavy-bodied sivatheres, which

were about the size of elephants and once roamed Africa and India.[25] Sivatheres had short necks and

elaborate horns (ossicones) resembling palmate or flat antler-like structures. Their bones were generally

only half as long as those of modern giraffes. A second branch of the sivathere group is hypothesized

to be the ancestor of the family Giraffidae, with the giraffe evolving as a separate lineage during the

Miocene epoch. Another animal proposed as the primitive ancestor of the giraffe is Samotherium, a

deer-like creature with slightly longer necks. It has also been suggested that giraffes evolved from

cervoids (superfamily: Cervoidea), which were deer-like animals with side toes, an anatomical feature

absent in giraffes. Probably, these side toes may have been lost during evolution.[9]

The only extant giraffid besides the giraffe is the rare, forest-dwelling okapi (Okapia johnstani), which

is confined to central Africa. It is also called the ‘forest giraffe’. The okapi has distinctive black and

white striped markings on its buttocks, thighs, and the tops of its forelegs, reminiscent of a zebra's

pattern. It is the only species in the genus Okapia.[4] The okapi and the giraffe are the two living members

belonging to the family Giraffidae. The primitive giraffe is thought to be a fast running, and somewhat

large animal similar to okapi, measuring approximately 1.6 meter at the shoulder. Like the giraffe, there

is no fossil evidence indicating evolution of okapi. Okapi is often regarded as a "living fossil" because

it has remained essentially unchanged as per fossil records for several millions of years.[9] It is the

closest and the only living relative of the giraffe.

In the absence of clear empirical evidence, evolutionary biologists have proposed various theories,

linking giraffes to several dissimilar animals. Despite the abundance of fossil remains of related species,

there is no conclusive evidence to support the existing speculations regarding the evolution of the

giraffe. Meanwhile, these speculations have led to numerous controversies, and the evolution of the

giraffe remains a debated and unresolved topic.[9]

Darwin’s work

It was Charles Darwin’s (1809 - 1882) five-year voyage on the Beagle that sparked his evolutionary

thinking. The expedition covered South America and the Galápagos Islands off the coast of Ecuador.

His one-month stay in the Galápagos Islands was probably one of the most significant and productive

periods of his life, as it helped him understand the distribution of diverse animal groups. This was

particularly striking in the Galápagos, where each island had its own unique yet closely related

assemblage of species. The fauna characteristic of each island was distinct and appeared different from

that of the mainland. In Galapagos islands, he observed finches with differently shaped beaks. These

observations led him to believe that “present species resembled past species and that different species

shared similar structures” and also that “one species had been taken and modified for different ends”

indicating evolution as the cause for the formation of the species.[6] Darwin was profoundly influenced

by reading an Essay on ‘The Principle of Population’, published by Thomas Malthus (1766 -1834), a

British economist. Malthus argued that while food supplies necessary to sustain a population increase

arithmetically, populations themselves grow geometrically, resulting in inevitable food shortages.

Famine, war, and disease, according to Malthus, act as natural controls to limit population growth. This

idea inspired Darwin to ponder that the tendency of species to reproduce more offspring than available

resources could sustain would lead to competition among individuals in animal populations. In this

"struggle for existence," animals with favourable variations would have a higher chance of survival and

reproduction, while those with unfavourable traits would be gradually eliminated through a natural

selection process. Over time, this mechanism could lead to the emergence of new species. Alfred Russel

Wallace, a naturalist who specialized in collecting various species of animals, independently arrived at

a similar theory of natural selection, after also reading Malthus's work. Wallace shared his theory with

Darwin, and in 1858, the two jointly published a short paper on natural selection in The Journal of the

Linnean Society. However, the publication received little response from the scientific community, likely

because it lacked substantial supporting evidence.[6] In 1859, Darwin published his landmark book, On

the Origin of Species, which provided extensive evidence and documentation supporting the concept of

evolution through natural selection.[26] This work was well-received by both naturalists and the general

public, marking a pivotal moment in the history of evolutionary biology.

Criticisms

The publication of Darwin’s book sparked enthusiasm among naturalists and made evolution a widely

debated topic in academic circles. However, theologians and clergy of the Anglican Church accused

Darwin of attempting to undermine belief in God, dismissing his ideas as a brutal philosophy designed

to tarnish Christianity. One notable debate occurred between Bishop Samuel Wilberforce and Thomas

Huxley, a passionate advocate of Darwinism. The discussion became personal when the bishop

mockingly asked Huxley whether his grandparents were descended from monkeys. Huxley retorted

that, if given the choice, he would unhesitatingly prefer to have an ape as his ancestor rather than a

bishop who misused his position to oppose scientific progress.[6]

While the scientific community largely accepted the concept of evolution that all living beings evolved

from a common ancestor through the development of favourable variations and their preservation by

natural selection, there were differing opinions regarding the specifics of Darwin’s process of natural

selection.[7]

The concept of struggle for existence

Malthus’s views on human population growth and its associated problems influenced Darwin, who

incorporated some of these concepts into his theory. For example, Darwin envisioned a struggle for

existence among individuals in animal populations, as a consequence of geometric population growth.

While sudden population increases are uncommon, occasionally such eruptions happen, such as pest

outbreaks. However, these are typically transient and have little impact on evolution. The dynamics of

animal populations depend on various characteristics and ecological factors. For instance, a direct

relationship exists between prey and predator populations. Every population has a self-regulating

system, preventing its size from exceeding the habitat’s carrying capacity.[27] Regarding the struggle for

existence, it can be assumed that successful populations tend to avoid competition through various

strategies, such as migrating to unexplored habitats or exploiting new food sources. This explains why

a single host plant can serve as food for multiple insect species. For example, more than 72 pest species

are reported for paddy, and over 109 pests for cotton.[28] Another example can be drawn from a study

on the feeding behaviour of giraffes. Research conducted by Cameron and du Toit (2007) reveals that

giraffes, by feeding primarily on the upper foliage of trees, leave the lower bushes and grasses available

for other ungulate browsers sharing the same habitat. This behaviour helps reduce competition for food

resources. [29,30]

In Darwinian theory, individuals within a population are tasked with competing for survival, with the

fittest prevailing while others perish. However, in reality, individuals play no direct role in evolution

beyond contributing desirable alleles to the gene pool. An individual organism cannot evolve into a new

species or produce offspring of a new species simply by engaging in and winning the struggle for

existence. In fact, it is the population that evolves into new species by shifting the frequency of

favourable mutated alleles over time.[31]

Darwin's concept of heredity

Darwin had no understanding of the mechanism of heredity, which made it difficult for him to explain

the variations observed among different animal groups. At that time, genetics as a branch of biology

had not yet been developed. In fact, Mendel published his findings on the mechanism of heredity in

garden peas in 1862, three years after Darwin published his book on evolution. Despite this, Darwin

proposed a theory called Pangenesis to explain variability among animals.[6] According to this theory,

each part of an organism releases minute particles called "gemmules" into the circulatory system. These

gemmules, representing the organism's traits, are believed to reach the gonads, where they are multiplied

and transferred to the offspring during reproduction. However, this theory was both unconvincing and

unnecessary and was ultimately rejected by the scientific community.

It was August Weismann (1834 -1914) who disproved the theory of pangenesis through an experiment.

[6,32] He amputated the tails of mice for 22 consecutive generations and demonstrated that the offspring

in each generation were born with fully intact tails. This experiment conclusively showed that the

inheritance of tail was not influenced by the presumed loss of "tail gemmules." Additionally, this

experiment disproved the concept of the inheritance of acquired characteristics proposed by Lamark.

Weismann also proposed an alternative theory of inheritance known as the Germ Plasm Theory. This

theory postulated that hereditary factors for the entire organism are transmitted exclusively through the

germ plasm; i.e., the reproductive tissues of the testes and ovaries. It further suggested that changes

occurring in the somatoplasm (i.e., non-reproductive tissues) are not inherited. This implies that traits

acquired through constant use or environmental influence in somatic tissues cannot be passed on to the

next generation.[32]

Darwin’s evidence for natural selection

Darwin lacked examples of natural selection in action and presented examples that he himself said were

“imaginary Illustrations”.[7] He, however, had some indirect evidence. For instance, selective breeding

of pigeons produced diverse breeds, all descended from wild rock pigeons.[6] The problem with artificial

selection is that it relies on human choice rather than natural processes. In this method, breeders select

parents with desirable traits to produce offsprings while culling or eliminating those with undesirable

traits. Through continued selection, offspring with the desired traits are consistently produced in almost

all cases. Darwin claimed that natural selection could similarly produce significant effects comparable

to or even greater than those achieved through artificial selection leading to speciation, given the longer

time scales associated with evolution. Meanwhile, there is another issue with natural selection. Darwin,

like most biologists of his time, subscribed to the concept of blending inheritance, which suggested that

heredity is a mixture of maternal and paternal contributions, akin to the blending of two colours. Critics

of natural selection argued that, according to this concept, new traits or adaptations arising through

natural selection would gradually blend away with each generation of interbreeding, thus undermining

the very mechanism of natural selection.[6] Furthermore, artificial selection cannot be equated with

natural selection, as natural selection operates on populations with random mating and does not

guarantee the inheritance of any specific traits in the offspring.

Industrial melanism

“Industrial melanism” demonstrated by Kettlewell in 1973, is considered the first scientific proof of

natural selection.[33] In the industrial city of Birmingham, he identified two distinct forms of the

peppered moth (Biston betularia) based on pigmentation: melanic (dark) and non-melanic (light) forms.

Before industrialization, the light-coloured moths were more abundant, as they were well-camouflaged

against the pale lichens on tree trunks where they rested. In contrast, the melanic forms were rare.

Following industrialization, the number of melanic moths increased. For his experiment, Kettlewell

captured a known number of both forms of moths, released them in both industrial and non-industrial

areas, and later recaptured them. His findings revealed that the proportion of melanic moths was

significantly higher in polluted industrial areas. This indicated that the melanic moths had a survival

advantage there, as they were better concealed against darkened tree trunks, making them less

noticeable to bird predators. Meanwhile, in non-industrialized areas, where trees were covered with

pale lichens, the light-coloured moths remained better protected. In other words, natural selection

favoured the melanic forms in industrial areas while favouring the non-melanic forms in unpolluted

regions. In this experiment, the non-melanic moths which were camouflaged for a light background,

were artificially exposed to predation in industrial areas, negatively impacting their survival. Therefore,

it can also be argued that industrial melanism reflects an organism's adaptive ability to remain

inconspicuous in its natural habitat rather than being solely an effect of natural selection.

Genetic variability and evolution

Morphological differences among organisms are, in general, referred to as variations. It is now known

that morphological variations are the outward expressions of genetic variability. It is also a fact that

genetic variability is a prerequisite for evolution and that evolution works on genetic variability i.e.,

without genetic variability there will be no evolution. The genetic variability is, in turn, caused by

mutations. Mutations produce different variants of a gene and these are called alleles. Evolution happens

when the frequency of individual alleles shifts to a particular pattern. In fact, any change in allelic

frequencies through generations can be considered an indication of evolution. Allelic frequencies

remain the same in nonevolving populations referred to as Mendelian populations. According to

Strickberger a population with little or no variability may become extinct within a short period if

exposed to harsh changes in environment or some ecological challenges.[6]

Continuous vs. discontinuous variations

A controversial topic in evolutionary biology was Darwin’s view of evolution as a slow and gradual

process driven by natural selection, often referred to as the gradualistic view. Even some staunch

supporters of Darwinism disagreed with Darwin’s emphasis on continuity in variation and the slow

progression of evolution. While Darwin acknowledged the existence of large, discontinuous variations

in natural populations, he regarded these as too rare to serve as the primary source of evolutionary

change.[13]

Huxley and Galton argued that for natural selection to be effective, it must act upon occasional large,

discontinuous variations.[7] This idea gained experimental support from Hugo de Vries (1848-1935),

who observed sudden appearances of traits in his stocks of the evening primrose (Oenothera) that were

previously exposed to radiation.[34] De Vries termed the mechanism responsible for these sudden

changes as ‘mutation’. He proposed that mutations in natural populations could produce abrupt

variations in traits, providing raw material for evolution. This concept became known as the mutation

theory of evolution, which introduced a sense of purpose and direction to evolutionary change.[34]

Mechanism of inheritance of traits

Further support for discontinuous variations came from Mendel’s breeding experiments with the garden

pea, Pisum sativum. Mendal’s experiments in garden pea involved contrasting traits such as round vs.

wrinkled seeds and axial vs. terminal flowers.[31] Mendel’s work established a convincing mechanism

for the inheritance of hereditary traits. According to Mendel’s theory, hereditary traits are transmitted

from parents to offspring through discrete units called “factors” (now known as alleles). Each trait is

determined by a pair of factors, one inherited from the male parent and the other from the female. Traits

differ in their expression, with some being dominant and others recessive. The combination of factors

in a zygote is random, and the expression of traits follows predictable ratios based on the dominant and

recessive nature of the factors. These ratios, known as Mendelian ratios, are derived following the rules

of probability.[31]

Populations: The units of evolutionary change

Mendelian inheritance patterns soon became essential for studying population characteristics, such as

allelic and genotypic frequencies. Mendelian ratios were modified and extended into a mathematical

framework known as the Hardy-Weinberg equation, which facilitated the understanding of genetic

composition in populations and the development of models to study changes in genetic equilibrium over

successive generations.[31] By this time, populations were considered the fundamental units of evolution.

As statistical and mathematical approaches expanded, the field of population genetics emerged as a

critical and significant branch of evolutionary studies.[31]

Other objections

Evolution through successional changes within a single lineage is called phyletic evolution. Darwin’s

emphasis on phyletic evolution, i.e., the transformation of a single species into another, invited

criticism.[6] Many evolutionists who supported Darwinism disagreed with this view, as it failed to

explain the splits and divisions within an ancestral lineage that lead to the emergence of more than one

species. Palaeontology also provides evidence supporting the origin of many new species during

evolution, rather than the mere transformation of one old species into a new one. Darwin did not address

the question of how a newly evolved species could be considered distinct, if it arose solely from gradual

transformation. He also failed to explain how a new species could possibly evolve in the same

geographical area occupied by the original species. Additionally, he overlooked the role of isolation

between groups and the sterility of hybrids, both of which were crucial for maintaining unique

adaptations in the process of speciation.[6]

Eldredge and Gould analysed fossil data from various vertebrates and molluscs and observed long

periods of fossil uniformity interrupted by brief periods of rapid speciation. They termed these periodic

bursts of evolutionary activity as punctuated equilibrium. [35,36] According to Gould, in such cases, the

evolutionary trend resembles climbing a flight of stairs rather than rolling something up an inclined

plane. Macromutations are probably the cause for the rapid occurrence of speciation, a process referred

to as macroevolution. Evolution through punctuated equilibria can be considered a form of

macroevolution. Although punctuated equilibrium is often viewed as contrary to Darwin's gradualistic

model of evolution, there is now a consensus among evolutionists that both gradual and rapid changes

occur during the evolutionary process.[6]

Limitations of natural selection

Darwin and his supporters were unable to provide a precise definition for natural selection. It may be

perceived as a virtual phenomenon that deals with the challenges in the survival of an organism. Natural

selection acts as an external force on variations, selecting only the favourable ones. But, in the

evolutionary concept involving natural selection, the adaptive ability inherent in the organism is not

adequately considered as a cause of evolution. According to Strickberger, natural selection can result in

the exclusion of a significant number of non-optimal individuals, even in long-established

populations.[6] He states, “If we consider genetic perfection as the elimination of all deleteriously

inferior gene combinations, there is little doubt that most, if not all, populations are imperfect.” The

loss of genetic variability due to natural selection may be particularly evident in the early stages of

evolution, as selection favours advantageous traits by eliminating individuals with less favourable

characteristics. For example, consider the evolution of the giraffe according to the theory of natural

selection. The ancestral population of giraffes initially had short necks, and over time, some individuals

developed slightly longer necks. These long-necked giraffes were then selected through natural

selection. As a result, the short-necked individuals decreased in number, as they were eliminated from

the population. In subsequent generations, the long-necked variants became more prevalent, eventually

outnumbering the original short-necked ones. This process of eliminating a significant portion of the

original population would inevitably lead to a loss of variability within the population.[6]

Another limitation of natural selection is that it addresses challenges in the present environment but

cannot predict future advantages. It is also unlikely that natural selection acts simultaneously on

multiple traits for a specific purpose; instead, it primarily focuses on individual traits. Evolution is

thought to occur incrementally, as chance events which may lead to useful adaptations. Darwin argued

that giraffes evolved through gradual accumulation of small, random changes over long periods.[7,13]

Neo-Darwinism

In the first decades of the twentieth century, evolutionists recognized Mendelian heredity as a regular

system of inheritance in natural populations and mutations as the source of variation. It appeared to

them that Mendelism and Darwinism were complementary and that a synthesis of these two concepts

would provide a better understanding of the evolutionary process. This idea came to be known as Neo-

Darwinism or the synthetic theory of evolution. Neo-Darwinism views evolution as a consequence of

changes in the frequencies of alleles introduced into the population gene pool through mutations.

Among the various forces driving these changes, natural selection is considered the most significant,

though not the only, factor. Advocates of Neo-Darwinism include R.A. Fisher, Sewall Wright, and

J.B.S. Haldane, who developed quantitative models to study the distribution of gene frequencies in

Mendelian populations.[7] They analysed the effects of various factors, such as selection, mutation,

dominance, epistasis, population structure, and polymorphisms, on gene frequencies. While they arrived

at similar conclusions regarding most quantitative aspects of evolution, they held differing views on the

mechanisms by which natural selection operates. Neo-Darwinism also allowed for the reinterpretation

of many unresolved concepts of Darwinism from the standpoint of genetics.[6,7]

Evolution without natural selection

At the beginning of the post-Mendelian period, evolutionary studies became more scientific and less

conceptual due to rapid advancements in genetics. New theories emerged that explained evolutionary

mechanisms without the involvement of natural selection.

Genetic drift

Sewall Wright (1889-1988), a renowned population geneticist, for the first time proposed that evolution

could occur without selection in small, isolated populations. [37,38] In such cases, chance plays a crucial

role in shifting allelic frequencies randomly over several generations, eventually leading to either the

fixation or elimination of specific alleles. Since this process operates without selection, it does not take

into account whether the alleles involved are beneficial, harmful, or neutral. Genetic drift reduces

genetic diversity within a population, as certain alleles may be completely lost. This phenomenon is

most effective in smaller populations where mating is random. The "bottleneck phenomenon,” occurs

when a population experiences a drastic reduction in size due to sudden catastrophic events or natural

calamities like floods or droughts.[6] A consequence of such a population crash is a smaller gene pool

with potentially different allelic frequencies compared to the original population. A closely related

concept is the "founder effect," observed when a small group of individuals from a larger population

migrates to a new place or by other means establish a new population elsewhere. The new population

often has significantly different allelic frequencies when compared to original population due to the

limited genetic diversity of the founding members. Populations shaped by the bottleneck phenomenon

or the founder effect are typically smaller, creating favourable conditions for genetic drift to occur.[6]

Neutral theory of molecular evolution

The genome comprises two major components: a functional component and a nonfunctional

component. The functional component consists of the coding sequences of nucleotides in genes

responsible for protein synthesis and the sequences associated with the spatiotemporal regulation of

gene expression. The nonfunctional component includes noncoding DNA, such as introns, tandem

repetitive sequences, junk DNA, and transposable elements. Mutations in the functional DNA are often

unfavourable or may even be deleterious. Meanwhile, mutations in the nonfunctional DNA do not

impose constraints related to any specific function affecting the organism's survival, thereby permitting

changes that do not cause deleterious effects. Even within the functional component of DNA, single

amino acid substitutions may not always have harmful effects. For example, the human alpha chain of

haemoglobin shows 75 differences when compared to the beta chain. Moreover, because an amino acid

is specified by more than one codon, a point mutation in the codon of a gene may not always result in

amino acid substitution. In such cases, the mutated codon may simply specify the same amino acid as

the original codon.[6,31]

Kimura and his collaborators, as well as King and Jukes, independently proposed the neutral theory of

evolution. According to this theory, much of the genetic variability in populations arises from mutations

that are neutral with respect to selection.[39] Since neutral mutations are not subject to selection, the

frequencies of mutated neutral alleles and the fixation of such alleles occur through random genetic

drift, provided the conditions for drift are met. As natural selection is not involved in these changes,

evolution by this mechanism is often referred to as "non-Darwinian evolution." The neutral theory of

molecular evolution is supported by the extensive and widely observed degrees of enzyme and protein

polymorphisms. Neutral mutations also explain the accumulation of "junk DNA" in the genome.[39]

Molecular clock and developmental evolutionary biology

Studies involving changes in base sequences of nucleic acids (DNA and RNA) and amino acid

sequences in proteins form the foundation of molecular evolution. It has been revealed that proteins

evolve at constant rates. Hence, the time scale required for changes in these molecules can be used to

construct molecular clocks to estimate divergence times between species. In other words, nucleotide

differences in DNA or amino acid changes in proteins between different species can be used to establish

molecular phylogenetics.[6] Phylogenetic trees constructed on the basis of such data are found to be

quite accurate and often agree with trees established using traditional criteria.

Developmental genes, such as the homeotic genes in Drosophila, are known to play an important role

in the determination of embryonic cells and pattern formation. Homeotic genes contain a highly

conserved sequence of nucleotides known as the homeobox, which is present in many species. The

homeobox is also found in mammalian genes, and these genes are referred to as Hox genes.

Developmental genes, including homeotic genes, have provided significant insights into embryonic

development and evolution. A relatively new area of study focusing on the role of developmental genes

in evolution is gaining importance, and this field is known as developmental evolutionary biology.[31]

Studies on developmental genes may help answer how genes involved in early development contribute

to large morphological differences between species. Not much is known about whether altered activity

of regulatory genes during embryonic development can drive rapid, large-scale modifications in

anatomy and morphology that may provide advantages in a changing habitat, as may have occurred in

the evolution of the abnormally long neck in giraffe.[23]

Conclusion

According to Darwin, the giraffe evolved into its present form through the accumulation of individual

random changes preserved by natural selection. However, major evolutionary changes due to

macroevolution, believed to have occurred in giraffes, may require a comprehensive set of functionally

coordinated adaptations.[35] While the elongation of the giraffe’s neck has provided certain advantages

such as access to food sources unavailable to most other browsers and an improved field of vision, these

benefits may not be the reason for its evolution. Despite these advantages, giraffes faced several

challenges that necessitated extensive anatomical and physiological modifications. They have

successfully overcome these challenges and thrived for the past one million years since their

appearance. Such an outcome is plausible only if there is a coordinated blueprint or a more structured

framework for evolutionary changes, as these modifications are interdependent. Several pertinent

questions remain unanswered. If evolution is a random process, it is unclear how natural selection could

be presented with an integrated package of adaptations necessary to produce a highly modified animal

like the giraffe.[11] Similarly, how could natural selection favour a modification that requires several

corrections or multiple adjustments? Last but not least, considering the vast possible combinations of

variations, how can natural selection identify and favour the best? Therefore, given the above-

mentioned complexities, it appears that evolution of neck in giraffe cannot be fully explained on the

basis of any single theory.

An examination of the history of science reveals several instances where certain theories and concepts,

postulated by acclaimed philosophers and thinkers, persisted for long periods and hindered scientific

progress. For example, Karl Linnaeus’s contributions to taxonomy are invaluable. He developed a

system of classification for animals and plants based on morphological similarities and dissimilarities.

To support this concept, he proposed that each species has a fixed morphology that does not change, a

concept known as the “fixity of species.” [40] This may be appealing to taxonomists since classification

is done based on the characteristic morphological features of a species. Meanwhile, the very idea of

“fixity of species” rules out the possibility of evolution. Regarding Darwinism, the theory of natural

selection was proposed without valid scientific evidence and at a time when no information about

genetics or hereditary mechanisms was available. As a result, the theory has inherent flaws. It is,

therefore, not reasonable to show too much inclination towards a century-and-a-half-old theory to

interpret modern findings, particularly those related to evolutionary genetics. Evolutionary biologists

must distance themselves from outdated concepts that lack scientific validation and adopt a new

framework grounded in modern genetics. Similarly, textbooks for undergraduate and postgraduate

students often give undue importance to the pre-modern, classical evolutionary theories. A common

example is the use of the girraffe’s neck elongation to explain evolutionary theories. While there is

nothing wrong in using this example to illustrate these theories, it should not be presented in a manner

that implies “one theory is correct and the other is wrong”.

Postscript:

The perspectives of the author on certain issues are shared in this paper, which are subject to discussion

and debate.

Acknowledgement

The author is extremely grateful to Dr. Harshini Sarojini for providing the figure.

Financial support and sponsorship

Nil.

Conflicts of interest

There are no conflicts of interest.

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