Slide 1
As we discuss the biological diversity found on earth - it is
useful to take a historical look at our understanding of biodiversity and the
evolutionary relationships between organisms.
Darwin envisioned the history of life on earth as a tree, as he sketched here in one of his notebooks. This tree shows the recognition that all life on earth shares a common ancestor.
The branches on the tree reflect groups of organisms and until fairly recently all organisms were grouped into 5 kingdoms - kingdoms animalia, plantae, fungi, bacteria (also known as monera), and kingdom protista which included most of the single-celled eukaryotic organisms.
As you learned in the introduction to the prokaryotes - this 5 kingdom system is outdated and we know classify organisms into three domains - the bacteria and the archaebacteria - which we have already looked at - and eukarya which includes all of the eukaryotes.
Slide 2
A modern tree of life is much more complicated than anything
Darwin ever imagined. The number of different species on earth is
immense but one aspect of this tree that is the same as Darwin's tree is the
recognition that all life shares a common ancestor.
Slide 3
As we have learned more about the different types of organisms -
the tree of life has become more complicated. This is due, in part, to
the fact that what used to be the kingdom Protista has now been broken down
into a number of kingdoms. This reflects the fact that the organisms
that were all grouped into kingdom Protista do not all share a most recent
common ancestor or shared characteristics.
Slide 4
To understand why kingdom Protista was split into multiple
kingdoms - we need to understand more about how classification systems are
developed and there are some terms that need to be defined.
The term paraphyletic refers to a group that does not include all of the descendents of a common ancestor.
For example, protists are paraphyletic because animals, plants and fungi arose from within protists but are not included in the group.
The term monophyletic refers to a group containing all of the descendants of a common ancestor
For example, the alveolates are monophyletic and are distinguished by the presence of small cavities just beneath the plasma membrane.
The term polyphyletic refers to a group which, through convergent evolution, contains individuals with similar characteristics but who have different ancestors.
For example, previous classification of the protists grouped them as animal-like, plant-like, fungal-like - based on convergence of ecology, not true evolutionary relationships.
The goal of developing a classification system is to base it on monophyletic groupings. Paraphyletic groupings and polyphyletic groupings do not accurately reflect the evolutionary relationships between different organisms.
Slide 5
Convergent evolution is the evolution of species from different
taxonomic groups toward a similar form; the development of similar
characteristics by taxonomically different organisms.
This illustration shows an example of convergent evolution in four different animals from around the globe. They may look similar, but it's not because they're close relatives. Instead, they've evolved similar adaptations because they occupy similar niches -- dining on ants, hunting in the high grass, or swimming in the dark -- although their evolutionary origins are quite different.
Slide 6
All of the protists were originally grouped together into a single
kingdom in large part because they superficialy appeared similar as a result
of convergent evolution.
We now recognize that splitting the protists into multiple kingdoms makes more evolutionary sense.
This area of study has not been settled so the classification system that we are going to discuss has not been agreed upon by all biologists. However, we will be discussing the archaezoa, the euglenozoa, the alveolata, the stramenopilans, the chlorophyta, and the rhodophyta. I will use the term protists casually to describe these organisms but recognize that this term does not have any real biological meaning.
Slide 7
The first group of organisms that we will be looking at are those
that have modified mitochondria. These organisms used to be grouped
together in a kingdom called Archaezoa - but we have since learned that there
are significant differences between the members of this group and the Kingdom
was not monophyletic.
There are two main groups of organisms with highly modified or missing mitochondria. The diplomonads and the parabasalids.
Slide 8
The diplomonads have relic mitochondria but these modified
mitochondria do not have an Electron transport chain and thus do not play a
role in cellular respiration.
These organisms are anaerobic (not having an electron transport chain means they have no need for oxygen) and they are recognized because they have two haploid nuclie and four flagella which gives them a face-like appearance.
The most well-known representative of this group is the organism Giardia which is a parasite that lives in freshwater and, if ingested, causes severe intestinal distress. Giardia is one important reason you want to filter your water if you drink from a stream or lake while you are hiking or camping.
In 2007, scientists sequenced the genome of this organism and discovered that it makes several unique proteins that its animal hosts do not make. These proteins may be a promising target of drugs to treat the infection.
Slide 9
The parabasalids are a group of flagellated single-celled
organisms, most of which are symbiotic in animals.
These include a variety of forms found in the guts of termites and cockroaches, many of which have symbiotic bacteria that help them digest wood. Some other species are human pathogens.
Parabasalids are anaerobic, and lack mitochondria, but this is now known to be a result of secondary loss, and they contain small hydrogenosomes which apparently developed from reduced mitochondria.
The function of the hydrogenosome is to help the anaerobic organism produce ATP from pyruvate.
The parabasalids in the genus Trichonympha are particularly well studied because of their role in the guts of insects such as termites. These organisms use endosymbiotic bacteria to help them digest cellulose.