>>13514283>how does the species have sense of its numbers dwindling and needing to evolve rather than staying as it is?It's not so much as it "senses its numbers dwindling" as it is that "the numbers dwindling cause perceivable changes in the environment". Think about stuff like the lack of the protection a big herd may give against predators, changes in the ratio of predators to individuals, finding a mate becomes harder because of the decreased population density, offspring has a harder time making it to adulthood, if the animal is social then by the sole virtue of being alone it will start experiencing stress. All of these factors will cause selection pressure towards whatever phenotype happens to be the most advantageous.
>>13514411>what suddenly decides it should change color?Again, it's not that the organism "decides" to change color. Where does color come from in the first place? Pigments. Where do pigments come from? Specific metabolic pathways that work thanks to proteins. Where do proteins come from? Genes.
Think about how human hair color works, as it is a more straightforward example. There's eumelanin (black/brown) and pheomelanin (red). Each has a certain metabolic pathway for its synthesis (pic related), and ANY mutation affecting ANY protein in said pathway may affect the ratio of pigments and alter the phenotype (hair color) accordingly.
Back to the parasyte in the OP's webm - if you wonder about the bands with different colors, it basically has to do with segmentation of the body and transcription factor gradients causing the differential expression of pigments across said segments
