One of the requirements for all living things is reproduction. In order to carry on the species and pass genetic traits down from one generation to the next, reproduction must happen. Without reproduction, a species could go extinct.
There are two main ways individuals can reproduce. These are asexual reproduction, which only requires one parent, and sexual reproduction, which is a process that needs gametes (or sex cells) from a male and a female made by the process of meiosis in order to occur. Both have advantages and disadvantages, but in the terms of evolution, sexual reproduction seems to be a better bet.
Sexual reproduction involves the coming together of the genetics from two different parents and hopefully producing a more "fit" offspring that will be able to withstand changes in the environment if necessary. Natural selection decides which adaptations are favorable and those genes will then get passed down to the next generation. Sexual reproduction increases the diversity within a population and gives natural selection more to choose from to decide which is the best suited for that environment.
There are different ways individuals can undergo sexual reproduction. The species preferred way to reproduce is often determined by what environment a population lives in.
The prefix "auto" means "self". An individual that can undergo autogamy can fertilize itself. Known as hermaphrodites, these individuals have fully functioning male and female reproductive system parts necessary to make both the male and female gamete for that individual. They do not need a partner to reproduce, but some may still be able to reproduce with a partner if the opportunity arises.
Since both gametes come from the same individual in autogamy, the mixing of the genetics like other types of sexual reproduction does not happen. The genes all come from the same individual so the offspring will still show traits of that individual. However, they are not considered clones because the combination of the two gametes does give the offspring a slightly different genetic make up than what the parent shows.
Some examples of organisms that can undergo autogamy include most plants and earthworms.
In allogamy, the female gamete (usually called an egg or ovum) comes from one individual and the male gamete (usually called the sperm) comes from a different individual. The gametes then fuse together during fertilization to create the zygote. The ovum and the sperm are haploid cells. This means they each have half the number of chromosomes that are found in a body cell (which is called a diploid cell). The zygote is diploid because it is a fusion of two haploids. The zygote can then undergo mitosis and eventually form a fully functioning individual.
Allogamy is a true mixing of genetics from the mother and the father. Since the mother only gives half the chromosomes and the father only gives half, the offspring is genetically unique from either parent and even its siblings. This unification of gametes through allogamy ensures there will be different adaptations for natural selection to work on and, over time, the species will evolve.
Internal fertilization is when the male gamete and the female game fuse to undergo fertilization while the ovum is still inside of the female. This usually requires some sort of sexual intercourse to happen between a male and a female. The sperm is deposited into the female reproductive system and the zygote is formed inside the female.
What happens next depends on the species. Some species, like birds and some lizards, will lay the egg and keep it incubated until it hatches. Others, like mammals, will carry the fertilized egg inside the female body until it is viable enough for a live birth.
Just as the name implies, external fertilization is when the male gamete and female gamete fuse outside of the body. Most species that live in water and many types of plants will undergo external fertilization. The female will lay usually many eggs in the water and a male will come along and spray their sperm over the top of the eggs to fertilize them. Usually, the parents do not incubate the fertilized eggs or watch over them and the new zygotes are left to fend for themselves.
External fertilization is usually only found in water because the fertilized eggs need to be kept moist so they do not dry out. This gives them a better chance for survival and they will hopefully hatch and become thriving adults that will eventually pass down their genes to their own offspring.