I don't understand how this works

I’m studying Biology and in Section 1.2 Reproduction, the textbook says that DNA containing genes is passed down to an organisms offspring, meaning the offspring will be in the same species as the parent. But in Biology section 1.2 Adaption the textbook says the great diversity of species is “a consequence of evolution by natural selection, which operates in the lineage of reproducing organisms” Later, when discussing the great variety of species, the textbook states (1.2, The Diversity of Life) “The source of this diversity is evolution, the process of gradual change during which new species arise from older species.” But if the DNA passed down from the parent to the child means the child is the same species as the parent, how can evolution occur?

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One common definition of a species is a group of organisms where two members can produce viable offspring. So for instance, horses and donkeys can mate to produce a mule, but this isn’t considered a viable offspring since mules can’t mate; horses and donkeys are different species. An elephant and a squirrel can’t mate to produce any offspring for… a few different reasons.

Imagine that there is a species of bird on two sides of a tall mountain range (so tall that the birds don’t fly over it so the two groups are isolated). These two groups of birds are two populations of the same species - if you capture a male from one side and a female from the other and mated them in captivity, they’d produce viable offspring. As time goes on, weather patterns change and the leeward side of the mountain becomes more and more arid. There is less food in this more arid environment which creates an evolutionary pressure for the average size of this population to become smaller.

These birds have multiple genes that control how big they grow. Some birds have genes that make them big and some have genes that make them small. With the evolutionary pressure to become smaller, over a few generations the bigger birds don’t mate as well and the frequency of the smaller size genes increases, so you see that, overall, the birds are smaller. Sometimes there are random mutations in offspring which might make other genes that make the birds even smaller; these genes would then also be seen more frequently in the population after a few generations.

Eventually, the birds on the arid side of the mountain are so small that they can’t mate (and create a viable offspring) with a bird captured from the other side of the mountain (which kept the same environment and didn’t have an evolutionary pressure to get smaller). At this point, those two populations would be considered different species.

Let me know if I can explain any of that better!

Thank you for replying. Wouldn’t the genes from the parent birds on the arid side of the mountain that are passed to the chicks prevent them from growing that small? There would be a decrease in size in the overall population, certainly. But I’m not sure that the smallest birds in the overall population would get any smaller. It would be the average size that changed. I drew a graph to show what I think would happen.


What do you think?

You are right and for a few generations that is exactly what would happen. I’d say that at that point we’ve already witnessed evolution in this population since we’re seeing a change in the characteristics of the population. However, there’s another factor at play: mutation. Every time DNA is copied there is a chance that there will be a mistake and the copy will not be exact. So offspring can end up with a mutated form of a gene that will sometimes show a novel phenotype (in this case a smaller sized bird).

In our example, there might be a mutation in the 2 oz size gene that makes the offspring only 1.5 oz. If that mutation happened around year 100 the average size would begin to go lower after that as the frequency of this 1.5 oz gene increases.

I found this resource that might also help with explaining speciation and the different types of speciation - it does go in depth and has some more examples! https://biology.oer.hawaii.edu/biology171/chapter/formation-of-new-species/

By the way, I like the graph :slight_smile: that was helpful

Ok, I could see how that would work. But how small would the birds have to be before they were unable to mate with the birds on the other side of the mountain? And while birds could change into other birds, I don’t see how, say, a dinosaur could change into a bird. (Dinosaurs are purported to be the evolutionary ancestor of birds, correct?) How would it sprout feathers? Or, if it already had feathers, why would their arms turn into wings? Thank you for the link, it was interesting.