There are differences in humans. The left conveniently looks past these differences though by propagating Species Problem Nominalism, whereas the right supports Species Problem Realism.
It's a pretty philosophical microtaxonomy debate, but essentially the left believes that the cutoff between species is subjective.
Take men and women, for example. Men don't have as big of breasts as women, they have a Y chromosome, they have penises, and so on and so on. These are sexually dimorphic traits that distinguish the two genders. The left sees these groupings of traits and thinks, "Wow, here is an outlier person who doesn't fit these definitions of man or woman. They have the body of a guy but have XX chromosomes. This must mean that the groupings of traits that define gender are social constructs (because this person has traits A, B, C, E out of the traits ABCD that define men) and as such this outlier person is whatever they identify as. If they identify as a man, they are. If they identify as a woman (because they have trait E out of the traits EFGH that define women), they are. Who's to say that because this person does not have pronounced breasts that they are not a woman?"
This logically would mean that the cutoff points between breeds of animals such as dogs are purely social constructs, and, because they are social constructs, dog breeds do not objectively exist. Following this line of reasoning, you can even believe that one species of animal is another species of animal (even if they can't currently reproduce), because at many points throughout history certain species have been able to reproduce with each other, only to lose that ability-- and then sometimes even regain it.
This line of reasoning leads to the insane Tumblr-esque people who identify as different species.
The right, on the other hand, says "This is absurd! There are clearly differences between men and women and between dog breeds, just open your damn eyes! A chihuahua is not a Great Dane! They have different traits, which is why one is tiny and one is huge, and they look completely different. Likewise, a dog is not a cat. How can't you see this? To say there are no differences between men and women would be analogous to saying there are no differences between dog breeds."
And we know there are differences between dog breeds, and just about any other type of species that has breeds/subraces.
That, and the left tends to say that genetic variation is extremely small between humans compared to other species, that humans share 99.9% of the same genes so we're all the same. This is not a good argument. Humans share anywhere from 95-99% of the same genetic makeup with chimpanzees, yet that 1-4% of variation makes ALL the difference. To quote a [Scientific American] article(https://www.scientificamerican.com/article/what-does-the-fact-that-w/)
It is worth noting that individual humans generally differ by about 0.1 percent genetically. Thus, chimps differ from humans by about 15-fold more, on the average, than humans do from one another. The 0.1 percent human divergence certainly results in significant variation in physical appearance and traits between different humans. Therefor, perhaps we shouldn't be so surprised that chimps could be 98.5 percent related to humans. Relatively small genetic changes can produce major phenotypic changes.
If you looked at Asian countries and simply observed that they on average tend to be smaller than say, people from the Netherlands, and as a result, Asian people are generally not as suited for playing basketball due to these height differences as people from the Netherlands, the left would try to brand you as a racist. Note that I never said ALL Asian people, but ON AVERAGE-- statistically, factually.
There are clearly differences in physical attributes between humans. The hugely controversial part is when someone posits that it is in fact possible that there are differences (ON AVERAGE) in intelligence between humans, correlating with physical traits, NOT caused by physical traits.