Reproduction & Variation

Look at any species and you’ll find some strikingly different individuals. From species of plants to people, bacteria to beetles, no group is made of entirely identical organisms. The variety does far more than keep things interesting. It’s critical for a species' long-term survival.

Sometimes differences are easy to pick out because two members of a species physically look different. Yet there are hidden differences too. They might make some individuals able to resist a disease, eat a different food, or thrive in a certain place.

Both mutation and recombination generate variation at random, not because of a need or desire to change. In fact, most variations are neutral. They quietly accumulate in a population over many generations. But the more variations exist, the more it’s possible that a few may one day be helpful.

Mutation is a normal part of reproduction. And that is a GOOD THING. It's the source of new alleles in all living things, whether they reproduce sexually or asexually.

An allele is a version of a gene. Alleles come about through mutation—imperfect copying of DNA. Each individual has only two alleles per gene. Yet if you sample a gene across a population, usually you’ll find many more than two alleles.

Take the genes that affect height. There’s no single allele for “tall” or “short.” Instead, most of these genes come in multiple alleles, or “flavors.” And each one has a small effect, nudging a person’s potential height slightly upward or downward. That’s why height, just like lots of other traits, falls across a range.

Mutation happens by chance. But some new alleles may end up being useful one day. Say a butterfly accumulates mutations in a handful of genes that together let it eat a toxic plant. If the butterfly ends up living in the same area as the plant, the mutations could be helpful.

New mutations can come about any time a cell in your body divides—but most of them can never pass to offspring. That’s because only mutations in reproductive cells (eggs or sperm) can be inherited.

Reproductive cells are only a tiny fraction of your total cells. If a mutation happens anywhere else, like your arm or your big toe, you can’t pass it on. Say a mutation happens as your toenail grows, and you make keratin (the protein that builds toenails) with fluorescent properties. While you now have a fun new hypothetical trait, the mutation is still a dead-end for introducing variation into the population. Sadly, your glowing toenail ends with you.

To make a new allele that does add variation to a species, mutation has to happen when reproductive cells are made. Only then can the allele pass to offspring. Since alleles come about at random, some will contribute to trait variation and others won’t. Still, over time, any new alleles can be passed down and disperse through a population.

Sexually reproducing organisms have a second way to increase variation: recombination.

This system works in living things where genes come in pairs. Every reproductive cell (sperm or egg) that a parent makes is unique, since it randomly gets one allele from the pair. That cell then joins another unique cell from a second parent to make an offspring. All this random shuffling makes offspring with unique allele combinations: they’re different from each other and from the parents.

Even though each individual only has two alleles, a population can have many more. The alleles are mixed again and again every time reproduction happens. Over generations, this leads to endless variation.

Living things that reproduce asexually have other ways of shuffling their genes. Usually this happens outside of reproduction.

Bacteria, for example, can share their genes by literally giving them to one another. Like recombination, this process increases genetic variation.

Viruses can share or recombine genes as well. It happens when two viruses infect a host cell at the same time. The viruses need to be related, but they don’t have to be exactly the same type. Genes from the two viruses combine to make “offspring” with new combinations of genes.

Mixing in asexually reproducing organisms happens less often than in organisms that reproduce sexually. But it still makes for a lot of variation. When it comes down to it, life of all types has ways to mix things up.