The remaining two lectures before the next midterm exam will be devoted to the topic of reproduction.
Reproduction -- biological processes by which existing organisms give rise to new organisms-- since no individual organism is immortal, reproduction is required if species are to persist through time -- extinction occurs when reproductive rates fall below mortality rates.
As we discussed earlier in the semester, there are two modes of reproduction:
1. Asexual reproduction -- only one parent required and the offspring are genetically identical to parent -- mitosis is the mechanism underlying this mode of reproduction.
2. Sexual reproduction -- permits greater genetic variation and evolutionary flexibility -- 2 parents each produce specialized haploid sex cells (gametes) by meiosis -- gametes either sperm or ovum -- different gametes may be produced by same individual (hermaphroditic species) or one individual may belong to one sex or the other -- gametes fuse in a process called fertilization and the resulting single-cell progeny is called a zygote -- zygote then undergoes a period of development.
Let's look at some general patterns of reproduction displayed by animals.
Asexual reproduction -- generally seen in lower animals.
1. budding -- part of individual undergoes repeated mitotic divisions to produce a new individual that breaks off -- e.g. hydra, sea anemone.
2. fragmentation -- part of parent falls off or is broken off -- fragment undergoes repeated mitotic divisions to produce an entire individual -- e.g. starfish.
Some species of animals have evolved from a sexual mode of reproduction back to an asexual mode:
Parthenogenesis -- development of new individuals from ova that are not fertilized by male gametes -- two types:
1. haplodiploidy -- unfertilized eggs develop into individuals that are haploid -- e.g. in honey bees, queens lay two kinds of eggs -- eggs that are fertilized develop into diploid female bees called workers, while unfertilized eggs develop into haploid males called drones.
2. unisexual reproduction -- females produce diploid eggs that develop without fertilization into more females -- species contain only female individuals -- some species of fish and reptiles show this mode of reproduction.
Sexual reproduction -- involves production of haploid gametes and formation of zygotes through fertilization -- zygote then undergoes development -- variety of ways that this is accomplished.
Fertilization may be external of internal to the animal.
External fertilization-- usually seen in aquatic animals such as fish -- male and females shed gametes into water at same time and place and fertilization occurs in the water -- probability of fertilization increased by production of huge numbers of gametes.
Internal fertilization -- increases probability that egg and sperm will meet and that fertilization will occur -- structural adaptations to facilitate passage of sperm into female reproductive tract.
Timing of reproduction: most sexually reproducing animals have a single breeding period per year -- individuals physiologically unable to reproduce at other times of the year-- offspring born at the time of year most favorable for their survival -- e.g. abundant resources, mild climatic conditions.
As mentioned earlier, species must have a reproductive rate that offsets losses due to mortality in order to avoid extinction -- species display reproductive strategies that have been shaped by evolution -- reproductive strategies reflect the environment where the organism lives and its role in its ecological community.
Let’s look at the components of reproductive strategies.
1. age at first reproduction -- usually related to how long individuals live -- animals having short life-spans reproduce at an early age -- e.g. certain bacteria can divide every 8 minutes; fruitflies are sexually mature 14 days after they are born; redwoods become sexually mature in 100 years.
2. number of offspring per reproductive effort -- tremendous variation seen -- one female oyster can produce 60 million eggs in one reproductive effort -- other species (e.g. our own) may have only a single offspring per effort.
3. number of lifetime reproductive efforts -- some species display a pattern called semelparity = only one reproductive effort in lifetime -- e.g. salmon return to the stream where they were born to spawn once and then die -- other species display iteroparity = multiple reproductive efforts per lifetime.
4. amount of parental care -- parental care increases survival of offspring and therefore increases reproductive success -- amount of parental care is inversely related to the number of offspring per reproductive effort -- little or no care in species having large reproductive efforts (e.g. oysters) -- much parental care in animals having small reproductive efforts (e.g. humans).
Now, let's consider the structure and function of the human reproductive system
Male Reproductive System:
1. testes -- male gonads (= gamete producing organs) -- paired organs contained in sac of skin called the scrotum -- two functions of testes are a) production of sperm ( by seminiferous tubules) and b) production of male sex hormone testosterone (by Leydig cells) -- sperm stored and mature in a highly coiled, tube-like structure called the epididymas.
2. vas deferens -- duct (tube) that passes from epididymas up and out of scrotum, over and behind urinary bladder and then merges with the urethra.
a. seminal vesicles -- contribute about 60% of seminal fluid volume -- fructose sugar secreted as energy source for sperm.
b. prostate gland -- alkaline secretions that neutralize acids in urethra -- secretions also contain sugars for energy and other chemicals for sperm activation.
c. bulbourethral glands -- paired glands that secrete a small amount of clear fluid into the urethra -- neutralizes acidic environment of urethra.
3. penis -- special accessory organ which facilitates transfer of sperm from male directly into female reproductive system -- cylindrical organ composed largely of spongy, cavity-filled tissue called corpora cavernosa -- during sexual arousal, blood flow to pelvis and the penis is increased -- cavities in this tissue fill with blood causing the normally flacid penis to become erect -- changes called erection.
Figure 38.14. Male Reproductive System.
Female Reproductive System:
1. ovaries -- paired female gonads -- ovaries contain follicles (400,000 at sexual maturity) -- follicles contain cells that will mature to become ova (eggs) -- one or several follicle produce mature eggs each month after female reaches reproductive age -- follicle ruptures and releases ovum (ovulation) -- ovary also produces the female hormones estrogen and progesterone.
2. oviducts (Fallopian tubes) -- tubes leading away from each ovary and joining to form a large cavity called the uterus (womb) -- tubes lined with cilia to help egg move -- fertilization normally occurs in oviduct.
3. uterus -- cavity where zygote will implant and develop during pregnancy -- opening at bottom called the cervix-- consists of three layers: 1) endometrium -- inner lining consisting of highly vascularized tissue -- sloughed off during menstruation unless pregnancy occurs; 2) myometrium -- muscular layer that produces labor contractions during the birth process; 3) perimetrium -- external covering of uterus.
4. vagina -- cavity below uterus that opens to outside -- "birth canal" -- highly elastic organ into which penis is inserted during intercourse -- surrounded by pelvic floor muscles -- cells of vagina secrete fluids for lubrication during intercourse -- opening to vagina surrounded by two folds of skin: labia majora (outer) and labia minor (inner).
5. clitoris -- knob of erectile (corpora cavernosa) tissue above opening to vagina -- extremely sensitive and has no known function other than to facilitate sexual arousal in the female.
Figure 38.18. Female reproductive system.
Next time: Human Reproduction