Like so many questions in the health and wellness space, the answer has to be: it depends.
If your goal is to get stronger - meaning, increasing muscle stability and strength - and you start with yoga, there are a few things to keep in mind:
What style of yoga are you practicing? Iyengar, for example, typically involves a great deal of strength, as it requires students to hold postures for several breaths. Vinyasa, likewise, can involve a great deal of strength and control, as it flows from one pose to another. Restorative yoga, on the other hand, that focuses on calming the nervous system or passive poses is not going to provide the same muscular strength benefits.
An intentional practice makes a difference. Even if you do show up to a physically demanding asana practice, the way you approach the practice will influence whether you build strength. If you're actively engaging your muscles, taking time to transition in and between postures, and controlling your active range of motion, you're likely to see greater increases in your strength.
If your goals are to visibly build muscle mass, yoga might not be enough.
That said, yoga can really emphasize the eccentric (lengthening) and isometrics (static) phases of muscle contractions - something that traditional strength training doesn't focus on as much unless you're actively working toward practicing a thing called 'time under tension' (which is exactly what it sounds like.)
Yoga postures are also typically pretty functional, too, which makes a physical asana practice a really great way to train for daily movements like walking, taking stairs, hiking, turning, bending down... there's a great deal within the asana practice that mirrors every day life.
That all said - if your goal is to "tone"... I'm both sorry and delighted to tell you that yoga might not be enough. Resistance training, or, adding a load, is a much more realistic way to build muscle size, which combined with moderate or low body fat percentage will result in a "toned" aesthetic (though, in my opinion, the benefits of both yoga and resistance training span farrrrr beyond aesthetics. Feeling strong as f*ck is something that's much harder to measure.)
Plus, resistance training and building balanced strength can prevent injury in the wild and in the yoga studio. So many yoga students find themselves flexible beyond control, which can turn into overly lengthened hamstrings, soreness around the glutes (read: yoga butt), and unsupported joints due to lack of surrounding muscle strength.
So what's the TL;DR version: you should do both ;) But you know I'm biased.
Yoga can improve your strength in many ways, but not all of them. My two cents: bring a little more awareness to how your body moves in your yoga practice, and supplement your asana with 1-2 days of resistance training.
Want to learn more? I built a resistance program specifically designed for yoga students to try out resistance training. You can check out Strength for Yoga here.
Or, drop me a note at email@example.com - I'd love to connect and hear your thoughts on yoga and strength.