1. Descent with modification, Fitness as a result of adaptation, and Struggle for existence.

2. Descent with Modification
The evolutionary term descent with modification is most often associated with human evolution, but it is actually a more general term than that would suggest.
Simply put, descent with modification means that traits are passed down from generation to generation and sometimes undergo changes or modifications over time
These changes may be caused by natural selection, or even a mutation of DNA.

3. Descent with Modification
The term became popular when Charles Darwin used it in his book “The Descent of Man”
On a microevolution level, descent with modification would indicate a change in DNA that leads to a new or different adaptation that could be favourable for a species in that given environment
On a macroevolution level, this term would describe how a species has changed over time and may have even become a new species or gave rise to an additional species through speciation.

4. Descent with Modification
Descent with modification can be used to describe changes in a species over generations. For instance, modern day elephants have the traits they have because of descent with modification from their ancestors. Primitive relatives of elephants were as small as a domestic dog at one point.
Other points throughout an elephant's evolutionary history had species with long, shaggy hair and extremely long and curly tusks. Through descent with modification, the elephant is now a large animal with smaller tusks and much less hair.

5. Descent with Modification VS Natural Selection
Natural selection was Darwin's explanation for how species changed over time. For example, in a cold environment where thick fur helps an animal survive, individuals with a gene for thicker fur may live longer, which gives them a chance to produce more offspring.
Offspring who inherit the thick-fur gene live longer, too, producing more descendants, until the thick- furred individuals dominate the population.
Just as a dog breeder selects for particular traits, the environment also favors traits that enable individuals to survive and breed.

6. Descent with Modification VS Natural Selection
Natural selection and descent with modification may sound like the same process, but they are not. Genetic modification(which is how descent with modification occurs) can happen for multiple reasons besides natural selection. Some modifications occur because of genetic drift or horizontal gene transfer.
These modifications of the organisms occurs independent of selective pressures.
Selective pressures: reason for an adaptation to be necessary to survive
Example: certain types of beaks were necessary in the Finches because of the selective pressure of food source.

7. Descent with Modification VS Natural Selection
One difference between natural selection and descent with modification is that natural selection doesn't appear to create variation or give rise to new genetic traits. Natural selection works with the genetic mix already in the population. The environment favors certain heritable traits over others and may even influence the expression of some genes, but it can't favor a heritable trait that doesn't exist.
Genetic modification includes mutation and horizontal gene transmission, which add new genetic strains to the population. Without them, the genetic range of a species would be much more limited.

8. Fitness as a result of Adaptation
An adaptation is a feature that is common in a population because it provides some improved function.
Fitness: describes how good a particular organism is at leaving offspring in the next generation relative to how good other organisms are at it.
If brown beetles consistently leave more offspring than green beetles because of their color, you'd say that the brown beetles had a higher fitness.

9. Fitness as a result of Adaptation
Fitness is a relative thing. An organism's fitness depends on the environment in which the organism lives. The fittest organism during an ice age, for example, is probably not the fittest genotype once the ice age is over.
The fittest individual is not necessarily the strongest, fastest, or biggest. An organisms fitness includes its ability to survive, find a mate, produce offspring — and ultimately leave its genes in the next generation.
So the phrase “Fitness as a result of adaptation” just means an organism is able to survive, find a mate, and reproduce due to an adaptation which was advantageous for it.

10. Struggle for existence
Continuous struggle for existence in nature, from which only the “fittest” will survive.
There are far too many organisms that are produced in comparison to the amount of resources available on the planet.
Therefore, to sustain an environment, there is a natural struggle for existence.

11. Struggle for existence
For Darwin, the inevitability of a struggle for survival was the key to evolution by ‘natural selection’. Any individual plants and animals that happened to vary in an advantageous way would be more likely to triumph over their competitors.
This struggle took many forms
Predators
Food/Territory
Competition for mate
Extreme weather

12. Struggle for existence
Predators
Struggle for existence when predators are always on the look out for prey. Organisms with a better chance of camouflaging, or escaping the predator have a better chance of surviving. Therefore, the won the struggle for existence
Food
Struggle for existence when the type of food sources available are better suited to one organism compared to another. The organism with the disadvantage loses the struggle and dies.

13. Struggle for existence
Competition for mate
Competition for mate can also be a struggle for existence. That is because some animals have to either fight a male competitor for a female, or have to be more impressive than the male rival. Therefore, whoever gets the female can then reproduce, and has won the struggle for existence
Extreme weather
Weather can also be a factor in this portion. If an organism resides in a hot and dry climate, they are more likely to survive if they aren’t furry, whereas if they reside in a cold environment, they would survive.