Evolution is caused by the action of forces on survival of Variation. Reproductive Isolation keeps the species distinct therefore the basic forces of Organic Evolution are: (i) Variation (ii) survival (iii) Isolation.
(i) Sources of organic variation
Variation arises in a private member of a population, and if favourable, spreads into the population through “differential reproduction” by the action of natural selection. Variations may occur by
1. Mutation, which may be a sudden genetic change. it's going to be a change during a single
gene (genic mutation or point mutation) or may affect many genes (chromosomal
mutation).
2. Genetic recombination, which occurs in sexually reproducing organisms at every reproduction. The chromosomes and thus genes of the oldsters mix randomly during zygote formation. That's why offspring of the same parents are different from each other as they need different combinations of parental genes. Variation is additionally brought about when the crossover occurs during gamete formation.
3. Gene flow is when there's a chance of mixing of genes of closely related species through amphimixis.
4. Genetic drift occurs in small populations when a neighbourhood breaks far away from an outsized population. Only representative genes of the massive population are present which undergo a change at a right time and therefore the small population may evolve into a replacement subspecies or species.
(ii) Survival
Natural selection considered to be liable for “differential reproduction of genes” which suggests that more favourable genes get reproduced during a population.
Many samples of survival in action are available now. Given below are three such examples.
Example 1: DDT resistant mosquitoes
About 50 years back, the mosquito population had been kept on top of things with the help of DDT. Thereafter, it had been found that mosquitoes couldn't be killed with DDT any longer. There appeared DDT-resistant mosquitoes. What had happened was that a point mutation (variation) had conferred (given) on the mosquito, the power to resist the effect of DDT. While DDT killed other mosquitoes, those with the gene mutation survived and slowly within a couple of generations DDT resistant mosquitoes replaced the DDT-sensitive ones. In other words, the DDT resistant mosquitoes ‘reproduced differentially’ by the action of survival.
(iii) Role of Reproductive Isolation
Once new species arise from the parental species thanks to the effect of variation and natural selection, reproductive barriers prevent the 2 species from exchanging genes through reproduction. Thus two related species cannot mate with one another and remain distinct. Isolation means separation and reproductive isolation simply means the 2 species are prevented from successful reproduction and kept genetically distinct from each
other.
Speciation
The evolution of the latest species is termed speciation. Speciation occurs within the
following ways and is termed accordingly.
1.Allopatric speciation takes place when a neighbourhood of the population becomes geographically separated (geographical isolation) from the parental population. For example a gaggle of birds lives at the bottom of the mountain, some members fly up and get geographically isolated. Variation and survival act differently on the two because the environment during which the 2 live is different. Gradually genetic changes render them to be reproductively isolated.
2.Sympatric speciation
Sometimes a genetic barrier (reproductive barrier) prevents reproduction between a section of a population of a species with other members. Such a neighbourhood of population usually arises in plants due to polyploidy. Polyploidy may be a mutation in which the traditional diploid number of chromosomes become doubled or trebled (2n becomes 3n, 4n, 5n etc) during a section of the population of a species thanks to certain irregularities during cellular division. The polyploid section of the population is then unable to interbreed (mate and reproduce) with their diploid ancestors and becomes a new species.
Models of speciation
There are two accepted models of speciation that have given rise to biodiversity.
1. Phyletic Gradualism model
Two species from common ancestor gradually become more and more structurally different acquiring adaptations unique to every other. Darwin also believed that evolution may be a slow and gradual process.
2. Theory of Punctuated Equilibrium
A new species arise through major changes within the beginning then remain constant for long periods before changing again. This model was suggested by palaeontologists (scientists who study fossils), Niles Eldredge and Stephen Gould.
Hardy-Weinberg Equilibrium
This is an idea associated with both genetics and evolution and was proposed by G. H. Hardy and W. Weinberg. A population of sexually reproducing organisms during which genes combine randomly due to random mating is named panmictic. In other words, a panmictic population is one during which mating partners aren't specifically selected. for instance, we humans usually don't search for specific blood type when a wedding is arranged so we are panmictic with reference to blood types.
The Hardy Weinberg Principle states that during a panmictic population if there's no pressure of mutation, selection, genetic drift etc. then the frequency of any pair of genes remains constant, generation after generation. For instance, a gene has two alleles, p and q within the population and no mutation or selection etc. takes place, then the frequency of those two alleles will remain constant generation after generation. this will be mathematically represented as:
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