Biology Reference
In-Depth Information
presence of a nucleus, microtubules, mitochondria, and a chimeric genome (
Katz
1998
). How all these parts were assembled remains controversial; some hypoth-
esize there was a single endosymbiosis event and others suggest that there were
two or more endosymbiosis events. It is possible that the original event that gave
rise to mitochondria occurred in the ancestor of all extant eukaryotes, which
could explain both the chimeric nuclear genome and the origin of mitochondria.
Genome duplication
is thought to be an important component of the evolution
of eukaryotic genomes (
Ohno 1970, Wagner 1998, Sankoff 2001
). Genome or gene
duplication is thought to be a common method by which new gene functions can
evolve, despite the fact that the vast majority of duplicate genes are expected to
become pseudogenes through mutations. The loss of a duplicate gene is expected,
because as long as one gene functions normally the other can accumulate del-
eterious mutations. However, many protein-coding genes belong to multigene
families, which likely evolved by gene duplication (
Friedman and Hughes 2001
).
Analysis of the complete genomes of
Drosophila melanogaster
, the nematode
Caenorhabditis elegans
, and the yeast
Saccharomyces cerevisiae
showed that
duplication of genomic blocks has occurred, although the duplications did not all
occur at the same time (
Friedman and Hughes 2001
). Some blocks could have been
due to an ancient polyploidization event, whereas others are more recent and
could have involved duplications of chromosome segments. However, as much as
one-third of eukaryotic genomes may consist of unique genes with no apparent
homologues in other organisms, suggesting that novel genes can evolve as well.
12.8 The Fossil Record of Arthropods
Insects have a relatively extensive fossil record beginning in the Cambrian, with
1263 families of fossil insects known (
Labandeira and Sepkoski 1993, Budd and
Telford 2009, Edgecombe 2010
).
Labandeira and Sepkoski (1993)
found 472 ref-
erences on fossils covering 1263 insect families, with all of the commonly recog-
nized extant orders of insects represented as fossils.
Edgecombe (2010)
indicates
that the earliest arthropod body fossils can be found in Stage 3 of the Cambrian
and that arthropods have been the dominant component of animal species
diversity for the past 520 million years, although
Grimaldi (2010)
suggests that
the earliest divergences of hexapods were perhaps in the Late Silurian.
Schaefer
et al. (2010)
argue that oribatid soil mites (Arthropoda: Chelicerata: Arachnida:
Acari) originated in the Precambrian (571 mya) and that the radiation of basal
groups coincides with the gap in the terrestrial fossil record between the
Cambrian explosion and the earliest fossil records on land. These authors sug-
gest that the colonization of land started
>
150 million years earlier than the
oldest fossils that are from terrestrial ecosystems.
