English for

Protist Diversity 
With the advent of DNA sequencing, the relationships among protist groups and between 
protist groups and other eukaryotes are beginning to become clearer. Many relationships that were 
based on morphological similarities are being replaced by new relationships based on genetic 
similarities. Instead of sharing a recent common ancestor, protists with similar morphological 

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characteristics may have evolved analogous structures in response to similar selective pressures. 
Convergent evolution is the name given to this phenomenon. It is one of the factors making protist 
classification difficult. The emerging classification scheme groups the entire domain Eukaryota into 
six “supergroups” that contain all of the protists as well as animals, plants, and fungi (Figure 5.10); 
these include the 
Excavata
, 
Chromalveolata
, 
Rhizaria
, 
Archaeplastida
, 
Amoebozoa
, and 
Opisthokonta
. The supergroups are thought to be monophyletic, meaning that all of the members are 
more closely related to one another than they are to other organisms. This is because each supergroup 
is thought to have evolved from a single common ancestor. Evidence for some groups' monophyly is 
still lacking. 
Human Pathogens 
Many protists are pathogenic parasites that depend on infecting and spreading other organisms 
in order to live and grow. The parasitic protozoa that cause malaria, African sleeping sickness, and 
waterborne gastroenteritis in humans are protozoa. Other protist pathogens feed on plants, resulting 
in widespread crop destruction. 
Plasmodium Species 
To complete their life cycle, members of the Plasmodium genus must infect both a mosquito 
and a vertebrate. In vertebrates, the parasite grows in liver cells before moving on to infect red blood 
cells. With each asexual replication cycle, the parasite bursts from and kills the blood cells. The four 
Plasmodium species that can infect people are: 
P. falciparum
accounts for 50 percent of all malaria 
cases and is the primary cause of disease-related fatalities in tropical regions of the world. In 2010, it 
was estimated that malaria caused between 0.5 and 1 million deaths, mostly in African children. 
During the course of malaria, 
P. falciparum
can infect and destroy more than one-half of a human’s 
circulating blood cells, leading to severe anemia. The host immune system mounts a massive 
inflammatory response in response to waste products released as the parasites burst from infected 
blood cells, causing delirium-inducing fever episodes as the parasites destroy red blood cells and 
release parasite waste into the blood stream. Humans contract P. falciparum from the African malaria 
mosquito, 
Anopheles gambiae
. Techniques to kill, sterilize, or avoid exposure to this highly 
aggressive mosquito species are crucial to malaria control. 

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