Answer:changes in environmental conditions
Explanation: The kingdom protista includes algae with chlorophyll and are autotrophic in nutrition.They may be Unicellular or multicelllular.some are colonial and others are filamentuous.some protista undergoes both sexual and asexual reproduction eg chlamydomonas and ulothrix.
In asexual reproduction,the cell divides by mitosis;first in the nucleus and chloroplast and later in the cytoplasm.
sexual reproduction occurs when conditions are quite unfavourable.This is essential because recombination of genes occurs during sexual reproduction ,thus leading to genetic variation.the zygote formed may remain in a dormant state until conditions becomes favourable
