Introduction and Background
Walruses, whales, dolphins, seals, and manatees are marine mammals. They all have
streamlined bodies, legs reduced to flippers, blubber under the skin and other adaptations for
survival in the water. Although mammals evolved on land, these species have returned to the
sea. But are their adaptations the remnants of traits from a common ancestor or a product of
convergent evolution? Did marine mammals evolve from a single land ancestor who returned to
the ocean, or were there different return events and parallel evolution? Although one cannot go
back in time to observe what happened, DNA sequences contain evidence about the past
relationships of living creatures. These DNA artifacts can illuminate the evolutionary history of
marine mammals.
In this research, the amino acid sequences from the hemoglobin beta protein available in
GenBank were used. The sequences were then processed through bioinformatics software to
build a cladogram showing a putative evolutionary relationship among the selected mammals.
Hemoglobin beta was used because it is a gene that all mammals share: hemoglobin is a good
test molecule since it shows both conservation across species (since it performs the essential
function of carrying oxygen in the blood), and variation between species. Species with unique
challenges such as holding their breath for long underwater dives, may have evolved changes
in their hemoglobin which improved their supply of oxygen. In addition, hemoglobin has been
studied by many evolutionary biologists, so sequences are available in GenBank from many
different organisms.
The marine mammals — walrus, minke whale (a baleen whale), dolphin (a toothed
whale), seal, and manatee — were compared to representative mammals in the major orders of
the class Mammalia. The research question asked is: Did marine mammals evolve from a single
ancestor who returned to the ocean, or were there distinct return events from separate
ancestors? The hypothesis tested is that marine mammals did come from a common land
ancestor.
Materials and Methods
Amino acid sequences for the beta chain of the hemoglobin protein were obtained from
GenBank. The sequences were imported into ClustalX for alignment. The aligned amino acid
sequences were then imported into PhyloWin and analyzed under Maximum Parsimony and a
cladogram was produced. (See Figure 2).
The representative marine mammals were walrus, minke whale (a baleen whale),
dolphin (a toothed whale), seal, and manatee. The representative land mammals were dog
(Canis familiaris, Order Carnivora, GenBank P60524), rat (Rattus norvegicus, Order Rodentia,
GenBank P02091), cow (Bos taurus, Order Artiodactyla representing the herbivores, GenBank
P02070), human (Homo sapiens, Order Primates, GenBank P68871), African elephant
(Loxodonta africana, Order Proboscidea, GenBank P02085). The red kangaroo (Macropus
rufus, Order Diprotodontia, GenBank P02107) served as the outgroup for the cladogram since it
is a marsupial. An outgroup is necessary since it represents the ancestral state for traits and
thereby roots the phylogenetic tree.
Results
The aligned amino acid sequences of the hemoglobin genes from the representative
mammals in this research project are presented in Figure 1. The taller grey bars at the bottom of
this chart represent the areas of the amino acid sequence that have been highly conserved
through evolutionary time and the shorter grey bars represent the areas of the amino acid
sequence that have experienced (and tolerated) genetic changes.
Figure 1: The aligned amino acid sequences of the hemoglobin beta protein from a variety of
marine and land mammals.
The cladogram of the evolutionary relationships between the chosen marine mammals
and the representative land mammals is presented as Figure 2. Seven sites on the tree are
marked in Figure 2. Site 1 shows the evolutionary grouping of baleen and toothed whales. Site 2
shows the grouping of the whales with the cow. Site 3 shows the evolutionary grouping of the
harbor seal, otter and walrus. Site 4 shows the evolutionary grouping of those marine mammals
with the dog. Site 5 shows the manatee which is not grouped with any land mammal or ay other
marine mammal. Site 6 equally shows the African elephant grouped alone. Finally, Site 7 shows
the red kangaroo outgroup on its own branch.
Figure 2: A cladogram representing the putative evolutionary relationships between marine
mammals and land mammals based on the amino acid sequence of the hemoglobin beta
protein.
Discussion
A scan through the aligned sequences in Figure 1 shows some interesting molecular
artifacts. It appears that there was a deletion event of the first triplet codon in all species except
rat and human. All other species are missing the first methionine amino acid. The highly
conserved regions of the protein are assumed to be areas that are critical to the molecule’s
oxygen transport function and therefore cannot tolerate mutations whereas the areas that have
accrued genetic changes were able to tolerate those changes and are therefore assumed to be
noncritical areas of the protein’s structure.
The phylogenetic tree (Figure 2) developed using the amino acid sequences of the beta
hemoglobin protein found in representative marine and land mammals supports the following
conclusions about the evolutionary history of the marine mammals.
Site 1 signifies that the whales, both toothed and baleen, come from a common
ancestor. In addition Site 2 indicates that the whales share a common ancestor with the land
mammal, the cow. This suggests several conclusions about the physiological and morphological
adaptations of these mammals. It would seem that having teeth is the ancestral condition and
that the baleen of the minke whale is a derived characteristic. In addition, since the cow is a
hoofed herbivore, it would seem that the ancestor of the whales likely had these characteristics,
so the carnivorous diet of the toothed whales appears to be a derived trait. Hence, these
changes in diet and morphology may take a relatively short period of time to a evolve.
Site 3 signifies that the walrus, otter and seal have a common ancestor, but a distinct
lineage from the whales and dolphins. Site 4 indicates that the dogs and other members of the
order Carnivora share a common ancestor with this group of marine mammals. This branch of
the cladogram already suggests that there were separate evolutionary events that produced
marine mammals.
Site 5 shows the manatee on its own branch sharing no common ancestor with any
representative marine or land mammal. This would suggest that manatee ancestors returned to
the sea in a separate evolutionary event and that the ancestors and land mammal relatives of
the manatee are now extinct. In parallel, Site 6 indicates that the elephants do not share a
common ancestor with any of the marine mammals. It was originally thought they might share a
lineage with the manatee. However this was not supported by the hemoglobin sequences. It
should be noted that the relative lengths of branches on this cladogram cannot be used to
suggest a timeline for the evolution of these species. The PhyloWin analysis of the aligned
sequences does not take time into consideration, so no conclusions can be made about the
relative timing of each branch point as compared to any other.
The cladogram offers an answer to the research question: Did marine mammals evolve
from a single ancestor who returned to the ocean, or were there distinct return events from
separate ancestors? It appears that the marine mammals represent at least three distinct return
events: the whales, the walrus/otter/seal group, and the manatee. Their ancestors are
represented by a range of modern day mammals from different orders. Consequently the
hypothesis in this research was not supported and the common morphological traits of the
marine mammals are examples of convergent evolution.
The cladogram analyzed in this research was based on the amino acid sequence of a
single protein chain. This research is just a beginning. In order to develop a more sound
understanding of the evolutionary relationships between marine and land mammals, more
research is recommended. Data need to be gathered from analyzing other protein and gene
sequences as well as evaluating fossil data.