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Transcript
Lesson 1.20: Reproduction and Variation Key Points: 1. Distinguish between sexual and asexual reproduction. 2. Identify examples of each type of reproduction. 3. Understand how variation is produced in each type. 4. Evaluate the benefits and drawbacks of each type in terms of adaptation to the environment and production of variation. Asexual Reproduction Strictly speaking “like begets like” only applies to organsisms that reproduce asexually. In asexual reproduction, a single individual is the sole parent and passes copies of all its genes on to its offspring. For example, single celled organisms can reproduce asexually by mitosis (cell division), in which DNA is copied and allocated equally to two daughter cells. The offspring are exact copies of the parent. Asexual Reproduction Some multicellular organisms are also capable of reproducing asexually. Hydra a relative of the jellyfish, can reproduce by budding (pic). Since the cells of the bud were derived by mitotic cell division from the parent, the “chip off the old block” is usually (99% of the time) genetically identical to the parent. Very rarely genetic differences caused by changes in the DNA of the parent (called mutations) cause small differences. Asexual Reproduction Some plants and other animals can reproduce asexually as well. For example potatoes produce “little eyes” that are actually buds – groups of cells that undergo rapid cell division and develop into new plants. When potatoes are not used the buds begin to sprout and grow. In plants asexual reproduction is called Vegetative Propagation. Asexual Reproduction Another example is the starfish. Starfish prey on oysters so divers would collect the starfish in an attempt to save the oyster beds. After the divers collect the starfish they would tear them apart and throw the pieces back in the water. A sea star arm however, can regenerate an entire sea star if part of the central body is attached to it. So in an effort to rid the oyster beds of predators the divers actually made their problem worse! Any individual that reproduces asexually gives rise to a clone, a group of genetically identical individuals. Other Examples of Asexual Reproduction Ameoba Mold Bamboo Sexual Reproduction (Lets keep our minds out of the gutter please!) Compared to asexual reproduction, sexual reproduction usually results in greater variation; two parents give rise to offspring that have unique combinations of genes inherited from both parents, for example half of your genes came from your mother and half from your father. In contrast to a clone, offspring of sexual reproduction vary genetically from their siblings and both parents. What are some advantages of both types of reproduction? Sexual Reproduction One way sexual reproduction generates genetic variation is shown in the figure below which color codes the chromosomes so that we can track them as they are packaged in gametes (eggs and sperm). The two colors are used to distinguish which chromosomes are inherited from the mother and from the father. So there is a fifty/fifty chance that a gamete produced by an offspring will have a maternal chromosome or a paternal one. Sexual Reproduction In the case of humans the number of possible combinations of maternal and paternal chromosomes in the resulting gametes is 223, or about 8million. Thus each gamete that a human produces contains one of 8 million possible assortments of chromosomes inherited from that individuals mother and father. Crossing over produces even more variation Some organisms use both methods Some organisms (like the paramecium) and most plants can reproduce using either method. This can be highly advantageous, Why?