SPECIATION
Minor genetic changes occurring in a population is called microevolution. They change common characteristics of a species. It cannot explain formation of new species.
Speciation is an evolutionary process of formation of new species.
If a population
splits into two such that they cannot reproduce with each other, they are
called two species. It can be explained using beetle population.
Consider
a huge beetle population spread over a mountain range. Here, each beetle feed
mostly on nearby bushes. They do not travel far. So, there will be
sub-populations in neighbourhoods.
Most reproduction
occurs within these sub-populations.
In rare
cases, some beetles might go from one site to another or a crow picks up a beetle
from one site to another. As a result, genes of migrant beetle enter new
population by reproduction and cause genetic drift (accidental genetic
variation in small population).
If a
large river comes between two sub-populations, they will be isolated. Thus gene
flow between them decreases.
Over
generations, genetic drift accumulates many changes in each sub-population.
Also, natural selection operates in these different geographic
locations. E.g., in one sub-population, crows are eliminated by eagles. Here, the
green beetle variation will not be selected. But in other sub-population,
number of crows are very high. So green variation will be strongly selected.
Genetic
drift & natural selection together makes isolated sub-populations more and more
different. Eventually, members of these two groups will be incapable to
reproduce with each other even if they happen to meet.
This can
be happened by many ways. If the DNA changes are very high (e.g. change in
number of chromosomes), the germ cells of the two groups cannot fuse. Or a new
variation emerges in which green females will not mate with red males, but only
with green males (natural selection for greenness). Thus, new species of
beetles are generated.
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