Evolution and Speciation

Question 1 Define speciation? Explain mechanism of speciation?

Question 2 Define evolution? Why are traits acquired during the lifetime of an individual not inherited?

Question 3 What is organic evolution and inorganic evolution?

Question 4 What is genetic drift? Give its significance in evolution?

Question 5 What is variation?

Question 6 What are inherited and acquired traits?

Question 7 What are the various ways of reproductive isolation?

Evolution

Evolution refers to a gradual change from one form to another. Such a change in elements with time is called inorganic evolution. Gradual change in living organisms with time since the beginning of life is termed organic evolution.

The concept of organic evolution was first given by Charles Darwin .The present-day complex organisms have originated from the earlier simpler forms of life during the course of ages by the process of gradual change.

So, evolution is the constant process of gradual change occurring in the organisms since the origin of life that has given rise to enormous variety of complex organisms on this earth.

 

Example

Let us imagine that a group of 12 red beetles live on green leaves of some bushes. These beetles have the natural tendency to increase in numbers by sexual reproduction and consequently generate variations. The crows are the natural predators of these beetles. The crows regulate the population of beetles by eating them and, therefore, fewer beetles are available for sexual reproduction.

Situation 1 : Let us assume that beetles develop colour variation during reproduction. In the progeny, one beetle develops green body colour instead of normal red colour. This beetle can inherit this variation in colour on to its progeny so that all its offsprings are green in colour. Crows can not locate green-coloured beetles on the green leaves of bushes but continue to eat red beetles which are easily located on green leaves by them. As a result, number of green beetles in beetle population increases in each generation.

Situation 2 : Colour variation arises in beetles during sexual reproduction and in the progeny, one beetle develops blue body colour instead of normal red. This beetle can also pass this colour variation to its progeny so that all its offsprings are blue coloured beetles. Crows can detect both red and blue coloured beetles easily on green leaves of bushes and eat them regularly. As the population of beetles expands, initially there are few blue coloured beetles among the majority of red coloured beetles. At this point, an elephant comes by and stamps on the bushes where the beetle live. Most of the beetles get killed. By chance, most of the survived beetles are now blue coloured. This population again slowly expands and will contain mostly blue coloured beetles over a period of time.

In situation 1, natural selection is exerted by the crows. More the crows are there, more red coloured beetles would be eaten and more will be the proportion of green beetles in the population. Natural selection is directing the evolution in beetle population in this situation. Green coloured beetles in the population are adapted to fit better in their environment than the red beetles.

Natural Selection

Natural selection is the phenomenon wherein nature, in the wild, selects traits favourable to the species in its environment.

In situation 2 colour variation in beetles gave no survival advantage to them in the existing environment as crows could easily detect and eat both red and blue beetles on green leaves. A simple accident (stamping by an elephant foot) killed majority of the red coloured beetles from the population. Survival of more blue beetles in the population changed the colour characteristic from normal red to blue over a period of time. Had the beetle population been very large, the accident would not have caused such major havoc. In small population accidents can change the frequency of some genes in a population, even if these give no survival advantage to the possessors. 

 

The elimination of the genes of certain traits from the small population when a section of the species population dies of natural calamity or migrates to other region is called genetic drift. It alters the gene frequency of the remaining population.

In situation 3 as the beetle population starts expanding by sexual reproduction, a plant disease spreads resulting in decrease in the amount of leaf material in the bushes. Expanding population of beetles is now poorly nourished.The average weight of adult beetles decreases compared to what it used to be when leaves in the bushes were in plenty. After a few years time and a few beetle generations of such scarcity, the plant disease is eliminated. Now, more leaves of bushes are available as food for the beetles.

We find that there are two kinds of traits in every organism:

(1) Inherited traits

These traits are controlled by specific genes and are passed on from one generation to another. Any alteration in the DNA will be passed on, through germ cells, to the progeny resulting in variations in them.

(2) Acquired traits

Certain traits are acquired by organisms in their life time.It involves changes in the non-reproductive tissues caused by environmental factors. It will not bring any change in the DNA. Therefore, even if some of the generations of beetles are low in weight because of starvation, this trait can not be inherited by the progeny over generations.

Speciation

Origin of new species from the existing one is called speciation.

Microevolution means that small but significant variations appear in individuals that simply change the common characteristics (traits) of a particular species only.

Let us consider a situation that the bushes on which the beetles live are widely spread over a mountain range and over a period of time beetle population becomes very large.

In the large beetle population, individual beetles normally do not travel far. They continue to feed on few nearby bushes only throughout their life time. Thus, the large population of beetles, is divided into sub-populations called demes in neighbourhoods. Most of the male and female beetles reproduce sexually only within the respective demes.

Rarely an adventurous beetle might go from one sub-population to another or alternatively it is picked up by predator crow from one site and dropped in the other site without being eaten. In either case, the migrant beetle gets an opportunity to reproduce with local inhabitant of this sub-population. As a result, genes of migrant beetle enter into the gene pool of this sub-population. Such kind of gene flow happens between sub-populations that are only partly separated.

 

After few generations, genetic drift will accumulate different variations in each of the two geographically separated sub-populations. Natural selection may also operate simultaneously in a different way in these geographically isolated sub-populations.

For instance, in one geographical region, crows are systematically killed by eagles whereas in the other geographical region no such change occurs and number of crows is very high. Consequently, green body colour variation will not be selected at the first geographical region while it will be strongly selected at the other geographical site. In this way, both genetic drift and natural selection make these geographically isolated sub-populations to become more and more different from each other.

Reproductive isolation occurs between individuals of these two groups. As a result, they are unable to reproduce even if they happen to meet somehow.

Reproductive isolation

Reproductive isolation refers to the mechanisms which check the populations of two different groups from interbreeding.

Reproductive isolation may happen in a number of way:

(1) Temporal Isolation

It is the failure of two groups of individuals to interbreed because they breed at different times (seasons) of the year.

(2) Behavioural Isolation

It prevents interbreeding of two groups due to differences in courtship and mating behaviour.

(3) Physiological Isolation

It is the functional incompatibility in mating, or in production and survival of gametes.

(4) Mechanical Isolation

It is the difference in size or structure of genitalia (i.e., mismatched genitalia) between two groups that make interbreeding difficult or impossible.