Qurra-tul-Aien Role of isolation in evolution

Contents Evolution Speciation leading to evolution Reproductive isolating mechanisms Types of reproductive isolation Role of isolation Extinction Role of isolating mechanisms

 Evolution: Changes in the genetic composition of a population with the passage of each generation lead to evolution. o Evolution is the gradual change of living things from one form into another over the course of time, the origin of species and lineages by descent of living forms from ancestral forms, and the generation of diversity.  Isolation: It is the quality or condition of being isolated. It consists of a separation of cases (persons or animals)

Types of evolution  1) Microevolution: Small genetic changes in a population such as the spread of a mutation or the change in the frequency of a single allele due to selection (changes to gene pool) that is not possible without genetic variability in a population.  2) Macroevolution: Long term large scale evolutionary changes through which new species are formed and others are lost through extinction.

 Evolution involves speciation. That may occur by isolation of one species from other or also by isolation of one group from other within species.  Speciation is the process by which daughter species evolve from a parent species. Populations of a species have a shared genetic history.

Speciation 1) Morphological Species Concept: Based on appearance alone 2) Biological Species Concept: A species is one or more populations of individuals that are interbreeding under natural conditions and producing fertile offspring, and are reproductively isolated from other such populations

Speciation leading to evolution 1) Allopatric speciation: – Speciation that occurs when 2 or more populations of a species are geographically isolated from one another – The allele frequencies in these populations change – Members become so different that can no longer interbreed

2) Sympatric speciation – Populations evolve with overlapping ranges – Behavioral barrier or hybridization or polyploidy

Reproductive isolating mechanisms Any heritable feature of body, form, functioning, or behavior that prevents breeding between one or more genetically divergent populations is called reproductive isolation.

Pre-zygotic isolation Mating or zygote formation is blocked by – Ecological Isolation – Temporal Isolation – Behavioral Isolation – Mechanical Isolation – Gamete Mortality

Post-Zygotic Isolation Hybrids don’t work – Zygotic mortality - Egg is fertilized but zygote or embryo dies – Hybrid infertility - Hybrid is fully or partially sterile – Hybrid inviability - First generation hybrid forms but shows low fitness

I) Ecological isolation Species occupy different habitats. Two species may live in the same area but in different habitats. Living in these different habitats (in water, living on land, or living in tree tops) effectively segregates these organisms from each other. Since there is little if any contact the possibility of successfully mating is drastically reduced. That is habitat or ecological isolation. It involves physical separation.

The lion and tiger overlapped in India until 150 years ago, but the lion lived in open grassland and the tiger in forest. Consequently, the two species did not hybridize in nature (although they sometimes do in zoos). So these two species living in same area but do not hybridize causing the chances of evolving new species. Hence reduce the rate of evolution.

II) Temporal isolation Species breed at different times leads to temporal isolation. Two species that breed at different times of the day, season, or year cannot mix their gametes. Since the breeding times are different there is no chance of reproductive contact. Temporal isolation is genetic isolation achieved due to temporal differences in breeding.

For example, one population would be temporally isolated from another if its breeding season was in the fall while the other's was in the spring. For example, In North America, five frog species of the genus Rana differ in the time of their peak breeding activity.

III) Behavioral isolation Species engage in distinct courtship and mating rituals. Species-specific signals and elaborate behavioral patterns are used by closely related species to insure contact with the proper mate.

Birds, mammals, and insects have pre-mating rituals that attract the proper mate. These signals can be chemical or physical in nature. Other organisms pay little or no attention to these behaviors or scents and remain isolate.

IV) Mechanical isolation When interbreeding is prevented by structural or molecular blockage of the formation of the zygote, mechanical isolation done. Mechanisms include the inability of the sperm to bind to the egg in animals, or the female reproductive organ of a plant preventing the wrong pollinator from landing. Anatomical incompatibility may prevent sperm transfer between two closely related species.

Example The absence of certain appendages or their modification may inhibit a male from grasping and successfully fertilizing the female. Difference in floral structure may prevent pollen from reaching the stigma of the intended flower.

V) Gametic Isolation If for some reason foreign sperm is introduced into a female there are several preventative measures to insure that there is no union between the sperm and egg, gametic isolation occurred. Internal environmental conditions may cause the sperm to die.

Gamete recognition sites on the sperm do not fit with the intended egg. If the two species differ in the type of fertilization (external and internal) there is no chance of the sperm ever contacting the egg. All of the above types of isolation prevent the formation of hybrid zygotes.

2) Post-zygotic isolating mechanisms  I) Zygotic mortality  II) Hybrid sterility  III) Hybrid inviability

I) Zygotic mortality Development of the zygote proceeds abnormally and the hybrid is aborted causing zygotic mortility. For instance, the hybrid egg formed from the mating of a sheep and a goat will die early in development. Hence causes barrier to evolving new organisms and prevent chances to overlapping of gene frequencies for the development of new species.

II) Hybrid sterility Some times the hybrid is healthy but sterile called hybrid sterility. For example, The mule, the hybrid offspring of a donkey and a mare, is sterile; it is unable to produce viable gametes because the chromosomes inherited from its parents do not pair and cross over correctly during meiosis This hybrid sterility reduces rate of evolution by arising new species.

III) Hybrid inviability First generation hybrid forms but shows low fitness or infertility appears in later generation called as hybrid inviability. As witnessed in laboratory crosses of fruit flies, where the offspring of second- generation hybrids are weak and usually cannot produce viable offspring. That further may go to extinction of later descendants.

Role of isolation When an animal or plant is isolated they evolve by their surroundings. For example, say tigers live on mainland but a few get separated by some act of nature. Millions of years down the line they will have evolved differently to adapt better to their little isolated island then their cousins on the main land. To develop new species by making the isolated groups change with their environment separated from others of their species.

Role of isolation Eventually the isolated group will change so much they won’t be able to reproduce with the others, making a new species. All these types of isolation favor speciation leading to evolution. With the process of speciation, isolation may affect evolution in negative way also by the extinction and less biodiversity.

Extinction If the environment changes rapidly and the species living in these environments do not already possess genes which enable survival in the face of such change and random mutations do not accumulate quickly enough then all members of the unlucky species may die.

Role of isolating mechanisms Is to increase the efficiency of mating. Where other closely related species do not occur, courtship signals can “afford” to be general, non specific and variable.

Role of isolating mechanisms Where other related species coexist, however, non specificity of signals may lead to wasteful courtship and delays, even where no heterospecific hybridization occurs under these circumstances there will be a selective premium on precision and distinctiveness of signals.

Role of isolating mechanisms As each species is a delicately integrated genetic system that has been selected through many generations to fit into a definite niche in its environment. Hybridization usually leads to breakdown of this system and results in the production of disharmonious types. It is the function of isolating mechanism to prevent such a breakdown and to protect the integrity of genetic system of species.

Role of isolating mechanisms Any attribute of the species that would favor the production of inferior hybrids is selected against, since it results in wastage of gametes. Such selection maintains the efficiency of the isolating mechanisms and indeed helps to perfect them. Isolating mechanisms are among the most important biological properties of species leading to evolution.