Birds do it, bees do it, even educated sawflies do it

Bumblebee mimic hoverfly mating (Volucella bombylans) - sexual reproduction is popular!
Birds do it, bees do it, even educated sawflies do it. Yes, sexual reproduction is popular, everywhere you look, living organisms are well and truly 'at it'. 

However some organisms manage perfectly well without it. 

'Consider the aphid' (as the bible verse doesn't go). They reproduce asexually by the process known as parthonongenesis. As a rule the blackfly and greenfly are all females. These females give birth to more females, then a third generation arrives. And so on.

Even, without a single male in sight the aphid population can expand exponentially. It's been calculated that a single aphid could produce 600 billion descendants in one season (if you take predation out of the equation). Try spraying those with your Aphid-Be-Gone (other made-up insecticides are unavailable).

Aphids reproduce pretty effectively without bothering with sexual reproduction
Unlike aphids most organism reproduce sexually, despite the considerable disadvantages.
  • Half the population are unproductive in terms of producing offspring (i.e. males) 
  • You have to go to a lot of time and effort to find and attract a mate (e.g. birdsong, flowers) 
  • Only half of your genes get passed on to your offspring 
  • Favourable genes can be prevented from being passed to offspring
You could even go as far as to say sex shouldn't exist; natural selection should favour asexual reproduction.

An interesting counterfactual might be to consider how different the patch might be if there were no sexual reproduction - how would the world be with just aphid-style, asexual reproduction?

The world would be much less colourful ...and less musical

Flowers probably wouldn't exist - they evolved to attract pollinators (usually insects) and so enable the exchange of genetic material inherent in sexual reproduction.
It's true that some now asexually-reproducing plants have flowers, however the evolution of flowers must have driven by the imperative to attract pollinators.

In a world without flowers it follows that we'd lose the pollinators - the butterflies, moths, bees etc.
In turn this would have a massive knock-on effect on organisms that feed on them - many small birds depend on caterpillars and many bats depend on moths.

The male Mandarin Duck wouldn't have its outlandish plumage! Indeed a lot of the most colorful spectacles we see in nature exist because of the need to attract a mate.
It's also debatable whether birds, or indeed any of the groups of species we see today, would exist in anything like their current form.  
Birds, if they existed at all, probably wouldn't sing as it's considered to have evolved in the context of sexual selection.  If you agreed with someone like Geoffrey Miller, that all human creativity also evolved as a result of sexual selection pressure, you could even speculate that there would be no art either (neither would there be 'Dad Dancing')1

Life on earth would probably be far less rich and complex - there would be less biodiversity

It is believed that sexual reproduction is more effective in driving evolution than asexual reproduction -  there is more genetic diversity for natural selection to work on. Evolution  - the creation of new species  - occurs more quickly therefore, resulting in the fantastic diversity of life  that we see to day.  
Evolution *does* occur in populations that reproduce asexually, an example being the acquisition of drug resistance in bacteria. All this is within the realms of speculation, however it's safe to assume that the world would be very different...and it's also quite likely that we wouldn't be around to appreciate it!

These are some of the 'educated sawflies' mentioned above...and they're definitely 'doing it'!

So to return to the puzzle of sexual reproduction...
  • Organisms need to produce sperm or eggs (or their equivalents)
  • They must find a member of the opposite sex
  • The potential partner's selection criteria must be met
  • Successful mating must take place
Sexual reproduction is complicated and in biological terms, expensive - but these Large Skippers are definitely doing it!
Why bother?  Why is sexual reproduction much more common than asexual reproduction if it's so complex and biologically expensive?

Sexual reproduction must offer a significant advantage or it wouldn't have evolved in the first place. Exactly what that advantage is - remains under debate

However some of the suggested advantages are:

The chance that, at least some, offspring of a parent will survive increases if there is variation amongst those offspring. Suppose, for example, a deadly infection were to occur in the population. there will be a greater chance that some of the population will survive if the variation is increased.

Creation of new forms
Sex combines genes from two individuals, so populations that reproduce sexually can more easily combine advantageous traits (via genes) than can asexual populations.

Increased resistance to parasites
This is sometime called sometimes called the Red Queens Hypothesis. Sexual hosts of parasites need to continually adapt ('run') to resist parasites ('stay in one place') like the Red Queen in Alice in Wonderland.

Removal of harmful mutations
Some mutations are advantagous but the majority are neutral or harmful. Sexual reproduction is thought to be more efficient than asexual reproduction in removing the harmful mutations from the population.

So the fact that sexual reproduction is so popular is proof that that the advantages do indeed outweigh the disadvantages.

Getting it on - there's no getting away from it!

As these Scarlet Rosemary beetle will attest - Getting it on - there's no getting away from it!

  1This viewpoint could be termed 'Hard Darwinism'. There are, of course many other ways of explaining the evolution and indeed the very existence of art
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