ArtsAutosBooksBusinessEducationEntertainmentFamilyFashionFoodGamesGenderHealthHolidaysHomeHubPagesPersonal FinancePetsPoliticsReligionSportsTechnologyTravel

How And Why Did Sex Evolve

Updated on October 23, 2018
Volvox - Frank Fox
Volvox - Frank Fox | Source

Why sex, when many species can do without?

The existence of sex is puzzling, because many organisms, such as certain plants, some reptiles (e.g. whiptail lizard; Family: Teiidae, in particular the genera Cnemidophorus and Aspidoscelis, but also the brahminy blind snake (Ramphotyphlops braminus)) and some insects manage to reproduce perfectly well without it.

It appears that the combination of genes with individuals of the same species (i.e. sex) occurred as a means to repair damaged DNA. But, there are other reasons, too, why sexual reproduction has an advantage over asexual reproduction.

The difference between asexual and sexual reproduction

The purpose of life is to pass an individual's genes on to the next generation. Although organisms have evolved many different strategies to make this happen, there are only two methods of reproduction: asexual and sexual.


  • Asexual reproduction:
    • the entire organism duplicates itself. Its offspring are exact copies.
    • the ova develop without the need to be fertilised causing the offspring to be an exact copy of the mother.
  • Sexual reproduction: Each individual produces sex cells, called gametes, which carry one copy of each chromosome, instead of two, as in normal cells. Gametes of the male have to fuse with those of the female for reproduction to take place. Because of mixing of the genes, the offspring is no longer identical to its mother, but has some genes from the mother and some from the father.

So, what are the advantages of asexual and sexual reproduction?

  • Asexual reproduction is more efficient, as asexual species do not need to spend time finding a partner to reproduce, and all of their genes are passed on to the next generation. It won't surprise you, therefore, that all bacteria, most plants and even certain animals, such as several insects and reptiles reproduce asexually at least some of the time.
  • Sexual reproduction is less efficient, as finding a mate takes time and energy, and gametes that are not fertilised, go to waste. Nevertheless, sexual reproduction evolved and has become abundant throughout the animal and plant world. Why would that be?

A whiptail lizard

Aspidoscelis tigris
Greg Schechter

Chlamydomonas reinhardtii - in the public domain
Chlamydomonas reinhardtii - in the public domain | Source

An important advantage of sexual over asexual reproduction

It has been suggested that sex started out as a way to repair damage to DNA. Bacteria sometimes exchange bits of DNA via tube-like structures that form between cells. This happens when certain genes (sex genes) are activated by damage of DNA and this phenomenon is therefore coupled with the DNA repair system.

Sex, which is very different from the above, as it involves the fusion of two sex cells, may also have started out as a means of DNA repair. It was found that this happens indeed in the multicellular green alga, Volvox carteri. Most of the time these algae reproduce asexually, but when the temperature rises to 42.5 °C, DNA damage triggered by the heat stress, caused them to resort to sex: females released eggs in the water and males sperm, which combined.

Heat stress causes the algae to produce highly reactive forms of oxygen, collectively called ROS, which are known to damage DNA. When the levels of ROS went up through a threshold, the algae switched to sexual reproduction. However, when the team of researchers added antioxidants to mop up ROS, the algae remained asexual.

A similar mechanism seems to play a role in the single-celled alga Chlamydomonas reinhardtii, where the stress factor leading to sexual reproduction is nitrogen depletion.

In multicellular organisms, such as animals and plants there is a second reason why sexual reproduction is advantageous over asexual reproduction: keeping up with parasites.

Sex maintains genetic variation and this increases the likelihood that some individuals will be resistant to a parasitic disease. Asexual populations can only evolve resistance to a disease if a mutation arises. These, however, are rare, so there is a greater possibility that such an asexual population will go extinct if the disease is particularly harmful.

So, why sex?

  1. recombination of genes results in DNA repair
  2. keeping up with parasites

A colony of Aphis nerii on Oleander - Luis Fernández García
A colony of Aphis nerii on Oleander - Luis Fernández García | Source

Some organisms take advantage of both asexual and sexual reproduction

Aphids reproduce most of the year asexually and bear only daughters, which allows the colony to grow very rapidly.

But under the influence of the shortening day lengths of Autumn, they start producing sexual daughters and sons, which mate and produce the overwintering eggs. In the Spring, daughters hatch from the eggs, which again reproduce asexually. So, aphids take the most of both forms of reproduction!

The brahminy blind snake - the smallest snake on Earth

It is sometimes confused with an earthworm!

Leave your comments here

    0 of 8192 characters used
    Post Comment

    • profile image

      anonymous 

      5 years ago

      Leuk stuk, Marlies. Heel interessant. Ik wist wel van de bijen uiteraard dat de koningin kan kiezen tussen bevruchte of onbevruchte eieren, maar van bladluizen wist ik het niet. Het 2e legsel hommels gaan ook bevrucht de winter in om het voorjaar eitjes te kunnen leggen. Ik vind de natuur prachtig!

    • reginaaj2gerica profile image

      reginaaj2gerica 

      6 years ago

      I'm liking squidoo more and more everyday, with so many talented people on here, you learned something new with each lens created. Excellent Lens!

    • boost3782 profile image

      boost3782 

      6 years ago

      You did very well with the topic. Thanks for a quality lens.

    • Steph Tietjen profile image

      Stephanie Tietjen 

      6 years ago from Albuquerque, New Mexico

      A very interesting lens. I'm thinking (but haven't researched it) that it's the same for plants. There are a lot of crested and monstrosed succulent plants which are only reproduceable via cuttings. When these plants (those that can) reproduce sexually, the offspring are often normal.

    • marlies vaz nunes profile imageAUTHOR

      Marlies Vaz Nunes 

      6 years ago from Amsterdam, the Netherlands

      @JoshK47: Thank you!

    • profile image

      JoshK47 

      6 years ago

      Interesting read.

    working

    This website uses cookies

    As a user in the EEA, your approval is needed on a few things. To provide a better website experience, hubpages.com uses cookies (and other similar technologies) and may collect, process, and share personal data. Please choose which areas of our service you consent to our doing so.

    For more information on managing or withdrawing consents and how we handle data, visit our Privacy Policy at: https://hubpages.com/privacy-policy#gdpr

    Show Details
    Necessary
    HubPages Device IDThis is used to identify particular browsers or devices when the access the service, and is used for security reasons.
    LoginThis is necessary to sign in to the HubPages Service.
    Google RecaptchaThis is used to prevent bots and spam. (Privacy Policy)
    AkismetThis is used to detect comment spam. (Privacy Policy)
    HubPages Google AnalyticsThis is used to provide data on traffic to our website, all personally identifyable data is anonymized. (Privacy Policy)
    HubPages Traffic PixelThis is used to collect data on traffic to articles and other pages on our site. Unless you are signed in to a HubPages account, all personally identifiable information is anonymized.
    Amazon Web ServicesThis is a cloud services platform that we used to host our service. (Privacy Policy)
    CloudflareThis is a cloud CDN service that we use to efficiently deliver files required for our service to operate such as javascript, cascading style sheets, images, and videos. (Privacy Policy)
    Google Hosted LibrariesJavascript software libraries such as jQuery are loaded at endpoints on the googleapis.com or gstatic.com domains, for performance and efficiency reasons. (Privacy Policy)
    Features
    Google Custom SearchThis is feature allows you to search the site. (Privacy Policy)
    Google MapsSome articles have Google Maps embedded in them. (Privacy Policy)
    Google ChartsThis is used to display charts and graphs on articles and the author center. (Privacy Policy)
    Google AdSense Host APIThis service allows you to sign up for or associate a Google AdSense account with HubPages, so that you can earn money from ads on your articles. No data is shared unless you engage with this feature. (Privacy Policy)
    Google YouTubeSome articles have YouTube videos embedded in them. (Privacy Policy)
    VimeoSome articles have Vimeo videos embedded in them. (Privacy Policy)
    PaypalThis is used for a registered author who enrolls in the HubPages Earnings program and requests to be paid via PayPal. No data is shared with Paypal unless you engage with this feature. (Privacy Policy)
    Facebook LoginYou can use this to streamline signing up for, or signing in to your Hubpages account. No data is shared with Facebook unless you engage with this feature. (Privacy Policy)
    MavenThis supports the Maven widget and search functionality. (Privacy Policy)
    Marketing
    Google AdSenseThis is an ad network. (Privacy Policy)
    Google DoubleClickGoogle provides ad serving technology and runs an ad network. (Privacy Policy)
    Index ExchangeThis is an ad network. (Privacy Policy)
    SovrnThis is an ad network. (Privacy Policy)
    Facebook AdsThis is an ad network. (Privacy Policy)
    Amazon Unified Ad MarketplaceThis is an ad network. (Privacy Policy)
    AppNexusThis is an ad network. (Privacy Policy)
    OpenxThis is an ad network. (Privacy Policy)
    Rubicon ProjectThis is an ad network. (Privacy Policy)
    TripleLiftThis is an ad network. (Privacy Policy)
    Say MediaWe partner with Say Media to deliver ad campaigns on our sites. (Privacy Policy)
    Remarketing PixelsWe may use remarketing pixels from advertising networks such as Google AdWords, Bing Ads, and Facebook in order to advertise the HubPages Service to people that have visited our sites.
    Conversion Tracking PixelsWe may use conversion tracking pixels from advertising networks such as Google AdWords, Bing Ads, and Facebook in order to identify when an advertisement has successfully resulted in the desired action, such as signing up for the HubPages Service or publishing an article on the HubPages Service.
    Statistics
    Author Google AnalyticsThis is used to provide traffic data and reports to the authors of articles on the HubPages Service. (Privacy Policy)
    ComscoreComScore is a media measurement and analytics company providing marketing data and analytics to enterprises, media and advertising agencies, and publishers. Non-consent will result in ComScore only processing obfuscated personal data. (Privacy Policy)
    Amazon Tracking PixelSome articles display amazon products as part of the Amazon Affiliate program, this pixel provides traffic statistics for those products (Privacy Policy)