Page 3 - LN

 They do not travel far. So, in this huge population of beetles, there will be sub-
populations in neighbourhoods.
 Since male and female beetles have to meet for reproduction to happen,
most reproduction will be within these sub-populations.
 Of course, an occasional adventurous beetle might go from one site to another.
Or a beetle is picked up by a crow from one site and dropped in the other site
without being eaten.
 In either case, the migrant beetle will reproduce with the local population.
 This will result in the genes of the migrant beetle entering a new population.
 This kind of gene flow is bound to happen between populations that are partly,
but not completely separated.
 There can be a number of ways by which this can happen. If the DNA changes
are severe enough, such as a change in the number of chromosomes, eventually
the germ cells of the two groups cannot fuse
with each other.
 Or a new variation emerges in which green females will
not mate with red males, but only with green males.
 This allows very strong natural selection for greenness.
 Now, if such a green female beetle meets a red male from the other group, her
behaviour will ensure that there is no reproduction between them. Effectively,
new species of beetles are being generated.

i) Speciation takes place when variation is combined with geographical isolation.
ii) Genetic drift: It is the random change in the frequency of alleles (gene pair) in a small
population over successive generations.

Genetic drift takes place due to:
→ Severe changes in the DNA
→ Change in number of chromosomes

(iii) Natural selection: The process by which nature selects and consolidate those
organisms which are more suitable adapted and possesses favourable variations.

(iv) Geographical isolation: It is caused by mountain ranges, rivers etc.
 Geographical isolation leads to reproductive isolation due to which there is no
flow of genes between separated groups of population.
 For e.g., Between two such sub-populations a large river comes into existence,
the two populations will be further isolated.
 The levels of gene flow between them will decrease even further. Over
generations, genetic drift will accumulate different changes in each sub-
population.
 Also, natural selection may also operate differently in these different geographic
locations.
 Thus, for example, in the territory of one sub-population, crows are eliminated by
eagles.

3