process. Male apomixis is rare but possible, in which the pollen or male gamete makes the genetic material of the entire embryo. As mentioned, some species have the ability to alternate between asexual and sexual reproduction. This is called heterogamy. Aphids will do this, depending on the environmental conditions. So can some bee species, reptiles, birds, and some amphibians. Daphnia is a freshwater crustacean that will undergo parthenogenesis under conditions of a sparse population but will move on to sexual reproduction when competition is great enough. Some animals engage in polyembryony. This is when the egg is fertilized but the early embryo breaks up into multiple identical clones. This is obligatory in some animals but a sporadic thing in other animals, such as mammals, including humans. Throughout evolution, there are some that have been successful in reproducing asexually throughout the course of evolutionary time. Bdelloid rotifers create only females and do so asexually. They have done this for millions of years. Stick insects engage in parthenogenesis only—also for the past many millions of years.
EVOLUTION OF SEXUAL REPRODUCTION When it comes to evolution, sexual reproduction came along after asexual reproduction. Many types of organisms participate in this process. A few organisms already mentioned have lost the ability to reproduce sexually or have developed things like parthenogenesis and apomixis in order to circumvent dependence on sexual reproduction. Prokaryotes developed sexual reproduction about 2 billion years ago, and eukaryotic organisms developed it as well from a common eukaryotic ancestor. In sexual reproduction, recombination occurs in the making of gametes, and the genotype of the offspring is a mixture of the genotypes of the parents. It does not create identical offspring as the parent, which is what happens in asexual reproduction. As you will see, there are evolutionary advantages to sexual reproduction, which is why it has persisted for so long in the evolutionary process. Even so, there are disadvantages to the process of sexual reproduction. There is a population expansion disadvantage. Because sexual reproduction requires a male and female member, only half of the population can carry the offspring, leading to a decreased number of offspring than can be gotten through asexual reproduction. This is referred to as the two-fold cost of this type of reproduction.
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