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In the density-limited growth examined thus far, the ecological effects of density fed back fast quickly enough that the population’s growth could adjust and the population could reach a carrying capacity, equal to −r /s . But if the growth rate is too fast compared with that feedback, the population can overshoot its carrying capacity, which can lead to highly complex outcomes.
Think about feedback in this way. Imagine driving down the road, keeping your eye on the road, instantly correcting any little deviations of your car from your lane, adjusting the steering wheel without even perceiving it, and with only normal blinking of your eyes. In this case there is very little delay in your feedback to the steering wheel, and you stay in the lane. Now suppose you close your eyes for one second at a time, perhaps every ten seconds. (Do not run this experiment; just think about it!) You may have drifted a bit to the left or right in that second and would have to turn the steering wheel further to get back in you lane. And now imagine shutting your eyes for 15 seconds every minute, then opening them and correcting your path down the road. You’ll start oscillating in your lane and precariously jerking back and forth, possibly visiting the ditch. The cause? The delay in the feedback between stimulus and response.
So it is with populations. Delays in sensing the carrying capacity can start oscillations. For example, a modeled insect population that grows and lays eggs one year and emerges the next year can suffer such oscillations. The insects are, in effect, “keeping their eyes shut” about how many insects will be produced the next year. This is in contrast to species like bacteria or humans, where the population grows more or less continuously.
It is difficult to mistake whiptail lizard copulation for anything else (Figure 7.1). A lizard mounting another lizard from behind certainly suggests sex. However, whiptail lizards of the genus Apidoscelis are all females and reproduce asexually. Eggs are produced through meiotic division, but then undergo a doubling of chromosomes to return to a full set of chromosome pairs. This type of asexual reproduction, in which embryos develop from unfertilized eggs, is called parthenogenesis (from the Greek, meaning “virgin birth”), and is seen in many plants and animals.
These lizards are doubly intriguing because they may exhibit a stereotypical mating behavior in fact, this behavior has been shown to increase the number of offspring the lizards produce (even though there are no sperm involved!).
Species like the whiptail lizards highlight the major problem with sex. There are many asexual organisms such as these lizards (figure 1), and they appear to be doing just fine without sexual reproduction. This ability to thrive without sex demands an answer to the question: why does sex exist? Why would any organism incur all the costs associated with sexual reproduction, if asexual reproduction has so many obvious advantages?
Figure 7.1 Female – female copulation in whiptail lizards (Apidoscelis uniparens). Note the scientific name: “uniparens” means “one parent.” Which female assumes the dominant “male” role and which female assumes a “female” role depends on which female is ovulating.
Life Cycles of Sexually Reproducing Organisms
Fertilization and meiosis alternate in sexual life cycles. What happens between these two events depends on the organism. The process of meiosis reduces the resulting gamete’s chromosome number by half. Fertilization, the joining of two haploid gametes, restores the diploid condition. There are three main categories of life cycles in multicellular organisms: diploid-dominant, in which the multicellular diploid stage is the most obvious life stage (and there is no multicellular haploid stage), as with most animals including humans haploid-dominant, in which the multicellular haploid stage is the most obvious life stage (and there is no multicellular diploid stage), as with all fungi and some algae and alternation of generations, in which the two stages, haploid and diploid, are apparent to one degree or another depending on the group, as with plants and some algae.
Nearly all animals employ a diploid-dominant life-cycle strategy in which the only haploid cells produced by the organism are the gametes. The gametes are produced from diploid germ cells, a special cell line that only produces gametes. Once the haploid gametes are formed, they lose the ability to divide again. There is no multicellular haploid life stage. Fertilization occurs with the fusion of two gametes, usually from different individuals, restoring the diploid state (Figure 7.2 a).
Figure 7.2 (a) In animals, sexually reproducing adults form haploid gametes from diploid germ cells. (b) Fungi, such as black bread mold (Rhizopus nigricans), have haploid-dominant life cycles. (c) Plants have a life cycle that alternates between a multicellular haploid organism and a multicellular diploid organism. (credit c “fern”: modification of work by Cory Zanker credit c “gametophyte”: modification of work by “Vlmastra”/Wikimedia Commons)
If a mutation occurs so that a fungus is no longer able to produce a minus mating type, will it still be able to reproduce?
Most fungi and algae employ a life-cycle strategy in which the multicellular “body” of the organism is haploid. During sexual reproduction, specialized haploid cells from two individuals join to form a diploid zygote. The zygote immediately undergoes meiosis to form four haploid cells called spores (Figure 7.2 b).
The third life-cycle type, employed by some algae and all plants, is called alternation of generations. These species have both haploid and diploid multicellular organisms as part of their life cycle. The haploid multicellular plants are called gametophytes because they produce gametes. Meiosis is not involved in the production of gametes in this case, as the organism that produces gametes is already haploid. Fertilization between the gametes forms a diploid zygote. The zygote will undergo many rounds of mitosis and give rise to a diploid multicellular plant called a sporophyte. Specialized cells of the sporophyte will undergo meiosis and produce haploid spores. The spores will develop into the gametophytes (Figure 7. 2 c).
Chapter 7: Introduction to the Cellular Basis of Inheritance
Figure 7.1 Each of us, like these other large multicellular organisms, begins life as a fertilized egg. After trillions of cell divisions, each of us develops into a complex, multicellular organism. (credit a: modification of work by Frank Wouters credit b: modification of work by Ken Cole, USGS credit c: modification of work by Martin Pettitt)
The ability to reproduce in kind is a basic characteristic of all living things. In kind means that the offspring of any organism closely resembles its parent or parents. Hippopotamuses give birth to hippopotamus calves Monterey pine trees produce seeds from which Monterey pine seedlings emerge and adult flamingos lay eggs that hatch into flamingo chicks. In kind does not generally mean exactly the same. While many single-celled organisms and a few multicellular organisms can produce genetically identical clones of themselves through mitotic cell division, many single-celled organisms and most multicellular organisms reproduce regularly using another method.
Sexual reproduction is the production by parents of haploid cells and the fusion of a haploid cell from each parent to form a single, unique diploid cell. In multicellular organisms, the new diploid cell will then undergo mitotic cell divisions to develop into an adult organism. A type of cell division called meiosis leads to the haploid cells that are part of the sexual reproductive cycle. Sexual reproduction, specifically meiosis and fertilization, introduces variation into offspring that may account for the evolutionary success of sexual reproduction. The vast majority of eukaryotic organisms can or must employ some form of meiosis and fertilization to reproduce.