Confusion regarding niches in evolution theory

Question

This is the definition of a niche from Crash Course:

An area of the environment that requires a special set of skills or traits to extract food and reproduce.

Obviously, this version of the definition has been dummed so that it appeals to mass audiences, but after some research, I believe that the general concept remains the same.

Here is what I've learnt about species living in a niche:

• The more the competition, the slower the evolutionary process.
• Lack of competition provides an ideal environment rapid evolution.

But these points seem non-intuitive to me, because:

• Why would a increased competition slow down the evolutionary process? Shouldn't it speed it up, as different species will try to evolve the fastest in order to beat adversaries?
• Why does a lack of competition promote rapid evolution? Won't it slow down the evolution, as there is no reason to evolve?

I know that there is a logical reasoning behind this, and I would very much like to understand it.

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My source is Crash Course, specifically this video. I've found it to be extremely reliable over the past, and highly doubt that they are supplying incorrect information. If anything, it is probably my inability to understand the concept.

Answer 1

I think part of the confusion is that these terms are very loosely defined. Also this discusses evolution in terms of taxanomic radiation, e.g. macroevolution at large spatial and temporal scales.

Problem 1 is that you haven't been clear about what kind of competition, inter or intraspecific competition? These will have opposing effects generally which can counter balance each other [1] [2]. When a species is free from interspecific competition, but under high intraspecific competition, that will promote an adaptive radiation, which concurrent morphological changes. The most famous example of this would be Darwin's finches as studied by the Grants [3]. With a lack of interspecific competition, the finches exhibited rapid morphological divergence. Following the logic of this, you'll see that lack of competition will lead the kind of rapid taxanomic radiation the video is discussing, especially over a longer time scale.

However increased interspecific competition will have two effects. It will reduce the number of available niches for an existing species to expand into [4], this generally will drive specialization of a species, which tends to limit genetic diversity which will lead to less "raw material" for evolution to work with. This specialization though is also technically evolution. However with fewer niches, the new species tend to arise via replacement of existing species in an existing niche, as opposed to an adaptive radiation.

So you can see how actually both conclusions are right depending on the type of evolution and competition being considered.

More interspecific competition -> More specialization (one type of evolution), less adaptive radiation (another type of evolution)

More intraspecific competition -> More adapative radiation, less specialization.

I hope that helps answer your question.