|Introduction to Organismal Biology (BIOL221) - Dr. S.G. Saupe; Biology Department, College of St. Benedict/St. John's University, Collegeville, MN 56321; firstname.lastname@example.org; http://www.employees.csbsju.edu/ssaupe/|
Reproduction: A Primer
| "an individual is the transient caretaker of the genetic instructions that must
be conveyed down the generations."
According to Harold Morowitz, one of my favorite science writers (hes a biochemist), reproduction can be viewed as the mechanism by which an individual passes genetic information through time. So, how do plants and animals convey the genetic instructions from one generation to the next?
II. Plants &
animals can reproduce sexually
Exactly what is "sex"? - biologically "sex" is defined as the process that involves meiosis and fertilization. Thus, during sexual reproduction meiosis ultimately results in haploid gametes that fuse or recombine during fertilization ( also called syngamy).
III. Plants & animals can also produce offspring without sex (asexual
"A" means without; therefore asexual literally translates into reproduction without sex. Thus, during asexual reproduction there is no production of gametes, no fertilization and hence, no genetic recombination. Obviously the offspring will be genetically identical to the parents (=a clone). Some points to ponder:
IV. Why Reproduce Sexually?
At first you might consider this a silly question, but consider:
So, why sex? Because "Sex is not Selfish". In other words, the result of sex is offspring that have different genes than their parents. The fact that offspring are genetically different than their parents is a major advantage because it provides for genetic diversity that is the raw material for evolution.
V. Evolutionary Strategies
A. Males vs. females
For most species, males and females have different strategies when it comes to sexual reproduction. First, we will make the assumption that both sexes are interested in producing the maximum number of genetically fit individuals. So how do males and females optimize this possibility?
Consider gametes - females produce few, large gametes whereas males produce numerous small ones. Since eggs are expensive and only a few offspring can be raised, the evolutionary pressure for a female is for "choosiness" in mate selection. In contrast, sperm are "cheap" and the evolutionary success of a male is limited by his ability to deliver sperm to the egg. Males have evolved to be "salesmen", attractive to females. They advertise good genes by holding territories, displays, courtship rituals, gifts, appearance.
Do humans follow these evolutionary ideas? There is considerable debate. Whadayathink?
B. Female Gamete Choice
Courtship and mating are not the only places females can exert control over breeding. The females of several species are able to control which sperm fertilize an egg AFTER copulation. These females have the ability to discriminate between sperm of different species as well as individual males of their own species (Birkhead, 2000). For example, female fruit flies that mate with another species will not produce fertile eggs even though her sperm stores are full. If artificially inseminated with the same sperm she will lay fertilized eggs. Similarly, if mated with a closely related male and a distantly related male of her species, she will preferentially produce offspring from the distantly related species.
C. Reproductive Output
An individual man can theoretically have more children than an individual female. In fact, the maximum number of children for a female is about 20 whereas the most prolific father in history is presumed to be Ismail the Bloodthirsty, a Moroccan emperor from 1672 1727, who sired 888 children (see Natural History, Sept 1999, p 23). This record is rather suspect even considering his harem of 500 women. It's virtually impossible for him to have fathered all of the children attributed to him when you consider:
To father all of these children in face of these statistics he would have had to mate with 4.8 women daily for four decades. This performance makes Wilt Chamberlain look like a slacker. As an aside, even though males have the potential to have more offspring, in practice there isnt great differences in human reproductive rates between males and females.
If males and females have such differing evolutionary strategies, why isn't monogamy less common? Presumably because a monogamous coupling is paternity insurance for the male, since he can never be sure that the offspring is his.
Are humans a monogamous species? Consider that for other monogamous species the females are usually more aggressive and larger than the males, are territorial, and show other features that we don't possess. As an aside, a study of blood types of parents and offspring done in the 1950's showed that about 10% of offspring born were fathered by someone other than the man presumed to be the father.
E. Gender is relative
Some plants in which there are separate male and female flowers on different individuals (e.g., Jack-in-the-pulpit) are able to change sex. In other words, one year the plant will produce male flowers and the next female flowers. Typically the first few years the plants are male and then about the seventh year they switch and produce female flowers. Subsequent years are determined by the growing season of the previous season. This allows the plants to maximize their reproductive output. Some fish are also able to change sex!
Last updated: April 16, 2009 � Copyright by SG Saupe