Weeds have evolved unique traits that allow them to thrive in agricultural and disturbed environments. They produce large numbers of seeds that can remain dormant for many years, and also spread rapidly through vegetative reproduction. Weeds exhibit genetic variation and can evolve quickly in response to changing conditions and human management practices. Their ability to disperse seeds widely, reproduce rapidly, and adapt to diverse habitats makes weeds highly successful at colonizing new areas.

1. INTRODUCTION
Since the advent of agriculture humans have encountered
plants that have frustrated their goal to manage their
environment. Today, we call the plant pests that interfere with
agriculture ‘weeds’. In the last few centuries, humans have
taken an increasing interest in preserving and otherwise
maintaining the biodiversity.
Here, too, plant pests frustrate human intentions. In such
situations, these plants are called ‘invasives’. Weeds and
invasives are problematic plants at ends of a continuum of how
intensively humans manage an ecosystem, with manicured
lawns and cultivated croplands at one end, through forest
plantations and rangelands, to natural, deliberately lightly
managed areas at the other end.
Thus, the distinction between weeds and invasives, though
often clear, is occasionally fuzzy or arbitrary.

2. FACTORS INFLUENCING EVOLUTION
These factors include:
•Ecological adaptation
•Genetic variation
•Human influence
•Responses to control measures
•Hybridization and genetic exchange
•Rapid evolution

3. ECOLOGICAL ADAPTATION
• Weeds have evolved to adapt to different environmental
conditions, such as changes in climate, soil types, and
disturbance regimes.
• They are often characterized by high reproductive rates,
short life cycles, and efficient dispersal mechanisms, which
allow them to colonize and thrive in disturbed habitats.
• Weeds can evolve to have competitive advantages, such as
rapid growth, high seed production, and efficient nutrient
uptake, which allow them to outcompete native or cultivated
plants for resources.

4. GENETIC VARIATION
• Weeds, like all plants, exhibit genetic variation within their
populations. This genetic diversity allows them to adapt to
changing environmental conditions through natural selection.
• Weeds with genetic traits that confer advantages in terms of
survival, reproduction, or resistance to control measures,
such as herbicides, may have higher fitness and be more
likely to reproduce and pass on their genes to the next
generation.
• Over time, this can result in the evolution of weed
populations with increased resistance to control measures,
making them more difficult to manage

5. HUMAN INFLUENCE
• Human activities, such as agriculture, landscaping, and
transportation, have had a significant impact on the evolution
of weeds.
• Agricultural practices, including the use of herbicides,
tillage, and irrigation, can create selective pressures on weed
populations, leading to the evolution of herbicide-resistant
weeds or weeds that are adapted to agricultural systems.
• Landscaping practices, such as the introduction of non-native
plant species, can result in the establishment of invasive
weeds that outcompete native plants.
• Transportation of weed seeds through human activities, such
as machinery, vehicles, and clothing, can also facilitate the
spread and evolution of weeds to new areas.

6. EVOLUTIONARY RESPONSES TO
CONTROL MEASURES
• Weeds can evolve in response to control measures used by
humans, such as herbicides, mechanical removal, and
biological control methods.
• Over time, weeds that are less susceptible to control
measures may have higher survival and reproductive rates,
leading to the evolution of resistant populations.
• This can result in the need for increasingly complex and
diverse control strategies to manage weeds effectively.

7. HYBRIDIZATION AND GENETIC
EXCHANGE
• Weeds can hybridize with other closely related plant species,
resulting in the formation of hybrid populations with unique
genetic traits.
• Hybridization can increase genetic diversity in weed
populations and potentially confer novel adaptations, such as
increased invasiveness or resistance to control measures.

8. RAPID EVOLUTION
• Weeds are known for their ability to rapidly evolve in
response to changing environmental conditions or human
intervention.
• Due to their short life cycles and high reproductive rates,
weeds can undergo multiple generations in a single growing
season, allowing for faster evolutionary changes compared to
many other plant species.

9. CHARACTERISTICS OFWEEDS
Nature has bestowed the following qualities on weeds:
1. PROLIFIC SEED PRODUCTION
• Produces larger number of seeds compare to crops
• In general, annual weeds are characterized by the
production of very large number of seeds.
• Seed production varies from 40 to 1,96,000 number of
seeds/plant.
• Most of the weed seeds are small in size and contribute
enormously to the seed reserves.
• Weed seeds germinate earlier and their seedlings grow
faster

10. 2.VEGETATIVE REPRODUCTION
The vegetative reproduction is another way by
which the weeds were able to spread.
Some of the perennial weeds multiplied by various
means
like:
 Tubers and nuts: nut grass
 Rhizomes and root stock: in grasses eg.
Bermuda grass & Johnson grass. In quack
grass(Agropyron repens) rhizomes are known
as soboles.
Bulbs: wild onion and wild garlic
Roots: Canada thistle (Cirsium arvense)
Stems: dodder (Cuscuta spp) and
 Offsets: Aquatic weeds (Pistia and Eichhornia)

12. 3. SEED DORMANCY
• It is an efficient survival mechanism of weeds.
• It is defined as a state in which a viable seed
fails to germinate even under favorable
conditions for plant growth.
• Seeds of the most of the weeds exhibit one or
other type of dormancy like inherent or innate or
natural dormancy or induced or secondary
dormancy or enforced dormancy by wild
mustard, black mustard and wild oats respectively.
• Avena fatua exhibit all the these kinds of
dormancy

13. 4. LONGEVITYAND VIABILITY
OF SEEDS
• Weed seeds do not lose their
viability for years even under
adverse conditions.
• Lotus seed collected from the
bottom of the Manchurian
lake bed have viability even
after 1000 years.
• Whereas, wild mustard and
Chenopodium remain viable
for 30 and 20 years,
respectively.
Chenopodium

14. 5. WEED SEED DISPERSALAND
GERMINATION
• Weed dispersal has no geographical
boundaries.
• Common agents of weed dispersal are wind,
water, animals, birds, organic manures,
agriculture implements and human
beings.

15. 6. WIDERADAPTABILITY
• They have ability to thrive under adverse
conditions due to morphological and
physiological conditions Eg. Trianthema (carpet
weed), Sorghum halepense and Saccharum
spontaneum

16. 7.MODIFIED STRUCTURE IN
ORDER TO COMPETEWITH CROP
PLANTS
• They are provided with
hooks, stiff hairs,
thorns and spines
• Eg. Argemone
mexicana , Xanthium,
Tribulus, and
Achyranthus spp.
Xanthium

17. Other characteristic features like
• They have the capacity to germinate under
varied conditions, but very characteristically
season bound.The peak period of
germination always takes place in certain
seasons in regular succession year after year.
• Germination synchronization between
weeds and crop plants.
• They flower earlier and mature ahead of
the crop they infest.

18. • Rapid establishment and growth of weed seedlings
• Tolerance to shading effects by the crops at the time of
establishment
• Quick response to available soil moisture and
nutrients.
• Most of the weeds possess C4 type of photosynthesis,
which is an added advantage during moisture stress
• Relative immunity to post seeding soil
disturbance practices.
• They possess extensive root system, which go deeper
as well as of creeping type

19. • Mimicry: Resembles the crop plants, morphological
characters are similar to the crop Plants. Wild oat in
wheat crop and Echinochloa colonum in rice.
Echinochloa colonum in rice.