4 Types of Reproduction

Sexual reproduction offers better odds for survival of the species

Getty Images/ Sven Schulze / Eyeem

One of the requirements for all living things is reproduction. To carry on the species and pass down genetic traits from one generation to the next, species must reproduce. Without reproduction, a species could go extinct.

Reproduction can happen in two main ways: asexual reproduction, which requires only one parent, and sexual reproduction, which needs gametes, or sex cells, from a male and a female made by the process of meiosis. Both have advantages and disadvantages, but in terms of evolution, sexual reproduction seems a better bet.

Sexual reproduction involves the coming together of genetics from two parents and hopefully producing a more "fit" offspring that can withstand changes in the environment if necessary. Natural selection decides which adaptations are favorable, and those genes are passed to the next generation. Sexual reproduction increases the diversity within a population and gives natural selection more to choose from in deciding which is the best suited for that environment.

Here are four ways individuals can undergo sexual reproduction. The species' preferred way to reproduce is often determined by a population's environment.

Autogamy

A segmented earthworm undergoes autogamy.
Getty/Ed Reschke

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 parts necessary to make both the male and female gametes for that individual. They don't need a partner to reproduce, but some may 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 in other types of sexual reproduction doesn't happen. The genes all come from the same individual, so the offspring will show traits of that individual. However, they aren't considered clones because the combination of the two gametes gives the offspring a slightly different genetic ​makeup from the parent's.

Organisms that can undergo autogamy include most plants and earthworms.

Allogamy

Sperm fertilizing an ovum.
Getty/Oliver Cleve

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 another individual. The gametes fuse together during fertilization to create the zygote. The ovum and sperm are haploid cells, meaning they each have half the number of chromosomes found in a body cell, which is called a diploid cell. The zygote is diploid because it's 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 father. Since the mother and father each gives only half the chromosomes, the offspring is genetically unique from either parent and even its siblings. This unification of gametes through allogamy ensures different adaptations for natural selection to work on. Over time, the species will evolve.

Internal Fertilization

Pregnant human couple on the beach.
Getty/Jade Brookbank

Internal fertilization occurs when the male gamete and female gamete fuse to undergo fertilization while the ovum is still inside the female. This usually requires some sort of sexual intercourse to happen between a male and 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, such as birds and some lizards, lay the egg and keep it incubated until it hatches. Others, such as mammals, carry the fertilized egg inside the female body until it's viable for live birth.

External Fertilization

Salmon swimming upstream to spawn.
Getty/Alan Majchrowicz

As the name implies, external fertilization occurs when the male and female gametes fuse outside the body. Most species that live in water and many types of plants undergo external fertilization. The female lays usually many eggs in the water and a male sprays sperm over the top of the eggs to fertilize them. Usually, the parents don't incubate the fertilized eggs or watch over them, so the new zygotes must fend for themselves.

External fertilization usually is found only in water because the fertilized eggs need to be kept moist so they don't dry out, giving them a better chance for survival. Hopefully they will hatch and become thriving adults that eventually will pass down their genes to their own offspring.