2. The species diversity of plants, birds, mammals, and many other taxa declines

Question

2. The species diversity of plants, birds, mammals, and many other taxa declines from tropical regions toward the poles. What hypotheses account for this latitudinal gradient? What evidence is there for and against these hypotheses?

3. A factor that might contribute to increasing species numbers over time is the evolution of increased specialization in resource use, whereby more species coexist by more finely partitioning resources. Discuss ways in which, using either fossil or extant organisms, one might test the hypothesis that a clade is composed of increasingly specialized species over the course of evolutionary time.

(A) FIGURE 3.18 (A) Perhaps the most famous exam- ple of allometry and is the largest of deer, the extinct lrish elk (Megaceros giganteus). Its antlers were larger, relative to body mass, than those of any other deer. (B) A plot of antler 80 60 40 size against a measure of body size for 20 species of deer shows that the lri elk (red point marked M) had the approximate antler size predicted from its body size. Although the antlers are larger, relative to the body, in larger species, the relationship is linear when the logarithms of the measurements are plot- ted. (A from Millais 1897; B after Gould 1974.) 20 40 Height at shoulder (inches) Heterotopy Heterotopy is an evolutionary change in the position within an organism at which a phe notypic character is expressed (from the Greek topos, "place"). Studies of the distribution of gene products have revealed many heterotopic differences among species in sites of gene expression. For instance certain species of deep-sea squid have organs on the that house emitting bacteria. The light is through small lenses made up of the same two proteins that compose the lenses of the squids' eyes (Raff 1996) Heterotopic differences among species are very common in plants For example, the major photosynthetic organs most plants are the but photosynthesis the stem in cacti and many other plants that occupy dry environments. In many unrelated species of (woody vines FIGURE 3,19 Monstera deliciosa is a creeping vine native roots grow along the aerial stem (Figure 3 In some an as these

Explanation / Answer

Answer 1 Antlers of Iris elk were an adaptive advantage mainly for defensive purpose and sexual display.Sexual selection resulted in the selection of large antlers Offspring of large antlers inherted the large antlers trait. Constantly increasing size of antlers eventually led to the extinction of species.

Answer 2 There could be various explanation for the latitudnal gradient

a) Geographical hypotheses states that more biome and area at tropics can support more species.

b)Species energy hypothesis states that more solar energy and water availability at tropics results in more productivity which in turn harbouring more species compared to temperate.

c)Climate harshness hypothesis states that extreme cold environment is tolerated by few species only.

d) Climate stability hypothesis: Climate at temperate is highly variable comared to tropics which do not support wider range of species.

e) Evolutionary rate is higher in tropical region.

f) Few simulation studies also predicted more diversity at the centre.

Answer 3 Chinese bamboo is such an example of specialised species which seed once in 120 years. This can be explained as its trick to save its seed from other animals which may eat them all.

Drocus Gazelle a deer in Africa dispell its uric acid as pellet thereby reducing the amount of water is an example of specialised species'

Cassowary a bird native of Australia can digest a highly toxic fruit cassowary plum is an example of speacialized species.

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