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UNIVERSITY OF ELDORET
DEPARTMENT OF BIOLOGICAL SCIENCES
ZOOL 121: GENERAL ZOOLOGY
a distinct species forms. It can also be defined as a process in which two populations become genetically different.
A species is a group of living organisms, that can interbreed or exchange genes and produce viable offspring.
Types of Speciation
There are four different natural ways by which speciation occurs. These are:
Allopatric Speciation, Sympatric Speciation, Peripatric Speciation, and Parapatric Speciation
1. Allopatric speciation
Allopatric speciation occurs when biological populations of the same species become isolated from each other by barriers such as mountain ranges, waterways, or a group of species migrating to a new location without the opportunity to return. The barrier makes interbreeding of species difficult to an extent that the free flow of alleles is prevented and thus no genetic interchange.
The isolated populations then undergo genotypic or phenotypic changes such as:
➢ they become subjected to dissimilar selective pressures, ➢ they independently undergo genetic drift, and ➢ different mutations may arise in the gene pools of two isolated populations.
When the barrier is later removed, the two populations may be unable to successfully mate/interbreed with each other since they are now two distinct species.
LECTURER: Joan Kiplagat
MECHANISMS OF ISOLATION AND SPECIATION How does one species become two? What types of isolation lead to the formation of new species?
SPECIATION Speciation is a process of evolution through which two different existing populations evolve and
Example of allopatric speciation
The different finch species of birds on the Galapagos Islands came from one parent species that first inhabited the islands millions of years ago. The populations of the parent species spread from one uninhabited island to the next, where they were geographically isolated from other populations. The different populations adapted to different ecological niches and eventually evolved into many descendant species such as insect eating bird species, wood peckers, seed eating bird etc.
2. Peripatric speciation
It occurs when a smaller population detaches itself from a larger population and there is no exchanging of genes with the original/main population.
The small population that branched off from the main population enters a distinct biological niche, like feeding on different food or surviving in a different environment. The distinctive features of
The slight differences in members of sub populations makes it difficult to interbreed with members at the extreme end.
For example, plants that live on boundaries between very distinct climates may flower at different times in response to their different environments, making them unable to interbreed
REPRODUCTIVE ISOLATION MECHANISMS
Reproductive isolation is when two individuals of the same species are unable to reproduce due to certain barriers between them.
It can also refer to when two groups of animals live close enough to one another to interact but are unable to interbreed with one another.
Reproductive isolation can be divided into two major divisions - pre-zygotic barriers and post- zygotic barriers.
Pre-Zygotic Barriers
Pre-zygotic barriers are obstacles that are present before an egg can be fertilized.
Some examples of pre-zygotic barriers include temporal isolation, ecological isolation, behavioral isolation, and mechanical isolation.
i) Temporal/seasonal isolation
The two potential mate species never come into contact with each other because they are not active at the same time, or they have different mating/breeding seasons. For example;
Wood frog ( Rana sylvatica ) and Leopard frog ( Rana pipiens ) are unable to mate because they have different seasons of mating i.e. wood frog usually mates in late march or early April when water temperatures are about 7.2 OC (45OF), while, the leopard frog usually mates in mid-April when the water temperature is 12.8OC (55OF).
ii) Ecological/habitat isolation
The potential mate species have the same range but do not come across one another because they are eating different foods, living in different habitats, or are mating in different areas.
For example; the red-legged frog does not mate with the bullfrog because the red-legged frog breeds in fast-moving streams and bullfrogs breed in ponds. Another example is riverine fishes living in different tributaries, which prevents interbreeding.
iii) Behavioral/Ethological isolation
Potential mates meet but cannot mate, due to differences in courtship displays or other specific signals that are necessary rituals before mating.
Examples of signals or rituals which stimulate the opposite sex for mating;
➢ Feather displays and dancing in male birds is necessary to attract the female. The color and shape of the feathers, as well as the display pattern, is so unique for each species that mating between two different species is not possible. ➢ Some insects emit odors that attract the opposite sex. ➢ some male birds perform dances for the female ➢ Male porcupines urinate on the females before mating.
iv) Mechanical isolation
The animals actually try to mate, but are physically unable. For example, a certain species of snail is unable to mate if the shell is not coiled in the same direction, so only right-coiled shelled snails can mate with right-coiled shelled snails; the same applies to left-coiled snails.
ZOOGEOGRAPHY
Zoogeography is a branch of zoology that deals with geographical distribution of animals on the planet, which occur in different regions of the world in a distinct pattern. Zoogeography attempts to understand the complexities as well as the simplicity in the distribution of animals in light of evolutionary and environmental influences. ANIMAL DISTRIBUTION Animals are not uniformly distributed on land and in water. They are restricted to certain places by several factors such as; climate, food, shelter, flora and fauna etc. Categories of animal distribution Animal distribution can be classified into three categories, namely; Continuous, Discontinuous and Bipolar distribution.
- Continuous distribution. This involves Eurytopic or wide-ranging animals that are characterized by their ability to live in a wide range of environmental conditions. These animals are not specific to any particular type of food, and they may have special power to cross barriers either by flight, swimming, and easily adapt to new environmental conditions. Many of the eurytopic animals have continuous distribution because they are companions of man and have travelled to long distances with him. Examples of wide-ranging/eurytopic animals include; rats, bats, cockroaches, flies, mosquitoes, lizards, snakes, dogs and man etc.
- Discontinuous distribution. This occurs when the continuity of distribution of a species is broken by uninhabited areas which are sometimes very large stretches of oceans. Reasons why animals are distributed in widely separated areas include; ➢ The species was earlier distributed continuously but the land masses in the intermediate areas submerged, breaking the species into widely separated populations. ➢ Continental drift separated the continents and carried some animals to long distances, isolating them from other areas. ➢ Animals may reach distant areas by several means such as; sweepstake routes (e.g insects, snails and rats), by rafting (e.g turtles), by floating and swimming, and by storms (e.g birds blown by storms).
Examples of animals distributed discontinuously include: ➢ Peripatus {velevet worm}has 75 species distributed in Southeast Asia, East Indies, Australia, New Zealand, Africa, South and Central America. ➢ The three surviving genera of lungfishes occur in three different continents; Protopterus in Africa, Lepidosiren in South America and Neoceratodus in Australia. ➢ The two elephant species surviving today, Loxodonta africana occurs in Africa while Elephas maximus is found in India, Burma, Thailand and Sri Lanka. ➢ The limbless amphibians occur in Africa, South America, Central America, South Asia and East Indies.
- Bipolar distribution. Some species because they are adapted to the cold arctic climate cannot migrate to the warmer areas and hence restricted to the Polar Regions. Examples of polar species include; Polar bears, arctic fox, lemmings, coyote and reindeers are found in the northern Polar Regions while penguins are restricted to the Antarctica region.
Means of animal dispersal All animals do not spread across the sea or other barriers with the same speed, some spread faster and others slowly and some do not cross the barriers at all. Dispersal/ Distribution of animals, therefore, depends on ; The animal’s body size, reproductive rate, locomotory organs, physiological endurance and some means to carry them to long distances across barriers.
FACTORS AFFECTING THE DISPERSAL OF ANIMALS
There are four main factors due to which animals are prevented from spreading to every possible area i.e.
1. Climate Animals are adapted to a combination of temperature and humidity that is affected by rainfall, examples; ➢ Lower temperature prevents majority of reptiles from migrating northwards into the temperate areas.
