Help needed Biology !!

I would like to know complete mechanism of Natural selection leading to evolution and the difference between the mechanism of speciation with Natural Selection as iam quite confused

I have exam in few days , Help's Much appreciated

Natural selection in evolution:

Variation [due to independent assortment of homologous chromosomes, crossing over, random fertilization, mutation ----> some individuals better adapted to environment/able to resist adversities -----> these "fitter" organisms survive [others die out] -----> fitter inidividuals reproduce ("survival of the fittest" principle) -----> over thousands/millions of years, the gene pool consists entirely of the advantageous alleles -----> new modified phenotype = evolution.

Speciation:

(A few different definitions of species) traditional one: if two members of a population (male and female) can reproduce to produce FERTILE offspring, they are of same species.



e.g. if two groups of a certain species are separated (over time) by the geographical development of a spur of elevated land or of water (fjord???) ----> organisms on either side evolve differently due to varying environment either side of spur -----> when genotypes/phenotypes become too different for them to meet definition of species (think of it as like extreme natural selection, so that e.g. male and female cannot recognize each other [sort of like very shy geeks sitting opposite each other and neither can make the first move []] [OR like the Specsavers ad where this girl kisses a stranger haha!] or when they cannot physically mate) ----> = allopatric speciation (look up sympatric speciation/tetraploidy)

Hope this helps!

M (specialist biology tutor)

Individuals in a population vary, usually in many ways. For example, they may show different traits in colour, behaviour or biochemistry.

Natural selection is the process by which some individuals survive and reproduce more successfully than others on the basis of these traits.

For example, the Peppered Moth has two colour forms - dark and light. During the day, they hide on tree trunks. If they are the same colour as the trunk, they are harder for birds to spot and eat. So in polluted areas, where tree trunks are darkened by soot, the dark form survives and reproduces better than the light form. The result is that Peppered Moth populations in polluted areas are nearly all dark.

When the basis for the variation is genetic, this means that the favoured form of the gene spreads, and the disfavoured form becomes rare.

In this way, natural selection creates pressure on the population in a polluted area to become genetically different to a population in an unpolluted area. As you can imagine, if this continues for many generations, you can reach a point where the two populations are so different that they can no longer interbreed. This means they have become two different species => speciation has taken place.

The process of evolution is essentially the accumulation of a series of such events, which over millennia may result in a great diversity of new species.

If you want a more detailed discussion of speciation, this 8 minute video is quite good.

Okay , So when does Mutation occurs? Like i saw in some past papers , for eg 2 Lemur species , they get seperated , Mutation occurs DNA of these so that they develop certain characteristics , these characteristics are adapted due to different selection pressure by species , The advantageous alleles are passed on to their offspring
and in the antibiotic resistance example , where there are different type of alleles present in their populations !
I just dont get it , like when is there a mutation in a species? and whether the selection pressure causes Mutation ! I just dont quite get it

The orthodox view is that natural selection does not make mutations occur, and has no effect on when or where mutations occur.

Mutations appear spontaneously, all the time, often during the process of DNA replication. Bear in mind that there are billions of base pairs in a typical eukaryote genome (for example, the human genome contains around 3 billion base pairs). Every one of these bases has to be replicated to produce just one gamete. The process of replication is incredibly efficient and accurate, but with this number of operations, it is not surprising that every once in a while a mistake is made. For example, a "T" is misread and copied into a "G". That mutation can be enough to result in a significantly different gene product. Even more significant mistakes can occur too, such as mutations which cause a homeobox gene to be accidentally duplicated or placed in the "wrong" position. These can cause very obvious changes in how the organism develops.

It is only after this that natural selection begins to act.

Hi again member of feline family (sorry could not resist that one!),

A mutation can occur at various stages of the life of an organism, mostly during meiosis - I don't think it is known (at least I don't know!) that it occurs due to selection pressure in bacteria rather it occurs randomly, but the bacteria that [by chance] possess e.g. the gene composition to produce a specific beta-lactamase (penicillin is one of a group of antibiotics called beta-lactam group [from its chemistry] - the other main beta-lactam group is the cephalosporins) - in the case of bacterial resistance, then those few bacteria that can destroy penicillin or prevent its action by e.g producing an altered carrier protein which does not allow entry of penicillin into the bacterial cell, will thrive and multiply into huge numbers; the rest will die THIS IS SELECTION PRESSURE! They will also pass on this resistance-imparting gene to other bacteria by e.g. conjugation = horizontal transmission.

I would like to know complete mechanism of Natural selection leading to evolution and the difference between the mechanism of speciation with Natural Selection as iam quite confused

I have exam in few days , Help's Much appreciated

Ahan alright , and what about selection pressure , i mean does it act before mutation or after it ?

Selection pressure acts all the time. It is simply the selection action of the environment on the organism.

To take the antibiotic resistance example, a population of bacteria may be exposed to an antibiotic. This antibiotic environment exerts a selection pressure because it destroys all bacteria without resistance. If no bacteria are "born" with a mutation for resistance, then natural selection simply acts to destroy the whole population.

However, what often happens is that every now and then, a mutation spontaneously arises in a few bacteria in the population which will confer resistance to the antibiotic. Natural selection is still operating to select for resistant individuals, and now we have some! The resistant individuals now form the starting point for a new population, all of which will all have the mutation for resistance. The result is that natural selection has produced an antibiotic resistant bacterial population.

Check out freesciencevideos on youtube helped me a lot

Ohhhhh Okayyy !!! Thank youuuu Literally this was the actual thing that was confusing me , I thought the wrong way of selection pressure , Thank you soo much Sir May god bless you

Okay I got it
So Selection pressure is actually the removal of organisms from a population having non-advantageous alleles?