This is the big question! Species evolve when three conditions are met.
1. Inheritance. Animals pass characteristics to their offspring.
2. Competition. There are more offspring than can survive.
3. Variation. Offspring have differences, and some are going to be more successful than others.
If all 2 conditions are met, each generation will change slightly, and species will evolve! This was all Darwin’s idea of course…nothing to do with me.
Firstly, our genes can experience chance mutations which alter the blueprints that instruct our bodies how to grow and develop into adulthood
These mutations can then be passed from parents to offspring and enter the gene pool. If these mutations cause an organisms to be better adapted for their
current environment then the organisms will be able to reproduce more successfully before they die. If an adaptation puts an organism at a disadvantage (prom prey or other environmental factors) then that organism may die before
it had the chance to reproduce. This is known as “survival of the fittest” and it’s how natural selection works!
If a species becomes isolated from the rest of its group (by a mountain chain forming, or through habitat loss for example) any mutations that occur and are retained in the isolated population
will not enter the rest of the population through sexual reproduction- this is often how speciation occurs! So new species with different adaptations arsing from mutations can form over time.
The biodiversity we see today, having stemmed from single-celled organisms has arisen through small incremental steps like I’ve described over about 2-3 billion years! It’s not hard to imagine
when you take those sorts of time frames into account!