Origin of Species | Zoology

The below mentioned article provides a quick note on the origin of species.

Origin and Accumulation of Ge­netic Diversity:

It is presumed that gene mutation caused by cosmic radiation and other mutagens, modifier genes, inversion of parts of chro­mosomes, translocation of genes from one chromosome to another, elimination or re­duplication of chromosomes, introduce ge­netic diversity in natural populations.

The accumulation and preservation of genetic diversity are believed to be the effects of different types of evolution in different kinds of populations (Simpson, 1944; Dobzhansky, 1951).

Perfection of the genotype and a refined adaptation towards the environment occurs in large and con­tinuous populations through ‘centripetal evolution/ while ‘centrifugal evolution’ leads to an increase in genetic diversity in small and partially divided populations.

Environmental Selection:

Selection is not the indiscriminate mass mortality of populations but the higher mor­tality of individuals with one genetic con­stitution than with another. The agents of selection are both abiotic (physical) and biotic (biological). Light, temperature, oxy­gen concentration, salinity, pH and others belong to the first category, while the second category includes predator, parasite, com­petitor, food and others.

The agents of selection affect the fecundity of individuals or cause their death before attaining sexual maturity. Selection leads to a greater num­ber of individuals of a genotype (with advantageous genes) than another.

(In a genetically heterogenous population, the possibility of reproductive success of some genotypes will be higher than the probabil­ity of success of the other. Thus certain kinds of genetic information will be more and more common in the gene pool of the population and other kinds will be less and less common).

Genetic Assimilation:

The pathways of development of organs in an individual are different and they tend to end in normal state. In cases, develop­mental hazards force a deviation in one or more pathways; the organs in the phenotype become somewhat different from that in the normal. Selection pressure stabilizes the changes in the genotype and makes those irreversible.

The processes of development leading to the appearance of functioning adult organ­ism from a zygote are interrelated and form a system, which has been termed epigenotype (Waddington). It is a branching system of developmental pathways, each of which leads to one of the components of the adult.

Each individual in a population in a given environment will have its own genotype and, therefore, its own epigenotype, which will eventually result in the adult pheno­type. Selection to preserve fitness in the given environment may eliminate genotypes producing imperfect phenotype leading to the selection for a well-canalised epigenetic system.

With the change in the environment some individuals may produce fit phenotypes without immediate genotypic change. In course of time, genotypic change occurs in the new environment and developmental paths are stabilized by selection.

If the or­ganisms are returned to their original envi­ronment, original phenotypes are not pro­duced due to the change in the genotype. The original phenotypic (epigenetic) response to changed environment has become incor­porated in the genotype. This is genetic assimilation.

The Isolation of Genetically Di­verse Groups:

Isolation of interbreeding population bear­ing advantageous genes from the rest of the populations is a prerequisite for further progress towards speciation. The term ‘iso­lating mechanism’ is used to include all the agents that curtail or stop the gene exchange between populations.

The chief isolating mechanisms are:

1. Geographic or spatial isolation:

The populations inhabit different territo­ries. The gene exchange in allopatric popu­lations is prevented because they live in different territories. Sympatric sexual popu­lations remain distinct, only if the gene exchange between them is hindered or pre­vented by their intrinsic, genetically condi­tioned properties.

2. Reproductive isolation:

Restriction or suppression of gene ex­change between populations due to geneti­cally conditioned differences.

a. Ecological isolation:

Due to different habits, potential mates from different popu­lations do not meet and interbreeding is prevented, though they live in the same general area, (i) In the water snake, Natix sipedon, the fresh water and salt water races are kept separate by their habitat, though they may come very close together.

b. Seasonal or temporal isolation:

Mat­ing period occurs at different seasons and prevent mating, (i) In North Eastern United States three species of Rana breed in the same pond. R. sylvaticus breeds first at water temperature 44°F (6.67°C), R. pipiens at 55° F (12.78°C) and R. clamitans at above 60°F (15.56°C). If crossing takes place, embryos fail to develop.

c. Sexual, physiological and ethological isolation:

Lack of mutual attraction and behaviour differences—courtship, song, scent, etc. play a major role to prevent random mating.

(i) The mating dances of salamanders and turtles, courtship motion in crabs, sexual behaviour in drosophila are selective and reduce the chance of mating between different populations.

(ii) Songs of birds, scents of butterflies lead to the same result.

(iii) In some oysters, the chemical substance released in water with gametes stimulate other individuals to discharge masses of sex cells.

(iv) The swarming of pololo worm Eunice vixidis in the south Pacific; the maturation of eggs and sperms of Echinus esculents in the Mediterranean Sea are related to the phases of the moon.

(v) The jelly coat around the egg containing fertilizin and also the vitelline membrane and cortical layer of the egg promote pen­etration of a sperm of the same species or inhibit cross-fertilization with other species. The mechanism ensures effective fertiliza­tion within a local population.

d. Mechanical isolation:

Physical non- correspondence of the genital organs. In many animals, specially in insects, the male and female genitalia do not match for mating and are considered as mismatching sex organs.

e. Gametic isolation:

Spermatozoa are not attracted by the eggs, (i) In the cross- insemination between the related species of Drosophila viridis, D. americana, the mobility of the sperm in the sperm receptacle of females of foreign species is lost rapidly.

f. Hybrid in-viability:

The zygotes are inviable or inferior to those of the parental species, (i) The hybrid in-viability is total in North American species of Rana pipiens, R. palustris, R. clamitans and R. catesbeiana. (ii) Sheep and goat hybrids appear to be normal in the early embryonic stages but die much before birth.

g. Hybrid sterility:

Failure of hybrids to produce F₂ generation, (i) The F₁ hybrids produced by crossing related species of Drosophila pseudoobscura and D. similis are fully vigorous like paternal species but the hybrid males are sterile.

h. Hybrid breakdown:

In-viability or adaptive inferiority, of all, or a part of the F₂ or back cross hybrids, (i) The robust and vigorous hybrids of F₁ in many species produce degenerate hybrids in F₂.

The Crystallization of New Spe­cies:

Every race of a species is not an incipient species. With the selection and isolation of a new genotype with advantageous charac­ters, a new species is in the process of formation.

The phenotypic characters can be fully exploited only after harmonisation of the genetic constitution with the environ­ment. The harmonisation is not a separate step, but continuous with genetic mutation, selection and isolation, finally leading to the formation of a new species.