Is "Evolution is improvement of the species, or survival of the fittest" true?

No. Evolution is not a ladder with humans on the top rung, and it is not a contest where only the strongest get to breed.

It is a change in heritable traits in a population over generations. Natural selection is one way that happens. Mutation, genetic drift, and gene flow also shuffle the frequencies. Fitness, here, means leaving offspring in a particular environment. It does not mean winning a fight.

The version that stuck was a gym-wall slogan: survival of the fittest. Next to it sat the ladder, "lower" animals becoming "higher" ones, with us as the obvious destination.

Both lines grade easily. Both turn a population process into a story about winners.

Darwin's own term was natural selection. Herbert Spencer coined "survival of the fittest" in 1864 and offered it as another way to say the same thing. Darwin put Spencer's phrase into the fifth edition of On the Origin of Species in 1869. He even retitled a chapter "Natural Selection; or the Survival of the Fittest."

He meant organisms best fitted to their circumstances. Everyday English heard gym-class fitness. That is the mix-up.

The slogan also leans too hard on staying alive. Dead organisms do not reproduce, true. Selection still tracks who leaves offspring. A trait can spread because it raises reproductive success even if it does not make life longer or easier.

A common textbook definition is change in allele frequencies in a population. Alleles are versions of a gene. Four processes move those frequencies. Mutation adds new variation. Gene flow is migrants carrying alleles in or out. Genetic drift is chance, and it bites hardest in small populations. Natural selection is some heritable variants leaving more offspring in that environment.

Only selection systematically produces adaptation, the fit between organism and environment. Drift can park a lucky trait. Mutation can be harmful, helpful, or neither. Change is not a synonym for improvement.

UC Berkeley's teaching site puts the slogan correction in one line: survival of the fit enough. Plenty of individuals who are not the single "best" still breed and pass genes on.

If you learned that evolution is a ladder or a strength contest, you learned a poster. Populations change. Selection is one reason. Fitness is about offspring in a given place and time, not a trophy for the species.