All New World monkeys have been found to have a single M/L photo-pigment gene on the X chromosome. This gene limits males to have dichromatic or monochromatic color vision. Only heterozygous females for the gene were trichromatic. Male howler monkeys have recently been found to have more than one M/L pigment gene, and both are uniformly trichromoatic. These observations show DNA similarity between all new world monkeys because they have the M/L gene that codes for pigment. Evolution is shown in the howler monkey because it has 2 of the genes and it always codes for trichromatic vision, unlike the other monkeys. This shows how the howler monkey has differently evolved.

Also, new world monkeys such as marmosets have the common gene WFIKKN1. This gene is turned on in New World Monkeys, specifically marmosets, that encodes for the twinning process in reproduction. This shows DNA similarity in new world monkeys and also how marmosets have evolved to have this occur more often and for the gene to be more active. 
http://www.sciencedirect.com/science/article/pii/S0042698998000789

http://www.scielo.br/scielo.php?script=sci_arttext&pid=S1415-47572004000300007

Chromosome numbers in New World monkeys range from 16 in the titi monkey to 62 in the wolly monkey. There are many close numbers between new world monkeys such as the silvery marmoset, pygmy marmoset, and Common Squirrel Monkey with 44 chromosomes and the Goeldi’s marmoset with 46 chromosomes. These similarities show how close new world monkeys are similar to each other. Because these four monkeys have such close chromosome number this shows that they are evolutionary similar. In addition, the wide range of chromosome numbers within new world monkeys (titi to wolly) shows that there is also a wide range of evolutionary relationships within the platyrrhines. The chromosme numbers show how New World Monkeys as a whole have evolved together, but also shows how specific monkeys have changed within the Platyrrhines.
https://openi.nlm.nih.gov/detailedresult.php?img=PMC1963484_1471-2148-7-S2-S11-3&req=4

Anatomy and physiology
Anatomical

New world monkeys have broad faces, widely spaced noses, long tails, and a vomeronasal organ. These similar traits have been passed down from older new monkeys in the platyrrhine. This shows evidence of evolution because these traits are similar through several generations and have been passed on to other platyrrhines. These structures such as the vomeronasal organ has shown evolution in new world monkeys from other organisms with their structure such as humans because it has evolved to become different in monkeys. 
Homologous
A homologous structure in platyrrhines is the skull structure. The organisms in platyrrhines have a distinct forehead, two eye sockets, distinct nasal cavity, and jaw structure going up the face. This is similar with all of the organisms in platyrrhine and this structure of the skull is shared with all of them.  The similar skull structure in New World Monkeys show how they have evolved together as a whole, so that they contain this similar trait.
http://www.ncbi.nlm.nih.gov/pubmed/22042751

Coevolution
Spider Monkeys have evolved to gain a characteristic that allows their sense of smell to determine what fruit is repined to perfection. When these fruits are ripened to their liking, the monkeys will eat them. At the same time that these monkeys were evolving, these tropical fruit trees were also adapting to their needs. The plants have evolved to ripen only when their seeds are ready to be properly dispersed for maximum reproduction. This helps both the monkeys with their nutrition and the fruits with their dispersal.
http://www.natureworldnews.com/articles/17600/20151018/coevolution-monkeys-distinguish-ripened-fruit-based-odor-researchers.htm
Evolutionary Mechanisms
Microevolution

Spider monkeys don’t have thumbs. They have four curved fingers that wrap around, which helps with life in the forest to grab on to trees and other objects. This is microevolution because Spider Monkeys are the only New World Monkeys with out thumbs. This evolution has only occurred in Spider Monkeys because it benefits them in their environment.
http://www.cabrillo.edu/~crsmith/monkeycomparisons.html
http://www.softschools.com/facts/animals/spider_monkey_facts/311/
Genetic Drift
The spider monkey and Capuchin monkey have a different allele frequency from the rest of the New World Monkeys for the L-M opsin gene (photopigment generegions in their DNA. The L-M opsin gene codes for what the Monkeys can see through their eyes and what colors they can see. Specifically in Capuchin and Spider Monkeys the allele frequency is much different in those regions than the rest of monkeys. This shows genetic drift because the allele frequencies greatly changed in these two new world monkeys. Furthermore, the spider monkey specifically had even more allele outliers in the L-M opsin gene region except for exons 1 and 6. This shows more genetic drift in Spider monkeys in the L-M opsin gene region. This would not have a huge impact if this population got even bigger because the colors that they monkeys see, don’t greatly affect how well they live. If in some instance it did, and this population got bigger, this could hurt the population because it will become less genetically fit to survive. 
http://mbe.oxfordjournals.org/content/27/2/453.full
Emigration
In White Headed Capuchin Monkeys, after around 4 years males start to leave their troops that they were born into. On the other hand, females tend to stay with the same group. This results in male genes in Capuchins leaving their troops and being dispersed throughout the Capuchins around the region in which they live in. This could result in significant genetic diversity between troops of Capuchin. The male Capuchins will be dispersing their genetic make up between troops once they spread randomly to different troops. This will create troops of Capuchins to be genetically different because the males are migrating to different troops. Females that stay will help retain the unique features between troops because their genes won’t leave their troops. 
http://animaldiversity.org/accounts/Cebus_capucinus/

Physical:
Relatively broad noses, side facing, widely separated nostrils, long tails, one child born at a time, and contain a unique vomeronasal organ. A vomeronasal organ is a sense organ that is located close to the vomer and nasal bones. It contains sensory neurons that detect chemical stimuli.
Behavioral:
Reproduce with one baby born at a time, they are arboreal, spend most of the day searching for food and sleeping at night. They form monogamous pair bonds and show substantial care of their young. Usually live in very large groups and these sometimes break into smaller troops. They are mostly herbivorous. Mostly eat leaves, fruits, nuts, gums, and small prey like insects. 
Habitat:
Limited to tropical forest environments of Southern Mexico, Central, and South America. In addition, they are mostly arboreal and herbivorous. They live throughout the Amazonian ecosystem and the neotropics. 

Each of these Cebidae monkeys have around the same number of chromosomes. The monkeys A,B, and D have 44 chromosomes total and the Goeldi’s marmoset has 46. This shows that they are all very closely related, but those three with 44 are even  more closely related. Also, the Goeldi’s marmoset has different sex chromosomes because it has both a X1 and X2 chromosome unlike the rest of the monkeys. This shows how this marmoset is less like the other monkeys. Human’s have 46 chromosomes and one X and Y chromosome. These karyotypes of monkeys show close similarities to Humans. The monkeys have either 44 or 46 chromosomes, which show striking similarities between these monkeys and humans and show that they are closely related ancestrally. The Goeldi’s marmoset is very similar because both the human and those monkeys have 46 chromosomes. They are different because humans have one X, while the marmosets have a X1 and X2 chromosome. The other three monkeys are close to humans because they have 44 chromosomes , which is very close to 46 and they have one X and Y chromosome just like humans.

Age: 20 million years ago

Fossil: Skull
http://digimorph.org/specimens/Tremacebus_harringtoni/

Time Period: Middle Miocene

Fossil: Skull
http://cryptomundo.com/cryptozoo-news/new-skulls/

Middle Miocene Time Period

Fossil: Right upper first molar
http://morphosource.org/index.php/Detail/MediaDetail/Show/media_id/7431

Less than 500,000 years ago

Fossil: Complete Skeleton
http://pages.nycep.org/rosenberger/pdfs/Halenar%20and%20Rosenberger%20JHE%2013%20(Cartelles).pdf

Less than 500,000 years ago in Pleistocene time period

Fossil: Carium and Mandible
http://fossilmatter.blogspot.com/2015/05/varonas-fossil-monkey-paralouatta.html