... the Scrum Team is organizing its development effort, and are choosing team members, or are assessing how to grow the team skill set.
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The Scrum Team is not able to work autonomously because it does not have all the skills required to complete a complex network of tasks. Depending on skills from people outside the team, the team cannot take ownership for finishing their tasks. It reduces the influence on the time it takes to finish and can tarnish the quality of the end result. The core lean principles of consistency and reduced rework depend on short feedback loops. Most complex development needs people with numerous talents from areas as diverse as human factors, engineering excellence, and quality assurance. It is rare that one finds all these talents in members of a single team, let alone in any given individual. Teams often organize around areas of competence: birds of a feather flock together. This is sometimes called a functional organization. Yet, it is costly to coordinate these functions across team boundaries because efficient communications take place between those who share the context of the current work — and that is usually the team members.
A complex product might require that the team has mastered numerous skills to develop Done functionality. When work calls for an additional individual for each required skill, the team will become too big to be effective. One might be tempted to not extend the skill set in the team and to instead introduce external dependencies. On the other hand, one might choose to give the work to the team so that they can develop and learn the required skill, but learning takes time.
Local learning can become local optimization, where a group of specialists develop practices and processes that optimize their work. Specialization, local practices and processes can all be sources of efficiency in an organization, but can also create problems at the group boundary. To attack these problems, an organization can define contracts about how to work with each other (e.g. service requests). Such contracts might specify the nature of the work that an organization is willing to do along with expected durations for responses to requests. Anyone needing the group’s specialization would have to use these contracts. However, this can slow development of the product as a whole even though it increases the efficiency of the local department. Again there may be a need for additional coordination groups within the organization to manage these boundary contracts, to negotiate exceptions or ensure all parties understand what is required, to make sure the department meets their obligations of these contracts.
New products create a new world for their customers and because you cannot know in advance what this new world will be like you must focus on learning and experimentation as you develop your products. The team must find its new lessons in its experience with actual customer use of the product rather than in a plan they are following. And the team must integrate these lessons across the product development process. Everyone recognizes the advantages of local flow, autonomy, and control that come from working as an individual within a step of the process or within a functional area. However, such a work structure moves everyone (except the person doing the last step) further from the end user and the broad insights that come from interactions at that boundary. This may result in sub-optimal local functions though with greater optimization across the entire product development process.
Each Scrum Team should include all talent necessary to deliver Done functionality.
It’s good to pay attention to skill-set coverage when initially creating the team, but it’s more important that the charter team members share enthusiasm for the Vision and they have a track record for learning new things. Because things change over time, it is unlikely that the team will be able to foresee all its long-term skill needs from the beginning.
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Instead of changing team membership as the need for new skills emerges, grow the people internally and strive for Small and Stable Teams. Over time, cross-train team members so each member grows their skill set to accommodate more and more competency areas (see Moderate Truck Number). This will increase the ability of the team to work autonomously. With Cross-Functional Teams it becomes easier to Distribute Work Evenly.
The team members now have all the opportunities to learn secondary skills. They can swarm on PBIs, which increases learning opportunities, and which optimizes flow and helps get functionality to Done fast. The development of secondary skills makes the team more flexible so any member can stand in for another that has become unavailable. The team always makes progress and is autonomous.
Scrum is silent on how to handle a missing competency. Let common sense prevail: for example, ask for help from another team, or subcontract large work increments that surprise the team once in a while. It is understandable if the team needs such help now and then. If the team finds that they frequently depend on external work, then they should view it as an impediment and take measures (such as training, reorganization, or hiring) to remedy the situation.
These teams naturally act like “feature teams” (see Conway’s Law) because most PBIs are feature-shaped: marketable elements of revenue-generating functional product increment. If Cross-Functional Teams develop the product, then handoffs naturally disappear from the Value Stream: the team itself can develop any feature without outside support or intervention. Involving multiple teams introduces delays in feedback loops, increases the waste (muda) of rework, and creates inconsistency (mura) between development stages in the Value Stream.
A study of two corporations published in the Harvard Business Review, one organized functionally and the other by product, suggests that cross-functional teams offer the best features of both organizational structures. 
Set-Based Design is a technique that keeps developers engaged in many disciplines and domains that may be relevant to the business, whether or not they ultimately make it through to the current product. Such practice broadens the expertise base of the team and enterprise and reduces the probability that the team will be surprised by the need to master some new discipline.
As the team integrates new lessons there will be new product ideas. Change will proceed quickly (and must be allowed to proceed quickly). Change will be the norm rather than the exception. This requires small organizations where everyone knows what is happening: organizations that can embrace change, work across specializations, regularly deliver value and are, for want of another term: agile.
Assemble several small teams who will compete in a game to make and fly paper airplanes. Each team member may make only one fold at a time, and then must switch to work on another plane. No plane may have more than15 folds. It must be at least 15 centimeters long and 8 centimeters wide. It must have a blunt tip at least 2 centimeters wide. To qualify as a quality product the plane must fly 3 meters horizontally when the tester tests it. The tester may test each plane only once.
Try the game, and apply Scrum patterns (hint: Swarming) to optimize the number of quality planes produced in a one-minute Sprint.
 Arthur H. Walker and Jay W. Lorsch. “Organizational Choice: Product vs. Function.” In Harvard Business Review 46(6), 1968, pp. 129-138. https://hbr.org/1968/11/organizational-choice-product-vs-function (accessed 8 November 2017).
Picture from: https://pixabay.com/en/hospital-people-surgery-3372996/ (under CC0 license).