Evolution of the human genome - Human Gene Evolution - page 62

with these findings, studies of fish (Bailey et al ., 1978) and Xenopus (Hughes and
Hughes, 1983) have both suggested that about 50% of newly duplicated genes are
retained after tetraploidization.
2.2 Mammalian genome evolution
The best estimates of divergence times for the various mammalian orders and the
other major vertebrate lineages have come from the use of extant gene sequences
to calibrate a 'molecular clock' of vertebrate evolution (Kumar and Hedges, 1998).
Gene-specific evolutionary rates often vary quite dramatically (see Chapter 7, sec-
tion 7.1.3) but the use of multiple genes to derive mean divergence times should
yield more accurate and reliable estimates. Kumar and Hedges (1998) therefore
employed 658 genes from 207 vertebrate species to derive a molecular timescale
for vertebrate evolution ( Figure 2.1 ). The divergence times corresponded well to
previous estimates based upon the fossil record. Thus the calculated divergence
time for the jawless fish (Agnatha) was 564 Myrs ago in the Precambrian era.
Interestingly, the molecular data indicated that at least five major lineages of pla-
cental mammals [Edentata (armadillos, anteaters and sloths), Hystricognathi
(porcupines and guinea pigs), Sciurognathi (squirrels), Paenungulata (hyraxes)
and Ferungulata (carnivores)] could have arisen in the early to middle Cretacious
between 130 and 90 Myrs ago. This represents an important revision of previous
estimates of the timing of the adaptive radiation of the mammals [estimated by
Novacek (1992) to have occurred between 100 Myrs and 65 Myrs ago]. Since this
now appears to have predated the Cretacious/Tertiary extinction of the dinosaurs
65 Myrs ago, the adaptive radiation of the mammals could not have been simply a
consequence of the filling of niches vacated by the departing super-lizards. Other
factors such as climatic change and the continental breakup must also have played
a role (Hedges et al ., 1996). This question notwithstanding, the adaptive radiation
of the mammals has been very successful, resulting in the emergence of >4600 liv-
ing species that occupy a very diverse range of habitats and environments.
It has been suggested that the rate of mammalian speciation may have been influ-
enced by the rate of karyotypic change (Bush et al ., 1977; Qumsiyeh, 1994; Wilson et
al ., 1977). However, mammalian genomes still contain significant regions of genetic
linkage that have been conserved through evolutionary time. Indeed, appreciation of
the phenomenon of linkage group conservation between different mammalian
species has led to the identification and chromosomal localization of novel genes in
the human genome. Such studies also promise to aid greatly our understanding of
mammalian genome evolution by, for example, revealing chromosomal inversions or
translocations, duplications of genes or gene regions, or more subtle lesions that may
have led to the functional divergence and specialization of proteins.
Genomic mapping projects are underway for a variety of mammals including
the mouse, rat, cow, sheep, and pig but by far the most data are available for the
mouse (Edwards et al ., 1994; Eppig, 1996; Nadeau et al ., 1995; Nadeau and
Sankoff, 1998; O'Brien et al ., 1993). Comparative mapping data for a range of
mammals including a number of primates are available at http://www.informat-
ics.jax.org/homology.html . The comparative analysis of the human and murine