A species, for most plants and animals, is a group of populations that can exchange genes with one another, producing offspring that can themselves have offspring. A new species forms when one population splits into two that stop exchanging genes. The mechanisms of evolution then change each part on its own.
Key idea
Speciation starts when gene flow stops. Often a barrier divides a population (geographic isolation): a river, a mountain range, a new sea. Without a barrier, populations can split by using different habitats or by timing, such as flowering in different seasons. In plants, a doubled chromosome set (polyploidy) can make a new species in one generation.
Why it works
Once two populations no longer exchange alleles, each one's mutations, drift and selection stay with it. Over many generations the differences pile up until, even if the two meet again, their gametes no longer combine well or their hybrids cannot survive or cannot themselves have offspring. Then they are separate species.
- Geographic: when the Isthmus of Panama rose, it split sea animals into Caribbean and Pacific populations, which have since become different species.
- Habitat: two groups of the same insect may come to live on different host plants in the same area.
- Timing: two plant populations that flower in different months cannot pollinate each other.
- Polyploidy: bread wheat has six sets of chromosomes (2n = 42), the result of hybrids between three ancestral species whose chromosome sets doubled.
Worked example: Polyploidy in numbers
A plant species has 2n = 14. A cell division error doubles every chromosome, making a plant with 28. Can it make normal offspring with the original species?
- The new plant makes gametes with 14 chromosomes; the original makes gametes with 7.
- A combination of 14 and 7 gives 21, an odd number with no partner for many chromosomes, so such offspring usually cannot themselves have offspring.
Answer: No: the 28-chromosome plants are a new species, isolated in one step.
Common mistake
Two misconceptions are common. One is that a new species appears suddenly from one big mutation in an animal. Most speciation is gradual, through many small changes. The other is that speciation always needs a physical barrier; habitat, timing and, in plants, polyploidy can split populations living side by side.