This document discusses the factors that can cause microevolution in a population. It defines microevolution as changes in a species' gene pool over a short period of time due to reproductive individuals. Five factors are described: 1) mutation pressure introduces new alleles, 2) immigration changes gene frequencies, 3) genetic drift impacts small populations, 4) non-random mating disrupts Hardy-Weinberg assumptions, and 5) selection pressure favors better adapted individuals who reproduce more. These factors disrupt genetic equilibrium and cause populations to evolve over multiple generations.

1. Factors initiating elementary
evolutionary
changes(Microevolution)

2. Microevolution
• Microevolution is the change in the genome,
or gene pool, for a given species in a relatively
short period of geologic time by the
alterations of successfully reproducing
individuals within a population.

3. • Some environmental conditions are more
harsh than others, and organisms may have to
adapt more to survive in that conditions.
• Areas where the environmental pressures are
stable, or the organisms have adapted to it,
exhibit non-evolving populations.
• In a non-evolving population, the allele
frequency, genotype frequency,
and phenotype frequency remain in genetic
equilibrium.

4. • This phenomenon was illustratedd by a
German physician, Weinberg, and a British
mathematician, Hardy, both working
independently in 1908. Their combined efforts
are now known as the Hardy-Weinberg
equilibrium model.

5. Hardy-Weinberg Equilibrium
• To understand the Hardy-Weinberg equilibrium,
assume G and g are the dominant and recessive
alleles for a trait where GG = green, gg = yellow,
and Gg = orange.
• In our imaginary population of 1,000 individuals,
assume that 600 have the GG genotype, 300 are
Gg, and 100 are gg. The allele and genotype
frequency for each allele is calculated by dividing
the total population into the number for each
genotype:

6. • GG = 600/1,000 = .6
• Gg = 300/1,000 = .3
• gg = 100/1,000 = .1
• The frequency of the allele in the first
generation of offspring.

7. • First, determine the total number of alleles
possible in the first generation. In this
imaginary case, because each organism has 2
alleles and there are 1,000 organisms, the
number of possible alleles in the first
generation of offspring is:
• 2 × 1,000 = 2,000

8. • For the G allele, both GG and Gg individuals
must be considered. Taken separately,
• GG = 2 × 600 = 1,200
• + Gg = 300
• 1,500

9. • The letter p is used to identify the allele
frequency for the dominant allele (.75) and q
for the recessive allele (.25).
• Note that p + q = 1.
• The frequency for the G allele is therefore:
• 1,500/2,000 = .75

10. • For the g allele, the calculation is similar:
• Gg = 300
• + gg = 2 × 100 = 200
• 500
• The frequency for the g allele is therefore:
• 500/2,000 = .25

11. • Hardy-Weinberg can also predict second-
generation genotype frequencies. From the
previous example, the allele frequencies for
the only possible alleles are p = .75(G) and q =
.25(g) after meiosis. Therefore, the probability
of a GG offspring is p × p = p2 or (.75) × (.75) =
55 percent. For the gg possibility, the allele
frequencies are q × q or (.25) × (.25) = 6
percent. For the heterozygous genotype, the
dominant allele can come from either parent,
so there are two possibilities:
• Gg = 2pq = 2(.75)(.25) = 39 percent.

12. • Note that the percentages equal 100, and the
allele frequencies (p and q) are identical to the
genotype frequency in the first generation!
Because there is no variation in this
hypothetical situation, it is in Hardy-Weinberg
equilibrium, and both the gene and allele
frequencies will remain unchanged until acted
upon by an outside force(s). Therefore, the
population is in a stable equilibrium with no
change in phenotypic characteristics.

13. The Hardy-Weinberg equation highlights the
fact that sexual reproduction does not alter
the allele frequencies in a gene pool.
• Five factors impact the Hardy-Weinberg
equilibrium and create their own method
for microevolution.
• 1.Mutation pressure
• 2.Immigration
• 3.Genetic drift
• 4.Cross breeding
• 5.Selection pressure

14. 1.Mutation pressure
• A mutation is an inheritable change of a gene by
one of several different mechanisms that alter
the DNA sequencing of an existing allele to create
a new allele for that gene
• A primary mechanism for microevolution is the
formation of new alleles by mutation.
Spontaneous errors in the replication of DNA
create new alleles instantly while physical and
chemical mutagens, such as ultraviolet light,
create mutations constantly at a lower rate.

15. • Mutations affect the genetic equilibrium by
altering the DNA, thus creating new alleles that
may then become part of the reproductive gene
pool for a population. When a new allele creates
an advantage for the offspring, the number of
individuals with the new allele may increase
dramatically through successive generations. This
phenomenon is not caused by the mutation
somehow overmanufacturing the allele, but by
the successful reproduction of individuals who
possess the new allele. Because mutations are
the only process that creates new alleles, it is the
only mechanism that ultimately increases genetic
variation

16. 2.Immigration
Gene migration is the movement of alleles
into or out of a population either by the
immigration or emigration by individuals or
groups. When genes flow from one population
to another, that flow may increase the genetic
variation for the individual populations.

17. 3.Genetic Drift
• Genetic drift is the phenomenon whereby chance
or random events change the allele frequencies
in a population. Genetic drift has a tremendous
effect on small populations where the gene pool
is so small that minor chance events greatly
influence the Hardy-Weinberg arithmetic. The
failure of a single organism or small groups of
organisms to reproduce creates a large genetic
drift in a small population because of the loss of
genes that were not conveyed to the next
generation

18. • Conversely, large populations, statistically
defined as greater than 100 reproducing
individuals, are proportionally less affected by
isolated random events and retain more stable
allele frequency with low genetic drift.

19. 4.Cross breeding
• The Hardy-Weinberg equation assumes that all
males have an equal chance to fertilize all
females. However, in nature, this seldom is true .
In fact, the ultimate nonrandom mating is the act
of self-fertilization that is common in some
plants. In other cases, as the reproductive season
approaches, the number of desirable mates is
limited by their presence (or absence) as well as
by their competitive premating rituals. Finally,
botanists and zoologists practice nonrandom
mating as they attempt to breed more and better
organisms for economic benefit.

20. 5.Selection pressure
• The process by which comparatively better
adapted individuals out of a heterogeneous
population are favoured by the Nature over
the less adapted individuals is called natural
selection.
• The process of natural selection operates
through differential reproduction.

21. • It means that those individuals, which are best
adapted to the environment, survive longer and
reproduce at a higher rate and produce more
offsprings than those which are less adapted.
• So the formers contribute proportionately greater
percentage of genes to the gene pool of next
generation while less adapted individuals
produce fewer offsprings. If differential
reproduction continues for a number of
generations, then the genes of those individuals
which produce more offsprings will become
predominant in the gene pool of the population.