natural selection????????

Question

most cactuses have spines, which are leaves modified to protect the plant. the spines cover juicy stem that stores water. explain how cactus leaves and stems might have changed through the process of natural selection.

Answer 1

Spines are leaves that can no longer photosynthesize or exchange gases the stem has taken those functions. Spines may be sharp now but originally the leaves were not the best adaptation for the xeric conditions. The shift from fine sensitive leaves to a photosynthetic water conserving stem was the driving force. As the selective pressure to feed and exchange gases came off the leaves they were open to new pressures. The leaves are semi-vestigal organs put to a new use. They offer shade and surface shelter to the cacti.
Remember there is no best answer. Each plant adapts from the range of variation it has when the selection pressures shift. many desert plants lack spiny leaves yet grow, survive, and reproduce in deserts as well as the spiny cacti. The aloe, the boojum tree, the resurrection plant, or the creosote bush are all examples.
All cactus produce leaves but most are microscopic. There are however the relict genus Pereskia that have non-succulent stems and broad, thin leaves. Their existence supports the hypothesis that cacti evolved to the leafless condition.
Cactus phylogeny
http://www.amjbot.org/cgi/content/full/92/7/1177

Answer 2

The first cacti would have been unprotected. Then maybe one of the offspring mutated to have some sort of covering for the stem. This trait would have been advantageous to the survival of the cacus and hence that cactus would live on long enough to reproduce and pass on its traits. If this catches on, then after millions of years and generations, gradual changes are made and the plants begin to evolve to have spines. Meanwhile, the more primitive plants without spines die out because they have less advantageous characteristics to survive in the enviornment.

Answer 3

You really should do your own homework but here are a couple of hints. Consider the spineless (jungle) cacti, which are the more primitive ones. Also consider the value of spines other than for making cacti more difficult to eat.

Answer 4

The evolutionary conversion of leaves to spines in cacti
http://www.sbs.utexas.edu/mauseth/ResearchOnCacti/Spines.htm

Evolution and Phylogeny of Cacti
http://cactus.biology.dal.ca/evolution.html

Answer 5

Cactii that didn't have spines to deter animals from eating them got eaten and thereby didn't reproduce as readily as cactii that began to develop defenses like spines.